Some Aspects on Identification, Decay Properties and Nuclear Structure of the Heaviest Nuclei
aa r X i v : . [ nu c l - e x ] F e b Some Aspects on Identification, Decay Properties and Nuclear Structure of theHeaviest Nuclei
Fritz Peter Heßberger , , ∗ GSI - Helmholtzzentrum f¨ur Schwerionenforschung GmbH, Planckstraße 1, 64291 Darmstadt, Germany Helmholtz Institut Mainz, Staudingerweg 18, 55128 Mainz, GermanyVersion: February, 17, 2021
Abstract
Synthesis of new elements at the upper border of the charts of nuclei and investigation of their decayproperties and nuclear structure has been one of the main research topics in low energy nuclear physicssince more than five decades. Main items are the quest for the heaviest nuclei that can exist and theverification of the theoretical predicted spherical proton and neutron shells at Z = 114, 120 or 126 andN = 172 or 184.The scope of the present paper is to illustrate some technical and physical aspects in investigation ofthe heaviest nuclei (’superheavy nuclei’) and to critical discuss some selected results, which from a strictscientific point of view are not completely clear so far, making partly also suggestions for alternativelyinterpretations.A complete review of the whole field of superheavy element research, however, is out of the scope of thispaper.
1. Introduction
First extensions of the nuclear shell model [1, 2] into regions far beyond the heaviest known doublymagic nucleus,
Pb ( Z = 82, N = 126), performed about fifty years ago lead to the prediction of sphericalproton and neutron shells at Z = 114 and N = 184 [3, 4]. Nuclei in the vicinity of the crossing of bothshells were expected to be extremely stabilized against spontaneous fission by fission barriers up to about10 MeV. Particulary for the doubly magic nucleus a fission barrier of 9.6 MeV and hence a partial fissionhalf-life of 10 years [5], in a preceding study even 2 × years [6], were expected. In an allegorical picturethese nuclei were regarded to form an island of stability, separated from the peninsula of known nuclei (theheaviest, safely identified element at that time was lawrencium (Z = 103)) by a sea of instability and soonwere denoted as ’superheavy’ (see e.g [7]). The theoretical predictions initiated tremendous efforts fromexperimental side to produce these superheavy nuclei and to investigate their decay properties as well astheir nuclear and atomic structure and their chemical properties. The major and so far only successfulmethod to synthesize transactinide elements ( Z > > several weeks), c) for fast and efficient separation of products from completefusion reactions from the primary beam and products from nuclear reactions others than complete fusion,d) of detector systems to measure the different decay modes ( α - decay, EC - decay, spontaneous fissionand acompanying γ radiation and conversion electrons), e) of data analysis techniques, and f) for modellingmeasured particle spectra by advanced simulations, e.g. GEANT4 [8]. Despite all efforts it took more thanthirty years until the first serious results on the production of elements Z ≥
114 (flerovium) were reported[9, 10]. However, these first results could not be reproduced independently and are still ambiguous [11].Nevertheless, during the past twenty years synthesis of elements Z = 113 to Z = 118 has been reportedand their discovery was approved by the International Union for Pure and Applied Chemistry (IUPAC)[12, 13, 14]. The decay data reported for the isotopes of elements Z ≥
113 that have been claimed to beidentified indicate the existence of a region of shell stabilized nuclei towards N = 184, but the center has notbeen reached so far. Data on the strength of the possible shells is still scarce.Tremendous efforts have also been undertaken from the theoretical side to make predictions on stability (’shelleffects’), fission barriers, Q α - values, decay modes, half-lives, spin and parity of the ground-state as well asof low lying excited states, etc. . For about thirty years the calculations were performed using macroscopic-microscopic approaches based on the nuclear drop model [15] and the Strutinsky shell correction method ∗ E-mail: [email protected] [16]. Although predicted shell effects disagreed considerably the models agreed in Z = 114 and N = 184 asproton and neutron shell closures (see e.g. [17, 18]). The situation changed by the end of the 1990tieswhen for the first time results using self-consistent models like Skyrme-Hartree-Fock-Bogoliubov (SHFB)calculations or relativistic mean-field models (RMF) were published [19, 20]. Most of the calculations predict Z = 120 as proton shell closure, while others predict Z = 114 (SkI4) or Z = 126 (SkP, SkM*). Skyrme forcebased calculations agree in N = 184 as neutron shell closure, while the RMF calculations favour N = 172.As a common feature all these parametrizations and also the macroscopic - microscopic calculations resultin a wide area of high shell effects. That behavior is different to that at known shell closures, e.g. Z = 50, N = 50, 82, where the region of high shell effects (or high 2p - and 2n - separation energies) is strongly localized.It is thus not evident if the concept of nuclear shells as known from the lighter nuclei is still reasonable inthe region of superheavy nuclei. It might be wiser to speak of regions of high shell stabilization instead.On the other hand, it has been already discussed extensively by Bender et al.[21] that the proton number Z and the neutron number N , where the shell closure occurs strongly depend on details in the descriptionof the underlying forces, specifically on the values for the effective masses m ∗ and the strength of the spin -orbit interaction. It also has been emphasized in [21] that the energy gap between the spin - orbit partners2f / and 2f / determines whether the proton shell occurs at Z = 114 or Z = 120. Under these circumstancespredictions of shell closures at different proton ( Z ) and/or neutron ( N ) numbers by different models maybe regarded rather as a feature of ’fine tuning’ of the models than as a principle disagreement. Having thisin mind superheavy elements represent an ideal laboratory for investigation of the nuclear (’strong’) force.More detailed knowledge of properties and structure of superheavy heavy nuclei is thus undoubtedly decisivefor deeper understanding of basic interactions. Therefore investigations of decay properties and structure ofsuperheavy nuclei will become in future even more important than synthesis of new elements.One has, however, to keep in mind, that the theoretically predicted high density of nuclear levels in a narrowenergy interval above the ground-state may lead to complex α -decay patterns, while on the other hand oftenonly little numbers of decay events are observed. Therefore it is tempting to take the average of the measureddecay data, which finally results to assign the measured data as due to one transition in case of the α -decayenergies, or due to originating from one nuclear level in the case of life-times. Thus fine structure in the α decay or the existence of isomeric levels might be overseen. Rather, a critical analysis and assessment of themeasured data is required.As already indicated above the expression ’superheavy nuclei’ or ’superheavy elements’ had been originallysuggested for the nuclei in the vicinity of the crossing of the spherical proton and neutron shells at Z = 114and N = 184. The establishment of deformed proton and neutron shells at Z = 108 and N = 162 [22, 23, 24, 25]resulted in the existence of a ridge between the ’peninsula’ of known nuclei and the ’island of stability’. Thusit became common to denote all purely shell stabilized nuclei as ’superheavy’, i.e. nuclei with liquid dropfission barriers lower than the zero - point motion energy ( <
2. Experimental Approach
Complete fusion reactions of suited projectile and target nuclei has been so far the only successful methodto produce nuclei with atomic numbers Z >
103 (see e.g. [26, 27]). Under these considerations a separationmethod has been developped to take into account the specific features of this type of nuclear reactions. Dueto momentum conservation the velocity of the fusion product, in the following denoted as ’compund nucleus’(CN) † can be written asv CN = (m p / (m p + m t )) x v p where m p , m t denote the masses of the projectile and target nucleus, and v p the velocity of the projec-tile. This simply means that a) fusion products are emitted in beam direction (with an angular distributionaround zero degree determined by particle emission from the highly excited CN and by scattering in thetarget foil) and b) that CN are slower than the projectiles. It seemed therefore straightforward to use thevelocity difference for separation of the fusion products from the projectiles and products from nuclear reac-tions others than complete fusion. Such a method has the further advantage of being fast, as the separationis performed in-flight without necessity to stop the products. So separation time is determined by the flight-time through the separation device. In the region of transactinide nuclei this separation technique has beenapplied for the first time at the velocity filter SHIP at GSI, Darmstadt (Germany) [28] for investigation of † it is common to denote the primary fusion products which represent in mass and atomic number the sum of projectile andtarget as ’compound nucleus’ (CN) which is highly excited.The final product after deexcitation by prompt emission of nucleonsand/or α particles is denoted as ’evaporation residue’ (ER).
165 166 167 168 169 170 171 172 173 174 175 176 177112113114115116117118 a 9.927a 10.19SFa 9.47-10.18a 9.97, 9.81a 10.63 a 9.15SF
Fl-284 Fl-285 Og-294 Ts-294Ts-293 Lv-293Lv-292Lv-291Lv-290 Mc-290Mc-289Mc-288Mc-287 Fl-289Fl-288Fl-287Fl-286 P r o t on nu m be r Neutron number
Cn-277 Cn-281 Cn-282 Cn-283 Cn-284Nh-278 Nh-282 Nh-283 Cn-285Nh-284 Nh-285 Nh-286 ~97 ms 0.82 ms 3.8 s 99 ms 29 s1.4 ms 73 ms 100 ms 0.97 s 4.2 s 7.9 s2.5 ms 0.15s 0.12 s 0.48 s 0.58 s 1.9 s32 ms 171 ms 0.33 s 0.75 s8.3 ms 18 ms 13 ms 57 ms22 ms 51 ms0.69 ms a 11.43, 11.16 a 10.31 SF a 9.54SFa 11.68 a 10.12 a 9.3SF a 10.41 a 10.02 a 9.818a 10.59 a 10.33-10.58 a 11.66 a 11.05a 10.625a 10.55 a 10.31a 1.16-10.54 a 10.74 a 10.533a 10.6-11.20
Fig. 1 : Excerpt from the charts of nuclei for the region Z ≥ Ti + , Pb,
Bi [29, 30] and for the identification ofelement Z = 107 (bohrium) in the reaction Cr +
Bi [31]. Separation times in these cases were in theorder of 2 µ s.As an alternative separation technique gas-filled separators have been developped using the different mag-netic rigidities B ρ of fusion products and projectiles as the basis for separation. Early devices used ininvestigation of tranfermium nuclei were SASSY [32] and SASSY II [33] at LNBL Berkeley, USA and HECK[34] at GSI.Due to their simpler conception and more compact construction, which allows for separation times below 1 µ s gas-filled separators meanwhile have become a wide spread tool for investigation of heaviest nuclei andare operated in many laboratories, e.g. RITU (University of Jyv¨askyl¨a, Finland) [35], BGS (LNBL Berke-ley, USA) [36], DGFRS (JINR, Dubna, Russia) [37], GARIS (RIKEN, Wako, Japan) [38], SHANS (IMP,Lanzhou, China) [39], TASCA (GSI, Darmstadt, Germany) [40].Separation, however, is only one side of the medal. The fusion products have also to be identified safely.Having in mind that the essential decay modes of superheavy nuclei are α decay and spontaneous fission(SF) detection methods suited for these items have been developped. After it was shown that suppressionof the projectile beam by SHIP was high enough to use silicon detectors [41] an advanced detection set-upfor investigation of heaviest nuclei was built [42]. It consisted of an array of seven position-sensitive silicondetectors (’stop detector’), suited for α spectroscopy and registration of heavy particles (fission products,evaporation residues (ER), scattered projectiles etc.). To obtain a discrimination between particles passingthe velocity filter and being stopped in the detector (ER, projectiles, products from few nucleon transfer)and radioactive decays ( α decay, SF) and to obtain a separation of ER and scattered projectiles or transferproducts a time-of-flight detector was placed in front of the stop detector [43]. Also the possibility to measure γ rays emitted in coincidence with α particles was considered by placing a Ge- detector behind the stopdetector. This kind of detection system has been improved in the course of the years at SHIP [44, 45] andwas also adopted in modified versions and improved by other research groups in other laboratories; examplesare GREAT [46], GABRIELA [47] and TASISpec [48].The improvements essentially comprise the following items:a) the detector set-ups were upgraded by adding a box-shaped Si - dector arrangement, placed upstream Bh Db Po Po Ra Fr c oun t s E a / keV Fig. 2 : Spectra of particles registered in an irradiation of
Bi with Cr at SHIP [54]; black line: particlesregistered in anticoincidence to the time-of-flight detectors, red line: particles registered during the beam-offperiod.and facing the ’stop detector’, allowing with high efficiency registration of α particles and fission productsescaping the ’stop detector’. This was required as the ranges of α particles and fission fragments in siliconare larger than the range of ER; so about half of the α particles and 30 - 50 % of the fission fragments willleave the ’stop detector’ releasing only part of their kinetic energy in it.b) the ’old’ Si detectors where positions were determined by charge division were replaced by pixelized de-tectors allowing for a higher position resolution and thus longer correlation times.c) effort was made to reduce the detector noise to have access to low energy particles (E <
500 keV) likeconversion electrons (CE).d) digital electronics was introduced to enable dead time free data aquisition and to have access to short-livedacitivities with half-lives lower than some microseconds.e) detecor geometry and mechanical components were optimized to use several Ge detectors and to minimizescattering and absorption of γ rays in the detector frames to increase the efficiency for γ ray detection.As it is not the scope of this work to present experimental techniques and set-ups in detail we refer for thisitem to a recent review paper [49].Another technical aspect concerns the targets. As production cross-sections for heaviest elements are small,highest available beam currents have to been used. Consequently a technology was desired to avoid destruc-tion of the targets, which led to the development of rotating target wheels [50]; performance of the wheelsand target quality were continuously improved.As an alternative method to produce superheavy nuclei (SHN) recently the idea of using multinucleon trans-fer reactions was resumed, see e.g. [51, 52]. Indeed, intensive studies of those reactions with respect to SHNproduction, e.g. U +
U or
U +
Cm, had been performed at the UNILAC at GSI already aboutforty years ago. A summary of these efforts is given in [53]. Heaviest nuclides that could be identified inthese experiments were isotopes of mendelevium ( Z = 101). A drawback of these studies, however, was theuse of radiochemical methods which restricted isotope identification to those with ’long’ halflives, T / >> ≤ µ s - range,but also a broad angular distribution of the reaction products. A more detailed discussion of this feature,however, is beyond the scope of this review. c oun t s / ke V c oun t s / ke V a) a spectrum between the beam bursts Db b) a decays follwing Db within dt < 250 s c) a decays follwing
Db within dt < 250 s in the same detector strip c oun t s / ke V b) a decays follwing Db within dt < 250 s within a position difference of – 0.3 mm c oun t s / ke V E a / keV Fig. 3 : α - α correlation analysis of the radioactive decay chain starting from Db. a) Spectrum of α events observed in an irradiation of Bi with Ti at SHIP [60] between the beam bursts; the region below7.6 MeV has been downscaled by a factor of five for better presentation; b) spectrum of the first α decaysfollowing the α decay of Db within 250 s; c) same as b), but requiring that both α decays occur in thesame detector strip; d) spectrum of correlated (daughter) α events, requiring a maximum position differenceof ± <
250 s.
3. Data Selection
Within the commonly used techniques particles passing the in-flight separator are implanted into asilicon-detector set-up. As separation of the ER from ’unwanted’ particles (scattered projectiles, scatteredtarget nuclei, products from few nucleon tansfer etc.) is not clean, there will be always a cocktail of particlesregistered, forming a background covering the energy range of the α decays of the nuclei to be investigated,and usually also the energy range of the spontaneous fission products. In cases of small production crosssections, typically < µ b, α decays of the ER are usually not visible in the particle spectra. Further cleaningprocedures are required. An often applied procedure is the use of transmission detctors in front of the’stop - detector’ and requiring an anticoincidence between events registered in the stop detector and thetransmission detector. In practise the efficiency of the latter is never excatly 100 %, therefore there will stillbe a residual background in the spectra. In cases of a pulsed beam, one can restrict to the time intervalsbetween the pulses.An example is shown in fig. 2. where the α spectrum taken in an irradiation of Bi with Cr (271 MeV)at SHIP is presented [54]. The black line represents the particle spectrum taken in anti-coincidence with thetime-of-flight (TOF) detectors; clearly products ( m, m Po, , m Fr,
Ra) stemming from few nucleontranfer reactions are visible, the ER,
Bh and its daughter product , m Db, however, are buried underthe background. In cases of pulsed beams a further purification is achieved by requiring a ’beam - off’condition. Thus the α decays of Bh, , m Db become visible (red line in fig. 2). Such a restriction,however, is not desirable in many cases as it restricts identification to nuclei having liftimes in the order ofthe pulse-lengths or longer.A possible way out is the use of genetic correlations between registered events; these may be correlations ofthe type ER - α , α - α , ER - SF, α - SF etc.
4. Data Treatment4.1 Genetic Correlations
To establish genetic relationships between mother and daughter α decays is presently a standard methodto identify unknown isotopes or to assign individual decay energies to a certain nucleus.Originally it was developped at applying the He - jet technique for stopping the reaction products and trans-port them to the detection system. As the reaction products were deposited on the surface of the detector,depending on the direction of emission of the α particle the latter could be either registered in the detector,but the residual nucleus was kicked off the detector by the recoil of the emitted α particle or the residualnucleus was shallowly implanted into the detector, while the α particle was emitted in opposite direction anddid not hit the detector. To establish correlations sophisticated detector arrangements were required (see i.e. [55]). The technique of stopping the reaction products in silicon surface barrier detectors after in-flightseparation from the projectile beam simplified the procedures considerably [41, 56]. Due to implantationinto the detector by ≈ (5 - 10) µ m the residual nuclei was not kicked out of the detector by the recoil of theemitted α particle and therefore decays of the implanted nucleus and all daughter products occured in thesame detector; so it was sufficient to establish chronological relationship between α events measured withinthe same detector [56]. The applicability of this method was limited by the decay rate in the detector, asthe time sequence of decays became accidential if the search time for correlations exceeded the average timedistance between two decays. The application of this technique was improved by using position sensitivesilicon detectors [42, 57]. These detectors deliver the position of implantation as an additional parameter.The position resolution is typically around 300 µ m (FWHM), while the range of α particles of (5 - 10) MeVis (40 - 80) µ m [58] and the dislocation of the residual nucleus due to the α recoil is < µ m. Thus all subse-quent decays of a nucleus will occur at the same position (within the detector resolution). The probabilityto observe random correlations is reduced significantly by this procedure.In these set-ups position signals were produced by charge division between an upper (top) and a lower (bot-tom) detector electrode (see e.g. [59] for an advanced version of such a detector set-up). In modern set-ups(see e.g. [48]) these position sensitve detectors have been replaced by pixeled detectors having vertical strips(a typical width is 1 mm) on one side and horizontal strips of the same width on the other side. The positionis then given by the coordinates representing the numbers of the horizontal and vertical strips. One advan-tage of the pixeled detectors is a somewhat better position resolution; taking strip widths of each 1 mm, oneobtains a pixel size of 1 mm ; for the SHIP - type detector [59] (5 mm wide strips, position resolution 0.3mm (FWHM)) taking in the analysis three times the FWHM one obtains an effective pixel size of 4.5 mm (3 x 0.3 x 5 mm ). More stringent, however, is the fact that the position resolution for a pixeled detectoris given solely by the strip numbers and is thus independent of the energy deposit of the particle and of therange of the particle (as long as it does not exceed the strip width).In position sensitive detectors low energy particles ( α particles escaping the detector, conversion electrons)deliver small signals often influenced by the detector noise and nonlinearities of the used amplifiers andADCs, which significantly lowers the position resolution. In many cases signals are missing at all, as theyare lower than the detection threshold. Another drawback is that at electron energies of around 300 keV therange in silicon becomes ≈ µ m and thus reaches the detector resolution, which then requires to enhancethe position window for correlation search.But also one drawback of the pixeled detectors should be at least mentioned: due to the small widths ofthe strips (typically 1 mm) already for a notable fraction of the implanted particles the energy signal is splitbetween two strips making sophisticated data analysis algorithms necessary to reconstruct the energy of theparticles. Also, the energy split between two strips also introduces some ambiguities in the determination ofthe position.An illustrative example for the benefit of including the position into the correlation search is given in fig.3. Here the α spectrum obtained in an irradiation of Bi with Ti at SHIP [60] (using the same set-upas in [59]) between the beam bursts is shown in fig. 3a. Besides
Db (9.0 – 9.4 MeV), produced in thereaction
Bi( Ti,n)
Db, and its decay products
Lr (8.35 – 8.50 MeV) and
No (8.1 MeV, EC decaydaughter of
Lr) also α lines from g, m At (7.68,7.90 MeV and
Po (7.45 MeV) are present; theseactivities were produced by few nucleon transfer reactions; in addition also the α line of Po (6.78 MeV)is visible, stemming from α decay of Ra (T / = 11.43 d), produced in a preceeding experiment. In fig.3b the spectrum of the first α particles following an α decay of Db (energy region is marked by the redlines in fig. 3a is shown. Besides the daughter products
Lr and
No, strong random correlations with Ds Hs Sg Db Og Fl Rg Bh No Fm Cf Po Th Th Po Po E r e c / k e V E a / MeV Fig. 4 : Recoil energies transferred to the residual nuclei by α - decay of heavy nuclei. The lines are to guidethe eye. Po,
Po,
Db, g, m At are observed; the random correlation can be significantly suppressed if inaddition the occurence of both α decays in the same detector strip is required, as seen in fig. 3c; the result ofthe position correlation analysis finally is shown in fig. 3d. Here, in addition the occurence of both α eventswithin a position difference of ± α decays is completely gone, and alsodetails in the energy distribution of the α events are visible; the α events at (7.7 - 7.9) MeV stem from decayof Md ( α - decay daughter of Lr) and those at 7.45 MeV are here from
Fm ( α -decay daughter of No, EC - decay daughter of
Md). The events at (8.7 - 8.8) MeV are from decay of g, m Lr, the α decay daughters of g, m Db, which was produced to a small amount in the reaction
Bi( Ti,2n)
Db. α -particle and Recoil - energies Implantation of ER into a silicon detector has consquences for measuring the energies of α particles. Oneitem concerns summing of the α particle energy and the energy transferred by the α particle to the residualnucleus, which will be in the following denoted as recoil energy E rec .The total decay energy Q (for a ground-state to ground-state transition) is given byQ = (m mother - m daughter - m α ) × c This energy splits in two componentsQ = E α + E rec = (1 + m α /m daughter ) × E α Here m mother , m daughter denote the masses of the mother and daughter nucleus ‡ , E α the kinetic energyof the the α particle. ‡ strictly spoken, the atomic mass, not the mass of a bare nucleus c oun t s / k e V E a / keVc)
50 75 100 125 150 175 200 225 2500510152025 b) c oun t s / ke V E g / keV + + [624]9/2 - [734] 9/2 - [734] No Rf a) A Fig. 5 : Energy summing of α particles and conversion electrons (CE); a) decay scheme of Rf [63]; b)spectrum of γ rays emitted in coincidence with α decays of Rf; c) energy distribution of α particles incoincidence with the E = 203.6 keV (full line) or E = 143.3 keV γ -line (dashed line).Evidently the recoil energy E rec = (m α /m daughter ) × E α is stronly dependent on the mass of the daugh-ter nucleus and the kinetic energy of the α - particle.This behavior is shown in fig. 4, where for some isotopes E rec is plotted versus E α . The black squares repre-sent the results for , , Po and , Th, which are often produced in reactions using lead or bismuthtargets by nucleon transfer or in so called ’calibration reactions’ (reactions used to check the performance ofthe experimental set-up), the red dots are results for ’neutron deficient’ isotopes in the range Z = (98-110),the blue triangles, finally, results for neutron rich SHN produced so far in irradiations of actinide targetswith Ca. Evidently the recoil energies for the polonium and thorium isotopes are by 15-30 keV higherthan for the Z = (98-110) - isotopes, while the differences between the latter and the ’neutron rich SHN’ aretypically in the order of 10 keV; specifically striking is the difference of ∆ E rec = 65 keV between Po and
Og, both having nearly the same α - decay energy.In practise, however, the differences are less severe: the measured energy of the α particle is not simplythe sum of both contributions as due to the high ionisation density of the heavy recoil nucleus part of thecreated charge carriers will recombine and thus only a fraction of them will contribute to the hight of thedetector signal, henceE α (measured) = E α + a × E rec with a <
1, giving the fraction of the contribution of the recoil energy, which can be considered to be in theorder of a ≈ A = 150. As ionization density increases for heavier nuclei (larger Z ) the recombination might be larger forSHN, thus a < α particle and conversion electron (CE) energies One more problem is connected with energy summing of α particles and conversion electrons (CE) incases where excited levels are populated decaying towards the ground state by internal conversion, leading Fig. 6 : alpha - decay spectrum of
No (in coincidence with the 279.5 keV γ - transition). The insert showsthe region (7200 - 8200) keV in expanded scale.to a shift of the measured α energies towards higher values [62].An illustrative example is shown in fig. 5 where the decay of Rf is presented. The decay scheme is shownin fig. 5a; α decay populates the 9/2 − [734] - level in No, which then decays by γ emission either into the7/2 + [624] ground-state (E γ = 203.6 keV ) or into the 9/2 + state (E γ = 143.3 keV (fig. 5b) [63]. The M1- transition 9/2 + → + is highly converted. In fig 5c we present the energy distributions of α particleseither in coincidence with the E γ = 203.6 keV (black line) or the E γ = 143.3 keV (red line). We observea shift in the α energies by ∆E = 38 keV, which is even larger than the CE energy (31 keV)[64], indicatingthat not only the CE contribute to the energy shift but also the energy released during deexcitation of theatomic shell (e.g. Auger electrons). α particles escaping the detector As the implantation depth into the detector is typically ≤ µ m and thus considerably smaller than therange of α - particles in silicon ( > µ m at E > E also α particles with energies of E - ∆ E will be registered. So it is a priori not possible to state if these eventsrepresent decays into higher lying daughter levels or if they are just α particles of energy E escaping thedetector with an energy E - ∆ E . However, some arguments can be given on the basis of the probability toobserve the latter events. As an illustrative example the α spectrum of No [65] is given in fig. 6.Here the α decays in coincidence with the 279.5 keV γ line are shown, which represents the transitionof the 9/2 − [734] level in Fm populated by the α decay, into 7/2 + [624] ground-state. In that case oneobtains a clean α spectrum of a single transition not disturbed by energy summing with CE.Besides the ’peak’ at E α = 8005 keV a bulk of events at E < α particlesare registered in the peak, about 32% are found at E < E =(7.2-8.2) MeV is expanded. It is clearly seen0that the number of α particles in the range, taken here somewhat arbitrarily as E mean - 570 keV is small.The ’peak’ is here defined as the energy region E > α particles from the ’bulk’ leaving the detector with nearly fullenergy loss. They rather stem from decays into excited daughter levels (but possibly influenced by energysumming with CE) § . α energy measurements in the region of SHN As the numbers of decays observed in specific experiments is usually quite small it seems of high interest tomerge data of different experiments to enhance statistics to possibly extract details on the decay properties, e.g. fine structure in the α decay. One drawback concerning this item is possible energy summing between α particles and CE as discussed above; another problem is the compatibility of the decay energies measured inthe different experiments and thus a consequence of calibration. This is not necessecarily a trivial problemas shown in fig. 7, where the α energies obtained for Bh in three experiments are shown.
Bh wasproduced in irradiations of
Am with Ca within the decay chain of
Mc, via
Am( Ca,3n) Mc α → Nh α → Rg α → Mt α → Bh α → Db. This decay chain has been investigated so far at three differentseparators, DGFRS at FLNR, Dubna, Russia [66], TASCA at GSI, Darmstadt, Germany [67], and BGSat LNBL, Berkeley, USA [68]. The energy distributions of the odd-odd nuclei occuring in the decay chainof
Mc are in general quite broad indicating decays into different daughter levels accompanied by energysumming of α particles and CE. Solely for Bh a ’quite narrow’ line is observed. The results of the differentexperiments are compared in fig. 7. To avoid ambiguities due to worse energy resolution of ’stop + box’ eventswe restricted to events with full energy release in the ’stop’ detector. Evidently there are large discrepanciesin the α energies: the DGFRS experiment [66] delivers a mean value E α (DGFRS) = 9.022 ± E α (TASCA) = 9.063 ± E α (BGS) = 9.098 ± E α (TASCA) - E α (DGFRS) = 41 keV, E α (BGS) - E α (TASC) = 35 keV, E α (BGS) - E α (DGFRS) = 76 keV, which are by far larger than calibration uncertainties in the range of 10-20 keV, whichmight be expected usually. That is a very unsatisfying situation. § We briefly want to point to an detector effect that may pretend lower energies. In cases where the detector has alreadysuffered from radiation damages the charge collection may be incomplete and so the signal might be lower than that for a ’fullenergy event’ even if the α particles was completely stopped in the detector. E a = 9.022 – 0.012 MeVE a = 9.098 – 0.022 MeVa) DGRS - data [66]c) BGS - data [68] E a = 9.063 – 0.014 MeVb) TASCA - data [67] eve n t s / ke V eve n t s / ke V eve n t s / ke V E a / MeV Fig. 7 : Energy distributions of
Bh; a) DGFRS experiment [66]; b) TASCA experiment [67]; c) BGSexperiment [68]
5. Discovery of elements Z = 107 (bohrium) to Z = 112 (copernicium) and their approvementby the IUPAC
The elements Z = 107 to Z = 112 where first synthesized at the velocity filter SHIP, GSI, in the period 1981- 1996. The corresponding isotopes where identified after implantation into arrangements of silicon detectorsby registering their α decay chains. Identification was based on decay properties ( α energies, halflives) of atleast one member of the decay chain, that had been either known from literature or had been synthesizedand investigated at SHIP in preceeding experiments. The latter is the main difference to elements Z ≥ Z = 107 - Z = 112 depict in some cases already the difficulties to unambiguously identify anisotope on the basis of only a few observed decays and also the problems evaluaters in charge to approvediscovery of a new element are faced with.In order not to overtop the banks in the following only the reports of the Tranfermium Working Groupof the IUPAC and IUPAP (TWG) (for elements bohrium to meitnerium) or the IUPAC/IUPAP JointWorking Party (JWP) (for elements darmstadtium to copernicium) concerning the GSI new element claimsare considered. Other claims on discovery of one ore more of these elements are not discussed. The first isotope of element 107,
Bh, was synthesized in 1981 in the reaction
Bi( Cr,n)
Bh [31].Altogether six decay chains where observed at that time. Prior to approval of the discovery by the IUPAC twomore experiments were performed. The complete results are reported in [69]: two states of
Bh decayingby α emission g Bh ( T / = 102 ±
26 ms (15 decays)) and m Bh ( T / = 8.0 ± Bh (10 decays) were observed. Thus approval of the discovery of element 107was based on a ’safe ground’, and it was stated by the TWG [70]: ’This work ( [31] ) is considered sufficientlyconvincing and was confirmed in 1989 [69] .’ Compared to bohrium the data for hassium on which the discovery was approved was scarce. In the firstexperiment performed in 1984 [71] three decay chains of
Hs were observed in an irradiation of
Pb with Fe. In two cases a full energy event of
Hs was followed by an escape event of
Sg, while in one case an2 missing a10.34 MeV a8.10 a5.8 MeVescape a 9.01 MeV a1.4 MeVescape a11.10 MeV a1.10 MeV escape a10.21 MeV Mt Mt Mt Bh Bh Bh Db Db Db No Lr Rf EC ECSF
Chain 1 (1982) Chain 2 (1988) Chain 3 (1988)
Fig. 8 : alpha - decay chains observed in SHIP experiments in 1982 and 1988 and attributed to the decay of
Mt [73, 75].escape event of
Hs was followed by a full energy event of
Sg. The α particle from the granddaughter Rf was measured in all three cases with full energy.In follow-up experiment only one decay chain of the neighbouring isotope
Hs was observed in an irradiationof
Pb by Fe [72]. The chain consisted of two escape events followed by an SF, which was attributed to
Rf on the basis of the decay time. Nevertheless discovery of element 108 was approved on the basis ofthese data and it was stated by the TWG [70]: ’The Darmstadt work in itself is sufficiently convincing tobe accepted as a discovery.’
Discovery of element 109 was connected to more severe problems. In the first experiment at SHIP,performed in summer 1982, only one decay chain shown in fig. 8. was observed [73] in an irradiation of
Biwith Fe.It started with an alpha - event with full energy, followed by an escape event and was terminated by anSF event. The latter was attributed to
Rf produced by EC decay of
Db. A thorough investigationof the data showed that the probability for the event sequence to be random was < − [74]. Among allpossible ’starting points’ (energetically possible evaporation residues) Mt was the most likely one [74]. Ina second experiment performed early in 1988 (january 31st to february 13th) two more decay chains, alsoshown in fig. 8 where observed [75]; chain number 2 consisted of four α events, two with full energy, andtwo escape events, attributed to Mt (first chain member) and
Db (third chain member); the two fullenergy events where attributed to
Bh (second chain member) and
No (forth chain member), whichwas interpreted to be formed by EC decay of
Lr. The third chain consisted of two α decays, which wereassigned to Bh and
Db on the basis of the measured energies, while the α particle from the decay of Mt was not observed. The non-registration of
Mt could have different reasons:a)
Bh was produced directly via the reaction
Bi( Fe, α n) Bh. This possibility was excluded as thisreaction channel was assumed to be considerably smaller than the 1n - deexcitation channel. And indeed ina later experiment, performed after the approval of element 109 by the IUPAC, twelve more decay chains of
Mt were observed, but no signature for an α n - deexcitation channel was found [76].b) Mt has a short-lived isomer decaying in-flight during separation. This interpretation seemed unlikely asin case of α emission the recoil of the α particle would have kicked the residual nucleus out of its trajectory,3so it would not have reached the detector placed in the focal plane of SHIP; similary in case of decay byinternal transitions one could expect that emission of Auger electrons following internal conversion wouldhave changed the charge state of the atom, so it would been also kicked out of its trajectory.c) a short-lived isomer may decay within 20 µ s after implantation of the ER, i.e. during the dead time ofthe data acquisition system, and thus not be recorded. Also for this interpretation no arguments were foundin the later experiment [76].d) The α particle from the decay of Mt escaped with an energy loss <
670 keV, which was the lowerdetection limit in this experiment. This was seen as the most reasonable case.To summarize: the three chains presented strong evidence for having produced an isotope of element 109,but it still may be discussed if the presented data really showed an unambiguous proof. However, the TWGdid not share those concerns, leading to the assessment ’The result is convincing even though originally onlyone event was observed’ and came to the conclusion ’The Darmstadt work [73] gives confidence that element109 has been observed’ [70]’.
After a couple of years of technical development, including installation of a new low energy RFQ - IH ac-celeration structure coupled to an ECR ion source of the UNILAC and construction of a new detector set-upexperiments on synthesis of new elements were resumed in 1994. The focal plane detector was surrounded bya ’box’ formed of six silicon detectors allowing to measure with a probability of about 80% the full energy of α - particles escaping the ’stop’ detector as the sum E = ∆ E(stop) + E residual (box) . The first isotope of element110,
Ds, was synthesized 1994 in the reaction
Pb( Ni,n)
Ds [44]; four decay chains were observed.In three of the four chains α decays with full energy were observed down to Rf or
No, respectivly. Forthe forth decay chain of
Ds only an energy loss signal was measured, while α decays of Hs and
Sgwere registered with full energy. Further members of the decay chain (
Rf,
No, etc.) were not recorded.In a later re-analysis of the data this chain could not be reproduced any more, similary to the case of decaychains reported from irradiations of
Pb with Kr at the BGS, Berkeley, and interpreted to start from anisotope of element 118 [77, 78]. This deficiency, however, did not concern the discovery of element 110 andit was stated by the JWP of the IUPAC/IUPAP ’Element 110 has been discovered by this collaboration’ [79].
The first experiment on synthesis of element 111 was performed in continuation of the element 110discovery experiment. In an irradiation of
Bi with Ni three α decay chains were observed. Theywere assigned to Rg, produced in the reaction
Bi( Ni,n)
Rg [80]. Two of these chains ended with α decay of Db; for the first member,
Rg, only a ∆ E signal was registered. In the third chain α decay was observed down to Lr and all α particles from the decay chain members ( Rg,
Mt,
Bh,
Db,
Lr) were registered with ’full’ energy. It should be noted that also
Mt and
Bh had notbeen observed before. The JWP members, however, were quite cautious in that case [79]. It was remarkedthat the α energy of Bh in chain 1 was quite different to the values of chain 2 and 3 and that the α energy E = 9.146 MeV of Db in chain 2 was in fact in-line the literature value (given e.g. in [81]), butwas quite different to the value E = 9.200 MeV in chain 3. Further it was noted, that the time difference∆t( Db -
Lr) = 66.3 s, was considerably longer than the known half-life of
Lr ( T / = 28 ± Pb( Cu,n)
Rg was applied[85].
Concerning discovery of element 112 the situation was even more complicated. In a first irradiation of4
Pb with Zn performed at SHIP early in 1996 two decay chains interpreted to start from
Cn werereported [86]. In chain 1 α decays down to Rf, in chain 2 alpha decays down to
No were observed.Both chains showed severe differences in chain members α (1) and α (2). (In the following α (n) in chains 1and 2 will be denoted as α (n1) and α (n2), respectively.)The α energies for Cn differed by 0.22 MeV, while the ’lifetimes’ τ (time differences between ER im-plantation and α decay were comparable) with E α = 11.65 MeV, τ α = 400 µ s and E α = 11.45 MeV, τ α = 280 µ s. For α (2) ( Ds) the discrepancies were more severe: E α = 9.73 MeV, τ α = 170 ms and E α = 11.08 MeV, τ α = 110 µ s. It seemed thus likely that the α decays of Ds (and thus also of
Cn)occurred from different levels. This was commented in the JWP report [79] as ’Redundancy is arguably andunfortunately confounded by the effects of isomerism. The two observed alphas from
112 involve differentstates and lead to yet two other very different decay branches in
Rf in chain 2 ( E = 8.52 MeV) differedby 0.24 MeV from the literature value [81]. Indeed it was later shown by other research groups [87, 88, 89]that two longlived states decaying by α emission exist in Rf with one state having a decay energy anda halflife of E α = 8.51 ± T / = 2.6 +0 . − . s [89] in-line with the data from chain 2 ( E α = 8.52MeV, τ α = 4.7 s). But this feature was not known when the TWG report [79] was written. Consequentlyit was stated ’The results of this study are of characteristically high quality, but there is insufficient internalredundancy to warrant conviction at this state. Confirmation by further experiments is needed to assignpriority of discovery to this collaboration.’One further experiment was performed at SHIP in spring 2000, where one more decay chain was observed,which resembled chain 2, but was terminated by a fission event [82].The latter was remarkable, as the fissionbranch of Rf was estimated at that time as b sf < b sf = 0.82 ± Pb with Kr at the BGS, Berkeley, and interpreted tostart from an isotope of element 118 [77, 78]. It was shown that this chain had been created spuriously [82].At least this finding could explain the inconsistencies concerning the data for
Cn and
Ds in chains 1and 2. On this basis the JWP concluded [83]: ’In summary, though there are only two chains, and neitheris completely characterized on its own merit. Supportive, independent results on intermediates remain lessthen completely compelling at that stage.’In the following years two more experiments at SHIP using the reaction Zn +
Pb were performed with-out observing one more chain [90], however, decay studies of
Hs and
Rf confirmed specifically the datafor
Rf [87], while the decay chains of
Cn were reproduced in an irradiation of
Pb with Zn at theGARIS separator at RIKEN, Wako (Japan) [91, 92].On this basis the JWP concluded in their report from 2009 [93]: ’The 1996 collaboration of Hofmann etal. [86] combined with the 2002 collaboration of Hofmann et al. [82] are accepted as the first evidencefor synthesis of element with atomic number 112 being supported by subsequent measurements of Morita[91, 92] and by assignment of decay properties of likely hassium imtermediates [87, 94, 95] in the decay chainof
6. Some Critical assessments of decay chains starting from elements Z ≥
112 and discussion ofdecay data of the chain members
The experiments on synthesis of the new elements with Z = 113 to Z = 118 reflect the extreme difficultiesconnected with identification of new elements on the basis of observing their decay when only very few nucleiare produced and decay chains end in a region where no isotopes had been identified so far or their decayproperties are only known scarcely. Nevertheless discovery of elements Z = 113 to Z = 118 has been approvedby IUPAC and discovery priority was settled [12, 13, 14], and also names have been proposed and acceptedso far: Z = 113: Nihonium (Nh) Z = 114: Flerovium (Fl) Z = 115: Moscovium (Mc) Z = 116: Livermorium (Lv) Z = 117: Tennessine (Ts) Z = 118: Oganesson (Og)Still there remain a couple of open questions and ambiguities concerning decay properties of several isotopes,which may have feedback to their final assignment. In the following we will discuss some selected cases and5 S.Hofmann et al.EPJ A 52:180 (2016) [97]S.Hofmann et al.EPJ A 48:62 (2012) [59]S.Hofmann et al.GSI Sci. Report 2010[96] Oganessian , UtyonkovNPA 944, 62 (2015) [27]Oganessian, UtyonkovNPA 944, 62 (2015) [27] SF1.3 +2.3/-0.5 h8.54 – 0.08 MeV1.6 +1.5 /-0.5 mina (58 – 23 %)9.31 – 0.02 MeV0.20 +0.18 /-0.06 s9.71 – 0.02 MeV0.21 – 0.04 sSF (89 +4/-6 %)10.03 – 0.02 MeV0.48 +0.14 /-0.09 s9.53 – 0.02 MeV9.33 – 0.06 MeV8.94 – 0.07 MeV4.2 +1.1 /-0.7 s10.74 – 0.07 MeV(10.50 – 0.02 MeV )19 +17 /-6 ms10.502 – 0.015 MeV29 ms
SFSF
210 – 20 MeV49.417 s 210 – 20 MeV49.417 s9.315 – 0.015 MeV356 ms9.315 – 0.015 MeV356 ms 0.244 MeV1.718 s9.707 – 0.015 MeV(Puls)4.048s9.707 – 0.015 MeV(Puls)5.766 s 10.029 – 0.015 MeV406 ms10.029 – 0.015 MeV406 ms10.502 – 0.015 MeV29 ms
SF aaaaaaaa aSF aaa aa aaaa aSFaaaaa Ds Hs Lv Fl Cn Ds Hs Lv Fl Cn Ds Lv Fl Cn Lv Fl Cn Ds Hs Sg Lv Fl Cn Ds Hs Sg Rf a) b) c) d) e) Fig. 9 : Assignments and reassignments of a decay chain observed in an irradiation of
Cm with Ca; a)decay data reported for
Lv and its daughter products [27]; b) decay chain as assigned in [96]; c)decaychain as assigned [59]; d) decay chain as assigned in [97]; e) decay data reported for
Lv and its daughterproducts [27].point to open problems that need to be clarified in further experiments.For illustrating the following discussion an excerpt of the charts of nuclei covering the region Z ≥
112 and N = (165 - 178) is shown in fig. 1. Fl - Fl As already briefly mentioned in sect. 3, the continuous implantation of nuclei, the overlap of low energyparticles passing the separator with the α - decay energies of the expected particles and efficiencies lowerthan 100 % of detectors used for anti-coincidence to discriminate between ’implantation of nuclei’ and ’decaysin the detector’ introduces a problem of background. It might be severe, if only very few decay chains areobserved, since at a larger number of events single chains containing a member that does not fit to the restof the data can be easily removed.An example to illustrate those related difficulties is given in fig. 9. The decay chain was observed atSHIP, GSI, in an irradiation of Cm with Ca at a bombarding energy E lab = 265.4 MeV [59]. A firstanalysis of the data resulted in an implantation of an ER, followed by three α decays. The chain was ter-minated by a spontaneous fission event [96] as shown in fig. 9b. It was tentatively assigned to the decay of Lv. After some further analysis, one more α decay (an event that occured during the beam-on period)placed at the position of Cn was included into the chain, but still assigned tentatively to the decay of
Lv [59] as shown in fig. 9c. However, except for
Lv the agreement of the decay properties ( α energies,lifetimes) of the chain members with literature data [27], shown in fig. 9a, was rather bad. Therefore ina more recent publication [97] the assigment was revised and, including a low energy signal of 0.244 MeVregistered during the beam-off period, but without position signal, into the chain at the place of Cn. Thechain is now assigned to the decay of
Lv (fig. 9d). A comparison with the literature data [27], presentedin fig. 9e, shows a good agreement in α decay energies for Fl,
Ds,
Hs and in ’lifetimes’ (i.e. timedifferences between consecutive decay events) for
Lv,
Fl,
Ds,
Hs,
Sg. Stringent differences,6
Oganessian et al. PRC 63, 11301(R) (2000) [102]Oganessian et al. PRC 62, 041604(R) (2000) [101] c) Ca +
Cm E = 240 MeVb) Ca +
Pu E = 236 MeVa) Ca +
Pu E = 236 MeV
Oganessian et al. PRL 83, 3154 (1999) [10] SF aa aaaa aSFaaa Fl Lv Fl Cn Ds Cn DsSF SF Fl Cn Ds Fl Cn Ds Hs Fig. 10 : Decay chains observed at the DGFRS in irradiations of
Pu or
Cm with Ca and assigned todecays starting from
Fl ((a) [10]),
Fl ((b) [101]), and
Lv ((c) [102]).however, are obtained for α energy of Lv and the lifetime of
Ds (the event observed in the beam-onperiod). The differences in the α decay energies of 240 keV principally can be explained by decay in differentdaughter levels. As in [27] only three decays are reported, it might be that the decay of the lower energysimply was not observed in the experiments from which the data in [27] were obtained. Such an explanationis principally reasonable. For E α = 10.74 MeV one obtains a theoretical α decay halflife of T α = 32 ms usingthe formula suggested by Poenaru [98] using the parameter modification suggested in [99] which has beenproven to reproduce α decay halflives in the region of heaviest nuclei very well [100]. The value is indeed ingood agreement with the reported half-life of T α = 19 +17 − ms [27]. For E α = 10.50 MeV one obtains T α = 139ms. This means, that one expects some 25% intensity for an α transition with an energy lower by about 250keV, provided that α decay hindrance factors are comparable for both transitions. More severe, however,seems the lifetime of Ds, which is a factor of twenty longer than the reported half-life of T α = 0.21 ± ≈ − . To conclude: it is certainly alluringto assign this chain to Lv, the assignment, however, is not unambiguous. As long as it not confirmed byfurther data, it should be taken with caution. , Fl The observation of a decay chain, registered in an irradiation of
Pu with Ca at E lab = 236 MeV atthe Dubna Gasfilled Separator (DGFRS), assigned to start from Fl was reported by Oganessian et al.[10]. The data presented in [10] are shown in fig. 10a. In a follow-up experiment two more decay chains withdifferent decay chracteristics were observed and attributed to the neighbouring isotope
Fl [101], while inan irradiation of
Cm with Ca at E lab = 240 MeV one decay chain, shown in fig. 10c was registered [102].Decay properties of members 2, 3 and 4 of the latter chain were consistent with those for Fl,
Ds, and
Hs. Consequently the chain (fig. 10c) was interpreted to start from
Lv.However, results from later irradiations of
Pu with Ca were interpreted in a different way [103].The activity previously attributed to
Fl was now assigned to
Fl, while no further events having thecharacteristics of the chain originally attributed to
Fl [10] were observed. It was now considered as a7 decay chains
Fl ’new’ assignment decay chains
Fl ’old’ assignment aa SF6.6 +9.0-2.5 s9.17–0.06 MeV19 +26-7 s9.83–0.04 MeV1.8 +2.5-0.7 s 9.93–0.03 MeV0.66 +0.04-0.10 sSF98 +20-14 ms SF a aa +9-6 sSF3 +15-1 ms8.73–0.03 MeV12.7 +4.0-2.5 sSF 0.89 +0.04-0.06 +0.7-0.4 s SFSF
SF16.5 min8.83 MeV1.6 min8.67 MeV15.4 min9.71 MeV30.4 s aaSFaaa Hs Fl Cn Ds Hs Fl Cn Ds Fl Cn Fl Cn Ds Fig. 11 : ’Old’ and ’new’ decay chains for
Fl and
Fl.possible candidate for
Fl [104]. But this chain was not mentioned later as a decay chain stemming froma flerovium isotope [105]. However, a new activity, consisting of an α decay of E α = 9.95 ± T / = 0.63 +0 . − . s followed by a fission activity of T / = 98 +41 − ms was observed. It was assigned to the decaysequence Fl α → Ds SF → . These ’new’ results were consistent with those obtained in later irradiations of Pu with Ca [106] and
Cm with Ca [59, 107] in other labs.As a summary the ’old’ and new results for , Fl are compared in fig. 11. The ’new’ results are takenfrom the recent review [27], the ’old’ results for
Fl are the mean values from the three decays reported in[101, 102] as evaluated by the author.It should be noticed for completeness, that Kaji et al. [107] observed also a chain consisting consisting ofthree α particles terminated by a fission event. The chain was not regarded as unambiguous and so α and the SF event were only tentatively assigned to Cn ( α ) and Ds (SF). In a more recent decaystudy of
Fl using the production reaction
Pu( Ca,4n)
Fl a small α - decay branch (b α ≈ Ds were confirmed [108].
Fl - Cn A couple of weeks after submission of [10] (recieved march 9,1999) another paper was submitted by Yu.Ts.Oganessian et al. reporting on synthesis of a flerovium isotope with mass number A = 287 [9] (received april19, 1999). The experiment had been performed at the energy filter VASSILISSA at FLNR-JINR Dubna, andthe decay chains (shown in fig. 12a) were observed in bombardments of Pu with Ca at E lab = 230 - 235MeV. Two chains consisting of an α - decay (in one case only an ’escape’ α - particle was registered) follwedby spontaneus fission were observed. Although lifetimes of the SF events were longer than those of two SFevents correlated to ER observed in a preceding irradiation of U with Ca at VASSILISSA [109] (fig.12b) they were attributed to the same isotope,
Cn and the α decays were attributed to Fl. In a laterirradiation of
U with Ca at the same set-up two more SF events attributed to
Cn were observed [110](fig. 12c). The production cross section was σ = 3.0 +4 . − . pb in fair agreement with the value σ = 5.0 +6 . − . pbobtained in the first experiment [109].Both acitivities, however, could not be reproduced in irradiations of U, Pu with Ca performed at8 Ds Cn Cn Fl Fl SF
558 s, 228 s (a)182 s, 52 s (b)180 s, 1458 s (c) (1) (2)
Fig. 12 : ’Old’ and ’new’ decay chains for
Fl and
Cn; fig. 12.1 results observed at VASSILISSA: (a)data from Ca +
Pu [9]; (b) [109], (c) [110] from Ca +
U irradiation; fig. 12.2 results observed atDGFRS [105], SHIP [112], GARIS II [113], BGS [114] in irradiations of
U or
Pu with Ca. See textfor details.the Dubna Gas-filled Separator (DGFRS) [111] (see fig. 12).
Fl was here interpreted as an α emitter of E α = 10.02 ± T / = 0.51 +0 . − . s, Cn as an α emitter of E α = 9.162 ± T / = 4.0 +1 . − . s.Most of the decay chains were terminated by SF of Ds, except in two cases: in one decay chain, observedin the irradiation of
Pu also α decay of Ds and
Hs was observed and the chain was terminated by SFof
Sg; in one chain observed in the irradiation of
U also α decay of Sg was observed and the chainwas terminated by SF
Rf. The previously observed chains at VASSILISSA were suspected to represent ales probaly decay mode [104], but not listed any more in later publications (see e.g. [105]). The ’DGFR -results’ were in-line with data for
Cn and
Fl data later obtained in the reactions U( Ca,3n)
Cninvestigated at SHIP, GSI Darmstadt [112] and at GARIS II, RIKEN, Wako [113], as well as in the reaction
Pu( Ca,3n)
Fl investigated at BGS, LNBL Berkeley [114], while the ’VASSILISSA - events’ were notobserved.It should be noted, however, that due to a more sensitive detector system used in [112] than that used in[111] in cases where the α decay of Cn was denoted as ’missing’ in [111], since fission was directly following α decay of Fl, the α decay of Cn probably was not missing, but fission occured from
Cn [112].The discrepancy between the ’DGFRS results’ and the ’VASSILiSSA results’ could not be clarified so far,but it should be noted that the latter ones were not considered any more in later reviews of SHE synthesisexperiments at FLNR - JINR Dubna [27, 105].However, the ’VASSILISSA results’ again were discussed in context with a series of events, registered in anirradiation of
Cm with Cr at SHIP, which were regarded as a signature for a decay chain starting froman isotope of element 120 [97]. It should be noted that a critical re-inspection of this sequence of eventsshowed that it does not fulfil the physics criteria for a ’real’ decay chain and the probability to be a realchain is p << α and ’SF’ would represent Fl and
Cn if thechain starts at α is recorded as an α particle escaping the detector, releasing only an energy loss of∆E = 0.353 MeV in it. Using the measured lifetime (20 s) and a hindrance factor HF = 104, as derived fromthe full energy α event (10.29 MeV) attributed to Fl in [9], the authors calculated a full α decay energy for9 Tab. 1 : Comparison of the ’event sequence’ from the Cr +
Cm irradiation at SHIP [97] and theVASSILISSA results for
Fl and
Cn. Data taken from [97].Isotope E α (SHIP) / MeV ∆t E α (VASSILISSA) / MeV T / / s( ± Og) 11.814 ± Lv) 10.698 ± Fl) 0.353 (10.14 +0 . − . )** 20.1 s 10.29 ± +9 . − . s( Cn SF 701 s SF 308 +212 − s* time difference to the closed possible evaporation residue* energy calculated from ∆t = 20.1 s for a hindrance factor HF = 104 [97]. S (E<10.4): 11S (E>10.4): 1 a) Mc from a decay of Ts d ecays / ke V d ecays / ke V d ecays / ke V d ecays / ke V a / keV E a / keV b) Mc from direct produktion
S (E<10.4): 5S (E>10.4): 7
S (E<9.8): 10S (E>9.8): 5 c) Nh from production of Ts S (E<9.8): 1S (E>9.8): 8 d) Nh from production of Mc Fig. 13 : Comparison of α decay spectra of Mc and
Nh for different ways of production; a)
Mc fromdecay of
Ts; b)
Mc from ’direct’ production
Am( Ca,2n)
Mc; c)
Nh produced in α decay chainsstarting from Ts; d)
Nh produced in α decay chains starting from Mc.0 T ( Mc) = 0.13+0.09/-0.04 sT ( Nh) = 0.70+0.48/-0.20 sT ( Rg) = 5.5+3.8/-1.6 sT ( Mc) = 0.42+0.29/-0.12 sT ( Nh) = 0.70+0.59/-0.20 sT ( Rg) = 6.0+4.1/-1.7 s E a ( N h ) / M e V E a ( Mc) / MeV T ( Mc) = 0.44+0.27/-0.12 sT ( Nh) = 5.69+3.45/-1.56 sT ( Rg) = 31.6+19.2/-8.7 s c oun t s / ke V c oun t s / ke V E a / MeV Fig. 14 : α - α - correlations between decays of Mc and
Nh; the insert shows the energy distributionof
Nh (E α = 9.75-10.0 MeV), either correlated to Mc E α > α < α of E = 10.14 +0 . − . MeV. Using the same procedure they obtained a full α energy E = 10.19 +0 . − . MeV for the E = 2.31 MeV - ’escape’ event in [9]. The time differences ∆ T( α - α ) and ∆ T( α -SF) resulted in lifetimes τ ( α = 20 +89 − s ( T / = 14 +62 − s) and τ (SF) = 12 +56 − min ( T / = 500 +233 − s) [97] and thus were inline with the’VASSILISSA - data’ for Fl ( E α = 10.29 ± T / = 5.5 +9 . − . s) and Cn ( T / = 308 +212 − s).This finding was seen as a ’mutual support’ of the data strengthen the (tentative) assignments of the chainsin [9, 97], although the authors in [97] could not give a reasonable explanation, why these data were onlyseen in the VASSILISSA experiment and could not be reproduced in other laboratories. As it was shownin [115] that the decay chain in [97] represents just a sequence of background events, it becomes clear, thatblinkered data analysis may lead to correlations between background events even if they are obtained indifferent experiments. Within the so far assigned superheavy nuclei, the decay properties of the
N-Z = 59 nuclei are of spe-cific importance and interest, as the acceptance of discovery of elements Z = 117 (tenessine) and Z = 115(moscovium) is based on them. The heaviest known nucleus of that chain, Ts, was produced by thereaction
Bk( Ca,4n)
Ts [116, 117, 118, 119, 120, 121]. A second entry point into this chain is
Mcproduced in the reaction
Am( Ca,2n)
Mc [66, 122, 123]. It was stated in [116] ’decay energies andhalf-lives of the nuclei → → Rg decay chains observed in the
Am + Ca reaction agreewithin the statistical uncertainties with the decay properties of the daughter nucleus of the
117 nucleusproduced in the
Bk( Ca, 4n)
117 reaction (10 events) [...]. Such agreement provides indirect identi-fication and consistency checks via cross - bombardment production of the same nuclei in different fusionreactions of
Am and
Bk targets with Ca projectiles’. The fourth IUPAC/IUPAP Joint Working Party(JWP) on the priority of claims to the discovery of the new elements Z = 113, 115 and 117 followed thatstatement and regarded it as a central issue to have met the criteria for the elements Z = 115 and Z = 117using the following phrasing [13]: a) Element 115: ’JWP ASSESSMENT: The 2010 [...] jointly with the1 comparisondecay Bh,
DbEPJA 43, 175 (2010) [54]’this work’Interpretation’Dubna’ & ’TWG’ [27]
SF9.30 MeV5.5 s9.8-10.0 MeV0.7 s10.49-10.55 MeV0.13 sSF31.6 s9.35-9.8 MeV5.69 s10.2-10.4 MeV0.44 s10.6-11.2 MeV22 msSF, 9.28 MeV17 s9.47-10.18 MeV4.2 s10.15-10.54 MeV0.33 s10.6-11.2 MeV22 ms
Lr(2)
Db(2) Md Lr(1)
Db(1) Bh Rg(2)
Nh(2)
Mc(2)
Rg(1)
Nh(1)
Mc(1) Ts Rg Nh Mc Ts Fig. 15 : Suggestion for decay schemes of
Ts,
Mc(1) and
Mc(2).2013 [...] collaborations of Oganessian et al. have met the Criteria for discovery of the element with theatomic number Z = 115 in as much as the reproducibility of the alpha chain energies and lifetimes of
115 and Ca +
Bk and
Am is demonstrated in the top of the precious table. Thus, the 2010 [...],2012 [...] and 2013 [...] jointly with the 2013 [...] collaborations of Oganessian et al. have met the criteriafor discovery of the elements with atomic numbers Z = 115 and Z = 117.’This assessment was critizised and a different interpretation of the results was suggested [124]. This issuewill be illuminated in the following discussion. A final solution of the problem, however, cannot be presentedon the basis of the available data.The so far published decay data for the members of the N-Z = 59 chain [27] are summarized in table 2. Itshould be noted, however, that they are based solely on the results from the DGFRS - experiments. Acomplete list of the decay data published so far for the
N-Z = 59 chain members
Ts,
Mc,
Nh and
Rg are given in table 3. Altogether eighteen decay chains assigned to start from
Ts are reported so far,sixteen from experiments at the DGFRS (Dubna) [116, 117, 119] and two from an experiment at TASCA(GSI) [121]. Evidently only one of the TASCA chains was complete, in the second one the first two members(
Ts,
Mc) were missing. Eleven chains starting from
Mc were reported, four from experiments per-formed at the DGFRS [66], seven from a TASCA experiment [123]. Also included in table 3 are three eventsobserved in an irradiation of
Am with Ca at the BGS, Berkeley [68], although they were not explicitelyassigned to
Mc.At first glance three items are striking:a) four of the decay chains interpreted to start from
Mc consist of an α - sf correlation, i.e. sf decay of Nh, while in none of the eighteen chains starting from
Ts fission of
Nh was observed;b) in chain no. D4 of [66]
Nh has an α - decay energy more 0.2 MeV higher than that of all other decayswhere the α particle was registered with full energy (23 cases), which all had values E α <
10 MeV.c) α decay of Rg was only observed in decay chains starting from
Ts.To obtain more detailed information on the decay properties of the isotopes assigned to the
N-Z = 59chain a closer inspection of the data listed in table 3 was performed, specifically the results from the ’en-try’ into the chain at
Ts (reaction Ca +
Bk) and ’entry’ into the chain at
Mc (reaction Ca +2
Tab. 2 : Summary of decay properties of
N-Z = 59 nuclei; data taken from [27].Isotope Decay mode E α / MeV half-life Ts α +8 − ms Mc α +120 − ms Nh α +1 . − . s Rg α ( ≈ +0 . − . ) 9.28 ± +6 − s Mt SF 5 +9 − ms Am) were compared. The resulting α spectra are shown in fig. 13. Before starting a detailed discussionit seems, however, necessary to stress some items that could cause confusion. First, as discussed in sect. 4.4(individual) α energies measured in the experiments performed at the different laboratories (DGFRS Dubna,TASCA Darmstadt, BGS Berkeley) vary considerably, eventually due to the calibration procedures applied.Differences ∆ E = 76 keV were found between the DGFRS and BGS results, and ∆ E = 41 keV between theDGFRS and TASCA results for Bh; as ≈
90 % of the data in Table 3 are from DGFRS or TASCA anuncertainty of ≈
50 keV in the absolute value may be considered. As will be shown, the energy differencesfrom the different production mechanisms are munch larger and thus cannot be attributed to calibrationdiscrepancies.Part of the α energies were obtained as sum events from ’stop’ and ’box’ detector, thus suffering from worseaccuracy due to worse energy resolution. A few decays (three events) from the DGFRS experiments wereregistered as ’box - only’ events (i.e. the α particle escaped the ’stop’ detector with an energy loss below theregistration threshold). That means, the measured energies are too low by a few hundred keV. Due to thelow number these events also this feature cannot be the reason for the differences in the energy distribtions.In the considered reaction on direct production of Mc also
Mc is produced; the assigment to
Mcwas based on the fact, that the properties of the decay chains did not fit to the decay chain of
Mc. Inprinciple a production of
Mc by the reaction
Am( Ca,4n)
Mc can also not be ruled out. However,the properties of the decay chains attributed to
Mc are not in-line with the decay proprties of
Mc andits daughter products (see e.g. [27]).As seen from fig. 13 the α energy spectra of Mc (figs. 13a,b) and
Nh (fig. 13c,d) exhibit significantdifferences for the different production mechanisms. In the production by α decay of Ts nearly all
Mcevents (in 11 of 12 cases) have energies E α < E α = (10.49-10.55) MeV. Also half-lives are different; for the group at E α < T / = 0.39 +0 . − . s, for the group at E α > T / = 0.11 +0 . − . s. An analogue situation is found for Nh; about two third (10 of 15 cases) of the α events from chainsstarting at Ts have energies E α < Mc only oneof nine events is located in this energy interval.This behavior is also evident in the α - α correlation spectrum (fig. 14); the E α > Mc is exclusively correlated to
Nh events in the energy interval E α = (9.8-10.0) MeV, while for Mcof E α < Nh decays in that energy interval. Butas seen in the insert of fig. 14 the energy distributions are not the same. The
Nh decays correlated to
Mc at E α > E(mean) = 9.91 ± Mc at E α < E(mean) = 9.86 ± Mc component (it also could be a differentisotope) having an energy E α = 10.53 ± T / = 0.11 +0 . − . s is produced which is notpresent in the decay chain of Ts, i.e. eventually an isomeric state is populated by deexcitation of thecompound nucleus in ’direct’ production reaction, which is not populated by α decay of Ts. That wouldbe nothing surprising, as a couple of such examples are known in the transfermium region, e.g.
No,
Rf.The assumption of an isomeric state in
Mc does not vitiate the JWG assassment, but it clearly shows,how fragile such conclusion may be on the basis of very low statistics.So we have to face the following situation considerating all decays listed in table 3: • Within the production
Mc via α decay of Ts the α particles of Mc are located in an energyinterval E α = (10.2 - 10.4) MeV; the resulting half-life is T / = 0.28 +0 . − . s; • Within the production of
Mc ’directly’ via the reaction
Am( Ca,2n)
Mc we observe two com-ponents in the α energies: one at E α = (10.2 - 10.4) MeV with a half-life of T / = 0.71 +0 . − . s; and one3 Tab. 3 : Summary observed decay chains starting from either
Ts or
Mc.
Ref. Ts Mc Nh RgE α /MeV ∆t/ms E α /MeV ∆t/s E α /MeV ∆t/s E α /MeV ∆t/s[119] 10.99 17.01 missing 9.72 (16.17) SF 40.19[119] 11.14 7.89 missing 9.52** (2.23) SF 4.25[119] 11.08* 4.60 10.34 0.0175 9.71** 1.17 SF 12.3[119] 10.91 53.0 10.25 0.5118 9.79 0.238 SF 31.66[119] 11.00 20.24 10.27 0.4244 9.48 13.49 SF 76.56[117] 10.90 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Tab. 4 : Summary of halflife measurements.Isotope E /alpha / MeV T / / s correlation Mc 10.51 ± +0 . − . corr. to Nh (9.8 - 10.0 MeV)10.49 - 10.55
Mc 10.20 - 10.40 0.42 +0 . − . corr. to Nh (9.8 - 10.0 MeV)
Mc 10.20 - 10.40 0.44 +0 . − . corr. to Nh (9.3 - 9.8 MeV)
Nh 9.8 - 10.0 0.70 +0 . − . corr. to Mc (10.49 - 10.55 MeV)
Nh 9.8 - 10.0 0.70 +0 . − . corr. to Mc (10.2 - 10.4 MeV)
Nh 9.97, 9.87, 9.76 1.02 +0 .xx − .xx corr. to Rg α decay (9.32 ± Nh 9.3 - 9.8 5.69 +3 . − . corr. to Mc (10.2 - 10.4 MeV)
Rg SF 5.5 +3 . − . corr. to Nh (9.8 - 10.0 MeV) and/orcorr. to
Mc (10.49 - 10.55 MeV)
Rg SF 6.0 +4 . − . corr. to Nh (9.8 - 10.0 MeV) and/orcorr. to
Mc (10.2 - 10.4 MeV)
Rg SF 31.6 +19 . − . corr. to Nh (9.3 - 9.8 MeV) and/orcorr. to
Mc (10.2 - 10.4 MeV) Rg α decays 1.8 +1 . − . E α = (10.49 - 10.55) MeV with a half-life of T / = 0.12 +0 . − . s; • The
Nh events from the production via Ts → Mc → Nh are spread over an energy range E α = (9.35 - 10.0) MeV and exhibit a half-life T / ( Nh) = 2.44 +0 . − . s; • The
Nh events from the ’direct production’ of
Mc via Mc → Nh are in the an energy range E α = (9.8 - 10.0) MeV (except the two events at 9.58 MeV and 10.18 MeV ) and exhibit a half-life T / ( Nh) = 0.76 +0 . − . s; • Taking into account in addition the α - α - correlations Mc → Nh, we tentatively can distinguishthree groups1.
Mc ( E α = (10.49-10.55) MeV) → Nh ( E α = (9.8-10.0) MeV), with T / ( Mc) = 0.13 +0 . − . s and T / ( Nh) = 0.70 +0 . − . s; the fission events terminating the decay chain have a half-life T / ( Rg) = 5.5 +3 . − . s;2. Mc ( E α = (10.2 - 10.4) MeV) → Nh (E α = (9.8-10.0) MeV), with T / ( Mc) = 0.42 +0 . − . s and T / ( Nh) = 0.70 +0 . − . s; the fission events terminating the decay chain have a half-life T / ( Rg) = 6.0 +4 . − . s;3. Mc ( E α = (10.15 - 10.4) MeV) → Nh ( E α = (9.35 - 9.8)MeV), with T / ( Mc) = 0.44 +0 . − . s and T / ( Nh) = 5.69 +3 . − . s; the fission events terminating the decay chain have a half-life T / ( Rg) = 31.6 +19 . − . s.Under these circumstances we tentatively can distinguish the following decay chains. • Mc ( E α = (10.4 - 10.6) MeV, T / = 0.13 +0 . − . s) → Nh ( E α = (9.8 - 10.0 )MeV, T / = 0.70 +0 . − . s) → Rg (SF, T / = 5.5 +3 . − . s); • Mc ( E α = (10.2 - 10.4) MeV, T / = 0.42 +0 . − . s) → Nh ( E α = (9.8 - 10.0 )MeV, T / = 0.70 +0 . − . s) → Rg (SF, T / = 6.0 +4 . − . s); • Mc ( E α = (10.2 - 10.4) MeV, T / = 0.44 +0 . − . s) → Nh ( E α = (9.35-9.8) MeV, T / = 5.69 +3 . − . s) → Rg (SF, T / = 31.6 +19 . − . s);This at first glance somewhat puzzling seeming decay pattern can qualitatively explained by existenceof low lying long lived isomeric states in Mc,
Nh and
Rg decaying by α emission or spontaneousfission. The existence of such states is due to existence of Nilsson states with low and high spins placedclosely at low excitation energies; the decay by internal transitions of such states is hindered by large spindifferences and thus lifetimes become long and α decay can compete with internal transitions. That is a wellknown phenomena in the transfermium regions. In direct production both states are usually populated; inproduction by α decay the population of the states depend on the decay of the mother nucleus. If there aretwo longlived isomeric states in the mother nucleus, also two longlived states in the daughter nucleus maypopulated, see e.g. decay of Db → Lr [125]; if there is only one α emitting state populated by thedeexcitation process two cases are possible; either only one state in the daughter nucleus is populated as e.g.in the decay Sg → Rf [126], or both long-lived states in the daughter nucleus may be populated, asknown for Bh → Db [54].Under this circumstances the puzzling behavior can be understood in the following way: decay of
Tspopulates one state
Mc(1) (10.2-10.4 MeV), while direct production populates two states
Mc(1) and
Mc(2) (10.4-10.6 MeV);
Mc(2) decays exclusively into one state
Nh(2)(9.8-10.0 MeV), which thendecays into
Rg(2) which undergoes fission and α decay. Mc(1) on the other side partly populates
Nh(2) and
Nh(1) (9.35-9.8 MeV) which then decays into
Rg(1) which undergoes probably nearlyexclusively spontaneous fission. The resulting tentative decay scheme is shown in fig. 15.In addition there might be other contributions, e.g. the chain marked as D4 in Table 3, which does notfit to the other ones.Also the very short chains in Table 3 consiting of α → SF seemingly may have a different origins Thehalf-lives of the α events, T / ( α ) = 0.069 +0 . − . s, and of the fission events and T / (SF) = 0.3 +0 . − . are5
102 104 106 108 110 112 114 116 118 12010 -5 -3 -1 Fe +
Pu --> *64
Ni +
U --> *54
Cr +
Cm --> * 48
Ca + act. targets (4n) Ca + act. targets (3n)’cold’ fusion (Pb,Bi-targ.) s / pb Z ER Fig. 16 : Systematics of maximum production cross-sections in cold fusion reactions and reactions usingactinide targetslower than the values for
Mc(2) and
Nh(2), but considering the large uncertainties they are not indisagreement. So they could indicate a fission branch of
Nh in the order of b SF ≈ / = 1.8 +1 . − . s of the α events assigned to Rg which is considerably shorter thanthe half-life of the fission events. Despite this fact we tentatively assign them to
Rg(1).The joint analysis of the data presented for the decay chains interpreted to start either from
Ts or from
Mc seem to shed some light into the ’puzzeling’ decay data reported so far and suggests a solution. Itshould be noted, however, the conclusions drawn here must be confirmed by more sensitive measurementsbefore they can be finally accepted.6
Chain no. 3
Chain no. 2Chain no. 1
SFSF Db Db Bh Bh Bh Mt Mt Mt Rg Rg Lr Nh Nh Nh Rg Db Md Fig. 17 : Decay chains attributed to start from
Nh [135]chain 1 chain2 chain 3Isotope E α /MeV T / E α /MeV T / E α /MeV T /
113 11.68 ± ± ± Rg 11.15 ± ± ± Mt 10.03 ± ± Bh 9.08 ± ± ± Db sf 40.9 s sf 0.787 s 8.63 ± Lr 8.66 ± Tab. 5 : Decay chains observed at GARIS, RIKEN in the reaction Zn +
Bi and interpreted to startfrom
113 [135]. ’esc’ denotes that the α particle escaped the ’stop’ detector and only an energy loss signalwas recorded. Nh The first report on discovery of element 113 was published by Oganessian et al. [127] in 2004. In anirradiation of
Am with Ca performed at the DGFRs three decay chains were observed which wereinterpreted to start from
113 of the new element 113 was thus produced as the α decay descendant of
113 chains observed in 2004 withthe α energies being in excellent agreement among most of the events. [...] However, the criteria (q.v. [128])have not been met as there is no mandatory identification of the chain atomic numbers neither through aknown descendant nor by cross reaction. Chemical determination as detailed in the subsequent profile ofZ = 115 where they are documented, serving the important role of assigning atomic number are insufficientlyselective although certainly otherwise informative’ [13].Instead credit for discovery of element 113 was given to Morita et al. on the basis of three decay chainsobserved in the ’cold’ fusion reaction Zn +
Bi [13].Although cold fusion had been the successful method to synthesize elements Z = 107 to Z = 112, due to7 Morita et al. El113 production [135] a )b ) c oun t s / ke V Wilk et al.
Bh production [136]012
Morita et al.
Bh productiontriple a-correlations
Bh -
Db -
Lr[135,137] c ) d )
Morita et al.
Bh productiondouble a-correlations
Bh - (
Db or
Lr)[135,137] e )
Morita et al.
Bh productiona(
Bh) - SF
Db correlations[135,137] E a / MeVf ) Qin et al.
Bh production [138] g )
Haba et al.
Bh productiona(
Bh) - SF(
Db) correlations[144] h )
Haba et al.
Bh production triple a-correlationsa(
Bh) - a(
Db - a(
Lr)[144]
Fig. 18 : α decay energies of Bh as reported by different authors; a) from decay of
Nh [135], b)production via
Bk( Ne,5n)
Bh [136], c) production via
Cm( Na,5n)
Bh [137, 135], α energies fromtriple α correlations α ( Bh) - α ( Db) - α ( Lr), d) production via
Cm( Na,5n)
Bh [137, 135], α energies from double α correlations α ( Bh) - α ( Db or
Lr), e) production via
Cm( Na,5n)
Bh[137, 135], α energies from α - SF correlations, f) production via Am( Mg,3n) Bh [138], g) productionvia Cm( F,5n)
Bh, α ( Bh) - SF(
Db) correlations [144], h) production via
Cm( F,5n)
Bh,triple correlations α ( Bh) - α ( Db) - α ( Db) correlations [144].8 a) Morita et al. El.113 production [135]
01 Morita et al.
Bh productiontriple a-correlations
Bh -
Db -
Lr[135,137]012 d) Morita et al.
Bh productioncorrelations a(
Db) - a(
Lr)
Bh not registered
012 e)
Qin et al.
Bh) productioncorrelationsa(
Bh) - a(
Db) - ( a(
Lr))[138] E a / keV g) Haba et al.
Db productioncorrelationsa(
Db) - a(
Lr)[140]012 c)b)
Wilk et al.
Bh production[136] c oun t s / ke V f) Dressler et al.
Db productioncorrelationsa(
Db) - a(
Lr)[139]
012 h)
Haba et al.
Bh productioncorrelationsa(
Bh) - a(
Db)) ora(
Db) - a(
Lr))[144]
Fig. 19 : α decay energies of Db as reported by different authors; a) from decay of
Nh [135], b)production via
Bk( Ne,5n)
Bh [136], c) production via
Cm( Na,5n)
Bh [137, 135], α energies fromtriple α correlations α ( Bh) - α ( Db) - α ( Lr), d) production via
Cm( Na,5n)
Bh [137, 135], α energies from double α correlations α ( Db) - α ( Db, with α decay of Bh) not recorded; e)production via
Am( Mg,3n)
Bh [138], f) production via
Cm( F,5n)
Db [139], g) production via
Cm( F,5n)
Db [140], h) production via
Cm( Na,5n)
Bh [144].9
Wilk et al.
Bh production [136] b) Morita et al. El113 production [135] a) c oun t s / ke V Morita et al.
Bh productiontriple a-correlationsa(
Bh) - a(
Db) - a(
Lr)[135,137] c) Morita et al.
Bh productiondouble a-correlationsa(
Db) - a(
Lr)[135,137] d) Qin et al.
Bh production [138] e) Haba et al.
Db production[140] E a / keVg) f) Dressler et al.
Db production [139] h) Haba et al.
Bh production [144]
Fig. 20 : α decay energies of Lr as reported by different authors; a) from decay of
Nh [135], b)production via
Bk( Ne,5n)
Bh [136], c) production via
Cm( Na,5n)
Bh [137, 135], α energiesfrom triple α correlatiions α ( Bh) - α ( Db) - α ( Lr), d) production via
Cm( Na,5n)
Bh [137,135], α energies from double α correlatiions α ( Db) - α ( Lr, with α decay of Bh not recorded,e)production via
Am( Mg,3n)
Bh [138], f)production via
Cm( F,5n)
Db [139], g)production via
Cm( F,5n)
Db [140], h)production via
Cm( Na,5n)
Bh [144].0 E a / MeV Haba et al. [144]
Morita et al.[135,137] Bh Db Lr Qin et al. [138]Wilk et al. [136]Morita et al., El.113 [135]
Fig. 21 : Comparison of all published triple correlations Bh α → Db α → Lr α → α /MeV E α /MeV E α /MeV | ∆E α | /MeV | ∆ E α | /MeV | ∆E α | /MeVChain 1 Chain 2 Chain 3 Ch.1 - Ch.2 Ch.1 - Ch.3 Ch.2 - Ch. 3
113 11.68 ± ± ± Rg 11.15 ± ± ± Mt 10.03 ± ± Bh 9.08 ± ± ± Tab. 6 : α energy differences of the individual chain members of the three decay chains interpreted to startfrom Pb with Zn [86] it seemed straightforward toattempt to synthesize element 113 in the reaction
Bi( Zn,n) α decay chains that could be attributed to start from an element 113 isotope were observed. Merging theprojectile doses collected in both experiments an upper production cross section limit σ ≤
160 fb was obtained[131].More intensively this reaction was studied at the GARIS separator, Riken, Wako-shi, Japan. Over a periodof nine years (from 2003 to 2012) with a complete irradiation time of 575 days altogether three dcay chainsinterpreted to start from
113 were observed [132, 133, 134, 135]. The collected beam dose was 1.35 × Zn - ions, the formation cross-section was σ = 22 +20 − fb [134].The chains are shown in fig. 17 and the data are presented in table 5. Chain 1 and chain 2 consist of four α particles and are terminated by a fission event, while chain 3 consists of six α decays. Already at firstglance the large differences in the α energies of members assigned to the same isotope is striking, especiallyfor the events α ( Rg) in chains 2 and 3 with ∆ E = 0.68 MeV and α ( Bh) in chains 1 and 2 with∆ E = 0.69 MeV. Although it is known that α - decay energies can vary in a wide range for odd - odd nucleiin the region of heaviest nuclei, as was shown, e.g., for Mt, where α energies were found to vary in therange E α = (10.456 - 11.739) MeV [76], the assignment of such different energies to the decay of the sameisotope or the same nuclear level can be debated ( see e.g. [11]), as specifcally concerning the latter case it isknown, that in odd - odd (and also in odd - mass) nuclei often low lying isomeric states exist, which decayby α emission with energies and halflives similar to those of the ground state (see e.g. [60] for the cases of Db,
Lr, and
Md). In the present case large α energy differences of ∆ E > Bh which acts as an anchor point for identification ofthe chains. The observation of α decay of this isotope has been reported by several authors who producedit in diffeent reactions:a) Wilk et al. [136] used the reaction Bk( Ne,5n)
Bh. They observed one event with an α energy of E α = 9.29 MeV.b) Morita et al. [135, 137] used the reaction Cm( Na,5n)
Bh; they observed in total 32 decay chains;20 of them were attributed (partly tentative) to the decay of
Bh; four decay chains consisted of three α particles, assigned as decays α ( Bh) - α ( Db) - α ( Lr); four decay chains consisted of two α par-ticles, interpreted as decays α ( Bh) - α ( Db or
Lr); twelve decay chains consisted of an α particlefollowed by a fission event, interpreted as α ( Bh) - SF(
Db)(possibly SF from
Rf, produced by ECdecay of
Db); in the case of four α energies of E α < Am( Mg,3n)
Bh. They observed four decay chains which theyassigned start from
Bh.Evidently there is no real agreement for the α energies of Bh; two of the three energies of
Bh fromthe
113 decay chains (fig. 18a) are outside the range of energies observed in direct production, whichis specifically critical for chain 3, as it is not terminated by fission, but α decay is followed by two more α events attributed to Db and
Lr and thus is the anchor point for identification of the chain. Someagreement is obtained for the events from the direct production followed by fission [137, 135] (fig. 18e), the α energy in chain 1 (fig. 18a) (also follwed by fission) and the results from Qin et al. [138] (fig. 18f), wheretwo groups at (9.05 - 9.1) MeV and (8.9 - 9.0) MeV are visible. Note, that in [137, 135] the events at E α < Bh, while in [138] all Bh α decays are followedby α decays.2Unclear is the situation of the events followed by α decays. As seen in figs. 18c, 18d, and 18f, there arealready in the results from [137, 135] discrepancies in the Bh energies from triple correlations (fig. 18c)and double correlations (fig. 18d). In the triple correlations there is one event at E = 8.82 MeV, three moreare in the interval E = (9.08 - 9.2) MeV, while for the double correlations all four events are in the range E = (9.14 - 9.23) MeV; tentatively merging the Bh α energies from events followed by α decay we find sixof eight events (75 per cent) in the range E = (9.14 - 9.23) MeV while only one of twelve events followed byfission is observed in that region. In this enery range none of the events observed by Qin et al. [138] isfound, which are all below 9.14 MeV, also none of the events from the decay of α decay energies of Bh reported from the different production reactions as well as fromthe different decay modes of the daughter products ( α decay or (SF/EC) vary considerably, so there is noreal experimantal basis to use Bh as an anchor point for identification of the chain assumed to start at
113 decay chains a half-life of T / = 2.2 +2 . − . s isobtained for Bh [135], while Qin et al. [138] give a value T / = 0.66 +0 . − . s. The discrepancy is alreadyslightly outside the 1 σ confidential interval. No half-life value is given from the direct production of Moritaet al. [135].The disagreement in the decay properties of Bh reported by different authors renders the interpretationof the α decay chain (chain 3) quite difficult. It is therefore of importance to check the following α decaysassigned to Db and
Lr, respectively, as they may help to clarify the situation. In order to do so, it isrequired to review the reported decay properties of these isotopes and to compare the results with the datain chain 3.It should also be remarked here that the differences of the α energies attributed to Bh followed by α decaysor by SF in [135] indicates that the assignement of these events to the same isotope is not straightforward,at least not the assignment to the decay of the same nuclear level.In a previous data compilation [81] three α lines of E α = 8.45 ± E α = 8.53 ± E α = 8.67 ± T / = 34 ± Db. Morerecent data were obtained from decay studies of
Bh [135, 136, 137, 138] or from direct production via thereaction
Cm( F,5n)
Db [139, 140]. The results of the different studies are compared in fig. 19.The energy of the one event from the
113 decay chain 3 is shown in fig. 19a. The most extensive re-cent data for
Db were collected by Haba et al. [140]. They observed two groups of α -decay energies,one at E α = (8.40-8.55) MeV (in the following also denoted as ’low energy component’) and another one atE α = (8.60-8.80) MeV (in the following also denoted as ’high energy component’) (fig. 19g). Mean α energyvalues and intensities are E α = 8.46 ± i rel = 0.70 ± E α = 8.68 ± rel = 0.30 ± T / = 33.8 +4 . − . s is given for both groups. A re-analysis of the data, how-ever, indicate different halflives: T / = 39 +6 − s for E α = (8.40 - 8.55) MeV and T / = 24 +6 − s for E α = (8.60 -8.80) MeV. A similar behavior is reported by Dressler et al. [139] 2 events at E α = (8.40 - 8.55) MeV and oneevent at E α = (8.60 - 8.80) MeV (see fig. 19f). Qin et al. [138] oberserved three events at E α = (8.40 - 8.55)MeV and one event at E α = 8.604 MeV, outside the bulk of the high energy group reported in [140](see fig.19e). A similar behavior is seen for the double correlations ( Db -
Lr), with missing
Bh from thereaction Na +
Cm measured by Morita et al. [135] (see fig. 19d). Three of four events are locatedin the range of the low energy component, while for the triple correlations all four events are in the highenergy group (see fig. 19c). This behavior seems somewhat strange as there is no physical reason why the α decay energies of Db should be different for the cases where the preceding Bh α decay is recordedor not recorded. It rather could mean that the triple ( Bh → Db → Lr) and the double correlations( Db → Lr) of [135] do no respresent the same activities. The α decay energy of the one event observedby Wilk et al. [136] belongs to the low energy group (see fig. 19b), the one event from the decay chainattributed to start from
113 does not really fit to one of the groups. The energy is definitely lower thanthe mean value of the high energy group, an agreement with that group can only be postulated consideringthe large uncertainty ( ±
60 keV) of its energy value (see fig. 19a).Halflives are T / = 44 +60 − s for the E α = (8.40 - 8.55) MeV component in [135], T / = 16 +7 − s for the E α = (8.55 -8.80) MeV in agreement with the values of Haba et al. [140], and T / = 52 +21 − s for the SF activity, whichis rather in agreement with that of the low energy component.To summarize: The assignment of the event α in chain 3 in [135] to Db is not unambiguos on thebasis of its energy, in addition also its ’lifetime’ τ = t α -t α = 126 s is about five times of the half-life of thehigh energy component of Db observed in [140]. One should keep in mind, that the probability to observea decay at times longer than five halflives is p < α /MeV i α E α /MeV i α ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Tab. 7 : Alpha decay energies reported for
Lr by Eskola et al. [141] and by Bemis et al. [142].Reference Isoptope analysis mode T / / s[135] Bh decay chain
Nh 2.2 +2 . − . [138] Bh 0.66 +0 . − . [144] Bh correlated to SF 12.8 +5 . − . [144] Bh correlated to α decay 7.0 +3 . − . [144] Bh all events 10.0 +2 . − . [81] Db 34 ± Db E = (8.38-8.52) MeV 44 +60 − [135] Db E = (8.55-8.80) MeV 16 +7 − [135] Db corr.
Bh - SF 52 +21 − [140] Db E = (8.38-8.52) MeV 39 +6 − [140] Db E = (8.55-8.80) MeV 24 +6 − [138] Db 26 +26 − [139] Db 26 +26 − [144] Bh corre.
Bh - SF 39 +15 − [144] Db E = (8.38-8.52) MeV 32 +22 − [144] Db E = (8.55-8.80) MeV 6.7 +16 . − . [81] Lr 3.9 +4 − [135] Lr triple corr.
Bh -
Db -
Lr 4.7 +4 . − . [135] Lr double corr.
Db -
Lr 3.3 +3 . − . [135] Lr all events 4.0 +2 . − . [140] Lr corr.
Db, E = (8.38-8.52) MeV 3.5 +0 . − . [140] Lr corr.
Db, E = (8.55-8.8ß) MeV 4.1 +1 . − . [140] Lr all events 3.5 +0 . − . [139] Lr 3.1 +3 . − . [144] Lr all events 3.6 +1 . − . Tab. 8 : Comparison of half-lives of
Bh,
Db, and
Lr published or analysed from published data bythe author in this work.4 Observation of
Lr was first reported by Eskola et al. [141] and later by Bemis et al. [142]. Thereported α energies and intensities slightly disagree [143]. The data are given in table 6. The energies givenin [142] are 30-50 keV lower than those reported in [141]. More recent data were obtained from decay studiesof Bh,
Db [135, 136, 137, 138, 139, 140]. The results are compared in fig. 20.The quality of the data is lower than that of
Db, but not less confusing. Haba et al. got a broad energydistribution in the range E α = (8.50 - 8.75) MeV with the bulk at E α = (8.60 - 8.65) MeV having a mean value E α = 8.62 ± T / = 3.54 +0 . − . s is given. Dressler et al. [139]observed all the events at E α ≤ T / = 3.10 +3 . − . s.Each one event within the energy range of the Haba - data was observed by Qin et al. [138] (fig. 20e) andWilk et al. [136] (fig. 20b). Contrary to the energies of Bh and
Db the α energies for Lr from the
Bh decay study of Morita et al. [135] are quite in agreement for the triple (fig. 20c) and double (fig. 20d)correlations, a bulk of five events at a mean energy of E α = 8.70 ± α = 8.59 ± E α = 8.80 MeV; half-lives are T / = 4.7 +4 . − . s for the eventsfrom the triple correlations, T / = 3.3 +3 . − . s for the events from the double correlations, being in agreementwithin the error bars. They may be merged to a single half-life of T / = 4.0 +2 . − . s. The one decay eventfrom chain 3 attributed to start from
113 [135] of E α = 8.66 ± τ = t α -t α = 3.78 s fairly fitsinto the decay properties reported for Lr.The dilemma is evident as seen from fig. 21 where all triple correlations Bh α → Db α → Lr α → reported so far [135, 136, 138, 144] are shown. None of the forteen chains agrees with any other one. Thisfeature may indicate the complicate α decay pattern of these isotopes, but it makes the assignment to thesame isotope speculative. In other words: the ’subchain’ Bh α → Db α → Lr α → of the decay chaininterpreted to start from
113 does not agree with any other so far observed α decay chain interpreted tostart from Bh. The essential item, however, is that this triple correlation was regared as the key point forfirst identification of element 113, to approve the discovery of this element and give credit to the discoverers.But this decision is based rather on weak probability considerations than on firm experimental facts. Theonly solid pillar is the agreement with the decay properties reported for
Lr, which might be regarded asrather weak. In other words, the assignment of the three decay chains to the decay of
Nh is probable, butnot firm.It should be reminded that in case of element 111 in the JWP report from 2001 it was stated: ’The resultsof this study are definitely of high quality but there is insufficient internal redundancy to warrant certitudeat this stage. Confirmation by further results is needed to assign priority of discovery to this collaboration’[79]. So it seems strange that in a similar situation as evidently here, such concerns were not expressed.A new decay study of
Bh was reported recently by Haba et al. [144] using the same production reactionas in [135]. Alpha decays were observed correlated to fission events, assigned to the decay of
Db and α decay chains Bh α → Db α → or Bh α → Db α → Lr α → . The α spectra of decays followed by fission isshown in fig. 18g, that of events followed by α decays of Db in fig. 18h. Evidently in correlation to fissionevents a concentration of events (’peak’) is observed at E α = 8.85 MeV, not observed in [135], while in therange E α = 8.9-9.0 MeV, where in [137] a peak - like structure was oberserved Haba et al. registered only abroad distribution. Also only a broad distribution without indication of a peak - like concentration in therange E α = 8.8-9.4 MeV is observed in correlation to α decays. However, two α decays at E ≈ α energy of Bh in the
Nh decay chain 3 [135]. A remarkable result of Habaet al. [144] is the half-life of
Bh; values of T / = 12.8 +5 . − . s are obtained for events correlated to SF, andT / = 7.0 +3 . − . s for events correlated to α decays. This finding suggests a common half-life of T / = 10.0 +2 . − . s as given in [144] despite the discrepancies in the α energies. That value is, however, significantly largerthan the results from previous studies [135, 138].For Db in [144] a similar α decay energy distribution is observed as in [140], as seen in figs. 19g and 19h,but again not in agreement with the results from [137] (figs. 19c and 19d) where the low energy componentE = (8.38-8.52) MeV is practically missing. Halflives of SF events assigned to Db are in agreement withthose of α events at (E = (8.38-8.52) MeV (see table 8), but again for the events at E = (8.55-8.80) MeV ashorter half-life (T / = 6.7 +16 . − , s) is indicated. Interestingly all events at E = (8.55-8.80) MeV are correlatedto Bh α decays E > Bh in [144] have extremely low correlation times of (0.92 s and 0.33 s), the
Db events havea half-life T / = 13.7 +11 . − . s. Despite the above mentioned differences the same feature is observed in [135].The data are summarized in table 8. A halflife T / = 22.5 − . − . s is obtained clearly lower than that of theSF events and α - events E = (8.38-8.55 MeV), corroborating the possible existence of two long-lived statesin Db. Although data are scarce two other features are indicated: a) all
Bh energies are above the5Reference E α ( Bh) / MeV ∆t / s E α ( Db) / MeV ∆t / s[135] 9.05 8.71 54.91[135] 9.12 8.74 13.76[135] 9.20 8.67 13.71[144] 9.12 0.92 8.70 10.29[144] 9.04 0.33 8.63 9.07
Tab. 9 : α - α correlations Bh (E > Db (E =(8.60-8.75 MeV) from triple correlations Bh α → Db α → Lr α → .’bulk’ of the α energy distribution of Bh and b) one obtains a half-life of T / = 3.4 − . − . s, lower than thevalue of 10 s extracted from all events. This can be regarded as a hint for the existence of two long-livedstates in Bh decaying by α emission, resulting in two essential decay branches Bh(1) → Db(1) and
Bh(2) → Db(2).The α spectrum for Lr measured in [144] is shown in fig. 20h. Essentially it is in-line with the oneobtained in [140].To summarize: the new decay study of
Bh delivers results not really in agreement with those fromprevious studies concerning the decay energies and delivers a considerably longer halflife for that isotope. Sothe results do not remove the concerns on the decay chains interpreted to start from
Nh. But it deliverssome interesting features: the α decays in the range (8.38-8.55) MeV and the SF events following α decayof Bh seemingly are due to the decay of the same state in
Db, the fission activity, however, may bedue to
Rf produced by EC of
Db. The α - decays of Db of E = (8.55-8,80) MeV eventually are fromdecay of a second long-lived level. There is also strong evidence that this level is populated essentially by α decay of a long-lived level in Bh, different to that populating the one in
Db decaying by α particles inthe range (8.38-8.55) MeV. Further studies are required to clarify this undoubtedly interesting feature.Discussing the items above one has of course to emphasize the different experimental techniques used whichmay influence the the measured energies. The important feature are the different detector resolutions whichdetermine the widths of the ebery distributions. So comparison of energies might be somewhat ’dangerous’.Another item is energy summing between α particles and CE. In the experiments of Wilk et al. [136] andMorita et al. [135], the reaction products were implanted into the detector after in-flight separation. Qin etal. [138], Dressler et al. [139] and Haba et al. [140, 144] collect the reaction products on the detector surfaceor on a thin foil between two detectors. The letter procedure reduces the efficiency for energy summingconsiderably. This could be the reason for the ’shift’ of the small ’bulk’ of the Bh α energy distributionfrom ≈ ≈
7. (Exemplified) Cross-checks with Nuclear Structure Theory
In the following some discussion on selected decay and nuclear structure properties will be presented.
Alpha-decay energies provide some basic information about nuclear stability and properties. Discussingthe properties one strictly has to distinguish two cases, a) α decay of even-even nuclei, and b) α decay ofisotopes with odd proton and/or odd neutron numbers.In even-even nuclei α transitions occur with highest intensities between the I π = 0 + ground - states of motherand daughter isotopes. Still, in the region of strongly deformed heaviest nuclei ( Z ≥
90) notable populationwith relative intensities of (10-30 %) is observed for transitions into the I π = 2 + level of the ground-staterotational band [81], while band members of higher spins (4 + , 6 + etc. ) are populated only weakly withrelative intensities of < α line of highest intensity represents the Q-valueof the transition and is thus a measure for the mass difference of the mother and the daughter nucleus. Itshould be kept in mind, however, that only in cases where the mass of the daughter nucleus is known, theQ-value can be used to calculate the mass of the mother nucleus, and only in those cases α -decay energiescan be used to ’directly’ test nuclear mass predictions. Nevertheless, already the mass differences, i.e. theQ-values, can be used for qualitative assessments of those models. Particulary, as crossing of nucleon shellsis accompanied by a strong local decrease of the Q α - values, existence, and by some extent also strength6
150 155 160 165 170 175 18078910111213 150 155 160 165 170 175 18078910111213
Z=120Z=118Z=114Z=106Z=110Z=112 Z=116 Q a / M e V Q a / M e V neutron number neutron number a ) Z=104Z=108 Z=104Z=106Z=108Z=110 Z=112 Z=114Z=116Z=118Z=120 b) Fig. 22 : Comparison of experimental Q α - values of even-Z elements Z ≥
104 with theoretical predictionsof R. Smolanczuk and A. Sobiczewski [17] (fig. 22a) and P. M¨oller et al. [18] (fig. 22b). In case of isotopeswith odd neutron numbers, the Q α - value was calculted from the highest reported decay energy.of such shells can be verified by analyzing systematics of Q α - values. That feature is displayed in fig. 22,where experimental Q α values for the known isotopes of even-Z elements Z ≥
104 are compared with resultsof two (widely used) mass predictions based on the macropscopic - microscopic approach, the one reportedby R. Smolanczuk and A. Sobiczewski [17] (fig. 22a), and the one reported by P. M¨oller et al. [18] (fig. 22b).The neutron shells at N = 152 and N = 162, indicated by the black dashed lines are experimentally andtheoretically verified by the local minima in the Q α - values. But significant differences in the theoreticalpredictions are indicated, those of [17] reproduce the experimental data in general quite fairly, while theagreement of those from [18] is significantly worse.Alpha-decay between states of different spins are hindered. Quantitatively the hindrance can be expressedby a hindrance factor HF , defined as HF = T α (exp) / T α (theo), where T α (exp) denotes the experimentalpartial α -decay half-life and T α (theo) the theoretical one. To calculate the latter a couple of (mostly em-pirical) relations are available. In the following will use the one proposed by D.N. Poenaru [98] with theparameter modification suggested by Rurarz [99]. This formula has been proven to reproduce experimentalpartial α -decay half-lives of even-even nuclei in the region of superheavy nuclei within a factor of two [100].A semi-empirical relation for the hindrance due to angular momentum change was given in 1959 by J.O.Rasmussen [145]. The change of the transition probability P (no angular momentum change) through thebarrier, which can be equated with the inverse hindrance factor, was given asP L /P = exp[-0.2027L(L+1)Z − / A − / ]with L denoting change of the angular momentum, Z and A the atomic number of the daughter nuclei.In the range of actinide nuclei where data are available ( Z ≈
90 - 102) one expects hindrance factors HF ≈ L = 2 and HF ≈ (5 - 6) for ∆ L = 4, with a slight decrease at increasing A and Z . The experimentalhindrance factors for α decay into the I π = 2 + and I π = 4 + levels for the known cases in the actinide region Z ≥
90 are shown in fig. 23. They exhibit a complete different behavior: for the ∆ L = 2 transitions theexperimental hindrance factor are comparable, but increase at increasing A and Z . For the ∆ L = 4 transitionsthe hindrance factors are considerably larger and a maximum is indicated for curium isotopes in the massrange A = (240 - 246). Interestingly this behavior can be related to the ground-state deformation as shown in7 transition 0 + + transition 0 + + F m - F m - F m - C f- C f- C f- C f- C m - C m - C m - C m - C m - P u - P u - P u - P u - U - U - Th - Th - U - H i nd r a n ce F ac t o r ( H F ) Fig. 23 : Hindrance factors for decays into I π = 2 + and I π = 4 + daughter levels of even-even actinideisotopes Z ≥
90. Alpha-decay data are taken from [81]. H i nd r a n ce F ac t o r ( + + ) t r a n s i t i on Quadrupole deformation b Fig. 24 : Hindrance factors for decays into I π = 2 + daughter levels of even-even actinide isotopes Z ≥
90 asfunction of the quadrupole deformation parameter β . The line is to guide the eye.8 H i nd r a n ce F ac t o r ( + + ) t r a n s i t i on Hexadecapole Deformation b Fig. 25 : Hindrance factors for decays into I π = 4 + daughter levels of even-even actinide isotopes Z ≥
90 asfuction of the hexadecapole deformation parameter β . The line is to guide the eye.
244 246 248 250 252 i ( + + ) i ( + + ) mass number (Z = 98) mass number (Z = 100) i ( + + ) i ( + + ) i ( + + ) i ( + + ) c) b) a) Hassanabadi [147]exp.Denisov [146]
248 250 252 254 f)e)d) exp.Denisov [146]Hassanabadi [147]
Fig. 26 : Comparison of experimental and theoretical [146, 147] α transition intensities into I π = 0 + , 2 + ,and 4 + daughter levels for californium (a-c)and fermium isotopes (d-f). black lines and diamond representexperimental values, red dashed line and squares represent the caluclations of [146], blue dashed - dottedlines and circles represent the calculations of [147].9
98 100 102 104 106 108 110 a) Fm Ds Hs Sg Rf No Cf D m c / M e V c) Liran et al. [159]M(cid:246)ller et al. [18]
Atomic number b) ( D m c p - D m c A M E ) / M e V ( D m c p - D m c t h e o ) / M e V Fig. 27 : (a) Experimental ground-state mass excesses of
N-Z = 49 and (b) comparisons with (previously)evaluated values [160] and (c) theoretical predictions by P. M¨oller et al. [18] (squares) or Liran et al. [159](circles).0fig. 24 and 25. In fig. 24 the hindrance factors for the I π = 0 + → I π = 2 + transitions are plotted as functionof the quadrupole deformation parameter β (taken from [18]). Evidently a strong increase of the hindrancefactor at increasing quadrupole deformation is observed. In fig. 25 the hindrance factors for the I π = 0 + → I π = 4 + transitions are plotted as a function of the hexadecapole deformation parameter β (taken from[18]). Here, a maximum at a deformation parameter β ≈ α halflives were calculated in the ’standard’ way asT / = ln2/ ν Pwith ν denoting the frequency of assaults on the barrier, and P being the penetration probability throughthe potential barrier using the semiclassic WKB method.In [147] the α - nucleus potential was parameterized as a polynominal of third order for r ≤ C t and as sumof Coulomb V C , nuclear V N and centrifugal V l + (¯ h l(l+1)/(2 µ r )) potential. For V C and V l ’standardexpressions’ were used, for V n the ’proximity potential’ of Blocki et al. [148]. C t is the touching configu-ration of daughter nucleus (d) and α particle ( α ), C t = C d + C α , with C − t denoting the Suessman centralradii (see [147] for details). V.Yu. Denisov and A.A. Khudenko [146] use the ’unified model for α decayand α capture (UMADAC). Their potential represents the sum of a ’standard’ centrifugal potential V l (seeabove), a Coulomb potential V C including quadrupole ( β ) and hexadecapole ( β ) deformations, and a nu-clear potential V N of Woods-Saxon type (see [146] for further details).Their results for the californium and fermium isotopes are compared with the experintal values in fig. 26.Obviously the calculations of Denisov and Khudenko do not well reproduce the experimental data for both,californium and fermium isotopes; the calculated (relative) intensities for the 0 + → + transitions (fig. 26c)are too low and hence too high values for the 0 + → + transitions (fig. 26c) and the 0 + → + transitions(fig. 26a) are obtained. The latter are even roughly an order of magnitude higher than the experimental datafor the respective transition. Quite fair agreement between experimental data is evident for the calculationsof Hassanabadi and Hosseini (blue lines and symbols).In odd-mass nuclei the situation is completely different as ground-state of mother and daughter nucleiusually differ in spin and often also in parity. So ground-state to ground-state α decays are usually hindered.Hindrance factors significally depend on the spin difference, as well as on a possible parity change and/or aspin flip. For odd-mass nuclei an empirical classification of the hindrance factors into five groups has beenestablished (see e.g. [149]). Hindrance factors HF < HF = (4 -10) indicate a favourable overlap between the initial and final nuclear state, while values HF = (10 - 100)point to an unfavourable overlap, but still parallel spin projections of the initial and final state. Factors HF = (100 - 1000) indicate a parity change and still parallel spin projections, while HF > α decay patterns are similar along the isotone lines (see e.g. [150]), while in odd-Z odd-mass nucleithis feature is evident along the isotope lines (see e.g. [151]). So, in certain cases, based on empiricalrelationships tentative spin and parity assignments can be established, as done e.g. in suggesting an α decaypattern for No by P. Eskola et al. [152], which later was confirmed by α - γ spectroscopy measurement[153, 154].Another feature in the case of odd-mass nuclei is the fact, that competition between structural hindranceand Q-value hindrance may lead to complex α - decay patterns. Nilsson levels identical to the ground-stateof the mother nucleus may be excited states located at several hundred keV in the daughter nuclei, e.g. , ina recent decay study of Sg it was shown that the 11/2 − [725] Nilsson level assigned to the ground-statein this isotope, is located at E ∗ ≈
600 keV in the daughter nucleus
Rf [155]. Therefore the advantage ofa low hindrance factor may be cancelled by a lower barrier transmission probability due to a significantlylower Q-value compared to the ground-state to ground-state transition. Consequently α transitions withmoderate hindrance factors into lower lying levels may have similar or even higher intensities than thefavored transition as it is the case in the above mentioned examples, No and
Sg.1 -2-101234567891011 Q a / M e V N = 124N = 126N = 128
78 80 82 84 86 88 90 92 S p / M e V Atomic number Z Q a / M e V N = 150N = 152N = 154
94 96 98 100 102 104 106 S p / M e V Atomic number Z
Fig. 28 : left side: Q α - values and 2p - binding energies (S p for even - even nuclei of N = 124 (diamonds),126 (squares), 128 (circles) around Z = 82; right side: Q α - values and 2p - binding energies (S p for even -even nuclei of N = 150 (diamonds), 152 (squares), 154 (circles) around Z = 100. Data are taken from [163].A drawback of many recent α decay studies of odd-mass nuclei in the transfermium region was the factthat the ground-state to ground-state transition could not be clearly identified and thus the ’total’ Q α valuecould not be established. Another difficulty in these studies was the existence of isomeric states in severalnuclei, also decaying by α - emission and having half-lives similar as to the ground-state, as in the casesof No [63] or
Rf [156], while in early studies ground-state decay and isomeric decay could not bedisentangled. Enhanced experimental techniques, applying also α - γ spectroscopy have overcome widelythat problem in the transfermium region. An illustrative example is the N-Z = 49 - line, where based onthe directly measured mass of
No [157], and decay data of
No [65],
Rf,
Sg [126],
Hs [158] and
Ds [44] experimental masses could be determined up to
Ds and could serve for a test of theoreticalpredictions [18, 159] and empirical evaluations [160], as shown in fig. 27. The masses predicted by M¨olleret al. [18] agree with the experimental value within ≈ Z = 106, while towards Z = 110 ( Ds)deviations rapidly increase up to nearly 2 MeV. A similar behavior was observed for the even-even nucleiof the
N-Z = 50 line [100], which was interpreted as a possible signature for a lower shell effect at N = 162. α values as signatures for nuclear shells Historically evidence for nuclear shells was first found from the existence of specifically stable nuclei atcertain proton and neutron numbers (
Z,N = 2, 8, 20, 28, 50, 82 and N = 126) which were denoted as ’magic’.Experimental signatures were, e.g. strong kinks in the 2p- or 2n - binding energies at the magic numbersand on the basis of enhanced nuclear decay data also by local minima in the Q α values. The existence ofnuclear shells was theoretically explained by the nuclear shell model [1, 2], which showed large energy gapsin the single particle levels at the ’magic’ numbers, which were equated with ’shell closures’. This item wasthe basis for the prediction of ’superheavy’ elements around Z = 114 and N = 184 when the nuclear shellmodel was extended in the region of unknown nuclei far above Z = 82 and N = 126 [3, 4]. As the shell gap isrelated to a higher density of single particle levels, compared to the nuclear average (expected e.g. from aFermi gas model) below the Fermi level and a lower density above the Fermi level, the large energy gaps atthe magic numbers go hand in hand with large shell correction energies, leading to the irregularities in the2p-, 2n- separation energies and in the Q α values.2In between Z = 82, N = 126 and Z = 114 and N = 184 a wide region of stronly deformed nuclei is existing.Calculations (see e.g. [161]) resulted in large shell gaps at N = 152 and Z = 100. Later theoretical studies alsoshowed in addition a region of large shell correction energies in between N = 152 and N = 184 [22, 23, 24, 25]the center of which is presently set at N = 162 and Z = 108.While the (deformed) nuclear shell at N = 152 is well established on the basis of the Q α - value as seen fromfig. 22 and there is, despite of scarce data, strong evidence for a shell at N = 162, the quest for a shell closureat Z = 100 is still open. It was pointed out by Greenlees et al. [162] that their results on nuclear structureinvestigation of Fm are in-line with a shell gap at Z = 100, but 2p - separation energies and Q α - valuesdo not support a shell closure. The item is shown in fig. 28. On the right hand side Q α values and 2p -binding energies ( S p ) are plotted for three isotone chains ( N = 124, 126, 128) around Z = 82. In all threecases a strong increase in the Q α values and a strong decrease in the S p values is observed from Z = 82 to Z = 84. On the right hand side Q α values and S p values are plotted around Z = 100 for N = 150, 152, and154. Here a straight increase for both is observed from Z = 94 to Z = 106, i.e. the data do not indicate ashell closure at Z = 100. This means, even if a gap in the single particle levels will be confirmed in furtherexperiments this feature does not prove a proton shell (or a ’magic’ number) at Z = 100 as claimed recently[49]. The analysis of α decay chains from SHN produced in reactions of Ca with actinide targets so faracted on the assumptions that the chains consisted on a sequence of α decays and were finally terminatedby spontaneous fission [27]. The possibility that one of the chain members could undergo EC - decay wasnot considered. Indeed, EC - decay of superheavy nuclei has been only little investigated so far. Mainlythis is due to the technical difficulties to detect EC decay at very low production rates of the isotopes.Consequently, only very recently EC decay has been investigated successfully in the transactinide region forthe cases of Rf [164] and
Db [100]. Two ways of identifying EC - decay turned out to be successful, a)measuring delayed coincidences between K X-rays and α decay or spontaneous fission of the EC - daughter,and b) measuring delayed coincidences between implanted nuclei and conversion electrons (CE) from decay ofexcited states populated by the EC or delayed coincidences between CE and decays ( α decay or spontaneousfission) of the EC daughter. The latter cases, however, require population of excited level decaying byinternal conversion, which is not necessarily the case.Evidence for occuring EC within the decay chains of SHE gives the termination of the decay chains of odd-odd nuclei by spontaneous fission. Since spontaneous fission of odd-odd nuclei is strongly hindered, it canbe assumed that it may not the odd-odd nucleus that undergoes fission, but the even-even daughter nucleus,produced by EC decay [165].The situation is, however, quite complicated. To illustrate we compare in fig. 29 the experimental (EC/ β + )- halflives of lawrencium and dubnium isotopes with recently calculated [166] EC - halflives.In general the agreement between experimental and calculated values is better for the dubnium, specifcallyfor A ≤ Md and
Db observation of spontaneousfission of an odd-odd isotope is reported.
Db seems, however, a less certain case (see discussion in [167]).In table 10 the ’fission halflives’ of
Db,
Db and
Md are compared with the values obtained for theirodd-mass neighbouring isotopes with
A-1 and
Z-1 , respectively. Evidently the resulting hindrance factors HF = T SF (Z,A)/T SF (Z,A-1) or HF = T SF (Z,A)/T SF (Z-1,A-1) are much lower for Db (21.7,20.0) thanfor
Md (482,1.9x10 ), These low values suggest that ’fission’ of Db originates indeed from the EC -daughter
Rf, the lower hindrances factors for
Db, although the case is debated, puts some doubts inthat interpretation. On the other hand it is quite common to take the ratio of the experimental fissionhalf-life and an ’unhindered’ fission half-life, defined as the geometric mean of the neighbouring even - evenisotopes (see [167] for more detailed discussion), but to estimate reliable hindrance factors the spontaneousfission half-lives of the surrounding even-even nuclei have to be known. In the region of , , Db only forone even-even isotope,
Sg the fission halflive is known, T sf = 58 s, while a theoretical value of T sf = 0.35s was reported [168], which is lower by a factor of ≈
254 256 258 260 262 264 266 268 270 272 274 2761E+001E+011E+021E+031E+041E+05 T / / s mass number (Z = 105) b+,ECb - exp.
252 254 256 258 260 262 264 266 2681E+011E+021E+031E+041E+05 T / / s mass number (Z = 103) exp.b + , ECb - Fig. 29 : Comparison of experimental and calculated [166] EC - halflives for lawrencium (upper figure)and dubnium (lower figure) isotopes. For the cases , , Db it was assumed that spontaneous fissionoriginates from the even-even EC - daughter. Full squares - experimental halflives, open squares - β + ,CE -halflives, circles - β − halflives.
265 266 267 268 269 270 27110 -1 T / / s mass number A T (exp) T EC Dubnium isotopes T a T SF * 165 (EC daughter) Fig. 30 : Comparison of experimental and calculated [166] EC - halflives, α halflives [98] of , , Db andtheoretical SF halflives [168] of the EC daughters , , Rf.4 -3 -2 -1 T / / s T EC T (exp) Z = 117 (Ts) -3 -2 -1 T / / s T EC T (exp) Z = 115 (Mc)
166 168 170 172 174 176 17810 -3 -2 -1 T / / s Neutron Number T EC T (exp) Z = 113 (Nh)
Fig. 31 : Comparison of experimental and calculated [166] EC - halflives for Z = 113, 115, 117 - isotopes.The full squares denote the theoretical EC - halflives, the open squares the experimental halflives. The linesare to guide the eye.5 -3 -2 -1 T / / s Z = 111(Rg) -4 -3 -2 -1 T / / s Z = 109 (Mt)Z = 107 (Bh)
154 156 158 160 162 164 166 168 170 17210 -3 -2 -1 T / / s Neutron Number
Fig. 32 : Comparison of experimental and calculated [166] EC - halflives for Z = 107, 109, 111 - isotopes.The full squares denote the theoretical EC - halflives, the open squares the experimental halflives. The linesare to guide the eye.6 -3 -2 -1 T / / s -1 T / / s Z = 114 (Fl)
166 168 170 172 174 176 17810 -4 -2 T / / s Neutron NumberZ = 116 (Lv) Z = 112 (Cn)
Fig. 33 : Comparison of experimental and calculated [166] EC - halflives for Z = 112, 114, 116 - isotopes.The full squares denote the theoretical EC - halflives, the open squares the experimental halflives. The linesare to guide the eye.7 -5 -3 -1 T / / s -3 -1 T / / s
152 154 156 158 160 162 164 166 168 170 17210 -4 -2 T / / s Neutron NumberZ = 110 (Ds) Z = 108 (Hs)Z = 106 (Sg)
Fig. 34 : Comparison of experimental and calculated [166] EC - halflives for Z = 106, 108, 110 - isotopes.The full squares denote the theoretical EC - halflives, the open squares the experimental halflives. The linesare to guide the eye.
Sg is predicted as stable against beta - decay.8 Isotope T SF /s HF Db 93600
Db 4320 21.7
Rf 4680 20.0
Db 103
Db 18.4 5.6
Rf 32.5 3.2
Md 2.75x10
Md 5700 482
Rf 1.5 1.9x10 Tab. 10 : Comparison of ’fission’ halflives of some selected odd-mass and odd-odd nuclei in the range Z = 101 - 105. The ’hindrance factor’ HF here means the ratio of fission halflives of the odd-odd nucleus andits neighbouring odd mass nuclei (see text).sections of some nanobarn, while in the considered SHE region production rates are roughly three ordersof magnitude lower. So technical effort to increase production rates and detection efficiencies are requiredto perform successful experiments in that direction. From phsysics side such experiments may cause bigproblems as seen from fig. 30, where experimental halflives of , , Db (red circles) are compared withthe calculated EC halflives from [166] (black squares). Calculated SF halflives from [168] for the even-evenEC - daughters , , Rf, are in the range of ≈
20 ms - ≈
20 s, so the technique for identification shouldbe applicable. The situation could be, however, unfavourable if there is a similar situation as in the case of
Sg, where the experimental SF halflife is a factor of 165 longer than the predicted one. These modified SFhalflives are shown in fig. 30 by the margenta triangles. For comparison in fig. 30 also the expected α decayhalflives for , Db based on the E α values calculated from the mass predictions of [18] ( E α ( Db) = 7242keV, E α ( Db) = 7076 keV) are presented. As for
Db a value of E α = 7721 keV is predicted in [18], whichis roughly 200 keV lower than the experimental value of E α = 7.90 ± , Db. The halflives were calculated using the formula from [98]. Results are shownas blue dots in fig. 30. For
Db the experimental α - decay half-life is given. Evidently the values for , Db are still about an order of magnitude higher, so non-observation of α decay of these isotopes so faris in-line with the expectations.Another interesting feature is to identify candidates for EC - decay within the α - decay chains. Withrespect to the quite uncertain predictions of EC - halflives that task is not trivial. As the experimentalEC halflives are longer than the calculated ones for the lawrencium and dubnium isotopes (see fig. 29) onetentatively may assume that conditions are similar for the heavier elements (it should be kept in mind thatthis item is not proven !). In other words, candidates for EC decay are isotopes for which the experimentalhalf-life is similar or even longer than the calculated [166] EC - half-life. In figs. 31-34, the experimentaland calculated EC halflives are compared for the known nuclei with Z ≥ Z = 108, Z = 108, and Z = 114-118; possible candidates are , Nh( N =172,174), , Cn ( N =171,173), , , Rg ( N =169,170,171), Bh (N=163), and
Sg. It should,however, stressed that these isotopes are just ’candidates’.An example for possibly having observed EC in α decay chains are the ’short chains’ registered in ir-radiations of Am with Ca [123], denoted as B1 - B3. As a possible explantion the decay sequence Mc α → Mc EC → Cn SF → was given in scenario 2. So far, this item is, however, is just an interestingfeature, which has to be investigated thoroughly in future. Spontaneous fission is believed to finally terminating the charts of nuclei towards increasing protonnumbers Z . The strong shell stabilization of nuclei in the vicinity of the spherical proton and neutron shells Z = 114 or Z = 120 and N = 172 or N = 184 leads also to high fission barriers and thus long fission half-lives.Qualitatively these expections are in line with the experimental results. For all nuclei Z >
114 so far only α decay was observed, while for Z = 112 – 114 only for five nuclei , Fl, , , Cn spontaneous fissionwas reported. The spontaneous fission half-lives of the even-even nuclei , Fl, , Cn,
Ds agreewithin two orders of magnitude, those for , Fl even within one order of magnitude with the predictionsof R. Smolanczuk et al. [168], which calculations also quite fairly reproduce the half-lives of the even-even9
244 248 25230405060200250300350400450
244 248 252 _ c ) a ) experiment ___+___ _ E * / ke VE * / ke VE * / ke V Es Es Es Es Es Es _ b ) (h ) HFB - SLy4 [172] + + E / ke V (f )(i )(f ) Es Esmacros.-micros. [174] Es Es Es Es Es Es Es Es Es Es Es experiment DE (9/2+)-(7/2+) D E((7/2-[514])-(7/2+[633])) D E((7/2-[514]-(7/2+[633]))) d ) E / ke V theory [18] e ) b b mass number Fig. 35 : a) experimental low lying Nilsson levels in odd-mass einsteinium iotopes (data taken from [151]);b) results of HFB - SLy4 calculations for odd mass einsteinium isotopes (data taken from [172]); c) resultsof macroscopic - microscopic calculations for odd mass einsteinium isotopes [174]; d) (upper panel) energydifferences between the 7/2 − [514] and 7/2 + [633] Nilsson levels, (lower panel) energy difference betweenthe 7/2 + [633] bandhead and the 9/2 + rotational band member; e) (upper panel) quadropule deformationparameters β for odd mass einsteinium isotopes [18], (lower panel) hexadecapole deformation parameters β for odd mass einsteinium isotopes [18].isotopes of rutherfordium ( Z = 104), seaborgium ( Z = 106), and hassium ( Z = 108). These results indicatethat the expected high stabilization against spontaneous fission in the vicinity of the spherical proton andneutron shells is indeed present. For further discussion of these items we refer to the review paper [167]. Detailed information on nuclear structure of heaviest nuclei provide a wide field of information for testingnuclear models with respect to their predictive power. Presently the situation, however, is not very satisfyingfor at least three major reasons;a) ’detailed’ decay studies using α - γ spectroscopy are essentially only possible for nuclei with Z ≤
107 dueto low production rates;b) for many isotopes only very few Nilsson levels have been identified, while the assignment is partly onlytentative;c) agreement between experimental data and results from theoretical calculations is in general rather poor.In [150] experimental data are compared with results from theoretical calculations for N = 151 and N = 153isotones of even-Z elements in the range Z = 94-106. Agreement in excitation energies of the Nilsson levelsis often not better than a few hundred keV and also the experimentally established ordering of the levelsis often not reproduced by the calculations. Thus, for example, the existence of the low lying 5/2 + [622] -isomers in the N = 151 isotones is not predicted by the calculations. These deficiencies, on the other hand,make it hard to trust in predictions of properties of heavier nuclei by these models.In this study the situation will be illustrated for the case of the odd-mass einsteinium isotopes (fig. 35).Experimentally only two Nilsson levels have been established in all presented isotopes, namely 7/2 + [633] and07/2 − [514]. In the heaviest isotopes, , Es also the 3/2 − [521] was assigned. While in , Es 7/2 + [633]was identified as ground - state [169, 170], in case of Es the ground-state was assigned as 3/2 − [521] [171].For the lighter einsteinium isotopes the situation is unclear. The Nilsson levels 7/2 + [633] and 7/2 − [514]have been established from α - γ decay studies of odd-mass mendelvium isotopes [151, 172]. However noground-state assigment was made as on the basis of the results for the heavier einsteinium isotopes as itcould not be excluded, that the 7/2 + [633] and 3/2 − [521] are close in energy and may alter as ground-state.Indeed a more detailed decay study of Md indicates that the 3/2 − [521] level may be the ground-state in Es, while the 7/2 + [633] is located at E ∗ = 10 keV [173].The experimental data are compared with theoretical calculations in figs. 35b und 35c. In fig. 35b theresults from a self-consistent Hartree-Fock-Bogoliubov calculation using SLy4 force (HBF - SLy4) are pre-sented (data taken from [172]), in fig. 35c the results from a macroscopic - microscopic calculation [174]. TheHBF - SLy4 calculations only predict the ground-states of Es correctly, for
Es they result in 7/2 + [633]as for Es. For the lighter isotopes the ground-state is predicted as 1/2 − [521], while the 3/2 − [521], forwhich strong experimental evidence exists that it is ground-state or located close to the ground-state, islocated at E ∗ ≈
400 keV, except for
Es. The macroscopic - microscopic calculations, on the other side,predict 7/2 + [633] as a low lying level but the 3/2 − [521] one in an excitation energy range of E ∗ ≈ (400-600)keV. As noted in fig. 35b, the 3/2 − [521], 7/2 − [514] and 7/2 + [633] Nilsson levels arise from the f / , h / and i / subshells located below the shell gap at Z = 114, while the 1/2 − [521] stems from the f / subshelllocated above it [161]. The 3/2 − [521], 7/2 − [514] and 1/2 − [521] decrease in energy at increasing deforma-tion, while the 7/2 + [633] increases in energy. At a deformation ν ≈ ≈ − [521] and 7/2 + [633] states are located below the predicted shell gap, whilethe 7/2 − [514] and 1/2 − [521] are located above it. From this side one can expect that the energy difference∆E = E(7/2 − [514]) - E(7/2 + [633]) gives some information about the size of the shell gap. Indeed the exper-imental energy difference is lower than predicted by the HFB-SLy4 calculations (typically ≈
400 keV) andby the macroscopic - microscopic calcultions (typically ≈
600 keV) as seen from figs. 35a - 35c, which hintsto a lower shall gap as preticted. Indeed this could explain the non-observation of a discontinuity in thetwo-proton binding energies and the Q α - values when crossing Z = 100 (see sect. 7.2).Two more interesting features are evident: in figs. 35d and 33e (upper panel) the energy difference∆E = E(7/2 − [514]) - E(7/2 + [633] is compared with the quadrupole deformation parameter β , while in thelower panels the energy difference of the 7/2 − [514] bandhead and the 9/2- rotational level is compared withthe hexadecapole deformation parameter β , both taken from [18]. Both, the experimental energy differences∆E = E(7/2 − [514]) - E(7/2 + [633]) (not so evident in the calculation) and the β values show a pronouncedmaximum at N = 152, while as well as the energy differences E(9/2 − ) - E(7/2 − as the β - values decreaseat increasing mass number or increasing neutron number, respectively. Md and Lr Predictions of level schemes in heaviest odd-odd nuclei are scarce so far. Only for a couple of casescalculations have been performed so far. Thus we will discuss here only two cases,
Md and
Lr forwhich recently new results have been reported [60].The ground-state of
Md was predicted by Sood et al.[175] as K π = 0 − and a long-lived isomeric statewith spin and parity K π = 7 − expected to decay primarily by α emission or electron capture was predictedat E ∗ = 80 ±
30 keV. Recently a longlived isomeric state at E ∗ = 123 keV was identified [60] in quite goodagreement with the calculations. However, no spin and parity asignments have be done for the groud stateand the isomeric state.The other case concerns Lr. Levels at E ∗ <
250 keV were recently calculated on the basis of a ’Two-Quasi-Particle-Rotor-Model’ [176]. The results are shown in fig. 36. The ground-state is predicted as K π = 1 + and an isomeric state K π = 4 + is predicted at E ∗ ≈
75 keV. Recently an isomeric state at E ∗ = 108 keV wasidentified in Lr. Tentative spin and parity assigments are, however, different. The ground-state wasassigned as K π = 4 + , the isomeric state as K π = 1 − (see fig. 36). This assignment was based on the assumedground-state configuration K π = 0 − of Db and the low α -decay hindrance factor HF ≈
30 for the transition g Db → m Lr which rather favors K π = 1 − than K π = 4 + as the latter configuration would require anangular momentum change ∆K = 3 and a change of the parity which requires a much larger hindrance factor(see sect. 7.1).Here, however, two items should be considered:a) the spin-parity assigmnent of Db is only tentative,b) the calculations are based on the energies of low lying levels in the neighboring odd mass nuclei, in therespecting case
No (N = 151) and
Lr (Z = 103). The lowest Nilsson levels in
No are 9/2 − [734] for1 - (K=K( p) -K( n )( p - [514] x n + [622]))1 - (K=K( p) -K( n )( p - [514] x n + [622])) + (K=K( p) -K( n )( p - [521] x n - [734]))4 + (K=K( p) -K( n )( p - [521] x n - [734]))5 + (K=K( p) +K( n ) ( p - [521] x n - [734]))8 + (K=K( p) +K( n )( p - [514] x n - [734]))1 + (K=K( p) -K( n )( p - [514] x n - [734])) e n e r y / ke V a) theory [176] b) experiment [60] Lr Fig. 36 : Predicted (a) ([176]) and (b) experimentally (tentatvely) assigned [60] low lying levels of
Lr.the ground-state and 5/2 + [622] for a shortlived isomer at E ∗ = 167 keV [126]. In Lr tentative assignmentsof the ground-state (7/2 − [514]) and 1/2 − [521] for a low lying isomer are given in in [125]. The energy of theisomer is experimentally not established, for the calculations a value of 30 keV was taken [176]. It shouldbe noted, however, that for the neighboring N = 152 isotope of lawrencium, Lr the ground-state hadbe determined as the Nilsson level 1/2 − [521], while 7/2 − was attributed to a low lying isomeric state atE ∗ = 37 keV [172]. Therefore, with respect to the uncertain starting conditions, the results of the calculationsalthough not in ’perfect agreement’ with the experimental results, are still promising, and may be improvedin future.It should be noticed the that existence and excitation energy of the isomeric state in Lr has been confirmedby direct mass measurents at SHIPTRAP [177], and there is some confidence that spins can be determinedin near future by means of laser spectroscopy using the RADRIS technique [178].
Although elements up to Z = 118 have been syntheszied so far, the quest for the location of the spher-ical ’superheavy’ proton and neutron shells is still open. Indeed synthesis of elements up Z = 118 in Cainduced reactions show a maximum in the cross sections at Z = 114, which might be seen as an indication ofa proton shell at Z = 114 (see fig. 16). Such an interpretation, however, is not unambiguous since a completeunderstanding of the evaporation residue (ER) production process (capture of projectile and target nuclei,formation of the compund nucleus, deexcitation of the compound nucleus, competition between particleemission and fission) is required to draw firm conclusions. Indeed V.I. Zagebaev and W. Greiner [179] couldreproduce cross-sections for elements Z = 112 to Z = 118 produced in Ca induced reaction quite fairly, butevidently a main ingredient of their calculations was quite uncertain. They approximated fission barriersas the sum of the ’shell effects’ (according to [18]) and a ’zero-point energy’ of 0.5 MeV, which resulted inquite different values than obtained from ’direct’ fission barrier calculations (see e.g. [180, 181, 182]). Dueto these uncertainties measured cross sections are not a good argument identification for a proton shell atZ = 114 ¶ .From this point of view it rather seems useful to take the α decay properties as a signature for a shell, as ¶ We want to note that recently Samark-Roth et al. [108] claimed on the basis of their results on decay studies of , Fland their daughter products that their is not real indication for a poton shell at Z = 114.
108 110 112 114 116 118 12067891011121314 Q a / M e V Z N = 172 N = 184
Fig. 37 : Predicted Q α values along the N = 172 and N = 184 isotones lines [17]. The experimental Q α valuesfor Fl and
Cn (data from [108]) are shown by the open squares.
164 166 168 170 172 174 176 178 180 182 184 1861E-81E-71E-61E-51E-40,0010,010,1
M(cid:246)ller et al. [18]Litvinova, RMF+VC [186]Litvinova, RMF [186]Cwiok et al., HFB-SLy4 [185]Typel, Brown, HFB-SKX [183]Smolanczuk, Sobiczewski [17] E a / M e V TASCA: ca. 0.7 m sSHIP: ca. 2.1 m sSeparation times T a / s neutron number Fig. 38 : Comparison of Q α values and halflives for element 120 isotopes from different models. See text fordetails.3discussed in sect. 7.2. However, one has to note that strictly spoken even - even nuclei have to be consideredsince only for those a ground-state to ground-state transition can be assumed a priori as the strongest decayline. However one is presently not only confronted with the lack of experimental data. The situation isshown in fig. 37 where predicted Q α values for the N = 172 and N = 184 are presented. Different to situationat Z = 82 (see fig. 28) calculations of Smolanczuk et al. [17] predicting Z = 114 as proton shell result only ina rather small change in decrease of Q α values when crossing the shell, even at the predicted neutron shellat N = 184 compared to the heavier and lighter isotones. At N = 172 there is practically no effect any more,one gets a more or less straight decrease of the Q α values. So probably Q α values could not be applied foridentifying Z = 114 as a proton shell even if more data would be available.A possibility to decide whether the proton shell is located Z = 114 or Z = 120 results from comparison ofexperimental Q α values and halflives with results from models predicting either Z = 114 or Z = 120. However,one has to consider a large straggling of the predicted values, so it is required to produce and to investigatenuclei in a region, where uncertainties of the predictions are larger than the results from models predictingeither Z = 114 or Z = 120 as proton shells. An inspection of the different models shows that element 120seems to be first one where the differences are so large that the quest of the proton shell can be answeredwith some certainty. The situation is shown in fig. 38, where predicted Q α values and calculated halflives arecompared. Despite the large straggling of the predicted α energies and halflives there is seemingly a borderlineat E α = 12.75 MeV evident between models precting Z = 120 als proton shell [183, 184, 185, 186] and thosepredicting Z = 114 as proton shell [17, 18] while halflives < − s hint to Z = 114, halflives > − s to Z = 120as proton shell. This feature makes synthesis of element 120 even more interesting than the synthesis ofelement 119. Suited reactions to produce an even-even isotope of element 120 seem Ti(
Cf,3n) Cr(
Cm,2n,4n) ,
120 (N=178,180). Expected cross-sections are, however small.V.I. Zagrebaev and W. Greiner [187] predicted cross sections of σ ≈
25 fb for Cr(
Cm,4n)
120 anda slightly higher value of σ ≈
40 fb for Ti(
Cf,3n) Ni +
U at SHIP, GSI with σ< Cr +
Cm at SHIP, GSI with σ< Ti +
Cf at TASCA, GSI with σ< Fe +
U at DGFRS, JINR Dubna with σ<
9. Challenges / Future
There are two major problems concerning the experimental techniques used in the investigation of super-heavy elements. The first is connected with the implantation of the reaction products into silicon detectorswhich are also used to measure the α -decay energy, conversion electrons and fission products. This simplymeans that, e.g. in the case of α decay not only the kinetic energy of the α -particle is measured but alsopart of the recoil energy transferred by the α particle to the residual nucleus. Due to the high ionisationdensity in the stopping process of the heavy residual nucleus and partial recombination of the charge carriers,typically only about one third of the recoil energy is measured [61]. It results in energy shift of the α -decayenergy by ≈
50 keV, which can be compensated by a proper calibration, and a deterioration of the energyresolution of the detector by typically 5 - 10 keV.A second item is more severe. It is connected with populating excited levels in nuclei decaying promptly(with life-times of some µ s or lower) by internal conversion. In these cases energy summing of α particles withconversion electrons (and also low energy X-rays and Auger electrons from deexcitation of the atomic shell)is observed [62]. The influence on the measured α spectra is manifold, depending also on the energy of theconversion electrons; essentially are broadening and shifting the α energies often washing out peak structuresof α decay pattern. An illustrative case is α decay of No has been investigated using the implantationtechnique [153] and the He-jet technique with negligible probability of energy summing [154]. Specifically,different low lying members of the same rotational are populated, which decay by practically completelyconverted M1 or E2 transitions towards the band-head, these fine structures of the α decay spectrum cannotbe resolved using the implantation technique (see also [150]). Although in recent years successful attemptshave been untertaken to model those influences by GEANT - simulations [65], direct measurement are pre-ferred from experimental side. First steps in this direction have been recently undertaken by coupling anion trap [191] or an MRTOF - system [192] to a recoil separator and the BGS + FIONA system, which wasused to directly measure mass number of Mc [193].Also mass number measurement is an interesting feature, the ultimate goal is a save Z and A identificationof a nuclide. This can be achieved via high precision mass measuremts, allowing for clear isobaric separation(ion traps and possibly also MRTOF - systems). Presently limits are set by the production rate.The most direct method to determine the atomic number of a nucleus is measuring characteristic X-rays inprompt or delayed coincidence with its radioactive decay. Such measurements are, however, a gamble as theyneed both, highly K - converted transitions (M1, M2) with transition energies above the K - binding energy.4The latter is not a trivial problem as energies raise steadily and are in the order of 180 keV at Z = 110.Such measurements have been applied so far up bohrium ( Z = 107 [158]). In the region of superheavy nuclei( Z > α decay chains starting from the odd-odd nucleus Mc ( Z = 115), but no positive resultwas obtained.Alternatively one can attempt to measure L - X-rays to have excess to lower energies and also to E2 tran-sitions. Such measurement have been performed successfully up to Z = 105 [194], but are more complicateddue to the more complex structure of the L X-ray spectra.An alternative method for X-ray identification is measuring the X-rays emitted during electron capture (EC)decay in delayed coincidence with α decay or spontaneous fission of the daughter nucleus. This techniquehas been recently for the first time successfully applied in the transactinide region [100], by measuring K α and K β X-rays from EC - decay of
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