New α-Emitting Isotope ^{214}U and Abnormal Enhancement of α-Particle Clustering in Lightest Uranium Isotopes
Z. Y. Zhang, H. B. Yang, M. H. Huang, Z. G. Gan, C. X. Yuan, C. Qi, A. N. Andreyev, M. L. Liu, L. Ma, M. M. Zhang, Y. L. Tian, Y. S. Wang, J. G. Wang, C. L. Yang, G. S. Li, Y. H. Qiang, W. Q. Yang, R. F. Chen, H. B. Zhang, Z. W. Lu, X. X. Xu, L. M. Duan, H. R. Yang, W. X. Huang, Z. Liu, X. H. Zhou, Y. H. Zhang, H. S. Xu, N. Wang, H. B. Zhou, X. J. Wen, S. Huang, W. Hua, L. Zhu, X. Wang, Y. C. Mao, X. T. He, S. Y. Wang, W. Z. Xu, H. W. Li, Z. Z. Ren, S. G. Zhou
NNew α -Emitting Isotope U and Abnormal Enhancement of α -Particle Clustering inLightest Uranium Isotopes Z. Y. Zhang,
1, 2
H. B. Yang, M. H. Huang,
1, 2
Z. G. Gan,
1, 2, ∗ C. X. Yuan, C. Qi, A. N. Andreyev,
5, 6
M. L. Liu,
1, 2
L. Ma, M. M. Zhang, Y. L. Tian, Y. S. Wang,
1, 2, 7
J. G. Wang, C. L. Yang, G. S. Li, Y. H. Qiang, W. Q. Yang, R. F. Chen, H. B. Zhang, Z. W. Lu, X. X. Xu,
1, 2
L. M. Duan,
1, 2
H. R. Yang,
1, 2
W. X. Huang,
1, 2
Z. Liu,
1, 2
X. H. Zhou,
1, 2
Y. H. Zhang,
1, 2
H. S. Xu,
1, 2
N. Wang, H. B. Zhou, X. J. Wen, S. Huang, W. Hua, L. Zhu, X. Wang, Y. C. Mao, X. T. He, S. Y. Wang, W. Z. Xu, H. W. Li, Z. Z. Ren, and S. G. Zhou
14, 15 CAS Key Laboratory of High Precision Nuclear Spectroscopy,Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China School of Nuclear Science and TechnologyUniversity of Chinese Academy of Sciences, Beijing 100049, China Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-Sen University, Zhuhai 519082, China Department of Physics, Royal Institute of Technology (KTH), Stockholm SE-10691, Sweden Department of Physics, University of York, York, YO10 5DD, United Kingdom Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan School of Nuclear Science and TechnologyLanzhou University, Lanzhou 730000, China Guangxi Key Laboratory of Nuclear Physics and Technology,Guangxi Normal University, Guilin 541004, China State Key Laboratory of Nuclear Physics and Technology,School of Physics, Peking University, Beijing 100871, China Department of Physics, Liaoning Normal University, Dalian 116029, China College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment,School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai 264209, China School of Physics Science and Engineering, Tongji University, Shanghai 200092, China CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics,Chinese Academy of Sciences, Beijing 100190, China Center of Theoretical Nuclear Physics, National Laboratory of Heavy-Ion Accelerator, Lanzhou 730000, China
A new α -emitting isotope U, produced by fusion-evaporation reaction W( Ar, 4n)
U,was identified by employing the gas-filled recoil separator SHANS and recoil- α correlation technique.More precise α -decay properties of even-even nuclei , U were also measured in reactions of Ar, Ca with , , W targets. By combining the experimental data, improved α -decay reducedwidths δ for the even-even Po–Pu nuclei in the vicinity of magic neutron number N = 126 werededuced. Their systematic trends are discussed in terms of N p N n scheme in order to study theinfluence of proton-neutron interaction on α decay in this region of nuclei. It is strikingly foundthat the reduced widths of , U are significantly enhanced by a factor of two as comparedwith the N p N n systematics for the 84 ≤ Z ≤
90 and
N <
126 even-even nuclei. The abnormalenhancement is interpreted by the strong monopole interaction between the valence protons andneutrons occupying the π f / and ν f / spin-orbit partner orbits, which is supported by a large-scale shell model calculation. Nucleon-nucleon interaction, which governs the exis-tence of nuclear system, plays a fundamental role inunderstanding of the properties of exotic nuclei far fromstability. Although the proton-proton ( p - p ) and neutron-neutron ( n - n ) correlations are well-known to be crucialfor explaining a wealth of experimental data, the proton-neutron ( p - n ) interaction has long been recognized asone of the essential driving forces for the shell structureevolution, the development of collectivity and the onsetof deformation in atomic nuclei [1–10]. In the lastdecades, thanks to the development of radioactive beamfacilities worldwide, enormous progress in the physicsof change of nuclear shell structure as a function ofproton and/or neutron numbers has been achieved inlight nuclei. However, the experimental knowledge forthe structure evolution in heavy nuclei around and belowthe neutron closed shell at N = 126 remains scarce at present [3, 11–15].It is well known that, because of large overlap ofthe radial wave functions, the attractive and short-range interaction between valence protons and neutronsoccupying orbits with the same number of nodes andorbital angular momenta (i.e., ∆ n = ∆ l = 0) becomesstronger than those in other categories, and eventuallytriggers the remarkable changes of closed shells (see [3, 4],and references therein). For instance, the π f / – ν f / interaction was shown to play an important role forstructure evolution of N = 34 isotones of Ca, Ti, Cr, andFe, culminating in the creation of new magic numbersat N = 32, 34 in , Ca (see Fig. 1 from Ref. [16] andRef. [17]). In the trans-lead nuclear region with
Z > N ≤ h / , 1 f / ,and 0 i / orbits, while the neutrons mainly occupy the2 p / , 1 f / , and 2 p / orbits [18, 19]. Therefore, one can a r X i v : . [ nu c l - e x ] J a n expect the monopole p - n interaction between the π f / and ν f / spin-orbit partner orbits to have a significantimpact on nuclear structure evolution in that region. α -decay spectroscopy has been proven to be a powerfultool to probe the nuclear structure in heavy nuclei [20–23]. There are analytical formulae to calculate the α -decay half-lives such as the new Geiger-Nuttall law[24, 25]. Typically, the α -decay process is described bythe two-step mechanism, involving the preformation of α particle followed by its penetration through Coulomband centrifugal barriers. The α -particle preformationprobability involves all the nuclear structure information,and can be weighed experimentally by α -decay reducedwidth δ [26] or the model-independent formationprobability | R F α ( R ) | [27, 28]. It is interesting to notethat the α -decay reduced widths of several Z ∼ N nuclei around Sn ( Z = N = 50) are enhancedby at least a factor of two relative to the benchmarknucleus Po and its neighbouring Po isotopes [29–32]. This enhancement was explained by the so-called“superallowed α decay” [20, 33] in relation to thefact that the valence protons and neutrons are in thesame single-particle levels, giving rise to a strong p - n interaction. In fact, the influence of p - n interactionon the absolute α -decay widths in Po and nearbynuclei was usually neglected in microscopic calculation,since the low-lying proton and neutron single-particlestates are very different from each other in these cases[21, 34, 35]. However, several theoretical treatments[36–38] pointed to the particular significance of p - n interaction in α decay for these nuclei.In this Letter, we report on the observation of a newisotope U and on more precise measurements forthe α -decay properties of , U ( N = 124 , π f / and ν f / spin-orbit partner orbits to alarge extent. Thus, such nuclei can provide a uniqueopportunity to test the influence of p - n interaction on α -particle clustering in heavy nuclear region.To produce , , U nuclei, a series of experimentswere performed at the gas-filled recoil separator, SHANS(Spectrometer for Heavy Atoms and Nuclear Structure)[39], at the Heavy Ion Research Facility in Lanzhou(HIRFL), China. For
U, the fusion-evaporationreaction of W( Ar, 4n)
U with a beam energy of184 MeV and a typical beam intensity of ∼
500 pnAwas used. The
W targets with a thickness of 300-350 µ g/cm were prepared by sputtering the materialonto 80- µ g/cm -thick carbon foils and then covered by10- µ g/cm -thick carbon layer. The recoiled evaporationresidues (ERs) were separated efficiently by SHANSand collected by three 16-strip position-sensitive silicondetectors (PSSDs), which were mounted side by side atthe focal plane of the separator. Each PSSD has an activearea of 50 ×
50 mm . Due to the shallow implantationdepth of ∼ µ m, the full-energy α particles emitted from ERs and/or their descendants were registered with anefficiency of ∼ α particles. For suchevents, the total α -particle energy was reconstructed byadding the deposited energies in PSSD and SSD. In orderto distinguish the α -decay events from the implantedproducts, two multi-wire proportional counters wereinstalled upstream from the PSSDs. A digital datareadout system including waveform digitizers was usedfor the data acquisition. Details of the detection systemand data analysis method were described in Refs. [14, 15,40, 41].The identification of U was performed by searchingfor the position-time correlated α -decay chains with thehelp of known α -decay properties of its descendants.An energy spectrum for α -decay events following theERs and a two-dimensional plot for the decay energycorrelation between mother and daughter nuclei (ER − α m − α d ) are shown in Fig. 1(a) and 1(b), respectively.The Pa, Th, and Ac isotopes were produced fromcharged-particle evaporation channels. Two decay eventsin Fig. 1(b) were assigned to the new isotope Uunambiguously. The details of these decay chains are C oun t s / k e V ) < 0.01 s m α t (ER - ∆ (a) Ac Ac Th Pa -particle energy (keV) α Mother - pa r t i c l e ene r g y ( k e V ) α D augh t e r (b) U U U Pa Pa Th Th Ac ) < 0.01 s m α t (ER - ∆ ) < 50 s d α - m α t ( ∆ FIG. 1. a) Energy spectrum for α -decay events followingrecoil implantations within a time window of 10 ms. b) Two-dimensional plot of mother and daughter α -particle energiesfor ER − α m − α d correlations in the Ar+
W reaction.Maximum search times for the ER − α m and α m − α d pairsare 10 ms and 50 s, respectively. The decay events from thenew isotope U are indicated by red arrows. E a : 7923 keV: 6.45 ms: -2.5 mm TP a a a a E a : 7287 keV: 0.145 s: -2.1 mm TPE a : 705 keV+6082 keV: 0.673 s: -4.7 mm TP E a : 8543 keV: 0.286 ms: -1.8 mm TP a EP imp : 9256 keV: -2.4 mm E a : 6197 keV: 50.0 s: -2.4 mm TP E a : 3618 keV+4364 keV: 3.51 ms: -21.2 mm TP Missing E a : 6643 keV: 8.71 s (sum of 3 and 4 decay times): -19.6 mm T a a P E a : 8522 keV: 1.228 ms: -19.3 mm TP E imp : 10171 keV: -19.2 mm P chain 1 chain 2 UThRaRnPo a a aa FIG. 2. Observed α -decay chains for U. For each chain,the implantation energy of ERs ( E imp ), the α -particle energy( E α ), the decay time ( T ), and the position ( P ) in the stripdetector are shown. The reconstructed energies for escaping α decays are given as the sum of the PSSD and SSD energies. displayed in Fig. 2. The measured decay properties ofdaughter products match well with the known data [42]for Th,
Ra,
Rn, and
Po. Based on thesemeasurements, the mean α -particle energy and half-life of U were determined to be 8533(18) keV and0 . +0 . − . ms, respectively, which are listed in Table I. Theuncertainties of half-life were estimated by the maximumlikelihood method described in Ref. [43]. The productioncross section for U was determined to be 10 +14 − pb.The properties of U, which was the lightest even-even uranium isotope known previously, were reportedin our previous work [44] and in Refs. [45, 46]. However,at most four decay chains from the ground state of
Uwere observed in each study, resulting in a relativelylarge uncertainty of decay half-life. In the presentinvestigation, the same experimental setup as for
Uwas used, but with a reaction of W( Ar, 4n)
U ata beam energy of 191 MeV. Thirteen decay chains wereassigned to the ground-state-to-ground-state (g.s.-to-g.s.)decay of
U. The deduced decay energy and half-life of
U are 8374(17) keV and 1 . +0 . − . ms, respectively.By combining all data from the present study and fromRefs. [44–46], the averaged half-life for the ground stateof U was deduced to be 2 . +0 . − . ms. The results arecompared with the literature data in Table I.In order to obtain more precise decay properties of U, two experiments with W( Ar, 4n)
U and W( Ca, α U reactions were carried out withbeam energies of 190 MeV and 206 MeV, respectively.Totally, 76 decay chains were assigned to the decay fromthe ground state of
U, leading to the determinationof E α = 8612(14) keV and T / = 0 . +0 . − . ms. Theuncertainties of half-life were improved in comparisonwith previous results [44, 47–49] (see Table I).To study the nuclear structure evolution in the N =126 region, the reduced widths δ for g.s.-to-g.s. decaysof even-even 84 ≤ Z ≤
94 nuclei are extracted by using Rasmussen method [26], see Fig. 3(a). The uncertaintiesof δ values are mostly due to the half-life uncertainties.The , , U values determined in this work are shownin column 4 of Table I and plotted by filled circles inFig. 3(a).In each of the Po, Rn, Ra, and Th isotopic chains,a sharp decrease of reduced widths at N = 126 iswell-established, indicating a notable neutron shell effect[26, 54, 55]. Our new data suggest for the first timethat the minimum decay width for U isotopes is likelyat U ( N = 126). This result is in contrast withour previous work [44], where only half a value of δ ( U) (34 +34 − keV) was reported. A shrinking of the δ enhancement between the N = 126 and N = 130 isotoneswith the increasing of proton number was attributed toa weakening of the N = 126 shell effect as suggested inRef. [12]. The nearly constant or even decreasing valuesfor the most neutron-deficient polonium isotopes wereexplained by the configuration mixing effect [11, 20].In Fig. 3(a), another important feature revealed by ournew data is that, while the decay widths at N = 122, 124,and 126 for Po–Th isotopes increase monotonously withincreasing proton number, an unexpected sharp increasewas observed from Th to U isotopes at the same neutronnumbers. This suggests that the α -particle formationprobability is enhanced in these U isotopes.In order to get a deeper insight into the behaviorof reduced widths, we studied the influence of the p - n interaction upon the α -decay process in this mass region.Given the fact that the N p N n scheme [56, 57] allows auniform description of structure evolution for a varietyof observables and highlights the importance of valence p - n interaction [1, 2, 58–61], the δ values are plottedagainst N p N n in Fig. 3(b). Here, N p and N n are the TABLE I. The g.s.-to-g.s. α -decay energies and half-livesof , , U measured in this work. The reduced α -decay widths δ , in column 4, are calculated by Rasmussenformalism [26] assuming the α -particle angular momentum,∆ L = 0. The data for , U are compared with literaturevalues.
Isotope This work Literature data E α /keV T / /ms δ /keV E α /keV T / /ms Ref. U 8533(18) 0 . +0 . − . +233 − - - - U 8374(17) 2 . +0 . − . +22 − . +4 . − . [44]8340(50) 3 . +8 . − . [45]8390(33) 2 . +3 . − . [46] U 8612(14) 0 . +0 . − . +7 − . +1 . − . [44]8612(9) 0 . +0 . − . [47, 48]8625(25) 1 . +7 . − . [49] a The value is deduced by combining all 21 decay events fromthis work and Refs. [44–46], and is also used for the decaywidth calculation for U. -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 9010
100 104 108 112 116 120 124 128 132 136 140 144 14810 N = Po U (b)N>126N<126 U U d ( k e V ) N p N n210 Po Rn Ra Th U Pu U Pu U Po U U d ( k e V ) Neutron numberRnPo Ra Th U N = (a) Th Ra Rn Po FIG. 3. (a) Systematics of reduced widths for g.s.-to-g.s. α decays of even-even 84 ≤ Z ≤
94 isotopes as a functionof neutron number. The decay properties are taken fromRefs. [11, 12, 42, 50–53]. The values for , , U from thiswork are shown by filled circles. The errors of reduced widthsare only determined by half-life uncertainties. (b) Same as (a)but against N p N n for even-even Po to U isotopes. The N p and N n values are calculated relative to Z = 82 and N = 126closed shells, respectively, with an exception of Po , forwhich N n = −
20, relative to the closest N = 82 neutron shell. numbers of valence protons and neutrons relative to thenearest closed shells: Z = 82 for proton and N = 126for neutron. It is striking to see that the N p N n plotdisplays a remarkable simplification for the systematicsof decay widths in this region. In the N >
126 region,the δ values increase rapidly until N p N n ≈
20, and thenconverge into a nearly constant value of ∼
150 keV (exceptfor
U). This “saturation” phenomenon might indicatethat the α decays in these nuclei are affected only slightlyby the p - n interaction, but are dominated by the p - p and n - n pairing interactions, as pointed out theoretically inRefs. [21, 34, 35]. In other words, it is the strong pairingforce among the protons and neutrons occupying high- j orbits (e.g., π h / and ν g / ) which leads to the large α -particle formation probability [11].In contrast, for the N <
126 nuclei, the δ valuesfor Po–Th isotopes show quite different behaviors,increasing exponentially with increasing the absolute N p N n quantity along a relatively compact tendency(except for , Po). The increasing trend can be partlyexplained by the increasing neutron and proton pairingcorrelations [11] as one moves away from N = 126 and
84 86 88 90 92 94012345670123456701234567 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 (b) i -p i -p i -f f -p f -p f -f h -p h -p Monopole matrix element (MeV) h -f Proton number pi P r o t on o cc upa t i on nu m be r ph pf N=122 isotones (a)
N=124 isotones N=126 isotones p N=126Z=822s n FIG. 4. (a) Calculated proton occupation numbers for the π h / (square), π f / (circle), and π i / (triangle) orbitsin N = 122, 124, and 126 even- Z isotones of Po–Pu. (b)Monopole matrix elements of p - n interaction calculated forthe Z >
82 and N ≤
126 nuclei. The inset in panel (b)shows the single-particle orbits near the
Pb doubly closedshells, and the strong interaction between the 1 f / protonsand 1 f / neutrons is marked. Z = 82. More importantly, considering that the N p N n value provides a reliable measure of interaction betweenthe valance protons and neutrons [1, 2], this specificfeature shown in Fig. 3(b) implies that the p - n interactioncan also play an essential role in the α -particle clusteringin this region.The δ values of , U, however, show strikingdiscrepancy with the unified trend established for 84 ≤ Z ≤
90 and
N <
126 nuclei. Regardless the relativelylarge uncertainties for
U, a significant enhancementby a factor of two is revealed for , U as shown inFig. 3(b). This new feature might be related to thepossible changes of occupancy of 0 h / and 1 f / protonorbits approaching Z = 92. Indeed, below Z = 92,it is expected that the 0 h / protons play a dominantrole, which is confirmed by, e.g., the 9 / − ground statesfor most of odd- A Bi, At, Fr, and Ac isotopes[42]. The 0 h / orbit is expected to be highly occupiedin U ( Z = 92) with an enhanced probability of protonoccupancy of the higher-lying 1 f / orbit. The later,combined with the neutron occupancy of 1 f / orbitaround N = 118–124, might lead to a strong monopole p - n interaction (see inset of Fig. 4(b)), which enhancesthe preformation probability in α decay.In order to verify this conjecture, we have performedlarge-scale shell model calculations for the 84 ≤ Z ≤
94 and N = 122, 124, 126 even-even nuclei. Thesame model spaces with the 0 h / , 1 f / , 0 i / , 2 p / ,1 f / , and 2 p / orbits were selected for protons andneutrons. The single-particle energies are fixed to thoseof Bi and
Pb. The p - p , n - n , and p - n parts oftwo-body interactions are taken from the Kuo-Herlingparticle interaction [62], Kuo-Herling hole interaction[63], and monopole based universal interaction [64] plusM3Y spin-orbit interaction [65], respectively. Giventhe computational limit, the restrictions, for whichthe π (2 p / , 1 f / , 2 p / ) orbits are fully empty forprotons and the ν (0 h / , 1 f / , 0 i / ) orbits are fullyoccupied for neutrons, were made. The calculated protonoccupation numbers for the π h / , π f / , and π i / orbits in N = 122, 124, and 126 even- Z isotones areshown in Fig. 4(a). It can be seen that, due to the pairingcorrelation effect [1], the valence protons occupy mainlythe 0 h / orbit with the increasing occupation probabilityof the 1 f / and 0 i / protons from Po to Pu isotopes.In particular, the effective proton occupation numbersfor the 1 f / orbit in U and Pu isotopes are almost equalto or even higher than one.The calculated monopole matrix elements between theproton and neutron orbits for the Z >
82 and N ≤
126 nuclei are shown in Fig. 4(b). The calculationsdemonstrate that all the p - n interactions involving 1 f / protons are about twice more attractive than thoseinvolving 0 h / and 0 i / protons. In particular, the π f / – ν f / interaction is by far the strongest onein this region of nuclei. Therefore, the strong p - n interactions related to the 1 f / protons, together withthe increased occupancy of the π f / orbit, would leadto the enhanced α -particle formation probability in the N = 122, 124, and 126 uranium isotopes.In summary, a new isotope U was identified andimproved α -decay properties of , U were measuredby employing the gas-filled recoil separator SHANS andrecoil- α correlation method. By combining the new andpreviously known data, we extracted the α -decay reducedwidths δ for the even-even Po–Pu nuclei with Rasmussenmethod. It is found that the δ systematics from Poto Th can be merged into two compact trends for the N <
126 and
N >
126 nuclei in terms of N p N n scheme.The behavior in the N <
126 region indicates a crucialrole played by p - n interaction in α decay. Meanwhile, itis strikingly found that the reduced widths of , U areenhanced remarkably by a factor of two relative to thesystematic trend of
N <
126 nuclei in the N p N n scheme.This might be explained as being due to the strongmonopole interaction between the valence 1 f / protonsand 1 f / neutrons combined with increased occupancyof the f / proton orbit, which was confirmed by thelarge-scale shell model calculations.As a possible outlook for the future studies in thisregion, it is expected that, in view of the continuously increasing proton occupancy of the 1 f / orbit and thefurther enhancement of p - n interaction, this effect mightbecome even stronger in the Pu isotopes. Thus, it isextremely intriguing to extend the δ systematics tohigher- Z nuclei.The authors would like to thank the acceleratorcrew of HIRFL for providing the stable beams.This work was supported by the Strategic PriorityResearch Program of Chinese Academy of Sciences(Grant No. XDB34010000), the National Key R&DProgram of China (Contract No. 2018YFA0404402),the National Natural Science Foundation of China(Grants No. U1732270, No. 11975279, No. 11775316,No. U1932139, No. 11961141004, No. 11735017,No. 12035011, No. 11965003, No. 11675225,No. 11635003, No. 11961141004, No. U1867212,No. 11805289, No. U1732139), the Chinese Academyof Sciences (Grant No. QYZDJ-SSW-SLH041),the Youth Innovation Promotion Association CAS(2020409, 2017456), the Natural Science Foundationof Guangxi (Grants No. 2017GXNSFAA198160 andNo. 2017GXNSFGA198001), and the Science TechnologyFacility Council (UK). ∗ Corresponding author: [email protected][1] R. F. Casten,
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