D. Vermeulen

Fusion probability of symmetric heavy, nuclear systems determined from evaporation-residue cross sections

Abstract Cross sections were measured for the formation of evaporation residues in 48 Ca-, 86 Kr- and 124 Sn-induced reactions with Yb, Sb and Zr isotopes. For the nearly symmetric systems, the energy dependence of the fusion probability in central collisions was determined. The fusion probability at the expected fusion barrier as calculated from a one-dimensional heavy-ion potential was found to be reduced by several orders of magnitude. The fusion cross sections increase gradually, and only at energies appreciably above the expected fusion barrier the major part of the cross section of central collisions leads to fusion. The observed hindrance of the fusion process is compared with model predictions.

Alpha decay properties of new protactinium isotopes

Abstract Evaporation residues produced in the reactions 40 Ar + 181 Ta and 40 Ar + 184 W were separated from the primary beam by the velocity filter SHIP and detected by a Δ E-E counter telescope. The technique of delayed coincidences was applied to individually identify the reaction products implanted into a Si surface barrier detector by their subsequent α-decays. The previously unknown nuclei 215 Pa and 218 Pa were identified by their known daughter decays. 215 Pa was found to decay with E α = 8.085±0.015MeV and t 1/2 = 14± 2 3 ms. For 218 Pa two α-lines were found at 9.535±0.015MeV and 9.614±0.020MeV with a corresponding half-life of 120 +40 -20 μs. The half-life of the 8.33 MeV α-decay of 217 Pa. was determined to be 4.9 +0.6 -0.4 ms. Furthermore a new isomer was found in 217 Pa which decays with E = 10.16±0.02MeV and t 1/2 = l.6 +1.0 -0.5 ms. The decay of this isomer is tentatively interpreted as being hindered by an angular momentum of about 10ħ of the emitted α-particle. The ground state α-decay energies of these very proton-rich isotopes agree well with systematics but deviate considerably from theoretical mass predictions.

Cross sections for evaporation residue production near theN=126 shell closure

In fusion reactions of40Ar with isotopes of Ho, Tm, Yb, Lu, Hf and Ta, cross sections for the production of proton-rich evaporation-residues near the 126 neutron shell were measured. This first comprehensive study of very fissile spherical residues reveals a surprisingly low stabilizing influence of the sphericalN=126 shell on the survival probability. The experimental results are compared with evaporation calculations. Conclusions for the production of superheavy nuclei are drawn.

Evidence for element 109 from one correlated decay sequence following the fusion of58Fe with209Bi

An experiment to synthesize element 109 is presented. Decay patterns characteristic of complete fusion products were searched for in an irradiation of209Bi targets with58Fe projectiles at specific incident energies of 4.95, 5.05, and 5.15 MeV/u. A total dose of 7 ×1017 particles was obtained. The experimental method involves in-flight separation of forward peaked reaction products with a static-field velocity filter, their passage through a time-of-flight device and their final implantation into position sensitive solid state detectors to measure their kinetic energy, approximate mass and their time and position of incidence. The subsequent decay of the narrowly localised reaction products by cascades of alpha particles and/or spontaneous fission is also registered in terms of the energies and times of all the emitted particles. One outstanding decay sequence that started with the emission of two alpha particles within subsequent time intervals of 5 ms and 22 ms and ended with spontaneous fission after 13 s was found at 5.15 MeV/u. The first alpha particle had a kinetic energy of (11.10±0.04) MeV. A detailed analysis of all the alternative interpretations of this observation, such as a purely random correlation of signals, the decay of a product from a transfer reaction or of any of the various energetically possible evaporation residues, shows that the isotope with mass 266 of element 109, i.e. the one neutron evaporation channel after complete fusion, is the statistically most significant assignment. The outlook for new element synthesis is also briefly discussed.

Study of evaporation residues produced in reactions of207,208Pb with50Ti

Evaporation residues produced in heavy-ion fusion-reactions of50Ti and207Pb,208pb have been investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into position-sensitive surface-barrier detectors, where their α decay or their decay by spontaneous fission was measured. The production of weakly excited compound nuclei in reactions of projectiles and targets close to the doubly magic48Ca and208Pb was confirmed by the observation of the 1n deexcitation channel in the reaction50Ti +208Pb.The spontaneous fission activities of255104 and256104 could be confirmed; improved halflives are (1.4−0.2+0.2)s and (7.4−0.7+0.9)ms. Furthermoreα-decay of255104 (ba=0.48 ±0.07) and an indication for a smallα-branch (ba= 0.022−0.018+0.073) of256104 was found.A total kinetic energy of (207±13)MeV released in spontaneous fission of256104 was measured.A statistical model analysis of the evaporation-residue cross sections is shown to give evidence for a dynamic hindrance to complete fusion in the system50Ti +208Pb, as well as for non-equilibrium components in the deexcitation mechanism.

The new isotopes247Md,243Fm,239Cf, and investigation of the evaporation residues from fusion of206Pb,208Pb, and209Bi with40Ar

The evaporation residues produced in heavy ion fusion reactions between40Ar and206Pb,208Pb, and209Bi, respectively, were investigated. They were separated in-flight using a velocity filter (SHIP) and identified after implantation into an array of position sensitive surface barrier detectors by measuring their decay characteristics.Three newα-emitting isotopes were found:247Md,243Fm, and239Cf.247Md was identified by correlation to its known daughter decay243Es, and243Fm and239Cf were observed as correlated decays. Moreover, all previously known deexcitation channels, characterized by spontaneous fission or byα decay, could be observed for the first time in one experimental setup. Our results for formation cross sections and decay characteristics are in agreement with known data.

The new isotopes258105,257105,254Lr and253Lr

Evaporation residues from the heavy-ion fusion reaction50Ti on209Bi were investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into an array of position-sensitive surface-barrier detectors by analyzing theirα-decay chains. Spontaneous fission was also observed.Four newα emitters,258105 (T1/2=4.4−0.6+0.9s),257105 (T1/2= 1.4−0.3+0.6s),254Lr (T1/2= 13−2+3s), and253Lr (T1/2=1.3−0.3+0.6s) could be identified. For the isotope257105 we obtained a spontaneous-fission branch of about 20%. A spontaneous-fission activity with a halflife comparable to that for theα decay of258105 was explained as fission of258104, formed by electron capture from258105.An excitation function for evaporation-residue production was measured for bombarding energies in the range ofECM=184.4 MeV toECM=196.6 MeV. Nearly all evaporation residues we observed, could be attributed to the 1n and 2n deexcitation channels. The maximum cross sections wereσ(1n)=c/2.9±0.3) nbarn, andσ(2n)=c/2.1±0.8) nbarn, respectively.We could measure the total kinetic energy of the fission fragments of258104 to be TKE=(220±15) MeV, a value that fits into empirical systematics based on aZ2/A1/3 dependence.

Hindrance of fusion in central collisions of heavy, symmetric nuclear systems

The energy dependence of the fusion probabilities for central collisions of the systems124Sn +92,94,96Zr and86Kr +123Sb was determined from neutron-evaporation-residue cross sections. Near the barrier as calculated from one-dimensional barrier-penetration models, the fusion probability was found to be reduced by several orders of magnitude.

Fusion-fission and neutron-evaporation-residue cross-sections in 40Ar- and 50Ti-induced fusion reactions

Abstract For the reactions 40Ar+ 165Ho. 169Tm, 174Yb. 115Lu, 176–180Hf, 181Ta, 208Pb and 50Ti + 208Pb, 209Bi the cross sections for the fusion-fission process were determined by measuring energy and time-of-flight of the reaction products. In addition, the neutron-evaporation-residue cross sections were measured by using the velocity filter SHIP. A σ versus 1/E analysis of the fusion-fission cross sections is used to determine fusion barriers and fusion radii. The evaporation- residue cross sections are used to extract in an approximate way barriers for compound-nucleus formation. These barriers are found to agree with the fusion barriers determined from the fission cross sections. For all systems investigated the neutron-evaporation-residue cross sections reach their maximum close to the fusion barrier as calculated from the Bass potential.

The velocity filter SHIP, performance and survey of current experiments

Abstract The velocity filter SHIP is used to investigate unslowed evaporation residues from heavy ion fusion reactions. The main subjects of investigation are α emitting and spontaneously fissioning nuclei very far from stability with halflives as short as microseconds and formation cross-section down to the nanobarn region. The efficiency of the filter is calculated by a Monte Carlo method taking into account momentum transfer from nucleons and α particles evaporated from the excited compound system as well as energy loss, scattering in the target, and the ionic charge distribution. The calculations are compared with experimental results. For the irradiation of low melting point targets with high beam currents a rotating target wheel has been developed. Light evaporation residues are identified by an ionisation chamber. α emitting and spontaneously fissioning nuclei are investigated by implantation into surface barrier detectors. The high background rejection and sensitivity of detection are demonstrated for the case of the identification of Fm and element 104.