Long lifetime components in the decay of excited super-heavy nuclei

For nuclear reactions in which super-heavy nuclei can be formed, the essential difference between the fusion process followed by fission and non-equilibrium processes leading to fission-like fragments is there action time. Quite probable non-equilibrium

Evidence for the Role of Proton Shell Closure in Quasifission Reactions from X-Ray Fluorescence of Mass-Identified Fragments

The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in Ti48+U238 reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at Z=82 is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the N/Z (neutron over proton ratios of the fragments) evolution in quasifission reactions.The authors acknowledge support from the Australian Research Council through Discovery Grants No. FL110100098, No. FT120100760, No. DP130101569, No. DE140100784, No. DP160 101254, and No. DP170102318. Support for accelerator operations through the NCRIS program is acknowledged. Two of us (C. S. and M. A.) acknowledge support from the Scientific Mobility Program of the Embassy of France in Australia. This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government

3D silicon microdosimetry and RBE study using C-12 ion of different energies

This paper presents a new version of the 3D mesa "bridge" microdosimeter comprised of an array of 4248 silicon cells fabricated on 10 µm thick silicon-on-insulator substrate. This microdosimeter has been designed to overcome limitations existing in previous generation silicon microdosimeters and it provides well-defined sensitive volumes and high spatial resolution. The charge collection characteristics of the new 3D mesa microdosimeter were investigated using the ANSTO heavy ion microprobe, utilizing 5.5 MeV He2+ ions. Measurement of microdosimetric quantities allowed for the determination of the Relative Biological Effectiveness of 290 MeV/u and 350 MeV/u 12C heavy ion therapy beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. The microdosimetric RBE obtained showed good agreement with the tissue-equivalent proportional counter. Utilizing the high spatial resolution of the SOI microdosimeter, the LET spectra for 70 MeV 12C+6 ions, like those present at the distal edge of 290 and 350 MeV/u beams, were obtained as the ions passed through thin layers of polyethylene film. This microdosimeter can provide useful information about the lineal energy transfer (LET) spectra downstream of the protective layers used for shielding of electronic devices for single event upset prediction

Comprehensive study of reaction mechanisms for the Be9+Sm144 system at near- and sub-barrier energies

The delayed x-ray detection technique was used to measure complete and incomplete fusion cross sections for the $^{9}\mathrm{Be}+^{144}\mathrm{Sm}$ reaction at sub- and near-barrier energies. Elastic and inelastic scattering for this system were also measured. Reaction cross sections were derived and the transfer cross sections of one neutron were calculated. The suppression of complete fusion above the barrier, of the order of 10%, is attributed to $^{9}\mathrm{Be}$ breakup and is considerably smaller than the value of 30% found for the $^{9}\mathrm{Be}+^{208}\mathrm{Pb}$ system

Mechanisms Suppressing Superheavy Element Yields in Cold Fusion Reactions

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of 48Ca,50Ti, and 54Cr with 208 Pb. Moving from 48Ca to 54Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity PCN (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for 50Ti and 15 for 54Cr in comparison to 48 Ca. The energy dependence of PCN indicates that cold fusion reactions (involving 208Pb) are not driven by a diffusion process.The authors acknowledge the Australian Research Council for support through Discovery Grants No. DP140101337, No. DP160101254, No. DP170102318, No. FL110100098, and No. DE140100784. Financial support from the NCRIS HIA capability for operation of the Heavy Ion Accelerator Facility is acknowledged. The authors acknowledge the support of the German Academic Exchange Service (DAAD) via funds of the German Federal Ministry of Education and Research (BMBF)

Competition between high-K states and rotational structures in 177Ta

High-spin states in 177Ta have been studied using the 170Er(11B,4n) reaction. New intrinsic states have been observed corresponding to 3-, 5-, and 7-quasiparticle high-K structures. Rotational bands built on most of the 3-and 5-quasiparticle states and some transitions above the 7-quasiparticle states have been identified. Several isomers have been found, the longest lived being the 49/2-, 7-quasiparticle state with a meanlife of 192 μs. Configurations for the observed intrinsic states have been assigned on the basis of gK values, alignments, and decay properties. While the properties of most of the bands are consistent with their proposed configurations, the behavior of some, including the one built on Kπ = 21/2- 3-quasiparticle isomer, is not well understood. Multiquasiparticle blocking calculations based on the Lipkin-Nogami method, are in good agreement with the excitation energies of the experimentally observed states and all the predicted states close to the yrast line up to spin 49/2 have been observed experimentally. The calculations predict the existence of a 9-quasiparticle 67/2- yrast trap at ∼ 8.5 MeV

Fusion reaction Ca 48 + Bk 249 leading to formation of the element Ts (Z=117)

The heaviest currently known nuclei, which have up to 118 protons, have been produced in 48Ca induced reactions with actinide targets. Among them, the element tennessine (Ts), which has 117 protons, has been synthesized by fusing 48Ca with the radioactive target 249Bk, which has a half-life of 327 d. The experiment was performed at the gas-filled recoil separator TASCA. Two long and two short α decay chains were observed. The long chains were attributed to the decay of 294 Ts. The possible origin of the short-decay chains is discussed in comparison with the known experimental data. They are found to fit with the decay chain patterns attributed to 293 Ts. The present experimental results confirm the previous findings at the Dubna Gas-Filled Recoil Separator on the decay chains originating from the nuclei assigned to Ts

Quasifission Dynamics in the Formation of Superheavy Elements

Superheavy elements are created through the fusion of two heavy nuclei. The large Coulomb energy that makes superheavy elements unstable also makes fusion forming a compact compound nucleus very unlikely. Instead, after sticking together for a short time, the two nuclei usually come apart, in a process called quasifission. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission. A systematic study of carefully chosen mass-angle distributions has provided information on the global trends of quasifission. Large deviations from these systematics at beam energies near the capture barrier reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and thus implicitly in hindering or favouring superheavy element synthesis

Ca 48 + Bk 249 fusion reaction leading to element Z=117: Long-lived a -decaying Db 270 and discovery of Lr 266

The superheavy element with atomic number Z=117 was produced as an evaporation residue in the Ca48+Bk249 fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay

Comprehensive study of reaction mechanisms for the 9 Be + 144 Sm system at near- and sub-barrier energies

The delayed x-ray detection technique was used to measure complete and incomplete fusion cross sections for the Be9+Sm144 reaction at sub- and near-barrier energies. Elastic and inelastic scattering for this system were also measured. Reaction cross sections were derived and the transfer cross sections of one neutron were calculated. The suppression of complete fusion above the barrier, of the order of 10%, is attributed to Be9 breakup and is considerably smaller than the value of 30% found for the Be9+Pb208 system