L. Krupa

New data on the ternary fission of 252Cf from the Gammasphere facility

Ternary fission of 252Cf was studied at Gammasphere using eight ΔE×E particle telescopes. Helium, beryllium, boron, and carbon light charged particles (LCPs) emitted with kinetic energy more than 9, 21, 26, and 32 MeV, respectively, were identified. The 3368-keV γ transition from the first 2+ excited state in 10Be was found and the population probability ratio N(2+)/N(0+) = 0.160 ± 0.025 was estimated. No evidence was found for 3368-keV γ rays emitted from a triple molecular state. For the first time, charge distributions are obtained for ternary fission fragments emitted with helium, beryllium, and carbon LCPs.

The Peculiarities of the Production and Decay of Superheavy Nuclei

The interest in the study of the fission process of superheavy nuclei mainly deals with the opportunity to obtain information about the cross‐section of the compound nucleus (CN) formation at excitation energies E*≈15–30 MeV. It allows one to estimate the survival probability of the superheavy composite system after evaporation of 1–3 neutrons, i.e. in “cold” or “warm” fusion reactions. However, in order to solve this problem deeper understanding of the coalescence processes between colliding nuclei, the competition between fusion‐fission and quasi‐fission processes is needed. The characteristics of both processes, their manifestation in the experimental observables and the relative contribution to the capture cross‐section in dependence on the excitation energies, reaction entrance channel etc were investigated for a wide range of target‐projectile combinations. Results of the experiments devoted to the study of the fusion‐fission and quasi‐fission processes in the reactions of the formation of the super...

Bimodal fission in binary and ternary spontaneous fission of 252Cf

The hot bimodal fission of 252Cf is reexamined with new high-statistics data. We constructed a γ-γ-γ coincidence cube for binary fission and LCP-gated γ-γ matrix for ternary fission. By identifying the secondary fission fragments from their γ-ray transitions, we measured the yields for various fission splits. The normal neutron yield distribution is found to be Gaussian for Xe-Ru. However, the binary fission split of Ba-Mo is found to exhibit a bimodal neutron distribution with the “hot mode” corresponding to ≈3.1% of the total yield. In α ternary fission, the first measurements of yields for specific fission splits are presented. The Te-α-Ru and Xe-α-Mo neutron yields fit well with a single mode, but the Ba-α-Zr split shows evidence for an enhanced hot mode with an intensity of ≈13.8% of the normal mode.

Fission dynamics in the proton induced fission of actinide nuclei at intermediate energies

A multi‐parameter correlation study of the reactions 232Th(p,f), 238U(p,f) and 242Pu(p,f) at Ep=13, 20, 40 and 55 MeV has been conducted. The fission fragment mass, total kinetic energy distributions, double differential neutron spectra and γ‐ray spectra have been measured. The three humped shape of mass distributions has been observed up to higher proton energy. Pre‐equilibrium, pre‐scission and post‐scission neutron and γ‐ray multiplicities measured in coincidence with primary fission fragments provided access to fission dynamics. Shell structure effects were observed in fission fragment mass distributions even at high excitation energy. Manifestation of the nuclear shell Z = 28 near fragment mass Afr = 78 has been detected.

MASS-SPECTROMETRIC METHOD TO STUDY THE PROPERTIES OF HEAVY NUCLEI

There is considered a further development of the mass spectrometer MASHA to adapt the gas catcher technique to the mass spectrometric analysis. The ion-optical parameters of the new installation are calculated. These parameters were derived from the simulation of the compound nucleus fusion reaction 48 Ca+ 238 U→ 286 112 * → 283 112+3n. The applicability of this technique for production and investigation of both the SHE nuclei and the heavy neutron-rich nuclei created in the multinucleon transfer (MNT) collisions between 48 Ca, 86 Kr, and 136 Xe as projectiles and 208 Pb as a target is substantiated.

Neutron Emission in Fission And Quasi-Fission of Hs

Mass and energy distributions of fission‐like fragments obtained in the reactions 26Mg+248Cm, 36S+238U and 58Fe+208Pb leading to the formation of 266,274Hs are reported. From the analysis of TKE distributions for symmetric fragment it was found that at energies below the Coulomb barrier the bimodal fission of 274Hs, formed in the reaction 26Mg+248Cm, is observed, while in the reaction 36S+238U at these energies the main part of the symmetric fragments arises from the quasi‐fission process. At energies above the Coulomb barrier the fusion‐fission is a main process leading to the formation of symmetric fragment for the both reactions. In the case of 58Fe+208Pb reaction the quasi‐fission process is the main reaction mechanism at all measured energies. The pre‐ and post‐scission neutron multiplicities as a function of the fragment mass have been obtained for all studied reactions.

Symmetric and asymmetric quasifission modes in reactions with heavy ions

Mass and energy distributions of fission and quasifission fragments in the reactions 22Ne+249Cf, 26Mg+248Cm, 36S+238U and 58Fe+208Pb leading to the formation of the Z = 108 composite nucleus are reported. In this series of experiments it is shown for the first time that the quasifission process leads to the formation of not only asymmetric fragments but also gives a contribution to the symmetric mass region of fission fragments, most probably caused by the influence of nuclear shells with Z = 50 and N = 82.

Fragment yields obtained at gammasphere for the ternary fission of 252Cf

In an experiment carried out at the 4π Ge detector facility Gammasphere the yields of ternary fission events of 252Cf accompanied by the emission of He, Be and C LCPs were measured. For the first time independent yields were measured for 59 pairs of secondary fragments of the He ternary fission and for many fission fragments detected together with the He, Be and C LCPs. With a reasonable assumption that the detected LCPs predominantly are 4He, 10Be and 14C their yields and the charge distributions of fission fragments were conjointly analyzed in terms of the “energy cost” concept first suggested by Halpern.

Experiment aimed at the study of 252Cf binary and ternary fission

A new experiment devoted to the fission of 252Cf is described. It continued a series of our experiments based on correlation measurements of γ rays emitted by fission fragment pairs. The measurements of γ-γ and γ-γ-γ coincidences were done at Gammasphere with closed 252Cf sources. The open source was used for the first time in the last experiment. Fission fragment detectors were arranged in the center hole of Gammasphere. Correlations between fission fragment masses, total kinetic energy, and γ rays were observed. The first, preliminary results of data analysis are discussed.

Capture and Fusion‐Fission Processes in Heavy Ion Induced Reactions

Results of the experiments aimed at the study of fission and quasi‐fission processes in the reactions 12C+204Pb, 48Ca+144,154Sm, 168Er, 208Pb, 238U, 244Pu, 248Cm; 58Fe+208Pb, 244Pu, 248Cm, and 64Ni+186W, 242Pu are presented. The choice of the above‐mentioned reactions was inspired by the experiments on the production of the isotopes 283112, 289114 and 283116 at Dubna using the same reactions. The 58Fe and 64Ni projectiles were chosen since the corresponding projectile‐target combinations lead to the synthesis of even heavier elements. The experiments were carried out at the U‐400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR, Russia), the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) and the Accelerator of the Laboratory of University of Jyvaskyla (JYFL, Finland) using the time‐of‐flight spectrometer of fission fragments CORSET and the neutron multi‐detector DEMON. The role of shell effects and the influence of the entrance channel asymmetry and the deformatio...