CNRS Université Bordeaux

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Fission properties of nuclei along the N=100 chain in the 180Hg region

Fission is one of the most complex phenomena in nuclear physics. The liquid drop model approach – historically the first successful attempt to understand fission – is able to describe only the division of a nucleus on fragments with similar masses, i.e., the symmetric fission mode. However, it is known experimentally that this mode is not the dominant one in fission of actinide nuclei, which preferably produces fragments with very unequal mass numbers. Nowadays, the production of fission fragments of unequal masses is understood to be driven by the microscopic effects in the fragments. In addition, the contribution and thus importance of these microscopic effects is known to evolve along the nuclear chart from nucleus to nucleus. In particular, fission of light actinides (A 226) becomes completely symmetric, which is also the case for nuclei below Pb (Z=82) around the stability line. Yet recent studies made with very neutron-deficient mercury (Hg, Z=80) isotopes have shown that the isotopes of 178-190Hg possess unexpected fission properties ; they namely demonstrate an asymmetric mass division as well as small sensitivity of the latter to the excitation energy, all that despite of presence of the closed shell in the 90Zr (Z=40, N=50) nucleus. Similar studies made with 179Au and 178Pt have proved that this exotic behavior is a typical property of the neutron-deficient nuclei in the region. Therefore, all the mentioned findings determine existence of a new region of nuclei – whose extension on the nuclear chart is currently unknown – where the nuclear matter fissions with production of fragments of unequal masses. Such a property is similar in result to actinides but is apparently driven by a different mechanism (i.e., not linked to microscopic effects in final fragments). The proposed thesis project deals with a detailed multi-parameter study of fission of exotic nuclear matter (large proton excess ; nuclei with N=100 below 180Hg : 179Au, 177Ir and 176Os). This exotic neutron-deficient matter has been produced in fusion reactions of the 35Cl nuclei (beam) with 144Sm, 142Nd and 141Pr (targets). The experiment was conducted in June-July 2019 at the Advanced Science Research Center (ASRC) of the Japanese Atomic Energy Agency (JAEA). The obtained data set is supposed to provide information on the influence of protons on the fission properties of neutron-deficient nuclear matter (in comparison to the classical actinide region). The other original point of the project is the simultaneous measurement of fission fragments and neutron and / or gamma-rays emitted from the latter. This should allow for a quantitative description of fragments’ excitation energy and, consequently, for better understanding and description of the scission point of proton-rich nuclei. On a more global scale, the project aims at probing the extension of the new region of asymmetric fission, by studying fission of nuclei along the N=100 line. The PhD student :
- Is expected to be a strongly motivated person, with good English communication skills and with a basic background in nuclear reactions and physics of fission ;
- Will have the analysis of the measured data as his principal task. This will imply the use of special scientific software packages for the data analysis and simulation (ROOT & GEANT4 : any knowledge of which will be a strong plus) ;
- Is expected to present the results of his work at scientific conferences at both national and international levels, as well as publish them in scientific journals ;
- Will join the international researcher team and take an active part in the ongoing experimental program conducted by the collaboration (fission of exotic short-lived actinide nuclei through the multi-nucleon transfer reactions, beta-delayed fission), as well as by the hosting group, at different research facilities worldwide (JAEA, RIKEN, CERN, GSI, GANIL).

Application deadline : June 1st, 2020 Starting date : October 1st, 2020 Contact : Pr. I. Tsekhanovich