Study of neutron-rich isotopes with neutron-induced-fission reactions
Dr Łukasz Iskra, IFJ PAN
During the seminar, results from various experiments will be presented where the neutron-rich isotopes have been produced in the neutron-induced-fission of actinide targets. The data have been collected at the Institute Laue-Langevin (Grenoble) and IJC Laboratory in (Orsay) with thermal and fast neutrons, respectively.
The first experimental campaign concerns the studies of sudden onset of the deformation in nuclei around A=100, N=60; the region where the most dramatic shape change in the nuclear chart occurs. We have investigated the structures of the neutron-rich Y isotopes produced in the fission of 233U and 235U active targets induced by cold neutrons from the reactor at ILL. The level scheme of 96Y nuclei has been established based on gamma-ray coincidences measured with the new highly efficient HPGe array called FIPPS. The results suggest that deformed structures appear just after the subshell closure at N=56 and evolve smoothly when passing through N=57-59 isotopes, to become the ground state structure at N=60.
The other important instrument at ILL is the LOHENGRIN mass spectrometer which allows to select nuclei with a specific mass number from hundreds of fission products. In a series of experiments, we have measured many lifetimes (in the picoseconds range) of nuclear states in 93-96Y and 96Rb isotopes, separated after neutron-induced fission of 235U target, using the fast-timing technique. Moreover, with the LOHENGRIN mass spectrometer we performed extensive tests of the diamond and silicon carbide detectors from the point of view of their application in the fission experiments.
Additionally, the nuclei lying in the yet unexplored, north-east region of doubly magic 78Ni, have been studied at the IJC Laboratory. During the measurement, fast neutrons from LICORNE high flux neutron source induced fission of 232Th target, while gamma rays from exotic species were recorded in a high-resolution ν-ball array. The level schemes of neutron-rich As and Ge isotopes have been established based on gamma-ray coincidences. New experimental results provide important information concerning the shell evolution across the isotopic chains and offer a good testing ground for various theoretical models.