Spectroscopy of neutron-rich $^{194,96}$Y isotope produced in fission induced by cold neutrons

The onset of the deformation in neutron-rich nuclei around A = 100 mass region has for many years remained one of the most interesting subjects for nuclear spectroscopy study. For the neutron number N = 60, a sudden onset of the deformation has been observed at the ground state, which is manifested by the presence of rotational bands (e.g. [1]). On the other hand the occurrence of shape coexistence in nuclei with N = 58 and 59, in this region (e.g. [2]), suggests that the evolution of the deformation is a more gradual process. In the yttrium isotopic chain, a rotational band above the 4-, 496-keV isomer has been observed in the N = 59, 98Y nucleus, while there was no evidence of the deformed structure in the Y isotopes with neutron number less than N = 59. Our goal was to investigate N = 57, 96Y isotope where only a few states were known from beta decay study of 96Sr [3] as well as the long 9.6-s (1140-keV) isomer [4]. Additionally, we decided to investigate whether deformed structures are still present in the 94Y nucleus which lies 5 neutrons away from the N=60 boundary. So far, very little spectroscopic information has been gathered on 94Y – what regards higher spin yrast excitations established was only the presence of the 1.35-μs, (5+) isomer at 1202 keV excitation energy [5]. This isomer could be used as a starting point for the identification of structures on top of it. The yttrium-94 and 96 isotopes have been produced by fission of 235U and 241Pu targets induced by cold neutron from the reactor at Institut Laue-Langevin. The level scheme up to excitation energies in excess of 5 MeV has been established based on multi-fold gamma-ray coincidence relationships measured with the EXILL spectrometer [6] which consists of up to 46 HPGe detectors. By exploiting delayed- and cross-coincidence techniques, extensive structure has been delineated. During the analysis, over 50 new gamma transitions which feed previously known low-spin states as well as the long 9.6-s, 8+ isomer in 96Y isotope have been identified. Moreover, a new isomeric state at 1655-keV excitation energy has been located with half-life of 201 ns. Angular correlation analysis allows to define spin-parity assignment for most of the identified levels, in particular (6+) for new isomer. By using the delayed-coincidence method it was possible to identify above the 201-ns state a few weak transitions, which seem to form a rotational band, in analogy to the structure above the 4- isomer in the 98Y isotope. In the case of 94Y isotope over 11 new gamma transitions, which feed the previously known (5+) isomer, have been identified. Angular correlation analysis supported by shell-model consideration allowed to propose spin-parity assignments for some of the new levels. The existence of the new isomeric state and the possible deformed band built on that isomer in the N = 57, 96Y isotope shed new light on the study of the onset of deformation in neutron-rich nuclei around N = 60. It shows that the deformed structures appear just after the subshell closure at N = 56 and evolve smoothly when passing through N = 57-59 isotopes, to became a ground state structure in the 99Y isotope, i.e., at N = 60. The findings are in line with the 94Y results where only spherical structures are present at low excitation energy.