Medium and High Spin Structure in the $^{94}$Y Isotope Produced in Fission Induced by Cold Neutrons

International audienceThe shape of a particular nucleus results from an interplay between the collective (macroscopic)and single-particle (microscopic) energies and, therefore, highly depends on both the atomicnumber Z and the neutron number N. The neutron-rich nuclei around Z=40 and N=60 provide oneof the best territories for the exploration of this sensitivity. Indeed, the sudden onset ofdeformation observed for neutron-rich nuclei with Z = 36–40 nuclei near N=60 is considered themost dramatic shape change in the nuclear chart. Nevertheless, the resent study of the yttriumisotopic chain reveal occurrence of shape coexistence in the 98Y isotope [1] and possible rotationalband above the 6- isomeric state in the 96Y nucleus [2]. This suggests that the evolution of thedeformation is a more gradual process. Therefore, we decided to investigate whether deformedstructures 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 spinyrast excitations established was only the presence of the 1.35-μs, (5+) isomer at 1202 keVexcitation energy [3]. This isomer could be used as a starting point for the identification ofstructures on top of it.The 94Y isotope has been produced by fission of 235U target induced by cold neutron from thereactor at Institut Laue-Langevin. The level scheme up to excitation energies of about 7 MeV hasbeen established based on multi-fold gamma-ray coincidence relationships measured with theEXILL spectrometer [4] which consists of up to 46 HPGe detectors. During the analysis, over 30 newgamma transitions, which feed the previously known (5+) isomer, have been identified. Angularcorrelation analysis allows to define spin-parity assignment for most of the identified levels. Theidentified transitions don’t show any pattern characteristic for deformed structures. Furthermore,no higher located isomer has been found, similar to the ones known in heavier yttrium isotopes onwhich the rotational bands are built. Those findings are in line with the expectations that in 94Y onlyspherical structures are present at low excitation energy.[1] S. Brant, G. Lhersonneau, and K. Sistemich, Phys. Rev. C 69, 034327 (2004)[2] Ł. W. Iskra et al., to be published[3] J. Genevey et al., Phys. Rev. C 59, 82 (1999)[4] M. Jentschel et al., EXILL technical paper, JINST (subm. 2015