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

Abstract : The 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 atomic number Z and the neutron number N. The neutron-rich nuclei around Z=40 and N=60 provide one of the best territories for the exploration of this sensitivity. Indeed, the sudden onset of deformation observed for neutron-rich nuclei with Z = 36–40 nuclei near N=60 is considered the most dramatic shape change in the nuclear chart. Nevertheless, the resent study of the yttrium isotopic chain reveal occurrence of shape coexistence in the 98Y isotope [1] and possible rotational band above the 6- isomeric state in the 96Y nucleus [2]. This suggests that the evolution of the deformation is a more gradual process. Therefore, 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 [3]. This isomer could be used as a starting point for the identification of structures on top of it. The 94Y isotope has been produced by fission of 235U target induced by cold neutron from the reactor at Institut Laue-Langevin. The level scheme up to excitation energies of about 7 MeV has been established based on multi-fold gamma-ray coincidence relationships measured with the EXILL spectrometer [4] which consists of up to 46 HPGe detectors. During the analysis, over 30 new gamma transitions, which feed the previously known (5+) isomer, have been identified. Angular correlation analysis allows to define spin-parity assignment for most of the identified levels. The identified 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 on which the rotational bands are built. Those findings are in line with the expectations that in 94Y only spherical 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)
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Contributor : Michel Lion Connect in order to contact the contributor
Submitted on : Tuesday, May 9, 2017 - 2:30:25 PM
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  • HAL Id : in2p3-01519872, version 1




L.W. Iskra, B. Fornal, S. Leoni, G. Bocchi, M. Jentschel, et al.. Medium and High Spin Structure in the $^{94}$Y Isotope Produced in Fission Induced by Cold Neutrons. Shapes and Symmetries in Nuclei: from Experiment to Theory Workshop (SSNET 2016), Nov 2016, Gif sur Yvette, France. ⟨in2p3-01519872⟩



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