Speaker
Description
The area of the nuclear chart lying northeast of the doubly magic nucleus 78Ni represents a key benchmark for shell-model–based theoretical descriptions. Although closed-shell nuclei in this region have been extensively investigated for decades, the double-magic character of 78Ni was only recently confirmed experimentally by Taniuchi et al. [1]. Their results also revealed a weakening of the N = 50 and Z = 28 shell closures beyond 78Ni, driven by the appearance of deformed configurations, in agreement with modern large-scale shell-model calculations [2]. These observations underscore the importance of further experimental studies in the vicinity of 78Ni.
In this context, the selenium isotopic chain, located six protons away from the closed shell, displays clear signatures of shape coexistence. In particular, the tentative identification of a 3+ state in 86Se [3] points to the emergence of collectivity. The present work investigates neutron-rich selenium isotopes northeast of 78Ni, with special emphasis on 87Se, for which spectroscopic information has so far been very limited, with only three known γ-ray transitions [4].
The 86,87,88Se isotopes were populated through thermal-neutron–induced fission of 233U and 235U targets at the Institut Laue–Langevin. In 87Se a level scheme extending up to 3.5 MeV in excitation energy was constructed from high-fold γ-ray coincidence data recorded with the high-efficiency HPGe array FIPPS [5]. Using cross-coincidence analysis, seven previously unobserved γ-ray transitions were identified and their relative intensities determined. Spin assignments for low-lying states were inferred from γ-ray angular-correlation measurements. The observation of an E3 transition connecting the (11/2-) and (5/2+) states suggests enhanced octupole correlations and motivates systematic searches for similar E3 excitations in neighboring even-mass selenium isotopes.
In addition, our analysis, validated the presence of an emerging collective band in 86Se, until now observed only in β- decay [3] and confirmed 589-keV gamma ray as deexciting the first 2+ state in 88Se. These findings and validations in 86,88Se isotopes contribute to more vivid picture of evolution of a collectivity along even-even Se isotopic chain.
References:
[1] R. Taniuchi et al., Nature 569, 53-58 (2019).
[2] F. Nowacki et al., Phys. Rev. Lett. 117, 272501 (2016).
[3] T. Materna et al., Phys. Rev. C 92, 034305 (2015).
[4] T. Rząca-Urban et al., Phys. Rev. C 88, 034302 (2013).
[5] C. Michelagnoli et al., EPJ 193, 04009 (2018).
[6] K. Gajewska et al., Acta Phys.Polon.Supp. 18 (2025) 2, 2-A41.
| Type of contribution | Regular Abstract |
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