Investigating the Structure of Deformed, Neutron-Rich, Odd-Odd Nuclei in the A ≈ 100 Mass Region

¹ Department of Nuclear Physics and Accelerator Applications, Research School of Physics, The Australian National University, Canberra, ACT 2601, Australia
² Institut Laue-Langevin, 71 Avenue des Martyrs, 38042 Grenoble, France
³ Center for Exotic Nuclear Studies, Institute for Basic Science, 55 Expo-ro, Daejeon 34126, Korea
⁴ Faculty of Physics, University of Warsaw, ulica Pasteura 5, 02-093 Warsaw, Poland
⁵ National Centre for Nuclear Research, 05-400 Swierk-Otwock, Poland

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Published online: 13 May 2026

Abstract

The A ≈ 100 mass region is a cornerstone of contemporary nuclear structure research, defined by a dramatic structural phase transition from spherical to strongly prolate-deformed shapes (β₂ ≈ 0.4) that occurs sharply between neutron numbers N = 58 and N = 60. Investigating the odd-odd nuclei in this area, specifically the Nb (Z = 41) and Tc (Z = 43) isotopic chains, is particularly complex due to the demanding requirement of correctly modeling the proton-neutron (p-n) residual interaction and coupling. A historical deficiency of reliable experimental data, coupled with J^π assignments often based purely on theoretical models, has led to inconsistencies in the literature. To address this, we conducted specialized y-ray and y-conversion electron coincidence spectroscopy experiments utilizing the LOHENGRIN recoil mass spectrometer at the Institut Laue-Langevin. This technique is designed to determine transition multipolarities via internal conversion coefficients, providing the necessary experimental foundation for unambiguous J^π assignments. Our systematic study, targeting ¹⁰⁰Nb, ¹⁰²Tc, and ¹⁰⁴Nb, is aimed at challenging current theoretical understanding of deformation and p-n coupling far from stability. Preliminary analysis of ¹⁰⁴Tc data from the initial campaign reveals new candidate y-ray transitions, confirming the need for fundamental revisions to the existing level schemes.