Speaker
Description
Abstract
The investigation of nuclei in the mass-90 region provides insight into various aspects of both single-particle and collective excitations. Large-scale shell-model calculations have demonstrated good agreement with experimental data across both low- and high-spin states. High-spin states in the mass-90 region have been observed with multiquasiparticle configurations. The $g_{9/2}$ orbital plays a crucial role in generating both low- and high-spin states. The lower energy part of the level scheme is primarily dominated either by the excitation of $fp$ protons to the $g_{9/2}$ orbital or by proton occupancy in this orbital. In contrast, the high-spin states are mainly driven by the coupled excitation of ${\nu}g_{9/2}$, particularly to
${\nu}d_{5/2}$, along with proton excitation across the Z = 40 shell gap. In the N = 50 isotones— $^{86}$Kr, $^{87}$Rb, $^{88}$Sr, $^{89}$Y, $^{90}$Zr [1], $^{91}$Nb [2], $^{92}$Mo [3], $^{93}$Tc, $^{94}$Ru, and $^{95}$Rh—shell-model calculations have successfully explained neutron excitations from the $g_{9/2}$ orbital to $d_{5/2}$.
The odd-odd nuclei in the mass 90 region are equally interesting because both the odd nucleons span the same Z$\sim$40, N$\sim$50 subshell space, providing a good testing ground to study the role of proton-neutron residual interaction and its influence on both the single-particle as well as collective motion. The odd-odd nucleus $^{90}$Nb, with one proton particle and one neutron hole outside the Z = 40 and N = 50 shells, respectively, can provide us
valuable information about the particle-hole interaction at low as well as high-spin states. In-beam gamma-ray spectroscopy of $^{90}$Nb was carried out using fusion-evaporation reaction $^{65}$Cu($^{30}$Si, 3n2p) at a beam energy of 120 MeV [4]. The gamma rays were detected using the Indian National Gamma Array (INGA [5]) having sixteen Compton-suppressed HPGe clover detectors at the TIFR, Mumbai. In the present study, 15 new transitions were found. The positive parity sequence was modified based on triple gamma-ray coincidence conditions. We found an E3 transition decaying from 11$^-$ to the ground state, 8$^+$. However, the experimental B(E3) = 0.020(4) W.u. indicates that the 11$^-$ is not collective.
The odd-odd, odd-even, and even-even nuclei, $^{90}$Nb, $^{91}$Nb, and $^{92}$Mo, were studied in the framework of shell-model with GWBXG interaction. The deviations of shell-model calculation with the experimental data suggest the scope for improvement in the interaction.
The experimental results for $^{90}$Nb and the shell model comparison for $^{90}$Nb, $^{91}$Nb, and $^{92}$Mo will be presented.
Acknowledgment
This work is supported by the Department of Atomic Energy, Government of India (Project Identification No. RTI 4002); the Department of
Science and Technology, Government of India (Grant No. IR/S2/PF-03/2003-II); and the U. S. National Science Foundation (Grant No. PHY-2310059).
References
[1] P. Dey et al., PRC 105, 044307 (2022).
[2] P. Dey et al., PRC 109, 034313 (2024).
[3] Vishal Malik et al., JPhysG accepted.
[4] Vishal Malik et al., under review.
[5] R. Palit et al., NIM A 680, 90 (2012).
