Compressional-mode resonances in the molybdenum isotopes: Emergence of softness in open-shell nuclei near A=90

TL;DR

This study investigates the compressional-mode resonances in molybdenum isotopes to understand the emergence of nuclear softness near A=90, revealing that softness begins to appear at A=92, bridging the gap between closed-shell and open-shell nuclei.

Contribution

The paper provides new experimental data on ISGMR in molybdenum isotopes and compares it with theoretical models, highlighting the onset of softness in open-shell nuclei around A=92.

Findings

Softness in ISGMR begins at A=92 in molybdenum isotopes.

Experimental results align with relativistic RPA calculations.

Identifies the transition from closed-shell to open-shell nuclear behavior.

Abstract

"Why are the tin isotopes soft?" has remained, for the past decade, an open problem in nuclear structure physics: models which reproduce the isoscalar giant monopole resonance (ISGMR) in the "doubly-closed shell" nuclei, $^{90}$Zr and $^{208}$Pb, overestimate the ISGMR energies of the open-shell tin and cadmium nuclei, by as much as 1 MeV. In an effort to shed some light onto this problem, we present results of detailed studies of the ISGMR in the molybdenum nuclei, with the goal of elucidating where--and how--the softness manifests itself between $^{90}$Zr and the cadmium and tin isotopes. The experiment was conducted using the $^{94,96,98,100}$Mo($\alpha,\alpha^\prime$) reaction at $E_\alpha = 386$ MeV. A comparison of the results with relativistic, self-consistent Random-Phase Approximation calculations indicates that the ISGMR response begins to show softness in the molybdenum isotopes beginning with $A=92$.