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

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 $A=92$.

Comments: Accepted for publication to Physics Letters B