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(α, α ) reaction at E α = 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.