Core Polarization Effects in Odd A ≈ 80 Nuclei

Abstract

The nucleus ⁸¹Sr provides one of the best examples of the effects of an unpaired nucleon in polarizing the nuclear core. Four different rotational bands can be seen in the level scheme [1–3] of ⁸¹Sr, shown in Fig. 1. Each is built upon a different single-particle configuration and each exhibits distinctive properties. The large signature splitting of the strongly decoupled g_9/2 Yrast rotational band is consistent with the nearly oblate shape (β₂=0.23, γ=-50°) predicted by Woods-Saxon cranking model calculations [1,4]. The measured B(E2) strengths (See Fig. 2) also imply |β₂| ≈ 0.22 in this band. The nuclear shape changes substantially after the proton quasiparticle alignment at ℏω ≈ 0.5 MeV. The signature splitting nearly vanishes and ΔJ=1 transitions become strong. A similar band crossing has been seen [5] in ⁸¹Kr, and a nearly identical one has recently been reported [6] in ⁷⁹Kr. Theoretically, the alignment polarizes the core towards a triaxial or more nearly prolate shape with γ ≥ -30°.