A Pairing Effect in γ—Soft Nuclei

Abstract

For the spectra of axially-symmetric nuclei, there is a well known phenomenon that the experimental energy levels with high angular momenta lie below what is expected from the J(J+1) rule.¹ To be more precise, the level spacings of high spin states are compressed because of the increase of the moment of inertia with spin, of which the driving force is the reduction of the pairing correlation due to faster rotation, i.e.,the Mottelson-Valatin effect.² This kind of deviation has been explained by various models. For instance, the deviated rotational spectra for strongly deformed nuclei can be described by the SU(3) limit of the Interacting Boson Model (IBM) and the Fermion Dynamical Symmetry Model (FDSM), where the reduction of pairing correlation in high-spin states can be incorporated perturbatively.³ The deviation in the rotational spectra has also been explained in terms of the Geometric Model of Bohr and Mottelson,⁴ such as the Rotational-Vibrational Model (RVM) ⁵ or the Variable Moment-of-Inertia Model (VMI),⁶ showing success in describing the yrast states before backbending.