Spectroscopy of the T = 2 mirror nuclei 48Fe/48Ti using mirrored knockout reactions

R Yajzey,F Recchia,A Gade,H Iwasaki,D Kahl,D Bazin,S Uthayakumaar,E Lunderberg,N Kobayashi,L Morris,B Longfellow,B Elman,T Haylett,R Wadsworth,J Belarge,D R Napoli ,M A Bentley ,E C Simpson ,P C Bender ,J A Tostevin ,S J Lonsdale ,P Davies ,S M Lenzi ,X Pereira-López ,D Weißhaar

doi:10.1016/j.physletb.2021.136757

Abstract

A sequence of excited states has been established for the first time in the proton-rich nucleus 48Fe (Z=26, N=22). The technique of mirrored (i.e. analogue) one-nucleon knockout reactions was applied, in which the Tz= ±2 mirror pair, 48Fe/48Ti were populated via one-neutron/one-proton knockout from the secondary beams 49Fe/49V, respectively. The analogue properties of the reactions were used to help establish the new level scheme of 48Fe. The inclusive and exclusive cross sections were determined for the populated states. Large differences between the cross sections for the two mirrored reactions were observed and have been interpreted in terms of different degrees of binding of the mirror nuclei and in the context of the recent observations of suppression of spectroscopic strength as a function of nuclear binding, for knockout reactions on light solid targets. Mirror energy differences (MED) have been determined between the analogue T=2 states and compared with the shell model predictions. MED for this mirror pair, due to their location in the shell, are especially sensitive to excitations out of the f7/2 shell, and present a stringent test of the shell-model prescription.

Highlights

The attractive strong nuclear force that acts between protons and neutrons is virtually invariant to the charge of the individual nucleons
The optimum recoil velocity, β = v/c, used for the Doppler correction, was determined by varying β until any angular dependence of the observed γ -ray energy was removed. This can depend on the effective lifetime of the states emitting the γ ray since fast decays that occur on sub-ps timescales will decay near the centre of the target, whilst longer-lived states of the order of > 10 ps will decay outside the target, with a lower recoil velocity
Experimental exclusive cross sections were compared for some specific pairs of analogue states, where isospin symmetry suggests that the cross sections should be similar

Summary

Introduction

The attractive strong nuclear force that acts between protons and neutrons is virtually invariant to the charge of the individual nucleons. Differences in excitation energies of IAS will result from the Coulomb interaction and from any charge-dependent components of the nucleonnucleon interaction These isospin non-conserving (INC) forces lift the degeneracy of analogue states, break isospin symmetry and may result in isospin mixing of the states in question The developments of the shell-model calculations have enabled reliable interpretation of MED in terms of electromagnetic phenomena, of both multipole and monopole origin, which are expected to provide the largest contribution to the MED [3] These studies have yielded detailed information on the effective isospin non-conserving interactions that break the symmetry of mirror nuclei [10]. A mirror pair that are well separated in Z (such as those studied here) have very different values of S and yet isospin symmetry implies that similar spectroscopic strength is expected to the two sets of analogue states. Knockout may help shed light on the question of suppression of spectroscopic strength, and such an analysis is presented here

Experimental details

Results

Conclusion