Structure and Collectivity of Highly Deformed Nuclei Along the N=Z Line

The lifetimes of the first excited 2+ states in the N = Z nuclei 80Zr, 78Y and 76Sr have been measured using the gamma-ray lineshape method following population via nucleon-knockout reactions from intermediate-energy rare-isotope beams. 80Zr, 78Y and 76Sr are the heaviest N = Z nuclei where such measurements have been made to date. The extracted reduced electromagnetic transition strengths yield new information on where the collectivity is maximised and provide evidence for a significant, and as yet unexplained, odd-odd vs. even-even staggering in the observed values. The experimental results are analysed in the context of state-of-the-art large-scale shell model and nuclear density-functional theory (DFT) model calculations. DFT calculations incorporating T=1 np pairing reproduce the measured B(E2) values of the even-even nuclei but fail to replicate the extent of the reduction in B(E2) seen with the odd-odd 78Y. The mirror energy differences (MED) of the 79Zr/79Y mirror pair have also been measured, again the heaviest nuclei where such measurements have been taken. This is the first observation of the excited yrast states of 79Zr. No-core configuration-interaction (NCCI) model calculations show good agreement with the remarkably small experimental MED up to J = ( 9/2+), with a deviation at higher spin. The success of the NCCI model provides a stepping-stone to using the same approach for future MED analysis.