Puzzling $B(E2;0^+\rightarrow 2^+)$ strength in the proton dripline nucleus $^{36}$Ca

TL;DR

This study investigates the unusual $E2$ transition rate in $^{36}$Ca, revealing that continuum coupling and the unbound nature of the $2^+$ state significantly influence its electromagnetic properties.

Contribution

The paper introduces Gamow shell model calculations that incorporate continuum effects to explain the enhanced $B(E2)$ rate in $^{36}$Ca despite its higher excitation energy.

Findings

Continuum coupling affects the $B(E2)$ strength in $^{36}$Ca.

The $2^+$ state in $^{36}$Ca is spatially diffused.

Unbound nature of the $2^+$ state influences electromagnetic transition rates.

Abstract

Recent measurements of the $E2$ transition rate from the ground state to the first 2$^+$ excited state of the proton dripline nucleus $^{36}$Ca show an unusual pattern when compared to its isotopic neighbor $^{38}$Ca: despite having a higher $E_x(2_1^+)$ excitation energy, the $B(E2; 0^+_1\rightarrow 2^+_1)$ rate in $^{36}$Ca is larger. The question that naturally arises is to what extent this observation can be attributed to the unbound character of the $2^+_1$ state. To understand the influence of the continuum space on the low-energy properties of $^{36}$Ca, we carried out Gamow shell model calculations that can account for the continuum coupling effects associated with the occupation of unbound $fp$ shells. We found that in the threshold $2^+$ state, $^{36}$Ca is spatially diffused, which impacts the observed $B(E2)$ trend.