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

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 $B(E2;0_1^{+}\to 2_1^{+})$ 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.

6 pages, 5 figures