Measurement of Quadrupole Deformation using E$2$ and M$1+$E$2$ Transitions in Heavy Isotopes in the Mass Range of $150<A<250$

Abstract

The measurement of a permanent electric dipole moment (EDM) in atoms is crucial for understanding the origins of CP-violation. However, accurate interpretation of the EDM in systems involving deformed isotopes requires the characterization of their deformation. While nuclear deformation is indicated in various structure models, there is substantial mutual disagreement between the theoretical models or between theoretical models and experimental values. Nuclear E$2$ transitions allow access to quantify quadrupole deformation, but these transitions are often mixed with M$1$ transitions. Both E$2$ and M$1$ transitions are well characterized by Weisskopf estimates, which rely on a single-particle approximation, but deviate due to collective nuclear deformations. Previously, Weisskopf estimates were only available for the mass range $A<150$, and in this work we have extended the Weisskopf estimates of both E$2$ and M$1$ transition lifetimes to the mass range of $150\le A\le 250$. We comprehensively studied the deviation of E$2$ and M$1+$E$2$ transition lifetimes from the newly established Weisskopf estimates in $91$ candidate isotopes, by comparing the transition lifetimes of the candidate isotopes to their nearest even-even counterparts, whose E$2$ transition strengths are very well understood. Estimates of collective nuclear quadrupole deformation in $67$ of these isotopes were thus obtained, either from E$2$ or M$1+$E$2$ transition lifetimes. In $32$ cases they were obtained from both types of transitions independently, and are mutually consistent, as well as following the trends established in theory. We thereby identify the isotopes $^{223,225}$Fr, $^{221,223}$Ra, $^{223,225,227}$Ac and $^{229}$Pa, where EDM measurements are foreseen and information on nuclear deformation is needed, for which no measurement of nuclear quadrupole deformation has been made.