First direct measurement of the intrinsic dipole moment in pear-shaped thorium isotopes

TL;DR

This study provides the first direct measurement of the intrinsic dipole moment in thorium isotopes, confirming theoretical predictions of octupole deformation and identifying potential candidates for electric dipole moment searches.

Contribution

It reports the first lifetime measurements of low-energy states in 228Th using a direct fast-timing method, enabling estimation of octupole deformation and intrinsic dipole moments.

Findings

Measured lifetimes of low-lying states in 228Th

Estimated octupole deformation consistent with theory

Identified 229Th and 229Pa as promising EDM candidates

Abstract

It is now well established that atomic nuclei composed of certain combinations of protons and neutrons can adopt reflection-asymmetric, or octupole-deformed, shapes at low excitation energy. These nuclei show promise in the search for a permanent atomic electric dipole moment, the existence of which has implications for physics beyond the Standard Model. Theoretical studies have suggested that certain isotopes of thorium may have the largest octupole deformation. However, due to experimental challenges, the extent of the octupole collectivity in the low-energy states in these thorium nuclei has not yet been demonstrated. This paper reports measurements of the lifetimes of low-energy states in 228Th (Z = 90) undertaken using the mirror symmetric centroid difference method, which is a direct electronic fast-timing technique. Lifetime measurements of the low-lying Jp = 1- and 3- states, which are the first for a thorium isotope, have allowed the B(E1) rates and the intrinsic dipole moment to be determined. The results are in agreement with those of previous theoretical calculations allowing the extent of the octupole deformation of 228Th to be estimated. This study indicates that the nuclei 229Th and 229Pa (Z = 91) may be good candidates for the search for a permanent atomic electric dipole moment.