  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.
