In-beam spectroscopy of²²⁴Th

Abstract

A rotational band with states of alternating parity has been populated in²²⁴Th via the²⁰⁸Pb(¹⁸O, 2n) reaction. Spins up to I^π=10^>+ (11⁻) are identified. Stretched El and E2 transitions compete in the deexcitation, the average ratio of the reduced transition probabilities being B(E1)/B(E2)=(1.5±0.4)×10⁻⁶ fm⁻². This ratio implies that²²⁴Th has one of the largest intrinsic electric dipole moments observed so far.

The low-lying odd-parity excitations in the neutron deficient even Ra and Th isotopes have long been our main evidence for the occurrence of octupole deformations in nuclei at the beginning of the actinide region. More recently, new evidence for strong octupole effects has emerged: With increasing rotation even and odd parity yrast states seem to merge into one single band of alternating parity where neighbouring levels are connected by fast El transitions. Such bands have been observed in^{218, 219, 220} Ra and^{220, 221, 222}Th (see refs. [1, 2]). To extend the knowledge of high-spin states into the expected center of the octupole region [1] we have studied²²⁴Th using the²⁰⁸Pb(¹⁸O, 2n) reaction.