Features of the low-energy level scheme of 229Th as observed in the α-decay of 233U

Abstract

The γ-radiation emitted following the α-particle decay of 233U has been studied using a variety of Ge(Li) spectrometers and a Si(Li) spectrometer system. Analysis of γ-singles and γγ coincidence measurements has established the existence of 91 γ-rays which can be definitely established as representing transitions in 229Th. From these data, a level scheme consisting of 28 excited states below ≈ 0.53 MeV in 229Th is proposed. The well known state at 29.16 keV is shown to have I π = 5 2 + , in contrast with several previous proposals. Additional states which are shown to be members of the rotational band associated with this state are the following (with the assigned I π values in parentheses): 71.82( 7 2 +) ; 125.45( 9 2 +) ; and 195.76 keV ( 11 2 +) . Evidence is presented which indicates that these four states are members of a K π = 3 2 + rotational band, built on the Nilsson orbital 3 2 + [633]. The I π = 3 2 + band head is shown from indirect evidence to lie within ≈ 0.1 keV of the 5 2 +[633] ground state of 229Th. Seven negative-parity states are proposed to lie between ≈ 146 keV and ≈ 237 keV; and their energies can be well described in terms of Coriolis coupling among the j 15 2 related Nilsson states 3 2 −[761] , 5 2 −[752] , 1 2 −[770] and 7 2 −[743] . Evidence is presented which strongly suggests that a significant component of the K π = 3 2 − octupole-vibrational state { 3 2 + [631], 0 −} is present in these low-lying negative-parity states.