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

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 approximately 0.53 MeV in 229Th is proposed. The well known state at 29.16 keV is shown to have Isup(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 Isup(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 Ksup(π)=3/2+ rotational band, built on the Nilsson orbital 3/2+[631]. The Isup(π)=3/2+ band head is shown from indirect evidence to lie within approximately 0.1 keV of the 5/2+[633] ground state of 229Th. Seven negative-parity states are proposed to lie between approximately 146 keV and approximately 237 keV; and their energies can be well described in terms of Coriolis coupling among the jsub(15/2) related Nilsson states 3/2-[761], 5/2-[772] and 7/2-[743]. Evidence is presented which strongly suggests that a significant component of the Ksup(π)=3/2- octupole-vibrational state (3/2+[631],0-) is present in these low-lying negative-parity states. (Auth.)