Nuclear levels in228Th populated in the decay of228Pa

Abstract

The electron-capture decay of 22Spa to levels in 228Th has been studied using mass-separated sources and high-resolution y-ray and conversion-electron spectroscopy. A level at 979.5 keV is assigned as 2 § member of a second excited K = = 0 + band, with the 0 § band head at 938.6 keV. The 2 + and 3 + members of a second excited K s = 2 + band at 1153.5 and 1200.5 keV, which decay by strong E0 transitions to the 969 keV y-vibrational band, are confirmed, in addition we tentatively propose a K~= 1 § band at 944 keV. The KS= 0-, 1-and 2members of the octupole quadruplet are confirmed, and the y decay of these levels is analysed in an approach, in which the mixing of the octupole bands by the Coriolis interaction is taken into account. It is suggested that octupole correlations might be important for the E1 transition moments. A total of 29 levels is observed between 1.4 and ~ 2.0 MeV, for which the nuclear structure, and the possible assignment to rotational bands, is unclear.

Spectroscopy ofTh230in the(p,t)reaction

“…The strong excitations of the first excited 0 + states in the (p,t) reaction, combined with all other available evidence (rather weak E2 transitions to the ground-state band, strong α decays leading to them, the strong Coulomb excitation of the associated collective bands) suggest that these states represent a new and stable collective excitation, different in character from the most common formulation of the pairing vibration as well as from the β vibration usually found in the deformed rare-earth nuclei. The second excited 0 + states in actinides (firmly assigned) demonstrate completely different features [18,32,33]. Weak excitation in the (p,t) reaction, relatively strong E2 transitions to the groundstate band and a small B(E1)/B(E2) ratio for transitions to 2 + and 1 − states give evidence that they could be the usual β vibrational states.…”

0+states in deformed actinide nuclei by the(p,t)reaction

“…The experimental infor-mation about the low-lying states of 228,230 Th and 232 U is summarized in Refs. [33][34][35] and several more recent experimental investigations [21][22][23][24]28,36,37], including a ͑p , t͒ study of the lowest states [23]. Addressed to theoretical understanding of these nuclei there are a number of recent publications which are related mainly to features of octupole collectivity [38][39][40][41][42][43][44], triaxiality [45,46], and to the description within interacting boson approximations [47][48][49][50][51][52].…”

“…In recent years a few more 0 + states had been identified [21][22][23][24], but the understanding of the higher excited 0 + states remains as a challenge for nuclear theory. Within their quasiparticle model (QPM) Soloviev and co-workers [25][26][27][28] provided microscopic understanding of low-lying 0 + states.…”