J. Manns

Collective E1 transitions in even-A Ra, Th, and U nuclei

Abstract Transitions between rotational states of the ground-state and lowest K π = 0 − bands in 226 Ra, 224–232 Th and 230,232 U were studied by gamma-ray and conversion-electron spectroscopy following (d,pn) and ( α , x n) reactions in beam, and following the (p, x n) production of radioactive Pa isotopes out of beam. Coincidence measurements allowed to establish the level schemes and to determine E1/E2 branching ratios which can be interpreted in terms of the ratios of intrinsic electric dipole to quadrupole moments, D 0 / Q 0 , of a rigid rotor. This ratio was found to be independent of spin from I = 8 to 19 for 226 Th, whereas there is an indication that it increases for 224 Th in the same spin region by almost a factor of two. For the transitional nuclei 226 Ra and 228 Th no spin-dependence was found for spins up to I = 11 and 13, respectively. For the reflectionsymmetric nucleus 230 Th the D 0 / Q 0 ratios are consistent with the rotational predictions if the Coriolis coupling to the K π = 1 − band is taken into account.

Long-lived isomers in 190Ir

Abstract Long-lived isomeric states in 190 Ir have been investigated with the 192 Os(p,3n) 190 Ir and 192 Os(d,4n) 190 Ir reactions using beams of 18–31 MeV protons and 27.8 MeV deuterons, respectively. A series of measurements, including excitation functions, half lives, e − γ coincidences and e − e − coincidences, was performed. Five new transitions were observed, and the results of e − γ and e − e − coincidences indicate that these transitions are fed by the 148.7 keV M4 transition that depopulates the 11 − isomer. The previous decay scheme is shown to be incorrect, and the results allow the ground state parity and mass excess to be determined.

The structure of the parity-doublet bands in 231Pa

Abstract The level structure of 231 Pa was investigated by means of the 231 Pa(d,d′), 231 Pa(p,p′) and 232 Th(p,2nγ) reactions and Coulomb excitation and by means of the electron-capture decay of 231 U to 231 Pa. Two low-lying opposite-parity bands based on the K π = 1/2 ± states were observed up to spins 31/2 − and 21/2 + , respectively. From the E1/E2 branching ratios the intrinsic electric dipole moment was determined to be | D 0 | ≈ 0.04 e·fm. Weak Coulomb excitation of other bands was observed. The structure of 231 Pa is compared to that of 229 Pa, in particular with regard to octupole effects. The level scheme and transition intensities in Coulomb excitation are analyzed in the framework of the quasiparticle + phonon model.

Nuclear levels in 190Ir studied with the 192Os(p,3nγ) and 192Os(d,4ne−) reactions

Abstract Nuclear levels in 190 Ir have been investigated with the 192 Os(p,3n γ ) and 192 Os(d,4n e − ) reactions using beams of 18–31 MeV protons and 27.8 MeV deuterons. A series of measurements, including excitation functions, γγ and e − e − coincidences, and life time measurements, was performed. From the singles measurements, a total of 140 γ rays were assigned as belonging to the 192 Os(p,3n) 190 Ir channel. The results of the coincidence measurements show that many of these lines are multiplets. Using information from a previous isomer study and single-nucleon transfer reactions, a level scheme is proposed comprising of 112 γ rays placed between 76 levels. It is shown that most of the γ intensity arises from bands having K π ≥4 + . Several negative-parity bands are also proposed. Calculations performed using values of ϵ and γ characteristic of the region reproduce qualitatively some aspects of the level scheme.

Nuclear levels in228Th populated in the decay of228Pa

The electron-capture decay of228Pa to levels in228Th has been studied using mass-separated sources and high-resolutionγ-ray and conversion-electron spectroscopy. A level at 979.5 keV is assigned as 2+ member of a second excited Kπ=0+ band, with the 0+ band head at 938.6 keV. The 2+ and 3+ members of a second excited Kπ=2+ band at 1153.5 and 1200.5 keV, which decay by strongE0 transitions to the 969 keVγ-vibrational band, are confirmed. In addition we tentatively propose a Kπ=1+ band at 944 keV. The Kπ=0−, 1− and 2− members of the octupole quadruplet are confirmed, and theγ 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 theE1 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.

Level structure of227Thinvestigated in the α decay of231U

The level structure of {sup 227}Th has been studied using the {alpha} decay of {sup 231}U. The use of mass-separated {sup 231}U sources and high-resolution {alpha}-particle and {gamma}-ray spectroscopy enabled the identification of 16 excited levels of {sup 227}Th. The angular distribution of {alpha} particles in the decay of low-temperature oriented {sup 227}Th is consistent with a {sup 227}Th ground-state spin of 1/2. The low-lying levels of {sup 227}Th can be interpreted in terms of a K=1/2{sup {plus_minus}} parity doublet and a K=3/2{sup +} band coupled with the 1/2{sup +} member of the parity doublet by the Coriolis interaction. {copyright} {ital 1997} {ital The American Physical Society}

Level structure of {sup 227}Th investigated in the {alpha} decay of {sup 231}U

The level structure of {sup 227}Th has been studied using the {alpha} decay of {sup 231}U. The use of mass-separated {sup 231}U sources and high-resolution {alpha}-particle and {gamma}-ray spectroscopy enabled the identification of 16 excited levels of {sup 227}Th. The angular distribution of {alpha} particles in the decay of low-temperature oriented {sup 227}Th is consistent with a {sup 227}Th ground-state spin of 1/2. The low-lying levels of {sup 227}Th can be interpreted in terms of a K=1/2{sup {plus_minus}} parity doublet and a K=3/2{sup +} band coupled with the 1/2{sup +} member of the parity doublet by the Coriolis interaction. {copyright} {ital 1997} {ital The American Physical Society}

Level structure of Th-227 investigated in the alpha decay of U-231

The level structure of {sup 227}Th has been studied using the {alpha} decay of {sup 231}U. The use of mass-separated {sup 231}U sources and high-resolution {alpha}-particle and {gamma}-ray spectroscopy enabled the identification of 16 excited levels of {sup 227}Th. The angular distribution of {alpha} particles in the decay of low-temperature oriented {sup 227}Th is consistent with a {sup 227}Th ground-state spin of 1/2. The low-lying levels of {sup 227}Th can be interpreted in terms of a K=1/2{sup {plus_minus}} parity doublet and a K=3/2{sup +} band coupled with the 1/2{sup +} member of the parity doublet by the Coriolis interaction. {copyright} {ital 1997} {ital The American Physical Society}