H. Baltzer

The nuclear structure of 229Pa from the 231Pa(p,t)229Pa and 230Th(p,2nγ)229Pa reactions

The level structure of the 229Pa nucleus has been investigated by means of the 231Pa(p,t)229Pa and 230Th(p,2nγ)229Pa reactions. Triton angular-distribution measurements were subjected to a CCBA analysis and combined with the results of in-beam conversion electron and γ-ray spectroscopy to establish a level scheme. Two low-lying bands of opposite parity were observed up to spins 232− and 172+, respectively. Rotational bands built on some 0+ excitations of the even-even core can be assigned. The lowest states of three further low-lying bands are observed. The level scheme is interpreted in terms of an octupole-deformed core with an unpaired proton. From the E1/E2 branching ratio the electric dipole moment can be deduced ⊢D0⊢ = (0.09 ± 0.04) e·fm.

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.

Experimental investigation of vibrational excitations in230Th

Nuclear levels in230Th have been investigated in theβ decay of230Pa, the230Th(d, pnγ) reaction and the232Th(p, t) reaction. TheKπ=01+, 21+, and 22+ bands with band heads at 635, 781, and 1010 keV were observed up to the 8+, 9+, and 7+ levels, respectively. A second excited 0+ level was identified at 1297 keV which might be interpreted as the usualβ shape-oscillation. The branching ratios of theE2 transitions from the 01+, 21+, and 22+ bands are explained in the framework of the rotational model by taking into account the coupling of these bands with the ground-state band, and the coupling between the 01+ and 21+ band. A strong enhancement ofE2 transitions from the 21+ to the 01+ band reported earlier is not confirmed. For the octupole vibrations withKπ=0−, 1−, and 2− theE1 branching ratios are analyzed in terms of the Coriolis coupling of these bands. An almost complete experimental set ofE1 transition moments from these negative-parity bands to the 0g+, 01+, and 21+ bands was obtained. It is suggested that octupole correlations might be important in explaining theseE1 moments.

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.

The low-lying levels in229Pa and parity doublets

The nucleus229Pa was studied using the reactions231Pa(p,t) and230Th(p,2nγ). A level scheme based on particle and conversion-electron-gamma spectroscopy data is suggested.

Investigation of224Ra in the226Ra(α, α′ 2n) reaction

Positive and negative parity bands have been followed up to 10+ (possibly 12+) and 11− in224Ra and are compared to the corresponding bands in the isotone226Th. If a constant value of the intrinsic quadrupole moment is assumed for allE2 transitions in224Ra theE1/E2 branching ratios are consistent with an intrinsic dipole moment of ¦Q1¦=0.032(3)e·fm. This small value, as compared to ¦Q1¦=0.30(2)e·fm for226Th, can be explained by an almost complete cancellation of large positive liquid-drop and negative shell-model contributions.