B. Ackermann

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.

In-beam spectroscopy of231Pa

Information on energy levels and onE 2 andM 1 matrix elements in231Pa has been obtained using conversion-electron and gamma-ray spectroscopy following the232Th(p, 2n)231Pa reaction and Coulomb excitation of the radioactive target231Pa by4He and32S ions. The results are analyzed in the framework of the rotational model, applied to the rotational band built on the 1/2−[530] Nilsson state whose 3/2− member forms the ground state of this nucleus. The deviations of the level energies from the rigid-rotor values can be described by Coriolis couplings. The analysis of the Coulomb-excitation process shows that a constant set of rotational parameters Q0, gR, gK, andb can fairly well account for the measured line intensities.

In-beam spectroscopy of thekπ=0− bands in230–236U

TheKπ=0− bands in even uranium nuclei were studied in the compound reactions231Pa(p, 2n)230U,230, 232Th(α,2n)232, 234U and236U(d, pn)236U. In-beamγ-rays were measured in coincidence with conversion-electrons, which were detected with an iron-free orange spectrometer. The negative-parity levels are observed up to intermediate spins (I<13−). In addition, the 1− and 3− levels in230U were confirmed by a decay study with an isotope separated230Pa source. For the heavier isotopes (A≥232) the properties of theKπ=0− bands (energies andγ-branchings) are consistent with a vibrational character of these bands. For230U theKπ=0− band lies at rather low energy (E(1−)=367 keV), and the level spacings within this band are very similar to those of the isotones228Th and226Ra, which might indicate the onset of a stable octupole deformation.

The rotational bands in the nuclei 229Pa and 231Pa

Experimental evidence is presented for the similarity between the rotational spectra built on the 12−[530] state in 231Pa, where the 32− member of the band forms the ground state, and in 229Pa, where this state lies within 20 keV of the ground state. Our findings are in contrast with earlier work invoking octupole deformations in the ground state to account for the different positions of low-lying Nilsson states in the two isotopes.

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.

Level structure of217Rn and221Ra investigated in the alpha-decay225Th →221Ra →217Rn

The nuclei221Ra and217Rn have been investigated in the α-decay chain225Th→221Ra →217Rn through γ-ray and conversion-electron studies. The short-lived225Th nuclei (T1/2=8min) were produced in the226Ra(α, 5n) reaction, and γ-rays and conversion electrons were measured — between the irradiation periods — in coincidence with αparticles. In221Ra the five lowest levels are interpreted as members of aK=5/2 paritydoublet with ΔEπ=104 keV. These levels, as well as a higher-lying Kπ=3/2+ band, are consistent with an octupole deformation of221Ra, as expected from theoretical considerations. In217Rn only three excited levels are observed, with a favoured α-decay to a 5/2+ excited level thus establishing positive parity for the ground state of221Ra.