R. Tolle

On the rotation-aligned coupling of the h112 proton-hole states in 191, 193Au

The rotational bands built on the excited 112− levels in 191Au and 193Au were investigated in (α,4n) reactions with in-beam γ-ray and conversion electron spectroscopy. Half-lives of the rotation-aligned levels in these bands were determined from e−e− time spectra, which were measured with a new double-orange spectrometer. In addition, some information on the magnetic moments of the rotation-aligned levels was obtained from integral perturbed angular distribution measurements. Finally, the previously unknown low-energy 312− → 272− transition in 191Au was identified from conversion electron measurements. From these data it is concluded that the rotation-aligned Hg cores have predominantly νvi1322 structure, contrary to the earlier suggestion of a πh1122 core structure.

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.

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.

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.

Investigation of203, 205Hg with the (d, pγ) reaction — Identification of thei13/2 neutron hole state in205Hg

Low lying levels in203Hg and205Hg were studied with the (d, p) deuteron break-up reaction atEd=14 and 18 MeV. Gamma-rays and conversion-electrons were measured in coincidence with protons. In addition delayed spectra were recorded with us and ms beam pulsing. Thei13/2 neutron hole state was identified in205Hg and confirmed in203Hg. The known level structure of205Hg below 2 MeV is confirmed and extended. For203Hg we observe theγ-decay of several levels below 1.5 MeV, which were previously known from transfer reactions.