V. Grafen

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.

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 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.

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.

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.