C. Günther

High-spin states in 224,226,228Th and the systematics of octupole effects in even Th isotopes

Abstract High-spin states in 224,226,228Th were investigated using conversion-electron and γ-ray spectroscopic methods following 226 Ra (α, x n ) reactions. The Kπ = 0+ and 0− rotational bands are observed, which in 224,226Th merge into a single band characteristic for reflection asymmetric shape. The systematics of the level energies and El transition rates of 220–230Th are discussed in terms of a stable octupole deformation.

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.

g-Factors of high-spin isomers in 194Hg and 196Hg

Abstract The g -factors of the 10 + isomeric states in 194 Hg and 196 Hg have been measured using the in beam IPAD method. The results g( 194 Hg ) = −0.24(4) and g( 196 Hg ) = −0.18(9) are in agreement with the value expected for an ( i 1 3 2 −2 ) neutron satructure and clearly contradict the previous assignment as ( h 1 1 2 −2 ) proton configurations. Cranking model calculations show that the neutron excitation energies in the rotating frame agree satisfactorily with the experimental energies and that the proton excitations are expected ≈2 MeV above the experimental yrast line

Investigation of level energies and B(E2) values for rotation-aligned bands in Hg isotopes

Abstract High spin states in 191, 192, 193, 195, 197, 199Hg were investigated by observing γ-rays and conversion electrons in the compound reactions 192, 194, 198 Pt(α, x n ) and 192 Pt ( 3 He , 4 n ) . In 197Hg the decoupled band built on the 13 2 + state and the semi-decoupled negative-parity band are observed up to I π = 41 2 + and 33 2 − , respectively. A careful investigation of 199Hg revealed no new high spin states above the previously known levels with I π = 25 2 + and 31 2 −. Half-lives were determined for the 10+, 7−, 8− and 16− states in 192Hg, the if 33 2 + states in 191,193 Hg and the (frc states in 191, 193, 195, 197Hg. The systematics of the level energies and B(E2) values for the positive-parity ground and 13 2 + bands and the negative-parity semi-decoupled bands in 190–200Hg is discussed.

In-beam investigation of high-spin states in actinides with (α, xn) compound reactions

Abstract The ground-state rotational bands in 230, 232 U and 236, 238, 240 pu were investigated in ( α , x n) reactions. Conversion electrons were measured with an iron-free orange spectrometer in order to suppress the background from fission. Levels up to I π ≈ 16 + were identified in e − γ coincidence measurements. The systematics of the ground-state rotational bands in the even actinides is discussed.

Identification of the missing rotation-aligned (νi132−2) 12+ state in 196Hg

Abstract The ν i 13 2 −2 rotation-aligned structure has been verified for 196Hg by the identification of the (ν i 13 2 −2 ) 12 + state. Form half-life measurements we find B(E2) values for the 12+ → 10+ and 10+ → 8+ transitions close to those in 198Hg. Evidence is also presented for the existence of this state in 194Hg.

Investigation of196,198Pt and202,204Hg with the (d, pnγ) reaction

Gamma-ray and conversion electron spectra were measured in the reaction of 25 MeV deuterons on174Yb,196,198Pt,202,204Hg, and232Th. The (d,pxn) deuteron break-up reactions were studied by the measurement ofpγ- andγγ-coincidences. Levels with spins up to approximately 10ħ are observed in the (d,pn) reactions, with a strong preference of the population of yrast states. In the Pt and Hg nuclei the ground bands are seen up to the 6+ states and in196,198pt the semi-decoupled 5−, 7−, and 9− yrast levels are populated most strongly. In202,204Hg we observe fairly strongly new levels with tentative assignments of 5− and 7−. In addition a number of previously unknown levels are identified in the Pt and Hg nuclei, for which no spin-parity assignments could be obtained. A discussion of the level structure in terms of the interacting boson model (196,198Pt) and the shell model (202,204Hg) is given.

Multiple coulomb excitation of202Hg and204Hg

Multiple Coulomb excitation measurements on202, 204Hg have been performed with 5 MeV/u208Pb projectiles. The ground bands are excited up to the newly discovered 6+ states and B(E2) values are derived for the 4+→2+ and 6+→4+ transitions. Whereas the 2+ and 4+ levels in the Hg isotopes with 196≦A≦204 have an almost constant energy, the 6+ levels increase in202Hg and204Hg, compared to the lighter isotopes, by approximately 100 and 300 keV, respectively. The relative B(E2) values in both nuclei show that the collectivity in the neutron rich Hg nuclei is of more complex origin than expected from the few proton and neutron holes with respect to the 82 and 126 major shells.

Electromagnetic transition rates and coriolis coupling in some deformed nuclei

Abstract The M1 and E2 transition rates between the [411 3 2 ] and [413 5 2 ] Nilsson states in 153 Eu, 155 Eu and 159 Tb, the [521 3 2 ] and [523 5 2 ] Nilsson states in 161 Dy, 163 Dy and 165 Er, and the [404 5 2 ] and [404 7 2 ] Nilsson states in 175 Lu, 177 Ta, 179 Ta and 181 Ta are studied. The Coriolis coupling matrix elements, derived from the B (E2) values, are approximately a factor of three smaller than the calculated values. The influence of the Coriolis coupling on the Ml transition rates in 153 Eu, 161 Dy and 173 Lu is studied.