J. Recht

High-spin states in 163–166Hf

Abstract High-spin states in 163–166 Hf, populated in 148,150 Sm ( 20,22 Ne , x n ) reactions, have been studied by in-beam γ-ray spectroscopy using the first four to six Compton-suppressed Ge detectors from the OSIRIS set-up. Levels up to I > 30 kh are identified. The results are interpreted within the framework of the cranked shell model. Good agreement is obtained between experiment and calculation for the two and three quasi-neutron alignments in all four isotopes. The nature of the second band crossing in the even- A isotopes is discussed.

High-spin structure in 169W and 170W

Abstract High-spin states in 169, 170 W have been populated in 154 Gd ( 20 Ne , x n ) reactions. In-beam γ-ray spectroscopic techniques with multi-detector set-ups, multiplicity filters and an anti-Compton shield have been used. Levels up to about spin 30 (tentatively up to 36) in 170 W and up to 57 2 (tentatively up to 61 2 ) in 169 W have been identified. The data are interpreted within the framework of a pairing-selfconsistent cranking model. The nuclear shape evolution with increasing spin is studied theoretically within a configuration-controlled shell correction approach and also pairing effects are studied. The behaviour of the yrast states around 28 + in 170 W can be related in a model-dependent way to a reduction of the neutron-pairing correlations.

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.

Decay of the ground state and the 292+ isomer in 217Ac and g-factor measurements from perturbed α-particle angular distributions

Abstract The nucleus 217 Ac was studied using the 205 Tl( 16 O, 4n), 206 Pb( 15 N, 4n) and 209 Bi( 12 C, 4n) reactions. The mesurements included αγ, γγ, α-ce and ce-ce coincidence experiments as well as γ-ray and α-particle perturbed angular distribution studies. Results are g( 217g Ac ) = + 0.85(1) and T 1 2 ( 217 g Ac ) = 69(4) ns , the shortest known α-decay of a ground state. 217m Ac decays mainly by a γ-cascade, but also by α-emission to the single-particle states π h 9 2 , f 7 2 , i f 13 2 of 213 Fr. Spins and parities of levels to the 29 2 + isomer at 2013 keV with T 1 2 = 740(40) ns and g = + 0.347(5) are determined. The level scheme of 217 Ac and the α-decay of N = 128 isotones are discussed.

The (vi2132)8+, 10+, 12+ triplets in 190–196Hg

Abstract Decays of isomeric states in the even 190–196 Hg isotopes are studied by in-beam conversion electron spectroscopy using the 181 Ta( 14 N, 5n) and 192, 194, 196 Pt(α, 4n) reactions. A superconducting solenoid was used to transport conversion electrons emitted by the recoiling excited nuclei to cooled Si(Li) detectors and singles e − as well as e − e − coincidence spectra were measured. The missing 12 + → 10 + transitions in 190, 192, 194 Hg were observed with transition energies of 24 keV, 28 keV and 52 keV. The B (E2) rates of the 12 + → 10 + and the 10 + → 8 + transitions are 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.

Levels, lifetimes and g-factors in 198Po and 200Po

Abstract 198,200 Po nuclei have been studied by in-beam gamma-ray spectroscopy with the 182,184 W( 20 Ne, 4n) reactions. In both nuclei 8 + , 11 − and 12 + isomers have been observed, and their lifetimes and g -factors were measured. The low-spin states show signs of a collective structure, while at higher spins very pure neutron and proton states occur with very little mutual interaction.

Delayed second band crossing in 170W

Abstract Discrete transitions between very high spin states in 170W were studied with the 154Gd(20Ne, 4n) reaction. The second band crossing occurs at a rotational frequency of ≈ 0.48 MeV, which is considerably higher than in other nuclei. The data are interpreted within a pairing selfconsistent HFBC model which suggests that the proton h 9 2 2 (ω = 1 2 ) alignment is predominantly responsible for the second band crossing.

High-spin rotational bands and pairing reduction in 166Hf

Abstract Rotational properties of the 166Hf nucleus have been studied by in-beam spectroscopic methods using the 150Sm(20Ne, 4n)166Hf reaction. The ground band was observed up to the Iπ = 18+ level, the s-band from the Iπ = 12+ up to Iπ = 22+ level; in addition two side bands were found. The results are interpreted in terms of the pairing-self-consistent Hartree-Fock-Bogolyubov cranking model. Quantitative agreement between theory and experiment is obtained for the crossing of the ground band and s-band. The calculations predict a reduction of about 50 % in the neutron pairing energy gap of yrast states in the crossing region. The kinematical ( I (1)) and dynamical ( I (2)) moments of inertia are analysed.