M. Guttormsen

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

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.

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.

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.

Cranking model interpretation of weakly coupled bands in Hg isotopes

Abstract The positive-parity yrast states of the transitional 189–198 Hg isotopes are interpreted within the Bengtsson and Frauendorf version of the cranking model. The very sharp backbendings can be explained by small interaction matrix elements between the ground and s-bands. The experimentally observed large aligned angular momenta and the low band-crossing frequencies are well reproduced in the calculations.

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.

Observation of the weakly deformedvi 13 2/2 band in188Hg

High-Spin states in188Hg have been investigated using the168Er(24Mg,4n) reaction. The rotation aligned vi132/2 band, which intersects the weakly oblate ground state band atI≈10, is observed up toIπ=20+ with an isomeric 12+ level with T1/2=134(15)ns. The coexisting strongly deformed band, built on the 825 keV 0+ level, is observed up to the 14+ state. The systematics of the vi132/2 rotation aligned bands is discussed.