A.M. Baxter

Configuration-dependent deformations in 171Re

Abstract The level scheme of 17175Re96, has been studied using (heavy ion, xnyp) reactions. Rotational bands associated with the one-quasiproton Nilsson configurations 5 2 + [402] , 1 2 + [411] and 9 2 − [514] and the “cross-shell” orbitals from the h 9 2 and i 13 2 protons (nominally 1 2 0 − [541] and 1 2 + [660] ) have been identified. Less extensive results for 173Re have also been obtained. Differing (configuration dependent) deformations are required to explain the frequencies and alignment gains in the neutron band crossings. The relative differences are consistent with predicted deformation changes in the “deformation-driving” h 9 2 and i 13 2 (proton) orbitals. Signature splitting in 9 2 −[514] and 5 2 + [402] bands at low spin suggests some γ-deformation. Competing in-band and out-of-band E2 decays in the region of the “real” crossing between the 1 2 + [660] and 5 2 + [402] bands are explained through particle-rotor band-mixing calculations with the ad hoc inclusion of ΔN = 2 mixing. Limited agreement between the observed 1-quasiparticle energies and predicted values underlines the limitation of currently accepted nuclear potentials in this region. Small alignment gains in the 5 2 + [402] and 1 2 + [411] bands, before the AB neutron alignment can be related to the low-spin anomaly in 172Os and explained using three-band mixing. The absence of a similar effect in the 9 2 − [514] band is discussed.

Mass dependence of the static quadrupole moments of the first 2+ states in the cadmium isotopes

Abstract The static quadrupole moments Q 2 + and the B (E2; 0 + → 2 + ) values of the first 2 + states m the even-mass Cd isotopes have been determined using the reorientation effect. The Coulomb excitation probabilities were measured by resolving inelastically and elastically backwardscattered 4 He and 16 O projectiles in an annular surface barrier detector. The results, in contrast to theoretical predictions and previous experimental work, indicate no significant variation of Q 2 + with mass number. Assuming constructive interference from higher 2 + states, we find: Q 2 + ( 106 Cd ) = −0.28 ± 0.08, Q 2 + ( 108 Cd ) = −0.45 ± 0.08, Q 2 + ( 110 Cd ) = −0.36 ± 0.08, Q 2 + ( 112 Cd ) = −0.39 ± 0.08, Q 2 + ( 114 Cd ) = −0.36 ± 0.08 and Q 2 + ( 116 Cd ) = −0.42 ± 0.08 e · emb. For 116 Cd , B( E 2; 0 + → 2 + ) = 0.532 ± 0.004 e 2 · b 2 , which is 20% lower than the previously adopted value.

Band crossings in 170Os

Abstract Excited states in the neutron-deficient nucleus 170 Os were identified up to spin (24 + ) in the yrast band and to spin (23 − ) in the lowest negative-parity band. Deformation systematics implied by the 2 + state energies for the very light osmium isotopes are compared with theory. Band-crossing frequencies, alignments and alignment gains are compared with cranked shell-model calculations. Deformation changes are required to obtain detailed agreement. A three-band mixing approach is invoked to explain the low-spin yrast anomaly in 172 Os and to reproduce the yrast band in 170 Os. The excitation energy of the postulated “intruder” band in 170 Os and 172 Os is deduced.

Non-yrast states and shape co-existence in light Pt isotopes

Abstract Low-lying states in the even-even light platinum isotopes 176Pt, 178Pt, 180Pt and 182Pt have been populated using β+ /EC decay from parent gold nuclei, created in (HI,xn) reactions. State energies, spins and parities and γ-ray branching ratios were determined using γ-ray and electron spectroscopy. Whereas non-yrast states were observed in 178Pt, 180Pt and 182Pt, none were seen in 176Pt. The excitation energies of the observed states are analysed in terms of a band-mixing model, yielding the moments of inertia of the unperturbed bands. Branching ratios and ground-state-band quadrupole moments are calculated and compared with experimental values. The results indicate that the two lowest-lying 0+ states in each of the light Pt isotopes are formed from the mixing of two intrinsic states of different deformation, and other low-lying states can be described as admixtures of rotational states built on these intrinsic states, and on γ-vibrational states.

Properties of 152− states in 215Ra and 217Th; evaluation of the 152− to 92+ E3 strength in N = 127 isotones

Abstract The lifetime of the yrast 15 2 − state in 215 Ra was measured using pulsed beams and γ-ray and electron techniques. Transition multipolarities were established from measured conversion coefficients. The B (E3) of the 15 2 − → 9 2 + transition is found to be considerably larger than previously reported. A candidate for the corresponding transition in 217 Th was also observed. The E3 strength of the 15 2 − → 9 2 + transition in the N = 127 isotones is evaluated in the light of these and other recent results. Interpretation in the framework of particle-octupole vibration coupling requires a systematic lowering of the core 3 − vibration as proton pairs are added to 208 Pb.

Electric quadrupole moments of the first excited states of 194Pt, 196Pt and 198Pt

Abstract Coulomb excitation of 194, 196, 198 Pt by 4 He, 12 C and 16 O projectiles has been used to determine the static electric quadrupole moments Q (2 1 + ) of the first excited states of 194,196,198 Pt, together with values of B (E2; 0 1 + → 2 1 + ). It is clearly established that Q (2 1 + ) is positive for each nucleus, having values of 0.48 (14) e · b and 0.66 (12) e · b for 194 Pt and 196 Pt, respectively, and 0.42 (12) e · b or 0.54 (12) e · b for 198 Pt, depending on whether the interference term P 4 (2 2 + ) is positive or negative. Results obtained for B (E2; 0 1 + → 2 1 + ) are 1.661 (11) e 2 · b 2 , 1.382 (6) e 2 · b 2 and 1.090 (7) e 2 · b 2 for 194, 196, 198 Pt respectively. The results are compared with the predictions of various nuclear models.

Interference between spherical and deformed states in 190Pb

Abstract The level scheme of 190 Pb identified in the 166 Er( 28 Si,4n) 190 Pb reaction includes several long isomers and exhibits structures which can be associated with co-existing spherical, oblate and prolate configurations. A chance near-degeneracy between 8 + states from the competing configurations causes mixing and fragmentation of the observed E3 and E2 decays from the yrast 11 − and 10 + states, through interference. The candidate identified for the prolate band is approximately 500 keV higher than that observed in 188 Pb.

Spectroscopy of 175Ir and 177Ir and deformation effects in odd iridium nuclei

Abstract Excited states in 175Ir and 177Ir have been identified using (heavy-ion, pxn) reactions and γ-ray spectroscopic techniques. Rotational bands based on intrinsic states arising from the odd-proton parents, h 9 2 , h 11 2 , i 13 2 and d 5 2 were established to high spin except for the 5 2 + [402] bands. Only the h 9 2 bands show sharp alignment gains compatible with alignment of ( i 13 2 ) 2 neutrons. The smooth apparent alignments of the i 13 2 proton bands can be attributed, at least qualitatively, to a much larger deformation. The complex alignment gains observed in the h 11 2 bands in these and other iridium isotopes in the range 173–181Ir are consistent with the effects of mixing with a deformed intruder (equivalently, a low-spin shape change). This conjecture is tested against the in-band decay properties using a three-band model. Examination of the band structure suggests a significant gamma deformation at very low spin, before the change to a more deformed configuration occurs.

Low-frequency band crossing in 171Re: a deformed intruder interpretation

The decay sequence based on the 5/2+(402) proton orbital has been identified in 171Re up to spin 41/2. An anomalous upbend in the alignment is observed below the expected backbend, a phenomenon similar to that observed in the yrast sequence of 172Os. The properties of the 5/2+(402) band can be understood in terms of two crossings, one at low spin with a band based on a deformed intruder configuration, the second at higher spin with a band composed of aligned i13/2 neutrons.

The static quadrupole moment of the first excited state of 198Hg

Abstract The first direct evidence of oblate nuclear deformation in the even-A mercury isotopes has been obtained by measurement of the static quadrupole moment of the first excited state of 198Hg using the reorientation effect in Coulomb excitation.