A.E. Stuchbery

Lifetimes of excited states in 196, 198Pt; Application of interacting boson approximation model to even Pt isotopes systematics

Abstract The lifetimes and lifetime limits of the low-lying excited states up to and including the 6 1 + levels in 196, 198 Pt were determined by the recoil-distance method (RDM). Gamma-ray angular distributions in 198 Pt were also measured. These states were populated by multiple Coulomb excitation using 220 MeV 58 Ni ion beams and the measurements were carried out in coincidence with back-scattered projectiles. The measured mean lives of the states and B (E2) values inferred for the transitions between levels are presented. These specific findings, and the observed structure systematics obtained from the combination of the present results and those of prior workers for the even 194–198 Pt isotopes, are critically compared with our structure calculations employing the interacting boson approximation (IBA) model incorporating a symmetry-breaking quadrupole force; evaluative comparisons are also made with boson expansion theory (BET) calculations.

Critical assessment of interacting boson model wave functions from measured gyromagnetic ratios of lowest eigenstates in even Os isotopes

Abstract The gyromagnetic ratios of the low-lying levels in the even 188−192 Os isotopes were measured. Of particular import, the ratios g (2 + 2 )/ g (2 + 1 ) were found to be 1.45 ± 0.18, 0.99 ± 0.14, and 0.72 ± 0.06 in 188 Os, 190 Os, and 192 Os, respectively. Although the E2 observables in these nuclides are accommodated quite well in interacting boson model descriptions (IBM-1 and IBM-2), both formulations fail to concurrently account for either the intra-nucleus or mass-dependent g -factor variations of the lowest eigenstates in these Os isotopes.

Disparity between the transient hyperfine fields for Pt and Os in Fe; an electron vacancy sharing interpretation

Precessions of the 21+ states in198Pt and192Os have been measured in the enhanced transient hyperfine magnetic field acting on these nuclei as they recoiled through thin polarized cobalt foils. Two separate targets consisting of contiguous layers of198Pt and192Os electrodeposited on ∼1 μm and ∼4 μm Co foils were employed. The levels of interest were Coulomb excited by 80 MeV32S and 220 MeV58Ni beams and the precessions of the 21+→01+ γ-ray angular distributions in both nuclides were measured simultaneously. The results of these studies are compared with recently reported similar studies in which ions of188Os and194Pt recoiled through thin polarized Fe foils. It is concluded that (i) the transient field acting on Pt in Fe is singularly anomalous, and (ii) the recent contention that the g-factors of the 21+ states in the event Pt isotopes may be substantially lower than had been reported in the literature cannot be sustained. This transient field discontinuity is examined in terms of possible molecular orbital electron vacancy sharing between Pt and Fe.

Gyromagnetic ratios of excited states in 198Pt; measurements and interacting boson approximation model calculations

Abstract The enhanced transient hyperfine field manifest at the nuclei of swiftly recoiling ions traversing magnetized ferromagnetic materials was utilized to measure the gyromagnetic ratios of the 2 + 1 , 2 + 2 and 4 + 1 states in 198 Pt by the thin-foil technique. The states of interest were populated by Coulomb excitation using a beam of 220 MeV 58 Ni ions. The results obtained were: g (2 + 1 ) = 0.324 ± 0.026; g (2 + 2 ) = 0.34 ± 0.06; g (4 + 1 ) = 0.34 ± 0.06. In addition, these measurements served to discriminate between the otherwise essentially equally probable values previously reported for the E2/M1 ratio of the 2 + 2 → 2 + 1 transition in 198 Pt. We also performed interacting boson approximation (IBA) model-based calculations in the O(6) limit symmetry, with and without inclusion of a small degree of symmetry breaking, and employed the M1 operator in both first- and second-order to obtain M1 selection rules and to calculate gyromagnetic ratios of levels. When O(6) symmetry is broken, there is a predicted departure from constancy of the g -factors which provides a good test of the nuclear wave function. Evaluative comparisons are made between these experimental and predicted g -factors.

Gyromagnetic ratios of excited states in 150Sm and 152Sm

Abstract The gyromagnetic ratios of the 2 1 + , 4 1 + , 6 1 + , 2 2 + , and 2 3 + states in 150 Sm and the 2 1 + , 4 1 + , 6 1 + , 8 1 + , 10 1 + , 2 2 + , and 2 3 + levels in 152 Sm have been measured using the transient field perturbed γ-ray angular correlation technique. States of interest were populated by multiple Coulomb excitation using 150- and 220-MeV 58 Ni beams. The present results display no significant deviation from constancy of the g -factors of all levels investigated in either isotope separately or of corresponding states in 150,152 Sm. These findings are compared with available prior measurements and assessed in terms of applicable theoretical model calculations.

Tests of interacting boson model wave functions from measured gyromagnetic ratios of states in the even Os isotopes

Abstract The gyromagnetic ratios of the 2 1 + , 2 2 + , and 4 1 + states in 188,190,192 Os were experimentally measured. It is shown that the hamiltonians of standard interacting boson models (IBM-1 and -2) fit to level spectra and B (E2) rates fail to account for either the intra-nucleus or mass-dependent g -factor variations of these states in these Os isotopes.

Variations of the gyromagnetic ratios of low-lying states in 192Os

Abstract The gyromagnetic ratios of the 22+ and 41+ states in 192Os were measured relative to that of the 21+ level. The thin foil, perturbed γ-ray angular distribution technique was employed utilizing the transient hyperfine magnetic field present at the nuclei of these ions as they swiftly recoiled through a thin, magnetized Co foil. The states of interest were Coulomb excited using a beam of 220 MeV 58Ni ions. The present measurements yielded ratios g(2 2 + ) g(2 1 + ) = 0.68 ± 0.08 and g(4 1 + ) g(2 1 + ) = 1.00 ± 0.12 . The sizable disparity in the measured g-factors of the levels of the two low-lying bands is compared with interacting boson approximation model-based calculations, as well as with a Nilsson basis single-particle model description.

The extraction of lifetimes of weakly-populated nuclear levels in recoil distance method experiments

Abstract Two analytic techniques are described which extend the conventional analysis of recoil-distance method (RDM) data. The first technique utilizes the enhanced counting statistics of the composite spectrum formed by the addition of all γ-ray spectra recorded at the different target-to-stopper distances employed, in order to extract the lifetimes of levels whose observed depopulating γ-ray transitions have insufficient statistics to permit conventional analysis. The second technique analyses peak centroids rather than peak areas to account for contamination by flight distance dependent background. The results from a recent study of the lowlying excited states in 196, 198 Pt for those levels whose lifetimes could be extracted by conventional RDM analysis are shown to be in good agreement with those obtained using the new methods of analysis.

Level structure and lifetimes of low-excitation states in 63Cu

Abstract The lifetimes of 9 states in 63 Cu up to an excitation energy of 2092 keV have been determined by means of the Doppler-shift attenuation method using the 60 Ni (α, p γ) reaction. A search was made for a suitable set of M1 and E2 single-particle matrix elements for use with shell-model eigenstates throughout the Ni, Cu and Zn isotopes. The theoretical predictions for the level spectroscopy and electromagnetic transitions and static moments in 63 Cu are in satisfactory agreement with the experimental findings.

Systematics of gyromagnetic ratios of the 2+-1- states in even Ge isotopes

The relative gyromagnetic ratios of the 2t states in 70,72,74,76Ge were measured simultaneously by me ns of the thin-foil, perturbed y-r yngular distribution technique utilising the transient hyperfine field manifest t nuclei of Ge ions rapidly traversing polarised Fe. The states of interest were Coulomb excited using 75-MeV 34S projectiles incident upon natural element l Ge t rget. The present results re comp red with previously reported measurements and collective model expectations.