I.G. Bearden

Compton suppression tests on Ge and BGO prototype detectors for gammasphere

During the last decade the application of arrays of 10--30 Compton Suppressed germanium (CSG) detectors in the field of {gamma}-ray spectroscopy has led to a number of significant advances in studies of nuclear structure. Despite the success of these instruments, most of them are limited to detecting two-fold (and in a few cases three-fold) coincidences from highmultiplicity cascades. To increase the detection sensitivity for weak cascades, proposals have recently emerged for construction of much larger arrays. In the USA, the proposed array, called GAMMASPHERE, is to have 110 large CSG detectors which will subtend a solid angle of almost 2{pi} steradians, sufficient to permit routine acquisition of up to 5-fold coincidences from high-multiplicity cascades. This together with other improvements, will yield a resolving power several orders of magnitude greater than currents arrays. In Europe, an array of similar size and power, called EUROGAM, is currently being constructed. In this paper, we report on measurements of the Compton suppression and overall P/T ratio of two Ge detectors in a BGO shield of the honeycomb pattern. These were the first prototype CSG detector assemblies for GAMMASPHERE

Superdeformed bands in 189,190Hg

Abstract Experiments using the 160 Gd ( 34 S , x n ) reaction at 159 and 162 MeV have revealed two γ-ray cascades which have properties similar to those of the previously reported superdeformed bands in the heavier Hg isotopes. These sequences have been identified as superdeformed structures in 189Hg and 190Hg. For the band in 190Hg, an average quadrupole moment Q0 = 18 ± 3 e · b was obtained from a DSAM measurement. The data are interpreted within the cranked Woods-Saxon formalism and the rise in the dynamical moment of inertia for both bands is linked to the presence of pair correlations and quasiparticle alignments. For the 189–194Hg superdeformed bands, the variations in the observed population intensity and in the rotational frequencies at which decay out of the bands occur are found to be consistent with the calculated change in the well depth of the superdeformed minimum with mass.

Proton excitations in the superdeformed well of 193Tl

Abstract Two superdeformed bands of 13 transitions each have been found in 193 Tl with the 160 Gd( 37 Cl, 4n) reaction. The dynamic moments of inertia for the two bands are found to rise with rotational frequency, as for all observed superdeformed bands in other nuclei in this region. The two bands can be interpreted as signature partners which exhibit some signature splitting for rotational frequencies above 0.2 MeV. The data are interpreted with cranked Woods-Saxon calculations and illustrate the role of the proton i 3 2 (Ω = 5 2 ) intruder orbital.

Gamma-ray studies of 119, 121, 123Sn isomers formed in deep inelastic heavy ion collisions☆

Abstract Yrast isomers in 119Sn, 121Sn and 123Sn have been identified among products of heavy ion collisions with 124Sn targets 10–15% above the Coloumb barrier. Isomeric decay schemes are reported, and further evidence for half-filling of the ν h 11 2 subshell at N = 73 is presented. For ( ν h 11 2 n seniority-3 states, observed level energies agree well with results of fractional parentage calculations.

Shape-driving effects in 193Tl from the spectroscopy of yrast and near-yrast states

Abstract The nucleus 193Tl has been populated with the 160Gd(37Cl,4n) reaction and its level scheme has been extended up to spin 41 2 h by using an array of 12 Compton-suppressed Ge detectors. Near-oblate band structures arising from a 9 − 2 state and markedly different, single-particle-like, structures above a 13 + 2 state have been established. The two structures are consistent with total routhian surface calculations indicating the existence of two minima in the total routhian surface of the nucleus which survive separately up to the observed spins. Details of the rotational-like part are discussed in terms of the cranked shell model, while the single-particle-like features of the nucleus are compared with non-collective structures in neighboring nuclei, in particular 191Hg.

Detailed band structures in 189Hg and 190Hg

Abstract The level structures of the 189 Hg and 190 Hg nuclei have been extended considerably from earlier studies by using the 160 Gd( 34 S, 4n and 5n) reactions in conjunction with an array of twelve Compton suppressed Ge detectors. Eight band structures have been delineated in 189 Hg and ten in 190 Hg. Most bands can be understood within the framework of cranked shell model calculations assuming an oblate collective shape and specific quasiparticle configurations are proposed. There is also some evidence for the onset of triaxiality. In both nuclei level structures which appear to be non-collective in character are interpreted in a band termination picture.

Superdeformation in the mercury nuclei

We shall first summarize the present experimental situation concerning {sup 192}Hg, the nucleus regarded as the analog of {sup 152}Dy for this superdeformation (SD) region in that gaps are calculated to occur at large deformation for Z = 80 and N = 112. Proton and neutron excitations out of the {sup 192}Hg core will then be reviewed with particular emphasis on {sup 191}Hg and {sup 193}Tl. The presentation will conclude with a brief discussion on limits of the SD region for neutron deficient Hg nuclei. 26 refs., 10 figs.

Search for entrance channel dependence in the population of superdeformed bands in 191Hg

The population intensity of some SD bands in the mass 150 region were observed to depend on the mass symmetry of the entrance channel in the fusion reaction. The authors raised the possibility that the population of SD bands had a memory of the entrance channel. To check this interesting possibility, we made measurements of the population intensities of superdeformed (SD) bands in the {sup 160}Gd({sup 36}S,5n){sup 191}Hg and {sup 130}Te({sup 64}Ni,3n){sup 191}Hg reactions. To ensure that any observed effect was not due to a simple angular momentum difference in the entrance channels, we also measured the average entry points and spin distributions of normal and SD states in {sup 191}Hg in the two reactions. The entry points and spin distributions for {sup 191}Hg are the same and, indeed, so are the SD intensities in the two reactions. Hence, no entrance-channel effect is observed in the population of the SD band in {sup 191}Hg, in contrast with data for SD bands in the mass 150 regions. We suggest that the effect observed previously in the mass 150 region is due to an angular momentum effect. A letter reporting our results was submitted for publication.

Instrumentation of double-sided silicon strip detectors for multi-particle detection

A compact, modular, low-cost, high-density readout system for double-sided silicon strip detectors (DSSD) has been developed. The system, composed of a customized high-density, multi-channel preamplifier unit combined with commercial CAMAC modules, provides for the individual readout of each of the 32 segments in a 16 × 16 crossed-strip DSSD. This system permits X–Y position sensitivity to be achieved for multiple-particle events, making double-sided silicon strip detectors powerful tools for the study of nuclear reactions leading to many-charged-particle final states.

Level structure of the odd-odd nucleus 22489Ac

Abstract The level structure of the odd-odd nucleus 224Ac has been studied by measuring radiations associated with the α-decay of the 22 h 228Pa isotope. The 228Pa activity was produced by the irradiation of Th metal foils with 45 MeV protons. The Pa fraction was subsequently chemically isolated. Alpha singles as well as αγ-, αe-, αγγ-coincidence measurements were performed with thin sources, and a partial level scheme was deduced for the 224Ac. Although the spins of the observed levels have not been uniquely determined, the data establish positive- and negative-parity bands which exhibit characteristics of parity doublets. In particular, two bands at 354 and 360 keV appear to be members of a parity doublet. The similarity in the hindrance factors of α-decays to the 354 and 360 keV levels, which have opposite parities, indicates that the wavefunctions of the two states are very similar, which is a signature for octupole deformation.