S.A. Forbes

Measurements on prototype Ge and BGO detectors for the Eurogam array

Abstract This paper presents the results of tests, both with sources and in-beam experiments, on prototype Ge and BGO detectors of the Eurogam array. Timing and energy resolution data are reported along with results for various methods of ballistic deficit correction. The first measurements with the new “shared suppression” technique are presented.

Collective and non-collective high-spin states in 115I

The 92Mo(27Al, 2p2n gamma ) reaction has been used to populate high-spin states in 115I. The dominant structures are associated with pi h112/(550)1/2- and pi g92/(404)9/2+ orbitals. At the highest spins, favoured states with Ipi =43/2- and 51/2-, associated with non-collective oblate configurations are yrast. The systematics of such states in the odd-A 115-121I isotopes are discussed.

Neutron excitations across the N = 86 superdeformed shell gap

Abstract Based on data obtained with the Eurogam spectrometer, a systematic investigation of superdeformed bands built on single neutron excitations across the N = 86 shell gap has been carried out for gadolinium, terbium, and dysprosium isotopes with 85 ≤ N ≤ 87. In each nucleus, pairs of bands corresponding to degenerate signature partners have been observed. The effective alignment is constant and close to zero for many of the bands, in agreement with cranked shell model calculations for the [402]5/2 and [514]9/2 orbitals. Several bands exhibit non-zero alignments, indicating in some cases the influence of pairing correlations even at high spin and in other cases the occupancy of the [521]3/2 orbital.

Measurement of transition quadrupole moments of high-spin states in the N=74 isotones 133Pr, 132Ce and 131La

Abstract The Doppler-Shift Attenuation Method has been used to extract transition quadrupole moments of high-spin bands in the N=74 isotones 133 Pr, 132 Ce and 131 La, produced in the 37 Cl + 100 Mo reaction. The results appear to be configuration dependent and, for 133 Pr and 132 Ce, the involvement of Ω=1/2 νh 9/2 and νf 7/2 intruder orbitals appears to enhance the collectivity at high spin ( I>25 ℏ ).

Determination of the intrinsic quadrupole moment of the superdeformed band in 143Eu

The intrinsic quadrupole moment, Q0, of the superdeformed band in 143Eu has been extracted from nuclear lifetimes, measured in a Doppler shift attenuation method experiment. Data were collected with the NORDBALL gamma-ray spectrometer following the reaction 110Pd (37Cl, 4n) 143Eu at a beam energy of 160 MeV. A centroid shift analysis of the data was carried out, using the Braune stopping powers to model the slowing down of the recoil nuclei in the target and backing materials. This gave a value of Q0 = 13.0 ± 1.5 eb for the band. The result is in good agreement with the theoretical prediction of Total Routhian Surface calculations, and supports the assigned single particle configuration, involving occupation of both neutron and proton i132 intruder orbitals from the Nosc = 6 oscillator shell.

Quadruple moment of the i13/2 intruder band in 139Gd

The Doppler shift attenuation method has been used to extract the mean quadruple moment of the nu i13/2 intruder band in 139Gd. From a centroid-shift analysis of the data, and using Braune stopping powers, a value for the intrinsic nuclear quadrupole moment of Q0 approximately=7.0 eb is suggested, corresponding to a deformation beta 2 approximately=0.35. Because of appreciable side-feeding into the band at each level, a lineshape analysis of the data has also been attempted, in which the time structure of the side-feeding has been allowed to vary. This analysis indicates that the side-feeding is significantly slower than the in-band cascade.

Lifetimes of low-lying states in 132Nd and 134Nd

Abstract Lifetimes of low-lying states have been measured in 132 Nd and 134 Nd using the coincidenceplunger technique. The reaction 32 S+ 105 Pd was used at a bombarding energy of 152 MeV. The measurement has been performed at the NSF Daresbury using the ESSA 30 array. The differential decay-curve method (DDCM) was used to analyze the recoil-distance Doppler-shift (RDDS) data. The experimental B (E2) values in 132 Nd are well described by the predictions of the rotational model and the IBM in the O(6) limit.

Shape coexistence in 138Sm and evidence for the rotational alignment of a pair of N=6 neutrons

The level scheme of 138Sm has been extended up to Ipi =(32+) using gamma -ray coincidence data collected with the Eurogam array. The 106Cd(35Cl,3p)138Sm fusion-evaporation reaction was used at a bombarding energy of 150 MeV. Several new bands have been observed and the known bands extended to higher spin. Band crossings and shape coexistence are discussed with the aid of Woods-Saxon cranking calculations (CMS and TRS). Evidence is presented for a neutron (i132/)2 pair alignment which is predicted to drive the nucleus to a prolate shape with enhanced quadrupole deformation beta 2=0.32. A strongly coupled band was also established and is interpreted as a collectively rotating oblate structure.

Prolate and oblate rotational bands in 136Sm

Six rotational bands have been established in 136Sm following heavy-ion induced reactions carried out at the Nuclear Structure Facility, Daresbury. The yrast band has been observed up to Ipi =(24+) and undergoes three band-crossings. A band was observed built on the low-lying 2+ gamma-vibrational state, in addition to two negative parity sidebands built on two-quasiproton excitations. A dipole band showing the characteristics of collective oblate rotation was also established. The results are discussed in terms of total Routhian surface and cranked Woods-Saxon model calculations.

High-spin studies of neutron-deficient A=138 nuclei: 138Eu and 138Gd

The level schemes of mass A approximately 138 nuclei have been extended using data collected with the Eurogam array. The 35Cl+106Cd fusion-evaporation reaction was used at a bombarding energy of 150 MeV. Results are presented for odd-odd 138Eu and even-even 138Gd. The high-spin behaviour is discussed in terms of quasiparticle alignments of h11/2 proton and neutron pairs.