H.R. Andrews

Gamma-ray spectroscopy of 126Ba

Abstract States of 126Ba up to spin 36+ were populated in the reaction 96Zr(34S, 4n)126Ba at 155 MeV and up to spin 20+ in the reaction 116Sn(13C, 3n)126Ba at 56 MeV. Gamma-ray spectroscopy was performed with the 8π spectrometer, an instrument comprising 20 Campton-suppressed HPGe detectors and 71 BGO ball elements. A level scheme organized into fifteen rotational bands is proposed on the basis on the γγ-coincidence and γ-ray angular correlation data. The competing π h 11 2 and v h 11 2 band crossings are investigated and interpreted in terms of cranked shell model and total routhian surface calculations. Amongst the topics discussed are (i) a comparison of DCO and spin orientation techniques for determining spins and multipolarities. (ii) the addivity property of quasiparticle energies or routhians, and (iii) analysis of B(M1)/B(E2, ratios between signature partner bands. The possibility of couplings between vibrational and aligned quasiparticle structures is explored.

A new series of beta-delayed proton precursors

Abstract The T z =+ 1 2 beta-delayed proton precursors 65 Ge, 69 Se, 73 Kr and 77 Sr have been identified and their decay energies, half-lives and proton branching ratios determined. The half-life of the previously unreported 77 Sr is 9.0± 1.0 s.

Tz = 12β-delayed proton precursors: (I). The decay of 69Se

Abstract The isotope 69 Se has been produced via the 40 Ca( 32 S, 2pn) 69 Se reaction at 100 MeV incident energy. Thirty-two β-delayed γ-rays, as well as delayed protons in the range 1.0 to 3.2 MeV, have been observed with a half-life of 27.4 ± 0.2 s. A proposed decay scheme is based on γ-γ, β + -γ, p-γ, p-X and γ-X coincidence measurements. The total decay energies of 69 Se and 69 As were measured to be Q EC = 6795 ± 52 keV and 4067 ± 50 keV, respectively. The proton spectrum and decay properties are compared with detailed statistical model calculations.

Intruder bands in 108Sn

Abstract The nucleus 108Sn has been populated via the 54Fe(58Ni, 4p) reaction channel at a beam energy of 243 MeV. The high-spin structure is dominated by three ΔI = 2 rotational sequences. These bands can be interpreted in terms of particle-hole excitations involving the proton g 7 2 , g 9 2 and h 11 2 orbitals and also aligned neutrons from the bottom of the h 11 2 shell. Lifet measurements have also been performed using the Doppler-shift attenuation method. These data have enabled quadrupole moments to be deduced for the two strongest bands. The results yield Q0 = 2.6±0.4 e·b for the positive-parity band and 3.4 ± 0.6 fse·b for one of the proposed negative-parity bands. These values yield quadrupole deformations of β2 = 0.20 and 0.26, respectively, for the two bands. The results obtained are discussed in terms of Woods-Saxon and total routhian surface calculations.

Collective properties of 48Cr at high spin

Abstract High-spin states of the nucleus 48 Cr have been studied via particle-γ-γ spectroscopy, following the 28 Si( 28 Si,2α) reaction. A 44-element particle-detector array was used to isolate 48 Cr residues and to reduce γ-ray Doppler broadening. The collective band built upon the ground state has now been firmly established to spin 16 + , the highest possible in the isolated f 7 2 shell, and lifetimes of the four highest states have been measured from Doppler shifts. Although some of the ground-state band properties are well reproduced by recent fp -shell model calculations, a sharp reduction in E2 transition rates at the backbend is not.

Studies of superdeformation in the gadolinium nuclei

Abstract We have used the 8π γ-ray spectrometer at the Chalk River TASCC facility to study superdeformed rotational bands in the chain of isotopes 145–149 Gd. The five bands already known have been extended to higher spin, while four new excited bands have been discovered. The use of very similar reactions, data-acquisition parameters and data-analysis techniques, combined with the enlarged set of data, has allowed us for the first time to approach the topic of superdeformation in the A = 150 mass region from a comprehensive point of view. Transition energies of neighbouring nuclei were compared to extract effective aligned spins of the added particle. Such a comparison requires knowledge of relative nuclear spins, which we have to assume. Nevertheless, we find that a very illustrative picture emerges making it possible to understand all nine bands in the Gd isotopes within a very simple scheme of orbital assignments.

Rotational bands in 238U

Abstract A thick foil of 238 U was bombarded with 209 Bi beams at 1130 and 1330 MeV, delivered by the TASCC facility at Chalk River Laboratories. Gamma-ray spectroscopy of states populated in multiple Coulomb excitation was performed with the 8π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. The event trigger required that 3 BGO elements and 2 HPGe detectors fire in coincidence. The experiment achieved a high degree of sensitivity, the weakest rotational band observed had about 0.16% intensity of the ground state rotational band. Several bands were observed to high spin for the first time, including the γ-vibrational band (spin 27 + ) and the octupole bands with K = 0 (spin 31 − ), K = 1 (spin 28 − ) and K = 2 (spin 25 − ). Results for positive and negative parity vibrational bands are compared with cranked RPA calculations. Although this theory can explain some features of the data, many puzzling aspects remain to be explored.

Spin dependence of the giant dipole resonance in Gd isotopes

Abstract The shape evolution of the Gd nuclei produced by the 124 Sn+ 30 Si reaction at a beam energy of 150 MeV has been studied as a function of spin by measuring the γ-ray decay of the giant dipole resonance. The nuclear deformation was extracted from the spectral shape and, independently, from the angular distribution with respect to the spin axis. Both measurements reveal that these nuclei remain prolate, with an increase in deformation as the spin increases. Sum-energy and multiplicity distributions have also been measured as a function of γ-ray energy.

High-spin states in 149Gd

Abstract High-spin states in 149Gd have been investigated via the 124Sn(30Si, 5n)149Gd reaction at a beam energy of 150 MeV. Measurements of γγ-coincidences and angular distributions with respect to the nuclear spin axis allowed the decay scheme to be extended up to J≈ 85 2 h and E ∗ ≈ 16 MeV . Single-particle configurations were assigned to many high-spin levels based on calculations performed with the deformed Woods-Saxon potential.

Gamma-ray spectroscopy of 127Ba at moderate spins

Abstract The near-yrast spectroscopy of 127Ba has been examined in detail up to spin 39 2 with the reaction 117Sn(13C, 3n) at 60 MeV incident energy. Gamma-ray spectroscopy was performed with the 8 π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. Evidence is presented for the identification of single-neutron quasiparticle bands corresponding to Nilsson orbitals [532] 5 2 , [523] 7 2 , [411] 1 2 and [402] 5 2 . The properties of the [523] 7 2 band suggest that the nucleus has a triaxial shape when that orbital is occupied. We draw attention to: (i) the signature splitting; (ii) staggering of B(E2) ratios; (iii) the mean Q 1 Q 2 ratio; and (iv) the comparison of the staggering in the B( M 1) B( E 2) ratios with the signature splitting. All these factors point to a γ-deformation in the neighbourhood of γ∼−20°. Based on the large B( M 1) B( E 2) ratios, the observed three-quasiparticle bands involve proton excitation. Of these bands, two have clear-cut structural assignments ν h 11 2 π( h 11 2 ) 2 and ν d 5 2 π( h 11 2 ) 2 . However, the third band presented difficulties and no assignment was consistent with all the data. The shift in crossing frequency for the π( h 11 2 ) 2 with a ν h 11 2 spectator is very large and corresponds to a shift in the routhian of about 220 keV. This we interpret to be due to a negative γ-deformation for bands involving ν h 11 2 . The opposite shift of ∼300 keV for the same crossing with a ν d 5 2 spectator cannot be explained as a shape polarization effect.