S. L. Tabor

Structure of 52,54Ti and shell closures in neutron-rich nuclei above 48Ca

Abstract The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

Octupole instability induced by rotation in the nuclei 146,148Nd

Abstract Levels in 146,148Nd have been populated in the 150 Nd (α, α′ x n ) reactions. Cascades of enhanced E1 transitions have been observed. B( E 1) B( E 2) branching ratios, of the order of 10−6 fm−2 as well as the presence of low energy negative-parity bands indicate strong octupole effects. Shell correction type calculations suggest an explanation in terms of octupole correlations at medium spins.

MoNA—The Modular Neutron Array

The Modular Neutron Array (MoNA), a highly efficient time-of-flight neutron detector, is being constructed for use at the National Superconducting Cyclotron Laboratory. The modular design of the detector provides significant flexibility and allows the bulk of the assembly and testing to be done by undergraduates at nine of the schools participating in the project.

Isomerism in 96Ag and non-yrast levels in 96Pd and 95Rh, studied in β decay

The β decay of 96Ag (Z=47,N=49) was investigated by measuring positrons, X rays as well as β-delayed protons and γ rays. The γ radiation was studied by means of germanium detectors and a NaI total-absorption spectrometer. Two β-decaying isomers in 96Ag were established with half-lives of 4.40(6) and 6.9(6) s and tentative spin–parity assignments of (8+) and (2+), respectively. For both isomers, the intensities of β transitions to low-lying levels of 96Pd (Z=46,N=50) and β-delayed proton decays to levels in 95Rh (Z=45,N=50) were measured. Several new 96Pd levels were firmly established. The level energies, their γ decays and the Gamow–Teller decay of 96Ag are compared to shell-model predictions. A new low-lying level in 95Rh was found at 680 keV excitation energy. Through a comparison with low-lying states of N=50 isotones, this level is interpreted as the first excited 7/2+ state built on the proton 9/2+ ground state. The assignments of further excited states in 95Rh are discussed.

The (3He, t) reaction at 197 MeV on 12C, 24Mg, 28Si and 40Ca

Abstract Spectra have been measured for the (3He,t) reaction on 12C, 24Mg, 28Si and 40Ca at Elab = 197 MeV and θlab = 15°. The analog of the giant dipole resonance (GDR) is strongly populated in the charge-exchange reaction for each of the targets used in this study. Differences between the spectral shape of the GDR known in 12C, 28Si, 24Mg and 40Ca from photonuclear work and their analogs in 12N, 28P, 24Al and 40Sc are discussed. No evidence is seen for the population of a compact isovector giant quadrupole resonance (GQR) in any of these targets. Angular distributions have been measured at this energy for the 40Ca(3He, 3He), 40Ca(3He, 3He′)40Ca and 40Ca(3He, t)40Sc reactions. The elastic 3He scattering from 40Ca is reasonably described by a volume Woods-Saxon optical potential that is very similar to one known from lower-energy studies. Collective DWA calculations using this potential reproduce the inelastic scattering to the 3− and isoscalar GQR states of 40Ca and an additional calculation using the Goldhaber-Teller model roughly reproduces the measured charge-exchange angular distributions of the GDR in 40Sc. Data for the inelastic-scattering and charge-exchange reactions to a few low-lying discrete states and the region including the GDR in mass 40 have also been examined in DWA calculations using a microscopic double-folding model with a realistic effective interaction and particle-hole wave functions. Where possible these results are compared with results for other probes and rough consistency is found. The decomposition of the strength observed in the GDR region via (3He, t) is considered in some detail and it is estimated that 30% of this strength is ΔS = 1.

Characterization of the first superdeformed band in the A ≈ 80 region

Abstract The recently discovered superdeformed band in one of the strontium isotopes has been identified as belonging to 83Sr from a study with the Gammasphere array and the Microball charged-particle detector system. From its decay pattern, a spin of (81 ± 2) 2 h is deduced for the highest level of this band. Evidence for very fast transition rates and agreement of the characteristics of this band with the theoretical predictions establish the yrast superdeformed nature of this band in 83Sr.

Multiple superdeformed bands in 81Sr

Abstract Four superdeformed bands extending over five to twelve transitions have been identified in 81 Sr from a study with the Gammasphere array and the Microball charged-particle array. One of the bands shows an upbend in the dynamic moment of inertia at a rotational frequency of 1.2 MeV and all bands exhibit a nearly constant moment of inertia below that frequency.

Excited intruder states in (32)Mg

The low energy level structure of N = 20 32 Mg obtained via β-delayed γ spectroscopy is reported. The level structure of 32 Mg is found to be completely dominated by intruders. An inversion between the 1p-1h and 3p-3h states is observed for the negative parity states, similar to the 0p-0h and 2p-2h inversion for the positive parity states in these N ∼ 20 nuclei. The intruder excited states, both positive and negative parity, are reasonably explained by Monte Carlo shell model calculations, which suggest a shrinking N = 20 shell gap with decreasing Z.

Projectile breakup and total reaction strengths in Li-induced reactions

Abstract Spectra and angular distributions have been measured for the light particles emitted from the reactions induced by 36 MeV 6 Li and 7 Li ions on targets of 12 C, 13 C and 27 Al. Some measurements were also performed at beam energies of 28 and 32 MeV. The spectra are dominated by broad peaks centered at energies corresponding to the beam velocity, a characteristic of projectile breakup. A simple breakup model roughly reproduces the shapes of the measured spectra and angular distributions. The total cross sections observed for projectile breakup average about 600 mb and are almost as large as the fusion cross sections. The 7 Li breakup yields are somewhat larger than those for 6 Li, the same trend that was previously observed for the Li-induced fusion cross sections on C. As a result the measured total reaction cross sections induced by 7 Li ions are about 200 mb larger than those of 6 Li on the same targets. This difference is not predicted by conventional optical model calculations.

Half-life and spin of {sup 60}Mn{sup g}

A value of 0.28{+-}0.02 s has been deduced for the half-life of the ground state of {sup 60}Mn, in sharp contrast to the previously adopted value of 51{+-}6 s. Access to the low-spin {sup 60}Mn ground state was accomplished via {beta} decay of the 0{sup +} {sup 60}Cr parent nuclide. New low-energy states in {sup 60}Mn have been identified from {beta}-delayed {gamma}-ray spectroscopy. The new, shorter half-life of {sup 60}Mn{sup g} is not suggestive of isospin-forbidden {beta} decay, and new spin and parity assignments of 1{sup +} and 4{sup +} have been adopted for the ground and isomeric {beta}-decaying states, respectively, of {sup 60}Mn.