A Johnston

High-spin states in 172W, 174W and 176W studied by (16O, xn) reactions: decoupled quasiparticle bands and backbending

High-spin states in 172W, 174W and 176W have been studied using gamma -ray techniques following (16O, xn) reactions on enriched Dy targets. The ground-state band has been shown to backbend in 174W and 175W, but not in 172W. Side-bands are populated in each nucleus, with preferential population of an odd-spin sequence in each case. The decay properties of these bands, which have negative parity, suggest that they are due to the partial decoupling of a pair of quasi-particles by the Coriolis force. The properties of the bands are used to estimate in an empirical way the properties of a rotation-aligned (i132/)2 neutron band in order to predict the systematics of backbending due to bandcrossing.

High-spin and single-particle structure in 173W and 175W

The level structures in 173W and 175W have been studied using (16O,4n) reactions. Rotational bands based on three different single-particle configurations are observed to high spin in each nucleus. From a comparison of their contrasting behaviour with the ground-state rotational bands in 172W and 174W, it is concluded that the decoupling of a pair of i13/2 neutrons causes the backbending in 174W. A single-quasiparticle Coriolis mixing calculation is able to reproduce the observed band structure up to high spin.

High-spin states and two-quasiparticle structure in 172Hf

Five rotational bands in 172Hf have been observed to high spin following the 160Gd(16O, 4n)172Hf reaction, using γ-γ coincidence techniques. The ground-state band is extended to spin 20+ without backbending, in contrast to the neighbouring even-even isotones. The most strongly populated side-band, consisting of separated odd-spin and even-spin sequences, appears to result from the partial decoupling of an i132 neutron. Three other bands are identified with the following band-heads: an 8− state at 2006 keV with 163 ns half-life; a 6+ state at 1685 keV with a half-life < 16 ns; and a level at 1857 keV with spin 4, 5, 6 or 7 and a half-life < 16 ns. The differences between the bands are discussed in terms of the influence of Coriolis forces and the two-quasiparticle level structure.

Lithium-induced reactions on carbon at 48 MeV

Abstract Angular distributions have been measured for transitions to states in 13N and 13C populated by the 12C(7Li, 6He)13N and the 12C(7Li, 6Li)13C reactions at E = 48 MeV. Elastic scattering of 7Li at 48 MeV on 12C and 12Li on 13C at 40 MeV were measured. Optical-model parameters extracted from fits to these data were used in the DWBA analysis of inelastic scattering to levels in 12C and 7Li and for particle transfers to levels in 13N and 13C. The calculations generally reproduced the data, except for the 3.09 MeV 1 2 + level in 13C, which was seriously out of phase. Spectroscopic factors were obtained from the transfer data and deformation lengths from the inelastic scattering to the 4.44 MeV 2+, and the 9.64 MeV 3− levels in 12C and to the 0.48 MeV 1 2 − level in 7Li.

Elastic and inelastic scattering of 6Li and 7Li by 54Fe at E = 36–50 MeV

Abstract Detailed angular distributions for the elastic and inelastic scattering of 7 Li at E = 36, 42, and 48 MeV and of 6 Li at E = 38, 44, and 50 MeV by 54 Fe have been measured. It is not possible to describe both sets of data with the same set of optical-model parameters. The ratio of U W is 0.8 for 7 Li and 1.4 for 6 Li at the strong absorption radius, implying stronger absorption for 7 Li than for 6 Li. No energy dependence in the optical-model parameters was necessary for either 6 Li or 7 Li. The inelastic scattering from the 54 Fe 2 + , 1.41 MeV state was well described by the DWBA and the extracted deformation length ( βR = 0.62) was the same for both 6 Li and 7 Li scattering. It was not possible to describe the 7 Li projectile excitation data with collective-model DWBA calculations showing that more detailed calculations for the projectile excitation are necessary.

The 118Sn(7Li, 6He)119Sb reaction at 48 MeV

Abstract Angular distributions have been measured for transitions to low-lying states in 119 Sb populated in the 118 Sn( 7 Li, 6 He) 119 Sb reaction at E 7Li = 48 MeV. Elastic scattering of 7 Li at 48 MeV and 6 Li at 42 MeV was measured and optical model parameters determined by fits to the angular distributions. The optical model parameters deduced were used in a finite-range DWBA analysis of the transitions to states at 0.0, 0.27, 0.66 and 1.35 MeV excitation energy in 119 Sb. Generally the data exhibited less structure than was predicted by the DWBA calculations, but the extracted spectroscopic factors are generally in good agreement with other results. The same types of discrepancies between calculations and data observed with heavier projectiles, are also observed for 7 Li induced reactions.

The 54Fe(d,t)53Fe reaction and the neutron configuration in 54Fe

The 54he(d,t)53Fe reaction has been used to study the levels populated in 53Fe in an attempt to establish the neutron configuration in 54Fe. The states observed show clear evidence for a 2p-4h admixture in 54Fe. In particular, the strength of the first 3/2- level relative to the 7/2- ground-state transition is 3-4 times that in neighbouring N=28 nuclei.

High spin and backbending in the light tungsten isotopes

High spin states in 172W, 174W 175W and 176W have been studied with (16O,xn) reactions. The ground-state bands in 174W and 176W backbend in contrast to the more regular GSB in the N=98 nucleus 172W. This behaviour and the anomalies in the odd nucleus 175W are discussed in terms of the influence of neutrons on backbending.

Coulomb excitation of 151Eu

The N=88 nucleus 151Eu has been studied by Coulomb excitation with 16O beams and from the decay of 151Gd using high-resolution gamma ray spectroscopy. A comprehensive level scheme including many new transitions, new levels and spin assignments is established. The results are compared with previous studies and with the predictions of a rotational-model calculation.

Selective population of three-particle states at high excitation energies in 15N

Abstract Selective population of states in 15N has been observed up to 24.5 MeV in excitation in 15N with the 12C(7Li, α) reaction at E( 7 Li ) = 48 MeV . Strong states are observed at 17.95, 18.70, 19.68, 20.93 and 24.75 MeV. The width of the 18.70 MeV state is 100 keV or less even though it is 4 MeV above the 12C + t breakup threshold. Comparison between the 12C(7Li, α) and previously reported 12C(α, p) data shows only the 17.95 MeV state to be strongly excited in (α, p).