D J G Love

A shape change along the yrast line in 132Ce

A rotational band has been found in 132Ce which has a high moment of inertia. This band decays by a series of discrete transitions in the spin range (20-40) h(cross) and is more strongly populated above 26 h(cross) than the yrast band known previously. These data are discussed in terms of the predicted shape change expected along the yrast line of 132Ce to a more deformed shape with epsilon 2 approximately 0.35.

Rotational band structures and lifetime measurements in 130Ce

The deformed nucleus 130Ce has been studied using the techniques of in-beam gamma -ray spectroscopy. Beams of 16O and 18O were used on targets of 116,117,118Sn to obtain excitation energies, mean lifetimes, spins, parities, mixing ratios and branching ratios. The gamma - gamma coincidence data necessary to determine the decay scheme were obtained using an array of five escape-suppressed spectrometers. The yrast band has been extended to Jpi =24+, or possibly Jpi =26+, with mean lifetimes measured for states up to Jpi =22+. Five new bands have been established and a 150 ns isomer has been identified at an excitation energy of 2454 keV. Backbending is observed due to the alignment of two h11/2 protons at the same rotational frequency in both the yrast band and the bands on the isomer. The gamma band based on the 834 keV (Jpi =2+) state can be identified with a triaxial shape corresponding to gamma approximately=-25 degrees . This band backbends at spin 10+ due to the alignment of two h11/2 protons showing softness of the nucleus and the coexistence of different shapes. The data may be interpreted in terms of the cranking model by assuming a prolate deformation with epsilon 2 approximately=0.25. The measured transition strengths in the yrast band generally show a decrease in epsilon 2 or a change in triaxiality from gamma approximately=0 degrees to gamma approximately=30 degrees . The transition rates measured for the 12+ to 10+ and 14+ to 12+ transitions are bigger than expected and are consistent with an increase in deformation to epsilon 2=0.30-0.35.

Band crossing and triaxiality in 129Ce

Gamma-ray decays from states in 129Ce have been studied using 16O-induced reactions on 116Sn and 117Sn targets. Previously known positive- and negative-parity bands have been extended to spins of 31/2 and 39/2, respectively. Band crossings are seen for the first time in 129Ce and occur near the same rotational frequency as the h112/ proton band crossing in 130Ce. The signature splittings in the bands are discussed in terms of the cranked shell model with a prolate deformation epsilon 2=0.25. The data show that the one-quasiparticle bands based on the h11/2 (523) 7/2 Nilsson orbit have gamma approximately -15 degrees (signature alpha =+1/2) and gamma approximately -30 degrees ( alpha =-1/2). After the band crossing, the three-quasiparticle bands (including two h11/2 quasiprotons) have gamma approximately 0 degrees for both signatures. The positive-parity states corresponds to gamma approximately 0 degrees for both the one-quasiparticle and three-quasiparticle bands. The branching ratios have been used to deduce the ratio (gK-gR)/Q0 as a function of rotational frequency. This ratio shows an increase when the backbending takes place.

Experimental investigation of the structure of 124Ce

Gamma-ray transitions have been observed for the first time in 124Ce using the Daresbury Recoil Separator. The excitation energy of the first excited state, 142 keV, implies a deformation epsilon 2 approximately=0.31. This value confirms the trend to higher deformation for the more neutron-deficient cerium isotopes, but it is larger than that predicted by recent calculations.

Unfolding the response function of high-quality germanium detectors

Abstract The form and unfolding of the response function of the germanium detectors in the γ-ray spectrometer TESSA2 or similar devices is discussed, both for singles-type spectra and γ - γ coincidence matrices. The reliable unfolding which is possible allows quantitative studies of the γ-ray continuum from (HI, x n) reactions to be made with high resolution germanium detectors.

High-spin structures in 127La and 129La

The odd-proton nuclei 127La and 129La have been investigated to high spin using gamma -ray spectroscopy. Seven new states have been found in 127La, while in 129La six new states were seen. In both nuclei, the yrast band has been extended to Jpi =47-/2 allowing a band crossing to be found at a rotational frequency near 0.5 MeV/h(cross). Angular correlation and linear polarisation measurements have shown that the positive-parity side-band has a spin sequence 21/2, 25/2, 29/2, etc., i.e. positive signature. In 127La a band crossing is also present in the side-band near h(cross) omega =0.5 MeV. The results can be explained in terms of the cranked shell model by assuming an axially symmetric prolate shape with epsilon 2 approximately 0.25. No evidence is found for triaxial shapes. Mean lifetimes were measured for the yrast states in 127La using both the recoil-distance method and the Doppler-shift attenuation method. The results show a marked drop in collectivity which is not understood.

Quadrupole moment of the superdeformed band in 131Ce

A mean lifetime measurement has been carried out on the states in the superdeformed band found in 131Ce using the Doppler shift attenuation method (DSAM). The measured intrinsic nuclear quadrupole moment is Q0 approximately=6 eb, assuming constant deformation, which corresponds to a quadrupole deformation beta 2 approximately=0.35. This is considerably smaller than the value deduced for 132Ce.

Lifetime measurements in the yrast band of 132Ce

Mean lifetimes have been measured for states up to Ipi =18+ in the yrast band of 132Ce using the recoil distance method. In the vicinity of the first backbend, caused by the alignment of a pair of h11/2 protons, a pronounced drop in the collectivity is observed. This contrasts with the anomalously high B(E2) values measured in 130Ce. The results are compared with the theoretical predictions of the MONSTER code.

Low-lying states in 138, 140Gd and 139Eu

Low-lying states have been investigated in the neutron-deficient nuclei 138, 140Gd and 139Eu. The states were populated using the 92Mo+50Cr and 92Mo+54Fe reactions. New energy levels are reported in each nucleus and mean lifetimes have been measured in 138Gd and 139Eu. The deformations deduced from the experiments confirm the predicted trend to higher deformation for the more neutron-deficient nuclei. The N=76 isotope (139Eu and 140Gd) show evidence for band crossing involving h11/2 neutrons and protons.

Energy levels in 131Pr

Excited states in the odd-proton nucleus 131Pr have been investigated to high spin using gamma -ray spectroscopy, finding 17 new energy levels. A negative-parity yrast band, based on the Ipi =(11-/2) state, is seen up to spin Ipi =(47-/2), revealing a band crossing at h(cross) omega approximately=0.4 MeV. A positive-parity sideband, with no backbend, is also found. The results are discussed in terms of the cranked shell model, assuming an axially symmetric prolate shape with epsilon 2=0.25, and compared with other N=72 isotones.