A H Nelson

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.

Near yrast discrete line gamma-ray spectroscopy in 156Dy up to spin 40kh

Abstract Rotational band structures in 156Dy have been populated via the reactions 148Nd(12C, 4n) and 124Sn(36S, 4n) at beam energies of 65 MeV and 155 MeV, respectively. High-spin states (I ≳ 40kh) have been observed and a detailed level scheme is presented. Two positive-parity bands have been observed to spins I = 42 and 36. Four negative-parity bands have been identified up to or above spin I = 36. New levels are also observed in octupole and gamma vibrational bands. The band structures are understood in terms of quasiparticle configurations using the cranked-shell model. The second and third i 13 2 neutron alignments, the systematics of the first h 11 2 proton alignments in even-even N = 90 isotones and the observation of a third discontinuity in the lowest energy positive- and negative-parity bands near spin 40 in 156Dy are discussed.

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.

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.

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.

Signature splitting in 128La and 130La

Band structures have been studied in the odd-odd nuclei 128,130La using gamma-ray spectroscopy techniques and heavy-ion-induced reactions. The states in 130La are reported for the first time. Signature splitting is seen in 128La both in transition energies and in B(M1)/B(E2) ratios, but not in 130La. The states are suggested to arise from a pi h11/2(X) upsilon h11/2 configuration, the signature splitting in 128La being evidence for a triaxial shape.

First experiments with TESSA at daresbury

SummaryThe properties of the Total Energy suppression Shield Array (TESSA) recently installed at Daresbury are briefly outlined. Preliminary results obtained for130Ce and158Er are presented, showing the power of that multidetector γ-ray array in the investigation of high-spin states in nuclei.RiassuntoSi descrivono brevemente le proprietà del TESSA (Total Energy Suppression Shield Array) installato recentemente a Daresbury. Si presentano risultati preliminari per130Ce e158Er, che mostrano la potenza di quella struttura multipla di rivelatori per raggi gamma nello studio degli stati a spin elevato nei nuclei.