A. Kirwan

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.

Electromagnetic transition probabilities in 130Ce

Abstract Lifetimes of excited nuclear states have been measured in 130 Ce using the coincidence-plunger technique. The employed nuclear reaction was 98 Mo( 36 S, 4n) 130 Ce at a beam energy of 150 MeV. The measurement has been performed at the NSF Daresbury using the ESSA 30 set-up. It was possible to determine lifetimes for the 4 + to 16 + states in 130 Ce. The experimental B (E2) values could be well reproduced by calculations using the triaxial rotor model with β = 0.26 and γ = 21° as well as the rotation vibration model ( β = 0.25).

Spectroscopy of 123La and configuration dependent proton pairing

Abstract Excited states in 123La have been established for the first time using γ-ray spectroscopy, thus extending the systematics of lanthanum isotopes further out from the line of β-stability. Bands built on the Nilsson orbitals [404] 9 2 , [550] 1 2 , and most likely also [422] 3 2 are observed. In the last two cases only the favoured signature is seen, while in the first case both signatures are observed in a strongly coupled band. States in 123La are observed up to a maximum spin of tentatively 59 2 . The configuration dependence of the first band crossing frequency indicates the presence of higher-order terms in the proton pairing. According to calculations, deformation effects are not sufficient to reproduce the observed shift in crossing frequencies.

Alignment processes in 119Cs, 121Cs and 123Cs

Rotational band structures have been observed in the odd-A Cs isotopes 119Cs, 121Cs and 123Cs. The previously known bands have been extended to higher spin values and several new bands have been es ...

The competition between collective and single particle behaviour in the odd-odd isotopes 128,130La

Excited states in the odd-odd nuclei 128,130La have been investigated to high spin using gamma -ray spectroscopy. Two distinct bands are seen, one based on a pi h11/2(X) nu h11/2 structure and the other on a pi h11/2(X) nu g7/2 configuration. Backbends are observed in both configurations for both nuclei and the nature of the particles responsible can be conclusively proved. Experimental variation in the energy-level signature splitting and B(M1)/B(E2) ratios are discussed in terms of the polarising effects of the odd proton and odd neutron on the soft collective core. The results are discussed in terms of the cranked shell model, odd-odd particle-rotor calculations and total Routhian surface calculations. They are also compared to neighbouring odd and odd-odd systems.

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.

Lifetimes of low-lying states in 132Nd and 134Nd

Abstract Lifetimes of low-lying states have been measured in 132 Nd and 134 Nd using the coincidenceplunger technique. The reaction 32 S+ 105 Pd was used at a bombarding energy of 152 MeV. The measurement has been performed at the NSF Daresbury using the ESSA 30 array. The differential decay-curve method (DDCM) was used to analyze the recoil-distance Doppler-shift (RDDS) data. The experimental B (E2) values in 132 Nd are well described by the predictions of the rotational model and the IBM in the O(6) limit.

Rotational band structures in 129La

High spin states in the odd proton nucleus 129La have been investigated using the techniques of in-beam gamma -ray spectroscopy. Nine rotational bands have been observed in 129La, seven for the first time. Various band crossings were observed at rotational frequencies in the range h(cross) omega =0.28-0.57 MeV. The data are mainly discussed within the framework of the cranked shell model. The rotational bands are assigned quasiparticle configurations originating from pi h11/2, pi g7/2, pi (h11/2)2(X) pi g7/2 and pi 11/2 (X) nu (h11/2g7/2) states, respectively. Four of the band crossings are thought to be associated with the rotational alignments of the first pair of h11/2 neutrons or protons.

Rotational bands in the doubly odd N = 73 nucleus 134Pm

Abstract The band structure of the doubly odd nucleus 134Pm has been investigated via the 92Mo(46Ti, 3pn)134Pm and 64Zn(74Se, 3pn)134Pm reactions at beam energies of 210 and 290 MeV, respectively. Two rotational structures showing both signature components have been established. One of these bands has minimal signature splitting and is thought to be built on the π[413] 5 2 + ⊗ v h 11 2 configuration. The second band has a small signature splitting and is consistent with a π h 11 2 ⊗ v h 11 2 configuration. A third doubly decoupled band has also been observed. Quadrupole moment measurements for this band show that it has a deformation of β2 ≈ 0.29. This band is interpreted as being based on a π h 11 2 ⊗ v h 9 2 configuration. The results are compared with the data from the neighbouring odd-odd N= 73 isotones 130La and 132Pr and with cranked shell model and total routhian surface calculations.