P. J. Bishop

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).

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.

Prolate and oblate rotational bands in 136Sm

Six rotational bands have been established in 136Sm following heavy-ion induced reactions carried out at the Nuclear Structure Facility, Daresbury. The yrast band has been observed up to Ipi =(24+) and undergoes three band-crossings. A band was observed built on the low-lying 2+ gamma-vibrational state, in addition to two negative parity sidebands built on two-quasiproton excitations. A dipole band showing the characteristics of collective oblate rotation was also established. The results are discussed in terms of total Routhian surface and cranked Woods-Saxon model calculations.

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.

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.

A superdeformed band in131Ce

A rotational band of 16γ-rays has been found in131Ce with a high moment of inertia indicating a deformation ε2∼0.38 and extending to spin ∼40ħ. Its intensity is ∼∼5% of the total in131Ce, confirming a difference in the intensity systematics for superdeformed states in Ce nuclei compared with Nd.

Band structures in134Nd

The decay scheme for134Nd has been extended to high spin and a new negative parity band discovered. These results are discussed in terms of particle alignments.