M J Godfrey

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.

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.

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.

Spectroscopy of the light rare-Earth nuclei 133Pm and 133Sm: Configuration-dependent pairing and evidence for strong residual proton-neutron interactions in neighbouring odd-N and odd-Z nuclei

Abstract The low-lying, rotational band structures of the highly neutron-deficient light rare-earth nuclei, 133Sm and 133Pm have been investigated using the 40Ca + 96Ru reaction at a beam energy of approximately 180 MeV. Five new bands have been observed in both nuclei. 133Sm possesss two strongly coupled bands which are thought to be built upon the v h 11 2 [523] 7 2 − and v d 5 2 [402] 5 2 + Nilsson configurations. In addition to the previously reported v i 13 2 intruder band, there is also evidence for a second decoupled, negative-parity structure in 133Sm based on the v h 9 2 [541] 1 2 − orbital. In 133Pm evidence is presented for both signature partners of the π h 11 2 band. The expected h 11 2 neutron alignment in these bands has not been observed. It is suggested that this provides evidence for a strong pn interaction. A strongly coupled band built on the π g 7 2 [413] 5 2 + configuration has also been established. Both signature partners of this band show clear evidence for the alignment of a pair of h 11 2 protons followed by a pair of h 11 2 neutrons. A strongly coupled three quasi-particle configuration has also been identified which feeds into the unfavoured signature of the π h 11 2 band. The results are discussed in terms of cranked shell model, total routhian surface calculations and the systematics of the A ∼ 135 region.

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.

The proton structure of the superdeformed bands in the N=73 isotones 130La, 131Ce and 133Nd

A superdeformed band has been identified in the odd-odd nucleus 130La. The nine transitions seen show a behaviour consistent with the deformation epsilon -0.4 corresponding to a prolate rotor with a 3:2 axis ratio. The behaviour of the moments of inertia in the superdeformed bands in the N=73 isotones 130La, 131Ce and 133Nd is discussed in terms of a few particles in high-N shells.

High spin spectroscopy of124Ba andh11/2 neutron alignment

The level scheme of the yrast band of124Ba has been extended up to spin 32ħ. Transitions in the two negative parity side bands are observed up to spin 27ħ and tentatively 20ħ. The second backbend observed in the S-band and the backbends in the side bands are explained as due to neutrons. The structure of the sidebands is discussed and compared with deformation self-consistent calculations, Total Routhian Surfaces (TRS).