High-spin structures in127La and129La

Abstract

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.