First Observation Of Collective Rotational Bands In Neutron-Rich La-142 And The Study Of Octupole/Triaxial Deformations In (142)'La-143

Collective rotational bands were observed for the first time in neutron-rich odd-odd La-142 by means of gamma-gamma-gamma and -gamma-gamma-gamma-gamma coincidence measurements of prompt fission gamma rays from Cf-252 using multi-detector array Gammasphere. Similarity between the yrast band of La-142 and those of neighboring La-144, Ce-144 was found and interpreted as quasiparticle excitations. PES calculations suggested triaxial deformation for the ground state of La-142. Considerable triaxial and near zero octupole deformations were deduced for the yrast band of La-142 by the TRS model calculations. The band-crossing observed in the yrast band of La-142 was interpreted to be caused by alignment of the (h(11/2))(2) proton pair, and the crossing frequency was best reproduced by the TRS calculations taking into account triaxial deformations. In contrast to the interpretations for the band crossing of La-142, alignments of the (i(13/2))(2) neutron pair were found to be responsible for the band-crossing of the yrast band in the neighboring even-N La-143, and triaxial degree, of freedom was found to play a more significant role than octupole deformations in the nucleus, although its beta(3) values deduced are large.