Low lying collective states in [sup 132]Ba

Abstract

The decay properties of low lying collective states in 132Ba were studied by means of γ-spectroscopy following the β-decay of the 2−-ground state (T1/2=4.8 h) and a 6−-isomer (T1/2=24.3 min) of 132La. The lanthanum nuclei were produced at the Cologne FN TANDEM accelerator using the reaction 122Sn(14N,4n)132La. The γγ coincidences and singles spectra were measured with the OSIRIS-cube spectrometer. Beside ground and quasi-gamma band many other low lying states were observed. The γγ angular correlations were analyzed to assign spins to the excited states, and to determine the multipolarities of the depopulating the γ-transitions. We also confirmed the expected dominant E2 character of transitions in the quasi-gamma band and from the quasi-gamma to the ground band but with a certain deviation: The decay 62+→61+ shows an unexpected large M1 fraction. Also the decays of the third, fourth and fifth 2+ states are dominated by M1 radiation. A level at 1660 keV could be identified as the 03+ state in 132Ba. The experimental data are compared with calculations using the Proton-Neutron Interacting Boson Model (IBM-2). Good agreement is reached in the vicinity of the O(6) limit. The data for the ground band and the gamma band agree rather well with large scale Monte Carlo shell model calculations.