Decay properties of low-lying collective states in 132Ba

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 ( T 1/2=4.8 h) and a 6 − isomer ( T 1/2=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 6 + 2→6 + 1 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 0 + 3 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.