Beta decays of ¹²⁸Ce and ¹²⁸La

Abstract

Low-lying states in ¹²⁸₅₇ La and ¹²⁸₅₆ Ba have been investigated by γ-ray spectroscopy after the β decays of ¹²⁸₅₈ Ce and ¹²⁸₅₇ La. For the EC+β⁺ decay of ¹²⁸Ce, a new halflife has been determined to be 4.1 ± 0.3 min. In ¹²⁸La a new β-unstable isomeric state with a halflife shorter than 2 min is suggested in addition to a previously known isomeric state with the halflife of 5.2 min. A tentative level scheme of ¹²⁸La at low excitation energies has been constructed from the γ−γ coincidence measurement after the β decay of ¹²⁸Ce. The γ−γ angular correlation for γ rays from ¹²⁸Ba after the β decay of ¹²⁸La suggests that the spin-parity assignment of I ^π = 4⁻ or 5⁺ is preferable for the 2424.9 keV level which is populated via the β decay of the 5.2-min isomeric state with a log ft value of 5.6. For the 2877.1 keV level observed with log ft = 5.8, a preferable spin-parity assignment is 5⁻ or 6⁺. On the basis of these decay properties and low-lying excited states in neighboring odd mass nuclei, the spins and parities of isomeric states in ¹²⁸La are proposed to be (1⁺ or 2⁺) for the low-spin state, while (5± or 6⁺) for the high-spin state, within the limit of zero order approximation to the proton-neutron couplings.