Beta Decays of 128ce and 128La

Low-lying states in 57 128 La and 56 128 Ba have been investigated by γ-ray spectroscopy after the β decays of 58 128 Ce and 57 128 La. For the EC+β + decay of 128 Ce, a new halflife has been determined to be 4.1 ± 0.3 min. In 128 La 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 128 La at low excitation energies has been constructed from the γ−γ coincidence measurement after the β decay of 128 Ce. The γ−γ angular correlation for γ rays from 128 Ba after the β decay of 128 La 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 128 La 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.