Beta decays of 128 Ce and 128 La

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 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.