BETA-DECAYS OF CE-128 AND LA-128

Low-lying states in La-128(57) and Ba-128(56) have been investigated b y gamma-ray spectroscopy after the beta decays of Ce-128(58) and La-12 8(57). For the EC+beta(+) decay of Ce-128, a new halflife has been det ermined to be 4.1 +/- 0.3 min. In La-128 a new beta-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 tenta tive level scheme of La-128 at low excitation energies has been constr ucted from the gamma-gamma coincidence measurement after the beta deca y of Ce-128. The gamma-gamma angular correlation for gamma rays from B a-128 after the beta decay of La-128 suggests that the spin-parity ass ignment of I-pi = 4(-) or 5(+) is preferable for the 2424.9 keV level which is populated via the beta decay of the 5.2-min isomeric state wi th a logft value of 5.6. For the 2877.1 keV level observed with logft = 5.8, a preferable spin-parity assignment is 5(-) or 6(+). On the bas is of these decay properties and low-lying excited states in neighbori ng odd mass nuclei, the spins and parities of isomeric states in La-12 8 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 a pproximation to the proton-neutron couplings.