Radioactive Decay ofPm148andPm148m

The decay of the two ${\mathrm{Pm}}^{148}$ activities produced by neutron irradiation of fission-product ${\mathrm{Pm}}^{147}$ has been studied. The half-lives of these two activities have been measured to be 5.39±0.06 days and 40.6±0.4 days. The energies and relative intensities of the gamma radiation emitted in the decay of the 5.4-day activity were found to be 0.550 MeV (1.00), 0.910 MeV (0.55), and 1.46 MeV (0.77). Beta-ray absorption measurements and beta-ray scintillation spectra indicated the existence of a beta branch of ∼2.6 MeV from the 5.4-day state in ${\mathrm{Pm}}^{148}$ to the ${\mathrm{Sm}}^{148}$ ground state. The decay of the 41-day activity gives rise to gamma rays in ${\mathrm{Sm}}^{148}$ whose energies and relative intensities are as follows: 0.098 MeV (0.032), 0.188 MeV (∼0.01), 0.286 MeV (0.11), 0.309 MeV (0.05), 0.412 MeV (0.19), 0.431 MeV (0.08), 0.498 MeV (0.08), 0.548 MeV (0.99), 0.596 MeV (-), 0.627 MeV (1.00), 0.723 MeV (0.33), 0.913 MeV (0.20), and 1.011 MeV (0.21). The results of gamma-gamma and beta-gamma coincidence experiments, together with the gamma-ray energies and relative intensities, indicate the existence in ${\mathrm{Sm}}^{148}$ of levels at 0.55, 1.18, 1.46, 1.59, 1.90, 2.02, 2.09, and 2.19 MeV. Gamma-gamma directional correlation measurements were made on all the prominent gamma-ray cascades and provided spin assignments for many of the observed states. The conversion-electron spectrum of 41-day ${\mathrm{Pm}}^{148}$ gives evidence for the existence of transitions of 60 and 75 keV in Pm and thus establishes that the 41-day activity is an isomeric state. A (6.5±1.7)% isomeric-transition branch was observed for this state. Properties of the low-lying states of ${\mathrm{Pm}}^{148}$ are discussed. Decay schemes for both ${\mathrm{Pm}}^{148}$ activities are proposed.