SPECTROSCOPIC STUDIES AND CRYSTAL-FIELD ANALYSES OF AM3+ AND EU3+ IN THE CUBIC-SYMMETRY SITE OF THO2

Fluorescence and excitation spectra of Am-3+ diluted in ThO2 are reported at room and liquid-helium temperatures along with fluorescence data for Eu3+ diluted in ThO2. The Eu3+ data Can be assigned primarily to magnetic-dipole transitions, but the Am-3+ data appear to be primarily phonon-assisted electric-dipole transitions. Earlier electron-paramagnetic-resonance (EPR) data on Pu-3+ diluted in ThO2 have set limits on the possible ratios of the crystal-field parameters B-0(4)/B-0(6) for this system. Assuming the same ratio should hold for Am-3+-ThO2, the observed transitions were assigned and the values for the crystal-field parameters B-0(4) = -6731 cm-1, B-0(6) = 714 cm-1 were obtained. The B-0(4) value is the same order of magnitude as found earlier in inelastic-neutron-scattering experiments for UO2 and NpO2, but B-0(6) for Am-3+-ThO2 is much smaller than determined in the neutron experiments.