Spectroscopic studies and crystal-field analyses of Am3+ and Eu3+ in the cubic-symmetry site of ThO2.

Abstract

Fluorescence and excitation spectra of ${\mathrm{Am}}^{3+}$ diluted in ${\mathrm{ThO}}_{2}$ are reported at room and liquid-helium temperatures along with fluorescence data for ${\mathrm{Eu}}^{3+}$ diluted in ${\mathrm{ThO}}_{2}$. The ${\mathrm{Eu}}^{3+}$ data can be assigned primarily to magnetic-dipole transitions, but the ${\mathrm{Am}}^{3+}$ data appear to be primarily phonon-assisted electric-dipole transitions. Earlier electron-paramagnetic-resonance (EPR) data on ${\mathrm{Pu}}^{3+}$ diluted in ${\mathrm{ThO}}_{2}$ have set limits on the possible ratios of the crystal-field parameters ${\mathit{B}}_{0}^{4}$/${\mathit{B}}_{0}^{6}$ for this system. Assuming the same ratio should hold for ${\mathrm{Am}}^{3+}$-${\mathrm{ThO}}_{2}$, the observed transitions were assigned and the values for the crystal-field parameters ${\mathit{B}}_{0}^{4}$=-6731 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, ${\mathit{B}}_{0}^{6}$=714 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were obtained. The ${\mathit{B}}_{0}^{4}$ value is the same order of magnitude as found earlier in inelastic-neutron-scattering experiments for ${\mathrm{UO}}_{2}$ and ${\mathrm{NpO}}_{2}$, but ${\mathit{B}}_{0}^{6}$ for ${\mathrm{Am}}^{3+}$-${\mathrm{ThO}}_{2}$ is much smaller than determined in the neutron experiments.