Th Isomer in a Solid-State Host*

The population dynamics of the ²²⁹Th low-lying isomeric state are studied in a solid-state crystal host under laser excitation. It is observed that the isomeric state can be relaxed back to the ground state via illumination by off-resonant vacuum ultraviolet (VUV) radiation through a photoquenching process. This photoquenching process is analyzed with a modified rate equation; with this analysis, the cross-section for this photoquenching process is measured. In addition, a theoretical model is presented, where photoexcitation of electronic states within the material bandgap opens an internal conversion decay channel. This model appears to reproduce the measured cross-section.