Mechanisms behind shortened 229mTh isomer lifetime in ThF4 thin films

Determine the physical mechanisms responsible for the shortened radiative lifetime (approximately 150 seconds) of the 229mTh nuclear isomer observed in thorium tetrafluoride (ThF4) thin films relative to the longer lifetimes in 229-doped LiSrAlF6 and CaF2 crystals by quantitatively assessing contributions from refractive-index-induced magnetic-dipole (M1) Purcell enhancement and host-induced quenching channels, including hyperfine-mediated electric-dipole (E1) decay via crystal-field-induced electronic hybridization, mechanical strain, interface-induced band bending and dipole layers, amorphous structure and impurities, and potential substrate effects.

Background

The authors measured isomer lifetimes of 150(15) s (Al substrate) and 153(9) s (MgF2 substrate) in ThF4 thin films, which are substantially shorter than those observed in 229:CaF2 and 229:LiSrAlF6 crystals (~600 s). They evaluated Purcell and superradiance effects and concluded substrate-induced Purcell modification is not the leading-order cause, suggesting other mechanisms must be involved.

They propose possible explanations including a high refractive index for ThF4 and host-induced quenching mechanisms mediated by electronic state hybridization, strain-related fields, and interface effects such as band bending and dipole layer formation, as well as amorphous structure and impurities. The precise combination and relative contributions remain unresolved.