Radioluminescence and photoluminescence of Th:CaF$_2$ crystals

TL;DR

This study explores thorium-doped CaF$_2$ crystals for optical spectroscopy of Th-229's nuclear isomer, analyzing luminescence properties to optimize detection and future optical clock applications.

Contribution

It provides detailed spectral and decay characteristics of Th:CaF$_2$ crystals, aiding in the development of nuclear transition detection methods.

Findings

Rich photoluminescence spectrum above 260 nm

Radioluminescence emission above 220 nm

Luminescence properties suitable for filtering nuclear signals

Abstract

We study thorium-doped CaF$_2$ crystals as a possible platform for optical spectroscopy of the Th-229 nuclear isomer transition. We anticipate two major sources of background signal that might cover the nuclear spectroscopy signal: VUV-photoluminescence, caused by the probe light, and radioluminescence, caused by the radioactive decay of Th-229 and its daughters. We find a rich photoluminescence spectrum at wavelengths above 260 nm, and radioluminescence emission above 220 nm. This is very promising, as fluorescence originating from the isomer transition, predicted at a wavelength shorter than 200 nm, could be filtered spectrally from the crystal luminescence. Furthermore, we investigate the temperature-dependent decay time of the luminescence, as well as thermoluminescence properties. Our findings allow for an immediate optimization of spectroscopy protocols for both the initial search for the nuclear transition using synchrotron radiation, as well as future optical clock operation with narrow-linewidth lasers.