On an attempt to optically excite the nuclear isomer in Th-229

TL;DR

This study attempts to optically excite the Th-229 nuclear isomer using specially developed crystals and tunable VUV radiation, aiming to measure its energy and lifetime, but faces challenges due to crystal photoluminescence.

Contribution

Development of Th-doped CaF2 crystals and experimental setup for direct optical spectroscopy of Th-229 isomer.

Findings

Excluded isomer energies between 7.5 and 10 eV at 95% confidence.

Sensitive to radiative isomer lifetimes between 0.2 and 1.1 seconds.

Observed strong photoluminescence hindered detection of the isomer.

Abstract

We aim to perform direct optical spectroscopy of the Th-229 nuclear isomer to measure its energy and lifetime, and to demonstrate optical coupling to the nucleus. To this end, we develop Th-doped CaF2 crystals, which are transparent at the anticipated isomer wavelength. Such crystals are illuminated by tunable VUV undulator radiation for direct excitation of the isomer. We scan a 5 sigma region around the assumed isomer energy of 7.8(5) eV and vary the excitation time in sequential scans between 30 and 600 seconds. Suffering from an unforeseen strong photoluminescence of the crystal, the experiment is sensitive only to radiative isomer lifetimes between 0.2 and 1.1 seconds. For this parameter range, and assuming radiative decay as the dominant de-excitation channel, we can exclude an isomer with energy between 7.5 and 10 eV at the 95% confidence level.