Measuring the Th-229 nuclear isomer transition with U-233 doped crystals

TL;DR

This paper proposes a novel method to measure the energy of the Th-229 nuclear isomer using U-229 doped crystals, potentially improving detection sensitivity over existing techniques.

Contribution

It introduces a new approach employing U-229 doped crystals to detect Th-229 isomer gamma emission, enhancing signal strength and measurement feasibility.

Findings

Expected signal at least 100 times larger than current methods

Detection possible even with low radiative decay probability

Background dominated by Cherenkov radiation from beta decays

Abstract

We propose a simple approach to measure the energy of the few-eV isomeric state in Th-229. To this end, U-229 nuclei are doped into VUV-transparent crystals, where they undergo alpha decay into Th-229, and, with a probability of 2%, populate the isomeric state. These Th-229m nuclei may decay into the nuclear ground state under emission of the sought-after VUV gamma, whose wavelength can be determined with a spectrometer. Based on measurements of the optical transmission of U:CaF2 crystals in the VUV range, we expect a signal at least 2 orders of magnitude larger compared to current schemes using surface-implantation of recoil nuclei. The signal background is dominated by Cherenkov radiation induced by beta decays of the thorium decay chain. We estimate that, even if the isomer undergoes radiative de-excitation with a probability of only 0.1%, the VUV gamma can be detected within a reasonable measurement time.