Controlling 229Th isomeric state population in a VUV transparent crystal

The radioisotope thorium-229 (Th-229) is renowned for its extraordinarily low-energy, long-lived nuclear first-excited state. This isomeric state can be excited by vacuum ultraviolet (VUV) lasers and Th-229 has been proposed as a reference transition for ultra-precise nuclear clocks. To assess the feasibility and performance of the nuclear clock concept, time-controlled excitation and depopulation of the Th-229 isomer are imperative. Here we report the population of the Th-229 isomeric state through resonant X-ray pumping and detection of the radiative decay in a VUV transparent Th-229-doped CaF2 crystal. The decay half-life is measured to 447(25) s, with a transition wavelength of 148.18(42) nm and a radiative decay fraction consistent with unity. Furthermore, we report a new "X-ray quenching" effect which allows to de-populate the isomer on demand and effectively reduce the half-life. Such controlled quenching can be used to significantly speed up the interrogation cycle in future nuclear clock schemes.

The version of record of this article, first published in Nature Communications, is available online at Publisher’s website: http://dx.doi.org/10.1038/s41467-024-49631-0