Controlling ²²⁹Th isomeric state population in a VUV transparent crystal

The radioisotope thorium-229 (²²⁹Th) 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 ²²⁹Th 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 ²²⁹Th isomer are imperative. Here we report the population of the ²²⁹Th isomeric state through resonant X-ray pumping and detection of the radiative decay in a VUV transparent ²²⁹Th-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.