Controlling ^229th Isomeric State Population in a VUV Transparent Crystal

The radioisotope Th-229 is renowned for its extraordinarily low-energy,long-lived nuclear first-excited state. This isomeric state can be excited byVUV lasers and the transition from the ground state has been proposed as areference transition for ultra-precise nuclear clocks. Such nuclear clocks willfind multiple applications, ranging from fundamental physics studies topractical implementations. Recent investigations extracted valuable constraintson the nuclear transition energy and lifetime, populating the isomer instochastic nuclear decay of U-233 or Ac-229. However, to assess the feasibility and performance of the (solid-state)nuclear clock concept, time-controlled excitation and depopulation of the^229Th isomer together with time-resolved monitoring of the radiative decayare imperative. Here we report the population of the ^229Th isomeric state throughresonant X-ray pumping and detection of the radiative decay in a VUVtransparent ^229Th-doped CaF_2 crystal. The decay half-life is measuredto 447± 25 s, with a transition wavelength of 148.18 ± 0.42 nm and aradiative decay fraction consistent with unity. Furthermore, we report a new“X-ray quenching” effect which allows to de-populate the isomer on demand andeffectively reduce the half-life by at least a factor 50. Such controlledquenching can be used to significantly speed up the interrogation cycle infuture nuclear clock schemes. Our results show that full control over the ^229Th nuclear isomerpopulation can be achieved in a crystal environment. In particular,non-radiative decay processes that might lead to a broadening of the isomertransition linewidth are negligible, paving the way for the development of acompact and robust solid-state nuclear clock. Further studies are needed toreveal the underlying physical mechanism of the X-ray quenching effect.