  The radioisotope Th-229 is renowned for its extraordinarily low-energy,
long-lived nuclear first-excited state. This isomeric state can be excited by
VUV lasers and the transition from the ground state has been proposed as a
reference transition for ultra-precise nuclear clocks. Such nuclear clocks will
find multiple applications, ranging from fundamental physics studies to
practical implementations. Recent investigations extracted valuable constraints
on the nuclear transition energy and lifetime, populating the isomer in
stochastic 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
$^{229}$Th isomer together with time-resolved monitoring of the radiative decay
are imperative.
  Here we report the population of the $^{229}$Th isomeric state through
resonant X-ray pumping and detection of the radiative decay in a VUV
transparent $^{229}$Th-doped CaF$_2$ crystal. The decay half-life is measured
to $447\pm 25$ s, with a transition wavelength of $148.18 \pm 0.42$ 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 by at least a factor 50. Such controlled
quenching can be used to significantly speed up the interrogation cycle in
future nuclear clock schemes.
  Our results show that full control over the $^{229}$Th nuclear isomer
population can be achieved in a crystal environment. In particular,
non-radiative decay processes that might lead to a broadening of the isomer
transition linewidth are negligible, paving the way for the development of a
compact and robust solid-state nuclear clock. Further studies are needed to
reveal the underlying physical mechanism of the X-ray quenching effect.
