Abstract:
Among all known isotopes, Thorium-229 has the lowest nuclear
excited state, only 8.4 eV above the ground state. This so-called
“isomer” is accessible to VUV laser excitation and a plethora of
applications at the interface of atomic and nuclear physics have
been proposed, including a nuclear clock, a gamma ray laser and a
sensitive detector for variations of fundamental constants. After
decades of attempts to determine the exact isomer energy and other
nuclear properties, we report on two experiments which resonantly
excite the isomer with a laser and spectroscopically resolve the
nuclear quadrupole splitting in a single crystal environment. This
allows us to determine the sensitivity of the nuclear clock
transition to variations of the fine structure constant, which
exceeds all schemes involving valence electron transitions by 3
orders of magnitude.