Lifetime measurement of the $^{229}$Th nuclear isomer

The first excited isomeric state of $^{229}$Th possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today's laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground-state (internal conversion, $\gamma$ decay and bound internal conversion), whose strengths depend on the charge state of $^{229m}$Th. We report on the measurement of the internal-conversion decay half-life of neutral $^{229m}$Th. A half-life of 7 $\pm$ 1 $\mu$s has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of $\approx10^4$ s of the photonic decay channel, gives further support for an internal conversion coefficient of $\approx10^9$, thus constraining the strength of a radiative branch in the presence of IC.