Lifetime Measurement of the 229Th Nuclear Isomer

The first excited isomeric state of Th-229 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 Th-229m. We report on the measurement of the internal-conversion decay half-life of neutral (229)mTh. A half-life of 7 +/- 1 mu s has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of approximate to 10(4) s of the photonic decay channel, gives further support for an internal conversion coefficient of approximate to 10(9), thus constraining the strength of a radiative branch in the presence of internal conversion.