Lifetime Measurement of the 229Th Nuclear Isomer

The first excited isomeric state of ²²⁹Th 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, γ decay, and bound internal conversion), whose strengths depend on the charge state of ^Thm₂₂₉ . We report on the measurement of the internal-conversion decay half-life of neutral ^Thm₂₂₉ . A half-life of 7 ±1 μ s has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of ≈1 0⁴ s of the photonic decay channel, gives further support for an internal conversion coefficient of ≈1 0⁹, thus constraining the strength of a radiative branch in the presence of internal conversion.