  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.
