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, γ 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 ± 1 µs has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of ≈ 10 4 s of the photonic decay channel, gives further support for an internal conversion coefficient of ≈ 10 9 , thus constraining the strength of a radiative branch in the presence of IC.229 Th is the only known nucleus providing an excited isomeric state of sufficiently low energy to allow for direct nuclear optical laser excitation [1]. The possibility to drive the transition with laser technology has led to the proposal of a multitude of interesting applications. The predicted spectroscopic properties of the 229m Th ground-state transition make it a promising candidate for a nuclear optical clock that may outperform today's existing atomic clock technology [2][3][4]. As other ultra-precise optical clocks, a nuclear clock could be a tool in the search for dark matter [5], gravitational waves [6] as well as for geodesy [7]. Such a nuclear clock promises ultra-high sensitivity for potential time variations of fundamental constants [8]. There exist also proposals towards a nuclear γ-ray laser [9] based on the 229m Th ground-state transition and a nuclear qubit for quantum computing [10]. However, to enable laser excitation, precise knowledge on the spectroscopic properties of the transition, such as the lifetime and the excitation energy, is required. Since the first proposal of the existence of a low-energy isomeric state of 229 Th in 1976 [11] several indirect energy measurements [12][13][14] have been performed. With steadily improved detector energy resolution, these measurements pinned down the energy to 7.8 ± 0.5 eV [15] (λ ≈ 159 ± 10 nm). A direct half-life measurement of a photonic decay channel [16] has been controversially discussed [17].Several different experimental approaches have been pursued, aiming for a measurement of the isomer's properties or for an optical laser excitation of the nucleus [18][19][20][21][22][23][24][25][26][27]. However, despite significant experimental effort, no conclusive measurement of the isomeric half-life has been reported so far.As a result of the low excitation energy, the isomer can decay via three decay channels to its ground state, whose occurrence depends on the electronic surrounding of the nucleus [28][29][30][31]: when the binding energy of an electron E B in the surrounding of the nucleus is lower than the excitation energy of the isomer E I , the isomer decays preferably via internal conversion (IC) by emit...