Observation of the Deexcitation of the 229mTh Nuclear Isomer

The Th-229 nucleus possesses the lowest-energy nuclear isomeric state. Two widely accepted indirect measurements of the transition energy place it within reach of existing laser capabilities. Direct searches for the isomer deexcitation have proven elusive despite extensive effort over the past couple of decades. There is now a growing interest in finding this unique transition because of its potential applications in nuclear, atomic, condensed matter, and optical physics, quantum information, metrology, and cosmology, including the development of a new type of clock based on this nuclear transition. In this Letter we report the first direct observation of the deexcitation of the lowest-lying isomeric state in Th-229. By collecting Th-229 recoils following the alpha decay of U-233 into MgF2 plates and measuring the subsequent light emission, we have isolated the isomer deexcitation and measured the transition's half-life to be 6 +/- 1 h. Through comparison measurements with U-235m isomer, we found that the observed Th-229m deexcitation signal originates from photon emission rather than internal conversion electron emission. This discovery lays the groundwork for optical and laser spectroscopy of Th-229m nuclear isomer and the development of a Th-229 nuclear clock.