The natural radioactivity of rhenium (rhenium-187) was investigated by the method of internal gas-tube low-level Geiger counting. A half-life of (1.2±0.4)×${10}^{11}$ yr was found for the direct beta decay of this isotope. Gas-tube proportional measurements suggest a value of about 3 kev for the maximum beta energy. This value of the endpoint energy, the lowest known, suggests that it may be a rhenium atom and not the nucleus alone which is decaying.

• Received 29 June 1961

DOI:https://doi.org/10.1103/PhysRev.125.307