THE DECAY OF Bi208

Abstract

It has been observed that Bi208 decays mainly by K- and L-electron capture to the 2.615-Mev first-excited level of Pb208. Decay to known Pb208 levels above the 2.615-Mev level has been shown experimentally not to occur in more than 3% of the Bi208 disintegrations, in agreement with an analysis of the mass differences in this region which indicates that such decay modes are energetically impossible. Positrons are not emitted in more than 5% of Bi208 disintegrations. The value of the ratio of K X-rays to 2.615-Mev γ-rays has been measured to be 0.23 ± 0.01. Assuming that the ground state of Bi208 is 5+ and that the theory of Brysk and Rose is accurate in the region near the K-capture threshold, this ratio measurement leads to a value of 2.807 Mev for the total decay energy of Bi208. This agrees with the value 2.75 ± 0.10 Mev obtained from reaction cycle analysis. New measurements of the Bi208 half-life have revealed an error in the previous measurement, the best value now being 7.5 × 105 years to within a factor of three. Incidental to the main experiment an upper limit of 0.5% has been measured for the positron decay of Bi207.