Long-Lived Isomer of RaE (Bi210)

The mass assignment of a long-lived bismuth alpha activity previously reported as being an isomer of ${\mathrm{Bi}}^{210}$ has been confirmed by means of electromagnetic mass separation. The half-life of the ${\mathrm{Bi}}^{210}$ (long) is now reported to be 2.6×${10}^6$ years. The alpha-decay energy of ${\mathrm{Bi}}^{210}$ (long) has been redetermined and found to be 5.031±0.020 Mev. Comparison with the alpha-decay energy of RaE places the ${\mathrm{Bi}}^{210}$ (long) 25 kev below RaE with, however, an uncertainty of the same order of magnitude. ${\mathrm{Bi}}^{210}$ (long) has measurable ${\beta }^{-}$ branching (1 part in 270) to give ${\mathrm{Po}}^{210}$, and this gives a partial half-life of 7.0×${10}^8$ years for this mode of decay. An unsuccessful search was made for ${\mathrm{Bi}}^{210}$ (long) as a decay product of RaD (${\mathrm{Pb}}^{210}$), resulting in a lower limit of 2×${10}^7$ years for the decay of RaD to this isomer. Also, an upper limit of 1.4×${10}^4$ years was set on the half-life for the transition from RaE to ${\mathrm{Bi}}^{210}$ (long). On the basis of decay data now available, the most probable designation for the ${\mathrm{Bi}}^{210}$ (long) state is deduced to be 4- although a somewhat higher spin number could be possible.