Formation of the Long-LivedBi210Isomer

The thermal activation cross section of ${\mathrm{Bi}}^{209}$ is shown to be definitely less than the absorption cross section, 19±2 mb as compared with 33±2 mb. In order to verify the high absorption value, previously obtained by pile reactivity methods, measurements were made with the long wavelength neutrons available with the Brookhaven slow chopper. The absorption cross section is obtained from the chopper measurements by subtraction of thermal inelastic scattering. The latter scattering is determined from the variation of measured scattering with sample temperature. The difference between the absorption and scattering, or 14±3 mb, is ascribed to the formation of the long-lived $\alpha$-emitting isomeric state of ${\mathrm{Bi}}^{210}$, leading to a half-life of (2.6±0.8)×${10}^6$ years. This long life is in agreement with the expected properties, relative to $\alpha$ and $\beta$ decay, of a high-spin state of ${\mathrm{Bi}}^{210}$.