A new primary standardization of 229Th

The National Institute of Standards and Technology (NIST) has certified a high-purity 229Th Standard Reference Material as SRM 4328C, based on live-timed 4παβ-γ anticoincidence counting (LTAC) of the equilibrium solution. The LTAC system was optimized to minimize the uncertainty in the result due to the two short-lived ground-states present in the decay chain. Confirmatory measurements were carried out by four other methods. Furthermore, the present absolute activity and measured γ-ray emission rates were combined to obtain γ-ray emission probabilities.