Standardization of U(X1 + X2): The 234Th + 234mPa + 230Th mixture

The not very well known intensities of the diverse beta-decay branches of both 234Th and 234mPa make the calibration of the genetic mixture 234Th + 234mPa difficult. We apply a new standardization method based on Cherenkov counting, which permits a verification of the primary and secondary intensities for 234mPa. We obtain 80% intensity for the primary branch, which contradicts the very weak gamma transition intensities observed for 234mPa. This disagreement between experimental and theoretical Cherenkov counting efficiencies can only be avoided by considering the 234mPa primary beta transition to be first forbidden non-unique. We also study the low-activity mixture 234Th + 234mPa + 230Th, which has practical interest in sediment studies. We separate this mixture by two different liquid scintillation methods, by alpha/beta discrimination and by pulse-height spectrum unfolding, which we compare experimentally.