Beta shape-factor function and activity determination of 241Pu

The Physikalisch-Technische Bundesanstalt (PTB) investigated the low-energy beta emitter 241Pu within the scope of an international key comparison on the activity concentration of the same solution. The activity concentration was measured by means of liquid scintillation counters with two and three photomultiplier tubes (PMT). The counting efficiencies were determined with two established techniques, which are based on a free parameter model. The free parameter is determined via 3H-efficiency tracing in systems with two PMTs, or it is derived from the triple-to-double coincidence ratio (TDCR) in a system with three PMTs. Both methods require an accurate computation of the beta emission spectrum of the first-forbidden (non-unique) transition. In this work, the experimental outcome of a recent measurement from with cryogenic magnetic calorimeters was used to determine a shape-factor function. The computed beta spectrum is in good agreement with the measured data when the shape-factor function C(W)=1-1.9582 W+0.96078 W2 and an end-point energy Eβ,max=21.6 keV are used. The activity concentrations determined with the two methods agree well when using the new shape-factor function, whereas a considerable discrepancy is found when assuming C(W)=1, as for an allowed beta transition. Consequently, the difference between the efficiency tracing method and the TDCR method, as observed by other researchers, could be resolved.