A new approach to quench correction when measuring 241Pu in the presence of Pu-α by supported-disc liquid scintillation spectrometry

It is widely recognized that the accurate measurement of β-emitting 241Pu in the presence of α-emitting Pu nuclides, by supported-disc liquid scintillation spectrometry and pulse shape analysis, necessarily involves a correction for the mis-classification of a small fraction of the α-induced events. What has not been clear is how best to correct for the quenching associated with the (almost) unavoidable variability in source quality of discs prepared by electrodeposition in the course of the routine analysis of environmental samples. The latter results in a reduction in the detection efficiency for 241Pu and an increase in the mis-classification of α events. It also gives rise to a shift of the α and β spectra to lower energies. In this paper, a novel approach is described in which the shift in the α peak, as measured by LS spectrometry, relative to that of an unquenched standard, represents the parameter by which both the α to β cross-talk and the β efficiency are determined using appropriate sets of quenched standards. The method has been tested with replicate samples of intercomparison materials and confirmed by thermal ionization mass spectrometry.