Quantification of the Alpha Wall Effect in Liquid Scintillation Counting

A long-standing uncertainty in liquid scintillation (LS) counting has been the possible loss of counting efficiency. 

A long-standing uncertainty in liquid scintillation (LS) counting has been the possible loss of counting efficiency due to interaction of particles with the surfaces of the detector volume. In particular for alpha counting, the LS efficiency was usually assumed to be exactly 100 %, because the efficiency loss was difficult to quantify, and presumed to be small. However, a report in 2002 from a well-respected group claimed evidence for an efficiency loss of 0.3 % (with no reported uncertainty), which would be very significant for metrology applications.  Over the past few years, the NIST team has been experimenting in this topic and has developed a technique for measuring this alpha wall effect with significantly smaller uncertainty than any previous estimate. Using ²⁴¹Am LS sources and the anti-coincidence counting system, the NIST team was able to directly measure this efficiency loss. After determining small correction factors based on further experiments and computer simulations, they reported an inefficiency of (6 ± 5)·10^-5. This value is consistent with older reports and upper limits. But unlike the 2002 report in literature, the present work has a meaningful uncertainty statement and indicates a much smaller inefficiency