“…Under the trigger threshold channel N 0 , the area of the spectra difference is null, and just above the level the difference forms a plateau at a constant level c 1,l-s , c 1,l-m , c 1,m-s The plateau level is extrapolated under the threshold to estimate the number of undetected low energy beta particles (c 0 ): c 0,l-s = N 0 × c 1,l-s . Then from analytical demonstration, the number of undetected non-ionizing particles is given by [12,13] When measuring a radioactive gas decaying by gamma transition (Xe-131m, Xe-133m) or electron capture (Xe-127), the counting efficiency is lower than unity, the efficiency has to be calculated using the decay scheme and the successive atomic desexcitation processes [13]. In these cases, a decaymodel uncertainty term must be also considered depending on the evaluated atomic and nuclear fundamental parameter uncertainties.…”