Uncertainties in TDCR measurement revisited: Contribution of optical effects

• Published in: Applied radiation and isotopes 2023-11, Vol.201, p.110992-110992, Article 110992
• Main Authors: Cassette, Philippe, Todorov, Vladislav, Sabot, Benoit, Georgiev, Strahil, Mitev, Krasimir
• Format: Article
• Language: English
• Subjects: Instrumentation and Detectors; LSC; LSC vials; Physics; PMT non-uniformity; TDCR
• ISSN: 0969-8043; 1872-9800
• DOI: 10.1016/j.apradiso.2023.110992

In the Triple to Double Coincidence Ratio method in Liquid Scintillation Counting, the detection efficiency is calculated from the value of a free parameter describing the intrinsic light yield of the counting system. This model is generally based on a Poisson distribution of the number of photoelectrons detected and the detection efficiency is obtained from the complement of the non-detection efficiency. In the classical free parameter model, the mean of the Poisson distribution, m, is a constant but some variability of this mean could be expected from optical effects due to internal reflections inside the LS source or from non-homogeneity of the detection efficiency of the photomultiplier tubes. Then, m becomes a random variable and the distribution of the photoelectrons becomes a compound Poisson distribution, with a random variable as mean value. This paper explores the implications of the variance of m, which were, to our knowledge, never considered previously in the uncertainty budget of TDCR measurements. •Internal light reflection and photomultiplier tubes inhomogeneity introduces variability of the free parameter model in LSC.•This variability causes additional variance on the counting rates.•The importance of this effect is studied for TDCR counting of 3H and 55Fe.•The effect mostly concerns triple coincidences and low efficiency detection.•Reporting experimental standard deviation of repeated measurement instead of standard deviation of the mean is recommended.