Original NIST author account in Journal of Research of NIST119(2014),371–396; DOI10.6028/jres.119.013. First-page byline verifies Denis E.Bergeron as first author. Section2.1 describes conversion of ionizing radiation to scintillation light and photomultiplier detection; section2.1.1 distinguishes efficiency from count rate. The cited section distinguishes the measured count rate from the activity inferred through a detection-efficiency model.