On the minimum detectable activity of β spectrometers and selective radiometers

A method of β-ray spectrometry is described that consists in measuring the activity of a particular radionuclide in a mixture of a few β-emitting nuclides. The activity of radionuclides simultaneously emitting β and γ rays is determined with a γ spectrometer, and their contribution to the total β spectrum is calculated. Formulas for finding the minimum detectable activity and the error of these measurements are derived. A frequently occurring specific case is considered in which the activity of ⁹⁰Sr radionuclide in equilibrium with ⁹⁰Y is measured against the background of ¹³⁷Cs and ⁴⁰K. The minimum activities that can be measured by the phoswich detector and a detector based on a plastic scintillator are compared. The main sources of errors inherent in this measuring technique are analyzed.