Determination of radioactive bulk and surface concentration by beta-detection: I. Quantitative analysis

The activity concentration of bulk and surface samples contaminated with l-emitting radioisotopes is difficult to measure without the "a priori" knowledge of the nature of the non-gamma emitting components. Beta-emitters cannot be identified from any measured spectral distribution. The counting efficiency of the measuring system changes significantly with 13-energy so it cannot be assumed to have a single value obtained with a standard source with known energy. Application of an energy selective 13-detector i s introduced for determining bulk and surface activity concentration. Samples of"infinitely thick" or"infinitely thin" natu re are to be prepared. The di stfibut ion of ~-ene rgy deposited in the detector is registered as counts in 8 or 16 energy intervals. No infomration is needed on the qualitative composition of the sample. Normalised integral distributions (intensity ratios) are derived from the count rates of the intervals. These distributions are then compared to calibrated intensity ratios obtained with suitable staudard sources. An average (effective) counting efficiency is generated from this comparison by a special algorithm. Activity concentration of an tutknown sample is then obtained using this average efficiency. Calibration and sensitivity data are presented for different types of bulk and surface samples. Duect measurement of the specific activity of bulk materials and of surfaces contaminated with ~3-emitting radionuclides is frequently needed in the practice of operational dosimetry. The main difficulty of this type of absolute activity measurement is that the counting efficiency of nuclear detectors changes significantly with the energy of the 13-radiation to be measured but the nature of the 13-emitting comPonents i.e., the~ energy of the 13-radiation is unknown. A new, empirical procedure is presented here which enables the surface or bulk concentration of any composition of unknown p-emitting radionuclides to be determined with sufficient accuracy. The method may also serve to obtain a first estimate prior to destructive radiochemical analysis of samples in order to determine non-gamma emitting radionuclides, e.g., 89Sr or 9~176