A coincidence system for radionuclide standardization using surface barrier detectors

A system for the standardization of alpha-gamma or electron-X radionuclide emitters is described. The system consists of one or two surface barrier detectors for alpha or electron detection which are coupled to thin-window NaI(Tl) crystals suitable for low-energy X- or gamma-ray detection. The performance of the system has been verified by the standardization of ²⁴¹Am, ¹³⁷Cs and ¹⁰⁹Cd solutions. The activity has been obtained using the extrapolation method applied to the 4π -γ and 2π e-X coincidence techniques. The surface barrier detection efficiency was varied by placing absorbers over the radioactive source or by changing the source-to-detector distance. The results were compared to those obtained using conventional absolute systems based on gas-flow and pressurized 4π proportional counters, or using radioactive solutions standardized in international comparisons sponsored by the Bureau International des Poids et Mesures, France. The expected and measured activities agreed within the experimental uncertainties, which were: 0.2% for ²⁴¹Am, 0.7% for ¹³⁷Cs and 0.6% for ¹⁰⁹Cd.