Evaluation of a Phoswich Detector for the in Situ Analysis of 90Sr

A Phoswich detector system which employs a 3.45 mm thick by 12.5 cm diameter CaF2(Eu) crystal coupled to a 6.4 cm thick NaI(Tl) crystal was evaluated for the detection of 90Sr(90γ) in the presence of 106Ru (106Rh) and 137Cs and for its ability to measure 238pu, 239pu, 241Am and 244Cm. The radioactive species were studied, both as surface contamination and contamination distributed in sediments, as a function of overburden thickness for different materials. The CaF2(Eu) crystal detects beta or low energy photon emissions while the NaI(Tl) crystal only detects higher energy photons from radionuclide decay or bremsstrahlung radiation. As little as 9 d/m/cm2 of 90Sr as surface contamination can be measured in a 1000 second counting period in the presence of comparable concentrations of 137Cs and 106Ru. The Phoswich system is specifically designed for the measurement of 90Sr(90γ) and does not detect the transuranic species well. The operating parameters and use of the Phoswich system for identification of specific radionuclides in shallow land waste burial environments are discussed.