Cesium-137 Determination in Fission Products by Direct Beta-Gamma Coincidence Counting

An instrumental method has been devised and tested for the direct measurement of ¹³⁷Cs in fission product mixtures. The method is based on beta-gamma coincidence counting, and involves beta counting of the mixture in a liquid or plastic scintillator while simultaneously measuring the gamma-ray spectrum with a NaI(Tl) detector-multichannel analyzer arrangement. Cesium-137 is unique among the long-lived fission products in that its decay (actually its ^137mBa daughter, T_1/2 = 2.8 min) does not involve a prompt beta-gamma coincidence. The ¹³⁷Cs is then measured from a noncoincidence gamma spectrum with comparatively little interference from the other fission products. The method is not directly applicable to recently irradiated fuels but has been applied to the quantitative measurement of ¹³⁷Cs in 8-month-, 1.3-year- and 2.0-year-old material with standard deviations of about 3, 0.8, and 0.7%, respectively.