The construction and performance of a coincidence/anticoincidence low-level gamma-ray spectrometer is described. The radioactive source (in a sealed, cylindrical, stainless steel container) fits into the well of a large plastic scintillator; a NaI(Tl) crystal (6 in. diameter × 4 in.) rests on top of the source inside the well. Electrical pulses from the photomultiplier of the crystal unit are routed via a coincidence/anticoincidence circuit to a multi-channel analyser. Only those pulses are analysed that are in coincidence/anticoincidence with selected pulses from the plastic scintillator. The flexibility of the system permits optimum discrimination in favour of, or against, particular radionuclides. In measurements of natural radioactivity, potassium and the uranium and thorium series can be separately assayed even at very low levels. Background, calibration and sensitivity data are given, together with the results of measurements of soil samples and cremated human remains.
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