An anticoincidence-shielded Ge(Li) gamma-ray spectrometer with high sensitivity for measurement of environmental radionuclides

Abstract

This paper describes a Ge(Li) detector system with anticoincidence shield designed for gamma-ray spectrometry of environmental samples. The system embodies three main differences from some previous designs. First, to minimise cost, a Ge(Li) detector made of standard materials was used. Second, the sample chamber is located in the anticoincidence shield so as to afford, depending upon source geometry, a greater efficiency for lower energy Compton scattered photons. Third, and more importantly for good sensitivity, a large sample may be accommodated in a re-entrant chamber. The system is based on a closed-end coaxial Ge(Li) detector of 75 cm3 volume with 2.0 keV fwhm resolution at 1332 keV. The anticoincidence shield consists of two cylinders of NE110 plastic scintillator each of 30 cm radius and 30 cm long. Calculations were carried out to determine for a small source the best position of the sample chamber to achieve, with acceptable sacrifice, improved Compton suppression at lower energies (100–200 keV) where backgrounds usually pile up but in which region there may be peaks of interest. Good agreement was obtained between expected and achieved suppression at different energies using a small 137Cs source. Background spectra under different shielding conditions are discussed, and the system's usefulness is illustrated using the spectrum produced by nuclear weapon test fallout on an air filter. Examples are given of calibration of the system using sources prepared from reference solutions. As an illustration of the sensitivity of the system, minimum detectable activities for two types of sample are calculated. Experience in operation verifies these results. In a bulk sample, an activity concentration of 12 fCi/g of 137Cs is measurable with an accuracy of ±10% (one standard deviation) in 1000 min. This performance is at least comparable with other more elaborate systems.