An Anticoincidence-Shielded Ge(Li) Gamma-Ray Spectrometer for High Sensitivity Counting

An anticoincidence-shielded Ge(Li) gamma-ray spectrometer for high sensitivity counting is described. The system consists of a 20 cc coaxial diode centered inside a 26" diameter by 24" thick plastic phosphor. The source is placed inside the shield and both the coincidence and anticoincidence spectra are recorded simultaneously. The anticoincidence shield reduces the 137Cs Compton edge by a factor of 7.5 to provide a peak to Compton edge ratio of 150:1 without reducing the photopeak efficiency by more than 3%. This system also provides a two to fourfold lower natural radionuclide background than similar sized diodes.