Background reduction of a HPGe-BGO anti-Compton system and its application to soil contamination monitoring

Accurate gamma spectrometry is widely performed by HPGe detectors. The limit of detection of gamma spectrometry system is determined by two main factors: the detector efficiency and its background. To obtain higher efficiencies, larger detectors have to be used, but their price become forbidding, thus it is common to try to reduce their background. For static laboratory systems optimal shielding can be used, but massive shielding is not practical for mobile systems intended to be operated outdoors, as for ground contamination survey. The main contribution to the background in an HPGe gamma spectrometer (except the background radiation) is due to the Compton scattering. When photons are Compton scattered in the detector, only a part of their energy is transmitted to the detector, and the pulse will not contribute to the main photo-peak, but will appear at a lower energy, as part of the background. A lower energy gamma peak will be superposed on this background, and its limit of detection will be higher. A practical way to solve the problem is to position an additional detector around the HPGe and operate it in anti-coincidence with the HPGe detector. Coincident pulses from both detectors, mostly from Compton-scattering, will not be counted, and the background will be reduced. This setup is called anti-Compton, and the characteristic of an anti-Compton system based on a HPGe detector, surrounded by a BGO scintillator, is presented in this work. Its application to ground monitoring is also presented