Compton suppression systems for environmental measurements

Anti-coincident gating of an MCA using an NaI(Tl) guard detector around a primary HPGe detector effectively reduces Compton continuum and virtually eliminates Compton edges. Practical considerations in specifying and using these systems are discussed. The timing electronics incorporate a TAC/SCA instead of a fast coincidence unit because of the simplified setup and the ability to directly view the timing spectrum. Timing requirements for Compton suppression utilizing HPGe detectors are described. Advantages of using n-type HPGe detectors, previously described in the literature, were confirmed by experiment. The peak-to-Compton (p/C) for 137Cs in n-type detectors is twice th p/C for p-type detectors. Availability of large volume n-type detectors provide better initial p/C ratios and correspondingly larger p/C ratios in the suppressed mode. Use of low background detectors and shielding further improves the obtainable measurement sensitivity. Vertical positioning of the HPGe detector within the annulus also augments the p/C by improving suppression of forward-scattered events. Typical p/C ratios for these systems range from 800:1 to 950:1 corresponding to a factor of 2 to 4 improvement in detection limits