Experimental Studies of ZnS Alpha Particle Counters and Methods for Minimizing Detector Background

The zinc sulfide scintillation counter employed at IBM for low-level alpha flux monitoring of device and packaging materials is described. This counter provides almost 100% counting efficiency for alpha particles of 1.5MeV or greater energy emitted from the surface of planar samples up to 130 cm2 in area, and exhibits a sea level background count rate on the order of 0.005 counts/cm2-hr. The contributions of electrical noise, cosmic radiation, and radioactive contamination to this background count rate have been studied experimentally. For near sea level operation, only the cosmic ray and radioactivity contributions are found to be significant. At 50°N geomagnetic latitude, the sea level contribution from cosmic ray interactions appears to be about 0.003 counts/cm2-hr. This component is strongly dependent upon altitude, increasing to about 0.1 counts/cm2-hr. at an altitude of 4.36 km. In the presence of very heavy external shielding, total backgrounds on the order of 0.002 counts/cm2-hr. may be realized, and this residual is attributed primarily to naturally-occurring radioactive contaminants which are present in the construction materials of the detector.