A Low Background Alpha Scintillation Counter

An alpha scintillation counter has been developed at this laboratory that exhibits unusually good characteristics for routine counting of low activity samples. The detector is a phoswich consisting of a .0008-inch thick CsI crystal mounted on .020-inch thick Pilot B discriminator. Optical coupling to the RCA 7746 photomultiplier is through a Lucite light pipe. The sensitive area of the detector is 2 inches in diameter, permitting use of a large sample pan. Counting efficiency, relative to that of a 2 ¿ windowless proportional counter, is about 86 per cent for a thin plutonium-239 source, and about 65 per cent for an infinitely thick alpha source (uranium-235 dispersed in lucite). Typical background for an uncontaminated detector ranges from 1/4 to 1/3 count per hour. The detector face can be decontaminated using tissue and. ethyl alcohol with no apparent deleterious effects. The electronic circuitry analyzes each pulse to determine rise time, peak current and integrated current. Since the ratio of peak to integrated current is a characteristic of the scintillator, and is quite different for pulses from the two materials utilized, it is possible to identify pulses occurring only in the CsI, where all of the alpha particles are stopped. The rise time circuit verifies absence of a simultaneous Pilot B pulse and aids in rejection of certain types of noise.