Small CdTe Gamma Backscatter Assay Meter

The prototype of a portable pocket-sized coal dust incombustibles meter has been developed for the U. S. Bureau of Mines safety program. Using 60-keV Am-241 gamma rays backscattered from the dust under assay, the device measures the count rate in a CdTe detector for 25 seconds and computes the fraction of incombustible material in the dust. The battery-powered meter is intended to occupy only a 330 cm3 volume, and have a battery life of 1,000 measurements. Stability and miniaturization were achieved by the use of a small CdTe solid-state gamma detector specially designed to operate on 9V, and by the incorporation of a single-chip CMOS microprocessor. The source-detector geometry has been optimized so that the meter reading is relatively insensitive to the assay material density. The technique can be used for real-time assay of many binary mixtures, and requires roughly 100 mCi of Am-241 to achieve a measurement accuracy of a few percent.