The Long-Range Alpha Detector LRAD

A variation of the gas-flow counter has been developed for the detection of alpha contamination. Common alpha particle detectors are limited by the short range of alphas in air. For example, the range of a 6-MeV alpha in air at normal temperature and pressure is about 46 mm. To circumvent this limitation, the LRAD does not measure the alphas directly. Instead, as shown schematically in Fig. 5.27, the ions created by the alphas in air are transported, with the help of airflow, and directed into an ion chamber. There, the current created by the ions is measured by an electrometer. Since the number of ions produced is proportional to the strength of the alpha source, the signal of the electrometer is also proportional to the alpha source strength. 

In principle, a similar detector could be developed for any particle that produces ions. However, particles like electrons, gammas, and neutrons generate a much smaller number of ions than alpha particles do, traveling over the same distance. For this reason, an LRAD-type detector would have a smaller sensitivity for these other particles than for alphas. Of course, an LRAD-type detector would operate satisfactorily for the detection of protons, deuterons, and other heavy ions. 

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