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Spark detector for alpha particles

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Uploaded on May 22, 2011

This video documents a simple but surprisingly effective detector for alpha radiation. (I'm indebted to Tim Raney of Richmond, Virginia for the hard work of re-discovering this concept from old technical literature, and for preliminary efforts to optimize its practical construction using modern techniques.)

Alpha particles and other ionizing particulate radiation of high linear energy transfer leave a dense wake of ionization following their passage through the air, which can trigger electrical breakdown (i.e. sparking) in the presence of suitably strong electric fields. In this detector, a close relative of the Geiger "point counter," an array of four thin tungsten wires at ground potential passes over an aluminum plate that is biased at ~6-8 kV negative with respect to ground. The wire-to-wire spacing is 0.2 inches, the wire-to-cathode spacing is 0.1 inches, and the tungsten wire is standard, unstraightened 0.003-inch (0.07-mm) diameter.

A current-limited power supply must be used that will not burn out the wires during discharges. I suggest a series resistance of 20 megohms in the cathode lead, made of resistors that are rated to handle the voltage of the supply (e.g. Caddocks). It is also important to limit the cable length attached to the wires (suggested length is one meter or less) so that capacitively-stored energy doesn't produce damaging sparks or damaging HV transients by reflection.

Sources of alpha radiation demonstrated in the clip include a 5-millicurie Po-210 source, a ~60-microcurie Am-241 source, and a 10-microcurie Ra-226 source. Other effective but less-dramatic sources include natural ambient radon (patience is required!) and uranium ore. Pure beta emitters, gamma emitters, and x-rays do not trigger this kind of detector.

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