A cylindrical cathode-coaxial anode proportional counter, designed for an undergraduate laboratory, is described. A collimated beam of α particles can be directed into the counter at any point along a narrow longitudinal window in the cathode wall and used to examine the electric field surrounding the anode structure at any point along its length. The anode wire has an enlarged section at one end and is surrounded at the other by an independently biased coaxial auxiliary electrode. The effectiveness of these two geometrical methods for controlling the production of secondary ionization can be studied, as can the effects of varying gas composition, pressure, and operating voltage. A variable output-2000-v power supply, pulse amplifying and counting equipment, a cathode ray oscilloscope, and a small mechanical vacuum pump are also needed for these experiments. Detailed information on construction of the counter and gas handling system is given.

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