CHARACTERISTICS OF A SINGLE-WIRE PROPORTIONAL COUNTER WITH VERY THIN RECTANGULAR CROSS-SECTION

As a first step to apply a very thin single-wire proportional-counter (SWPC) to focal plane detectors, we have constructed a 3 mm thick detector with an area of 40 x 160 mm2 and investigated the pulse-height distributions from Am-241 alpha-rays and Fe-55 X-rays, using an argon-methane mixture (P-10). We found a double-humped dependence of the pulse-height on the distance of the position of incidence from the anode wire (twin-peaks effect). A simple expression of the electric field strength with the distance was derived using the present data and a formula of conformal mapping, from which the maximum allowable detector height was concluded to be five times the thickness in order to avoid recombination.