Abstract: A cylindrical gas -flow proportional counter with a side window is described in this paper. Which can be used to measure soft X rays in the energy range of 183 ~ 933eV. Its counter gases are P -10 or helium isobutane gases. The pressure is 0.11MPa. The counter has a inner diameter of 25mm and window diameter of 0.3mm made of 80-90μg ·cm² formvar.
The specialities of this countes are as following: （1） Its window is thin, and has a high transmission for soft X rays. （2）It works in gas-flow form, and has a long lifetime. （3）It has a high energy resolution. （4） Its counting efficency is high up to 1 x 10¹⁴s^-1. （5） It can work in a wide energy range of 0.183-10keV. （6 ）Its window material, thickness and diameter can be easily replaced.