Point anode proportional counters for the detection of soft X rays in microanalysis

An end-window (point anode) counter is described which has been designed for detecting soft X rays in an electron-probe microanalyser. An experimental evaluation of the counter showed that it had good resolution for the X-ray wavelength range encompassing CK, OK and AlK, provided that the needle-to-window distance and the X-ray aperture size were both suitably selected. The deterioration in counter performance when the optimum geometrical settings were not used was found to be due to a spread in mean pulse energies, which could reach 40%, arising from ionizations across the counter axis. The best resolution figures achieved during practical operation of this end-window counter, and a larger model of similar design, were σ/ P = 20%, 42% and 64% for AlK, OK and CK X rays respectively, as compared with 15%, 35% and 46%, measured on a standard side-window design. These values are far inferior to suggested theoretical resolutions and methods of improving the counters are considered. It is concluded that a good side-window counter, with its superior resolution, is desirable for quantitative microanalysis using the network method, but other cases are discussed where the end-window counter may be preferred on account of its convenient geometry.