Electron counting using proportional counters.
2015-11-19T09:17:16Z (GMT) by
The X-ray energy resolution achieved by conventional charge signal measurements using a gas proportional counter is determined by statistical fluctuations in the primary number of electrons and in the charge multiplication process. Conventional X-ray energy resolution thus increases as (E)-1/2 where E is the X-ray energy. The proportional counter, therefore, gives a very unsatisfactory X-ray energy resolution for X-ray energies below 1 keV. By eliminating the statistical fluctuations due to the charge multiplication, it is possible to improve the X-ray energy resolution by a factor of 2 over that achieved by conventional charge signal measurements. The implementation of this concept requires an 100% detection efficiency for the electrons present in the primary clusters. The present work is based on electron counting method which uses the charge signals due to single electrons avalanching at the anode wire. The main aim of this work was to determine the maximum possible electron counting efficiency. This required a detailed examination of the parameters relevant to the operation of an electron counting system. An experimental chamber consisting of a uniform field drift tube and a coaxial proportional counter was constructed. Experimental work was carried out to determine electron loss mechanisms such as electron loss by capture, electron loss below the discriminator threshold of the electron counting electronics and electron loss due to the finite resolving time of the electron counting electronics. This involved the measurements of electron mobility and electron lifetime at very low drift fields (Ed/p 0.02 V/cm Torr) for a number of different counter gas mixtures. Single electron response was also examined for these counter gas mixtures at a wide range of charge gains. It was found possible to achieve 89.0% electron counting efficiency at 1.49 keV using A-CH4(50%). The corresponding X-ray energy resolution was found to be 19.5%FWHM, compared to 28.0%FWHM achieved by the conventional charge signal measurements.