Fano factor and the mean energy per ion pair in counting gases, at low x-ray energies

The mean energy per ion-pair (Wi) and the Fano factor (F) are provided with high accuracy (2% and 3%-4%, respectively), in C2H6, C3H8, i-C4H10, CH4, DME, Ar/C2H6(20:80), Ar/i-C4H10(20:80) Ar/DME (20:80) and Ar/Xe/i-C4H10(66.6/16.7/16.7), in the x-ray energy range of 0.11–1.5 keV. These parameters were extracted from precise measurements of the number and temporal distribution of x-ray induced electrons, accompanied by extended simulations of the detection process. A decrease in these parameters with increasing x-ray energy was observed, accompanied by sharp increases at x-ray energies just above some atomic shells. The effect is discussed in relation to Auger electron emission. A Penning process in Ar/C2H6(20:80) and Ar/i-C4H10 (20:80) is observed on the basis of comparative measurements of Wi and F in these mixtures and in the pure hydrocarbons. Ways are proposed for further improving the accuracy provided by the electron counting technique to better than 1%.