Experimental study on ion mobility in Ar-CH₄ mixtures

In this paper we present the results of the ion mobility measurements made in gaseous mixtures of argon (Ar) and methane (CH₄) for pressures ranging from 5 to 8 Torr and for low reduced electric fields (in the 17 Td to 43 Td range), at room temperature. The time of arrival spectra of the several mixture ratios studied revealed that the relative abundance of the ions formed depend on the mixture ratios. For CH₄ concentrations in the 2.5–10% range three well defined peaks were observed which were attributed to single-carbon ions (CH₅⁺), 2-carbon ions (C₂H₄⁺ and C₂H₅⁺), and 3-carbon ions (C₃H₄⁺, C₃H₅⁺, C₃H₆⁺ and C₃H₇⁺). The time of arrival spectra for CH₄ concentrations of 2.5% (P-2.5), 5% (P-5), 10% (P-10) and the reduced mobilities of the ions obtained from the peaks observed are presented in this paper. The ion mobility study was performed for typical reduced electric fields used in gaseous detectors (E/N > 15 Td)