engleski

The mean energy required to form an ion pair and the Fano factor in gas mixtures with an emphasis on argon-isobutane mixture

This work presents the results of both the theoretical and the experimental investigation of interaction of low-energy electrons and photons with gas mixtures. Calculation of the W value (the mean energy required to form an ion pair) and the Fano factor in a regular mixture of Ar and H2 by the Spencer-Fano method shows that both the W value and the Fano factor are smooth functions of the mixture composition, and that the contribution of the high-energy electrons to both the ionization yield and the variance yield is higher in H2 than in argon. Comparison of the experimental W values in regular mixtures with the W values calculated from the W values for pure gas components shows that different total ionization cross sections of the gas components determine the energy partition in gas mixtures. A similar formula for calculating the Fano factor in a regular mixture from the Fano factor for the pure components is proposed and tested on several gas mixtures for which the experimental Fano factors are available. The measured W value and the Fano factor in the isobutane-based tissue-equivalent gas mixture agree well with the calculated values. The W value and the Fano factor for the isobutane-based TE gas are determined here for the first time. A linear relation between the Fano factor and the ratio W/I (I is the first ionization potential) for high-energy electrons is obtained. The relation is tested on several examples. In the experimental part of the work various aspects of interaction of 5.89 keV photons (55Fe source) with the argon-isobutane mixture are studied. The pulse height distribution of the single electrons (SE) and that of 55Fe photons have been measured by using the specially designed proportional counter. The relative variance f of the SE spectra is calculated by fitting the spectra to the Polya distribution. The W value is calculated from the mean values of the SE and Fe spectra. The counter resolution at 5.89 keV has been measured, and the Fano factor calculated after substracting the instrumental resolution. It is found that the W value, the Fano factor, and the counter resolution, as well as the relative variance f of the SE spectra, depend on both the concentration fraction of isobutane in the mixture and on the total pressure of the mixture, i.e., they depend on the partial pressure of the admixture. When the partial pressure of isobutane is higher than 60 mbar, the mixture behaves as the regular one. When the partial pressure of isobutane is between 20 and 60 mbar, the W value and the Fano factor decrease, and the relative variane f increases. The expected improvment of the energy resolution in irregular mixture due to lower values of W and F has not been observed because of the faster increase of the f value.

Mean energy required to form an ion pair; Fano factor; gas mixture; Argon-isobutane; SE spectra

Rad izrađen na Institutu Ruđer Bošković, te djelomično na Physikalisch-Technische Bundesanstalt (PTB) u Braunschweigu, Njemačka, te u Argonne National Laboratory (ANL), Argonne, Ill, USA.