GAS AMPLIFICATION AND IONIZATION COEFFICIENTS IN ISOBUTANE AND ARGON-ISOBUTANE MIXTURES AT LOW GAS-PRESSURES

Measurements of the mean gas amplification factor in isobutane and in argon-isobutane mixtures are performed at total gas pressures of 10, 2 0, 30, 60 and 90 kPa over a range of reduced electric field strength 5 x 10(4) < S-a < 1.4 x 10(6) V m(-1) kPa(-1). At low partial pressure of isobutane deviations from an exponential growth of gas gain with ap plied high voltage are observed. The gas gain at which this over-expon ential effect occurs, increases as the isobutane partial pressure incr eases. The over-exponential growth of gas gain is explained by the con tribution of secondary avalanches started by photons due to non-effici ent quenching. The number of additional electrons produced per electro n in an avalanche depends on the isobutane partial pressure. The depen dence of the reduced gas gain on the reduced electric held strength in isobutane indicates that electrons are not in equilibrium with the el ectric held for S-a values above about 3 x 10(5) V m(-1) kPa(-1). In m ixtures, the S-a value at which the non-equilibrium effect is observed decreases with decreasing percentage of isobutane in the mixture for the same total pressure. The gas gain data were fitted by an analytica l model that takes into account non-equilibrium effects. A reduced fir st Townsend ionization coefficient of the form alpha/P = A exp(-B*/S- a) was derived. The fitting parameters A and B* increase when: (i) th e pure isobutane pressure decreases, (ii) the isobutane concentration in the mixtures increases at constant total pressure, and when (iii) t he total pressure decreases at constant isobutane concentration. The a lpha/P for isobutane is compared with other experimental and calculate d values which are all obtained for equilibrium conditions. An interes ting behavior of the ionization coefficient curves for argon-isobutane mixtures is found: at low S-a the alpha/P increases when the isobutan e concentration decreases, while at higher S-a the opposite behavior i s observed. Both the reduced electric field strength and the ionizatio n coefficient where the multiplication starts (M = 2) increase with th e decrease of the total pressure, as well as with the increase of isob utane concentration in the mixture at constant total pressure. (C) 199 8 Elsevier Science B.V. All rights reserved.