Gas amplification and ionization coefficients in isobutane and argon–isobutane mixtures at low gas pressures

Abstract

Measurements of the mean gas amplification factor in isobutane and in argon–isobutane mixtures are performed at total gas pressures of 10, 20, 30, 60 and 90 kPa over a range of reduced electric field strength 5 × 10 4 < S a < 1.4 × 10 6 V m −1 kPa −1. At low partial pressure of isobutane deviations from an exponential growth of gas gain with applied high voltage are observed. The gas gain at which this over-exponential effect occurs, increases as the isobutane partial pressure increases. The over-exponential growth of gas gain is explained by the contribution of secondary avalanches started by photons due to non-efficient quenching. The number of additional electrons produced per electron in an avalanche depends on the isobutane partial pressure. The dependence of the reduced gas gain on the reduced electric field strength in isobutane indicates that electrons are not in equilibrium with the electric field for S a values above about 3 × 10 5 V m −1 kPa −1. In mixtures, 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 analytical model that takes into account non-equilibrium effects. A reduced first Townsend ionization coefficient of the form α/P=A ∗ exp(−B ∗/S a ) was derived. The fitting parameters A * and B * increase when: (i) the pure isobutane pressure decreases, (ii) the isobutane concentration in the mixtures increases at constant total pressure, and when (iii) the total pressure decreases at constant isobutane concentration. The α/ P for isobutane is compared with other experimental and calculated values which are all obtained for equilibrium conditions. An interesting behavior of the ionization coefficient curves for argon–isobutane mixtures is found: at low S a the α/ P increases when the isobutane concentration decreases, while at higher S a the opposite behavior is observed. Both the reduced electric field strength and the ionization coefficient where the multiplication starts ( M=2) increase with the decrease of the total pressure, as well as with the increase of isobutane concentration in the mixture at constant total pressure.