Secondary effects on electron multiplication in pure isobutane

The presence of secondary processes in electron multiplication under high uniform electric fields at atmospheric pressure in pure isobutane was investigated. The experimental setup consists of a Resistive Plate Chamber-like cell with the anode made of a high resistivity glass (2×1012Ωcm) and a metallic cathode, on which photoelectrons are produced by the incidence of a pulsed laser beam. In particular, the dependence of the first Townsend coefficient (α) on the repetition rate and the intensity of the UV laser pulses was studied. The E/N range considered spanned from ≈145 to ≈200 Td. The α coefficient was determined by measuring both the primary ionization and the avalanche currents with the help of an electrometer, directly connected to the cathode. Of all the investigated secondary effects, only the ohmic drop across the resistive glass has been found to be non-negligible in the present experimental conditions and has been corrected for. The obtained values are compared with Magboltz simulation results and presented in tabular form.