Ionization of counting gases and ionizable gaseous additives in proportional chambers by UV lasers

Abstract The ionization induced in argon-methane proportional counters by lasers emitting at 337 nm and 266 nm has been investigated. The ionization is due to impurities in the gas which can be removed by appropriate filters. Observations are consistent with a two-photon absorption mechanism. Two gaseous additives (trimethylamine and tetramethylphenylenediamine) are studied which are easily ionized at λ = 266 nm by a two-photon process. They can be used with concentrations at the ppm level to simulate minimum ionizing charged particle tracks. The ionization density is proportional to the concentration of the additive trimethylamine and to the square of the energy density of the laser beam.