Electron emission properties of two-phase argon and argon-nitrogen avalanche detectors

Electron emission properties of two-phase Ar avalanche detectors are studied. The detectors investigated comprised a liquid Ar or Ar+N-2 layer followed by a multi-GEM multiplier operated in the saturated vapour at 84 K. Two components of the electron emission through the liquid-gas interface were observed: fast and slow. In Ar, the slow emission component dominated even at higher fields, reaching 2 kV/cm. In Ar+N-2 on the contrary, the fast emission component dominated at higher fields, the slow component being disappeared. This is explained by the electron backscattering effect in the gas phase. The slow component decay time constant was inversely proportional to the electric field, which is compatible with thermionic emission model. The electron emission efficiencies in two-phase Ar and Ar+N-2 were estimated to be close to each other.