  Proportional electroluminescence (EL) in gaseous Ar has for the first time
been systematically studied in the two-phase mode, at 87 K and 1.00 atm. Liquid
argon had a minor (56 ppm) admixture of nitrogen, which allowed to understand,
inter alia, the effect of N2 doping on the EL mechanism in rare-event
experiments using two-phase Ar detectors. The measurements were performed in a
two-phase Cryogenic Avalanche Detector (CRAD) with EL gap located directly
above the liquid-gas interface. The EL gap was optically read out in the Vacuum
Ultraviolet (VUV), near 128 nm (Ar excimer emission), and in the near
Ultraviolet (UV), at 300-450 nm (N2 Second Positive System emission), via
cryogenic PMTs and a Geiger-mode APD (GAPD). Proportional electroluminescence
was measured to have an amplification parameter of 109+-10 photons per drifting
electron per kV overall in the VUV and UV, of which 51+-6% were emitted in the
UV. The measured EL threshold, at an electric field of 3.7+-0.2 kV/cm, was in
accordance with that predicted by the theory. The latter result is particularly
relevant to DarkSide and SCENE dark matter search-related experiments, where
the operation electric field was thereby on the verge of appearance of the S2
(ionization-induced) signal. The results obtained pave the way to the
development of N2-doped two-phase Ar detectors with enhanced sensitivity to the
S2 signal.
