  A two-phase Cryogenic Avalanche Detector (CRAD) with electroluminescence (EL)
gap, operated in argon doped with a minor (49$\pm$7 ppm) admixture of nitrogen,
has been studied. The EL gap was optically read out using cryogenic PMTs
located on the perimeter of the gap. We present the results of the measurements
of the N$_2$ content, detector sensitivity to X-ray-induced signals, EL gap
yield and electron lifetime in the liquid. The detector sensitivity, at a drift
field in liquid Ar of 0.6 kV/cm, was measured to be 9 and 16 photoelectrons
recorded at the PMTs per keV of deposited energy at 23 and 88 keV respectively.
Such two-phase detectors, with enhanced sensitivity to the S2
(ionization-induced) signal, are relevant in the field of argon detectors for
dark matter search and low energy neutrino detection.
