  We study the performance of two-phase avalanche detectors based on Gas
Electron Multipliers (GEMs) and operated in an electron-avalanching mode in Ar
and Xe. Emission, gain, energy resolution and stability characteristics of the
detectors were studied. Rather high gains, reaching 5000, and stable operation
for several hours were observed in the two-phase Ar avalanche detector using a
triple-GEM multiplier. The signals induced by X-rays, beta-particles and
gamma-rays were successfully recorded. Preliminary results were obtained in the
two-phase Xe avalanche detector: the maximum gain of the triple-GEM in
two-phase Xe and Xe+2%CH4 was about 200. The results obtained are relevant in
the field of two-phase detectors for dark matter searches, coherent neutrino
scattering, PET and digital radiography.
