  The performances of THGEM multipliers in two-phase Xe avalanche mode are
presented for the first time. Additional results on THGEM operation in gaseous
Xe at cryogenic temperatures are provided. Stable operation of a double-THGEM
multiplier was demonstrated in two-phase Xe with gains reaching 600. These are
compared to existing data, summarized here for two-phase Ar, Kr and Xe
avalanche detectors incorporating GEM and THGEM multipliers. The optical
readout of THGEMs with Geiger-mode Avalanche Photodiodes (G-APDs) has been
investigated in gaseous Xe at cryogenic temperature; avalanche scintillations
were recorded in the Near Infrared (NIR) at wavelengths of up to 950 nm. At
avalanche charge gain of 350, the double-THGEM/G-APD multiplier yielded 0.07
photoelectrons per initial ionization electron, corresponding to an avalanche
scintillation yield of 0.7 NIR photons per avalanche electron over 4pi. The
results are compared with those of two-phase Ar avalanche detectors. The
advantages, limitations and possible applications are discussed.
