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Gas proportional scintillation counters are room-temperature, general-purpose X-ray detectors, which are used in many applications due to their good energy resolution, which can be better than standard proportional counters by a factor of /spl sim/2. However, for energies higher than /spl sim/20 keV, the experimentally measured energy resolution is found to deviate from the usual 1//spl radic/E law. Under these circumstances, it is of great interest to understand the mechanisms involved in the detection of higher energy X-rays. Since the photoelectrons will then carry most of the absorbed energy, we study in this work the response of xenon detectors to electrons with energies up to /spl sim/200 keV, using a Monte Carlo simulation technique. Distributions of the number of primary (subionization) electrons produced per parent electrons with energy E/sub e/ are calculated, and results are presented for the Fano factor, the w-value and the intrinsic energy resolution as a function of E/sub e/ in the range 20-200 keV. The influence of an applied reduced electric field on the results is assessed, showing that for 200 keV electrons an increase in the field from 0.1 to 0.8 Td causes an increase as high as 35% in the intrinsic energy resolution.