We present the results of a study of an argon gas proportional scintillation counter instrumented with a CsI-coated microstrip plate placed in the argon envelope. Although the measured light amplification gain and the photoelectron collection efficiency are 70% and 30-40% higher than those obtained in a similar xenon detector, the charge gain achieved in argon for the avalanches produced by the photoelectrons at the microstrip anodes is a factor of ten lower than in xenon. In both cases, these gains are limited by optical positive feedback. The energy resolution achieved for 5.9 keV X-rays was 14.8% compared to 12% in xenon. r

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The gas proportional scintillation counter/microstrip gas chamber (GPSC/MSGC) hybrid detector for X-ray spectrometry is described. The detector uses a CsI-coated microstrip plate placed in direct contact with the gas-filling as the photosensor readout for the GPSC scintillation substituting for the photomultiplier tube (PMT). Usable photosensor maximum gain is limited by optical positive feedback due to the additional scintillation produced in the electron avalanche process at the MSP anodes, in the absence of quenching. A low-photoelectron collection efficiency is achieved in the gas atmosphere, resulting in a scintillation conversion efficiency that is about a factor of 5 lower than that achieved with PMT-based GPSCs. However, energy resolutions of 11% for 5.9 keV X-rays are achieved with this detector. r

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The performance for X-ray spectrometry of neon-xenon gas proportional scintillation counters using a CsI-coated microstrip plate in direct contact with the scintillation gas as a VUV photosensor is investigated for different neon-xenon mixtures. At best operation conditions, the detector gain can reach values about 50% higher than those achieved with pure xenon filling. The highest gains and the best energy resolutions are achieved for xenon contents around 40%. However, the achieved energy resolutions are similar to those achieved with pure xenon. As in pure xenon and argon-xenon mixture gas fillings, the detector performance is limited by optical positive feedback resulting from additional scintillation produced in the electron avalanche processes around the MSP anodes. The best energy resolutions are achieved for positive feedback gains in the range of 1.1 to 1.2. The performance achieved with neon-xenon mixtures is inferior to that achieved with argon-xenon mixtures.Index Terms-CsI-photocathode, gas scintillation counter, microstrip gas chamber (MSGC), X-ray.