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A triple gas electron multiplier (GEM) detector has been built and characterized in a collaboration between ENEA, INFN and CEA to develop a soft X-ray imaging diagnostic for magnetic fusion plasmas. It has an active area of 5×5cm2, 128pixels and electronics in counting mode. Since burning plasma experiments will have a very large background of radiation, this prototype has been tested with contemporary X-ray, neutron and gamma irradiation, to study the detection efficiencies, and the discrimination capabilities. The detector has been preliminarily characterized under DD neutron irradiation (2.45MeV) up to 2.2×106 n/s on the detector active area, showing a detection efficiency of about 10−4, while the detection efficiency of X-rays is more than three orders of magnitude higher. The detector has been also tested under DT neutron flux (14MeV) up to 2.8×108 n/s on the whole detector, with a detection efficiency of about 10−5. The calibration of the γ-rays detection has been done by means of a source of 60Co (gamma rays of energy 1.17MeV and 1.33MeV) and the detection efficiency was found of the order of 10−4. Thanks to the adjustable gain of the detector and the discrimination threshold of the electronics, it is possible to minimize the sensitivity to neutrons and gamma, and discriminate the X-ray signals even with very high radiative background. [ABSTRACT FROM AUTHOR]