The results of a microwave investigation of gaseous plasmas produced by the radiation field within the core of a water-cooled nuclear reactor are reported. It is shown that the plasmas thus formed are applicable in reactor instrumentation and are quite suitable for investigating basic plasma phenomena such as various collision cross sections and electron-loss processes. Radiometric electron-temperature measurements indicate that, at adequately high gas pressures (≥50 Torr) in the light noble gases used, the electron gas temperature is at or near the host gas temperature and is independent of the reactor power. The dependence of the electron temperature on the gas pressure has been utilized to determine the electron—neutral collision frequency in neon at different electron energies and in helium at room temperature. Preliminary measurements give a value of 2×10−9 cm3·sec−1 for the recombination coefficient in helium at room temperature.