Radiation Stability of Counting Gas Mixtures Containing CF4

Several noble gas mixtures containing CF4 were irradiated at high neutron and gamma dose rates to determine their radiation stability for neutron flux monitoring applications where the total radiation dose can be large. Five test detectors (ionization and proportional counters employing either 3He or 235U as the sensitizing material) containing CF4 gas mixtures were irradiated in a neutron flux of ~1011 n·cm-2·s-1 and a gamma field of ~2 × 108 R/h. All detectors exhibited moderate to total degradation of pulse response at fluences from 1015 to 1016 n·cm-2, with more rapid degradation occurring in the fission detectors. The pulse height degradation was probably caused by electron attachment on irradiation products such as C2F6, C3F8, and CF3OF, which were detected at 100-to 6000-ppm levels in subsequent mass spectrometric analyses.