Remote sensing of radiation

The distance at which gamma-ray or neutron radiation from moderately intense radiation sources can be detected in air by conventional detectors is limited to less than about 100 m. We have investigated the possibility of increasing this range by measuring the effect of the radiation on the air near the source. The radiation ionizes the surrounding air causing chemical reactions to occur which form new molecular species that might be sensed with remote sensing optical techniques. We have calculated the molar fraction as a function of time of the species formed in air by irradiation with a 113 Ci source of 60Co. After 104 s, we find the dominant ones are O3, 5×10−7; N2O, 4×10−7; HNO3, 3 × 10−7; HO2NO2, 2 ×10−7; and NO2, 5 × 10−8. All of these species have complex absorption curves in the infrared (IR) range 2–12 μm. In addition, O3 has a broad absorption band in the ultraviolet (UV) range at 257 nm. The IR absorption spectrum is particularly interesting because the low attenuation by air of the interrogating IR radiation might allow detection of the generated chemical species in air near a radiation source at large distances using differential absorption lidar (DIAL). The higher attenuation over the UV range limits its useful range to perhaps 100 m. We describe our setups to measure the molar fraction by measuring the optical absorption with IR and UV spectrometers. We present responses or limits that are below the calculated values for the dominant species and suggest some reasons. We conclude that for moderately intense sources of gamma rays and neutrons, currently available optical techniques, while sensitive, are not sensitive enough for this application.