Remote detection of radioactive materials using a near-terahertz gyrotron

Summary form only given. We are studying new concepts for remote detection of concealed radioactive materials. The research is a balanced effort consisting of theory, simulations and experiments. Radioactive materials emit gamma rays which ionize the surrounding air. High energy free electrons produced by the gamma rays rapidly participate in subsequent ionization events producing a cascade of secondary electrons with temperatures down to thermal level (less than 1 eV). Such thermal electrons attach to oxygen molecules in ~10 nanoseconds forming negative O2- ions. In the vicinity of radioactive material, the densities of negative ions can be greatly elevated compared to the background (ambient) levels. For example, one gram of cobalt-60 can increase the density of negative ions at a distance of 4 meters by several thousand times. The extra electrons on the negative oxygen ions will be detached by collisions after a lag time (~1 microsecond) . When a free electrons is released into a focused high-power near-THz wave beam an avalanche breakdown of the air can be initiated resulting in an exponential rise in electron density and spark formation. The lag time delay for spark formation is a function of negative ion density and therefore provides a means for detecting the presence of concealed radioactive material. This scheme can be realized with a standoff range up to 100 m using a near-terahertz source with peak power of 200 kW and pulse duration on the order of 10 microseconds. We are developing a 670 GHz gyrotron oscillator to study the air breakdown phenomenon in the presence of negative ions and aerosols. Initial operation of a 670 GHz gyrotron with a 28 T pulsed solenoid produced a power at about the 80 kW level in a 7 microsecond pulse. These experiments are in progress.