Remote Detection of Radioactive Material on the Basis of the Plasma Breakdown Using High-Power Millimeter-Wave Source

Abstract

This thesis demonstrates the possibility of remote detection of radioactive material in long distance using high-power millimeter/THz wave source, gyrotron. It has been encouraged by a limitation of the current technologies which can only detect in short distance, about few meters, from the source. This proposed novel idea is based on ambient free electron density in the vicinity of the radioactive source. There are only a few free electrons in normal atmospheric air condition (~ 10 electrons/cm3). However, the electron density increases exponentially with radioactive material owing to the Compton scattering among the radiation from the source and air molecules. High density of average free electrons indirectly indicates the presence of the field enhancing factor such as radioactivity. The focused high-power millimeter-wave beam near the source induce the plasma avalanche ionization. As the millimeter-wave source, our laboratory-made gyrotron operates at 95 GHz and tens of kilowatts with 20 μs of pulse duration. In the experiments, we concentrate on the plasma avalanche delay time, from an appearance of a seed electron up to the emission of the light induced by the breakdown discharge. Beginning with the argon (Ar) discharge phenomena, plasma image, velocity, and spectroscopy were observed without external source. Based on the Paschen curve, which indicates the required electric field for the breakdown, the reduced threshold electric fields were experienced with a radioactive source. Besides, measurements of the eliminated statistical delay time lead to the existence of the radioactive material. Interestingly, even though the induced electromagnetic (EM) field was much lower than the threshold field, the attenuated radio frequency (RF) signal was measured in atmospheric air condition. These new experimental results were sufficient evidence to indicate the possibility of the remote detection of radioactive material. By measuring the breakdown discharge induced by the high-power pulsed millimeter-wave radiation, the detection range of our method can be extended to the kilometer range involving with the proper size of an antenna and simply weak turbulence. Furthermore, the detection sensitivity of the method is 4000 times higher than that suggested by theoretical calculations; this is achieved by measuring the plasma breakdown delay time with plasma on/off phenomenon. Our results provide a technical breakthrough in the remote sensing of radioactive material, which should not only be useful in the development of high-power EM wave sources but also directly affect the security aspects of modern life.