Year
2013
Abstract
The objective of this work is to develop a method to detect special nuclear materials at long stand-off distances, defined as hundreds of meters to kilometers, the realization of which requires the development of techniques that do not depend on directly sensing the gamma-ray and neutron radiation emitted by the source. We probed the secondary ionization in the air that surrounds the radioactive materials as an indirect measure of their presence. The fast electrons and ions facilitate the formation of the weak plasma, the density of which increases when radiation-induced tracks are present, compared to their formation in the pristine or polluted air. We established the conditions under which it is feasible to detect SNM at long range, passively using either UV fluorescence or radar sensing to image the ionization in the air about the source, and we conducted experiments to probe the ionization environment. The results show that competing ground-clutter, in the case of radar sensing, and solar-based air-glow, in the case of UV fluorescence, make source sensing infeasible for highly-enriched uranium (HEU); thus, we investigated various probe-and-detect modalities, in which high electric fields are delivered at long range to enhance the ionization signal. The enhancement of the free-electron concentration via microwave pulsing, although possible in principle, is prevented by the difficulty in focusing a microwave antenna in order to realize the high field-densities necessary. One can instead using shorter wavelengths, in the form of fast laser pulses, to induce intensely bright plasma sparks that are detectable at long range and whose presence is sensitive to the degree of ionization in the air