Nuclear Resonance Fluorescence Applied to Materials
Detection and Imaging

William Bertozzi
Massachusetts Institute of Technology
Monday, May 19, 2003

Abstract:
One of the greatest threats to the security of our Nation is the influx without inspection of millions of shipping containers and trucks. They could contain anything from benign commercial materials to contraband of the most dangerous kind. They are an inexpensive and very accurate form of delivery vehicle. The regular flow of these containers is necessary to maintain the efficient process of commerce, a vital component of our economy. The problem is how to inspect these containers quickly and with accuracy to prevent the influx and movement of high explosives, nuclear materials, "dirty bombs" and for that matter, contraband of any importance. We review the physics of nuclear resonance fluorescence to demonstrate the unique and ubiquitous signals it presents us for elemental identification. The photon energies used range from about 2 MeV to 7 MeV and the selection rules for electromagnetic excitation limit the number of signals to a few for each nucleus. Even with heavy nuclei such as lead and uranium there are only a handful of signals and confusion for identification purposes is almost nonexistent. The energies of the photons are in the region of minimal absorption and are most penetrating allowing inspection of densely filled containers throughout their volumes. With collimation, the inspection can produce three dimensional images with reasonable voxel sizes for almost any element, hydrogen and helium being two of the few exceptions. The dominance of the nuclear absorption over the standard electronic processes makes elemental sensitivity available for two dimensional transmission imaging as well. Because of the identification of elements with three dimensional imaging, identification of dangerous materials can be automated. We will discuss physical aspects of the process and its potential for quantitative and unique identification of materials.