Detection of Special Nuclear Materials Using Monoenergetic Gamma-ray Radiography

The detection of special nuclear materials (SNM) or an assembled nuclear device smuggled in commercial cargo represents a significant challenge in nuclear security, due to both the difficulty of detecting such materials and the large volume of cargo traffic. Additionally, a practical cargo inspection system must limit the radiation dose to which cargo, port workers, and stowaways are exposed. The strong variation in the photon interaction cross section as a function of the atomic number Z of a material may be leveraged to extract both material density and average elemental composition from radiographic measurements at different beam energies. This allows SNM to be identified by the high Z value of such materials. In particular, radiography with monoenergetic photons maximizes this leverage, allowing significant reduction in the radiation dose applied to the cargo. This paper describes the development of a nuclear reaction-based monoenergetic gamma ray radiography system. The system utilizes 3 MeV deuterons incident on a boron target to produce monoenergetic 4.4 and 15.1 MeV photons via the 11B(d,n?) 12C reaction. The transmission spectra of these photons through various material samples were acquired using an array of NaI detectors. Results from data taken at the MIT-Bates Research and Engineering Center indicated that this system is capable of producing basic 2D images of the density and effective Z of scanned materials. Work is now ongoing to expand the capabilities of this system and explore possible improvements. The development and validation of a detailed simulation of this system will be described, along with results from a new analysis of the existing data which utilizes the simulation model to increase the accuracy of materials identification. Additionally, plans for future experiments using different nuclear reactions and compact accelerator systems will be discussed.