Uncovering Special Nuclear Materials by Low-energy Nuclear Reaction Imaging

Abstract

Weapons-grade uranium and plutonium could be used as nuclear explosives with extreme destructive potential. The problem of their detection, especially in standard cargo containers during transit, has been described as “searching for a needle in a haystack” because of the inherently low rate of spontaneous emission of characteristic penetrating radiation and the ease of its shielding. Currently, the only practical approach for uncovering well-shielded special nuclear materials is by use of active interrogation using an external radiation source. However, the similarity of these materials to shielding and the required radiation doses that may exceed regulatory limits prevent this method from being widely used in practice. We introduce a low-dose active detection technique, referred to as low-energy nuclear reaction imaging, which exploits the physics of interactions of multi-MeV monoenergetic photons and neutrons to simultaneously measure the material’s areal density and effective atomic number, while confirming the presence of fissionable materials by observing the beta-delayed neutron emission. For the first time, we demonstrate identification and imaging of uranium with this novel technique using a simple yet robust source, setting the stage for its wide adoption in security applications.

Experimental demonstration of multiple monoenergetic gamma radiography for effective atomic number identification in cargo inspection

“…With sufficient position resolution, however, the presence of very high-Z (>80) material can be flagged for this configuration. This suggests that monoenergetic gamma ray radiography may be paired with a secondary technique (such as a system designed to detect induced photofission neutrons 6,22 ) to disambiguate such situations. Additionally, future work will explore the resolving power of radiography using multiple projections for mixed material configurations.…”

“…20,21 A parallel effort by other groups, using Cherenkov detectors, has used the same reaction to pursue a similar goal. 22 The prior work, however, did not achieve a precise determination of the atomic number or areal density of the scanned objects. This work demonstrates the ability to infer the effective atomic number (Z) and areal density (q A ) of a given spatial pixel across a cargo sample, providing essential information for the identification of materials present in the cargo.…”

High-energy γ rays resulting from low-energy nuclear reactions in light nuclei

“…The source is collimated using a combination of lead, HDPE, and two rows of large concrete blocks with a separation of 2.54 cm. These multiple layers of collimation result in a fan beam spanning the height of a cargo container for potential active interrogation experiments [22]. The copious amounts of shielding and collimation provide a relatively low room-return environment for the detectors placed approximately 9 meters from the boron target.…”

Hydrogenous content identification in heterogeneous cargoes via multiple monoenergetic neutron radiography

“…Radiographic techniques, on the other hand, use measurements of transmitted particle beams to infer the areal density and in some cases, the effective atomic number Z eff of the cargo [5,6]. Imaging of the cargo based on Z eff , in addition to its utility in locating actinides, can also be used to differentiate between medium-Z and low-Z cargoes, thus providing additional information to the operators in their effort to detect various contraband.…”