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

Abstract

The smuggling of special nuclear materials (SNM) through international borders could enable nuclear terrorism and constitutes a significant threat to global security. This paper presents the experimental demonstration of a novel radiographic technique for quantitatively reconstructing the density and type of material present in commercial cargo containers, as a means of detecting such threats. Unlike traditional techniques which use sources of bremsstrahlung photons with a continuous distribution of energies, multiple monoenergetic gamma radiography (MMGR) utilizes monoenergetic photons from nuclear reactions, specifically the 4.4 and 15.1 MeV photons from the 11 B(d,nγ) 12 C reaction. By exploiting the Z-dependence of the photon interaction cross sections at these two specific energies it is possible to simultaneously determine the areal density and the effective atomic number as a function of location for a 2D projection of a scanned object. The additional information gleaned from using and detecting photons of specific energies for radiography substantially increases the resolving power between different materials. This paper presents results from the imaging of mock cargo materials ranging from Z ≈ 5-92, demonstrating accurate reconstruction of the effective atomic number and areal density of the materials over the full range. In particular, the system is capable of distinguishing pure materials with Z 70, such as lead and uranium -a critical requirement of a system designed to detect SNM. This methodology could be used to screen commercial cargoes with high material specificity, to distinguish most benign materials from SNM, such as uranium and plutonium.

High-Contrast Material Identification by Energetic Multiparticle Spectroscopic Transmission Radiography

“…A summary of the Z estimation for each of the R-values for all materials is provided in Table III; across all R-values agreement to the true Z was within 3σ of the expected value. Single-mode gamma-ray radiography can also be performed using the d-BN source [9,10,15], which produces two prominent gamma-ray energies, 4.4 and 15.1 MeV. Interactions at these photon energies are dominated by different mechanisms, which scale differently with atomic number.…”

“…Neutrons are continuously produced until the deuteron energy drops below both of the Coulomb thresholds of target nuclei -1.7 MeV for 11 B. Table I lists the two main neutron-producing reactions from the deuteron-BN radiation source and the resulting gammaray energies [13,14] Recently, a similar source based on deuteron stopping in natural boron was used to perform material identification by dual-energy photon transmission radiography [9,10,15]. In this past work, atomic numbers of various objects and their areal densities were determined by spectroscopic photon radiography using two principal gamma rays produced in the reaction.…”

“…A well-characterized monochromatic-photon source permits the selection of specific energies which provide the most meaningful data for a given application. In dual-energy monoenergetic photon transmission radiography [9,10], photons of two distinct energies are used to exploit the dependence of the mass attenuation coefficient on both the photon energy and the atomic number. It is desirable for the photon energies to be adequately separated to ensure that a different photon interaction mechanism (photoelectric effect, Compton scattering, or pair production) dominates at the two chosen energies, since the cross sections of these mechanisms scale differently with atomic number and thus provide the elemental contrast.…”

Analysis of the Frequency and Detectability of Objects Resembling Nuclear/Radiological Threats in Commercial Cargo

“…This suggests that systems with penetration similar to the of the 6 MeV scanner analyzed here are necessary, but given the extremely low frequency of cargo with density >30 cm-steel equivalent systems with higher energies, penetration, and radiation dose are likely not necessary in this context. While the CAARS program and other systems 5 Cargo Advanced Automated Radiography System 6 Vehicle and Cargo Inspection System under development have focused on identifying nuclear materials by their high atomic number [49], which would further increase the sensitivity of a radiography system to nuclear threats, given the capabilities of existing radiography systems for threat detection suggested by this analysis expanded deployment of standard radiography systems may be advisable as more advanced systems are considered. A detection regime based on this technology would be inadequate for small samples of SNM, such as the 100 cm 3 sample targeted by CAARS, and thus careful consideration is required regarding the amounts of material that inspection schemes are required to detect.…”

Multiple monoenergetic gamma radiography (MMGR) with a compact superconducting cyclotron

“…Most traditional x-ray radiography methods make use of bremsstrahlung beams: the polychromatic nature of these beams, however, are their main limitations, as the lack of energy-specificity reduces their sensitivity to the Z of the cargo. Monoenergetic beams are more advantageous as the gammas have well defined energies, allowing for much better sensitivity to Z and thus improved reconstructions, as can be seen, for example, in the work of Henderson et al 7 While radiographic techniques are primarily based on photons, techniques involving other particles do exist such as using neutron beams. 8,9 For a high-level discussion of other active interrogation methods and bremsstrahlung radiography, see Runkle et al 10 and Sec.…”