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Smuggling of special nuclear materials and nuclear devices through borders and ports of entry constitutes a major risk to global security. Technologies are needed to reliably screen the flow of commerce for the presence of high- Z materials such as uranium and plutonium. Here, we present an experimental proof-of-concept of a technique that uses inelastic (p , p ′) nuclear reactions to generate monoenergetic photons, which provide means to measure the areal density and the effective- Z (Z eff ) of an object with an accuracy surpassing that achieved by current methods. We use an ION-12 SC superconducting 12 MeV proton cyclotron to produce 4.4, 6.1, 6.9, and 7.1 MeV photons from a variety of nuclear reactions. Using these photons in a transmission mode, we show that we are able to accurately reconstruct the areal densities and Z eff of a test object. This methodology could enable mobile applications to screen commercial cargoes with high material specificity, providing a means of distinguishing common cargo materials from high-Z materials that include uranium and plutonium. [ABSTRACT FROM AUTHOR]

The smuggling of special nuclear materials (SNMs) 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 utilizes monoenergetic photons from nuclear reactions, specifically the 4.4 and 15.1 MeV photons from the 11B(d,nγ)12C 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. [ABSTRACT FROM AUTHOR]

Cargo containers passing through ports are scanned by non-intrusive inspection systems to search for concealed illicit materials. By using two photon beams with different energy spectra, dual energy inspection systems are sensitive to both the area density and the atomic number of cargo contents. Most literature on the subject assumes a simple exponential attenuation model for photon intensity in which only free streaming photons are detected. However, this approximation neglects second order effects such as scattering, leading to a biased model and thus incorrect material predictions. This work studies the accuracy of the free streaming model by comparing it to simulation outputs, finding that the model shows poor atomic number reconstruction accuracy at high- Z and suffers significantly if the source energy spectra and detector response function are not known exactly. To address these challenges, this work introduces a semiempirical transparency model which modifies the free streaming model by rescaling different components of the mass attenuation coefficient, allowing the model to capture secondary effects ignored by the free streaming model. The semiempirical model displays improvement agreement with simulated results at high- Z and shows excellent extrapolation to materials and thicknesses which were not included during the calibration step. Furthermore, this work demonstrates that the semiempirical model yields accurate atomic number predictions even when the source spectra and detector response are not known exactly. Using the semiempirical model, manufacturers can perform a simple calibration to enable more precise Z reconstruction capabilities, which has the potential to significantly improve the performance of existing dual energy radiographic systems. [ABSTRACT FROM AUTHOR]

Detection of cross-border smuggling of illicit materials and contraband is a challenge that requires rapid, low-dose, and efficient radiographic technology. The work we describe here is derived from a technique which uses monoenergetic gamma rays from low energy nuclear reactions, such as 11 B(d,nγ) 12 C, to perform radiographic analysis of shipping containers. Transmission ratios of multiple monoenergetic gamma lines resulting from several gamma producing nuclear reactions can be employed to detect materials of high atomic number ( Z ), the details of which will be described in a separate paper. Inherent in this particular nuclear reaction is the production of fast neutrons which could enable neutron radiography and further characterization of the effective- Z of the cargo, especially within the range of lower Z . Previous research efforts focused on the use of total neutron counts in combination with X-ray radiography to characterize the hydrogenous content of the cargo. We present a technique of performing transmitted neutron spectral analysis to reconstruct the effective Z and potentially the density of the cargo. This is made possible by the large differences in the energy dependence of neutron scattering cross-sections between hydrogenous materials and those of higher Z . These dependencies result in harder transmission spectra for hydrogenous cargoes than those of non-hydrogenous cargoes. Such observed differences can then be used to classify the cargo based on its hydrogenous content. The studies presented in this paper demonstrate that such techniques are feasible and can provide a contribution to cargo security, especially when used in concert with gamma radiography. [ABSTRACT FROM AUTHOR]

The determination of hydrogenous content in commercial cargoes is an important challenge in cargo security. Prior work has shown the feasibility of hydrogenous cargo classification in radiographic applications. This result was achieved by using the significant material to material differences in the energy dependence of their neutron scattering cross-sections. The work presented here details the application of this technique to multiple-monoenergetic neutron transmission measurements of several heterogeneous cargo mock-ups with the goal of quantifying hydrogenous content. It demonstrates the ability to determine the hydrogenous content of a cargo container by performing analysis of pulse-height data of transmitted neutrons. The set-up used for these feasibility studies was able to detect and quantify areal densities of up to 30 g/cm 2 of high density polyethylene (HDPE), even when mixed with metallic materials. Accurate determination of the hydrogenous content of the cargo has two important uses: it can allow for better imaging and discrimination of conventional contraband during cargo screening; it may also allow the unfolding of the individual elemental contributions from an effective atomic number, Z eff , as determined from multiple monoenergetic gamma radiography (MMGR). This combination of capabilities could make multiple monoenergetic neutron radiography applications a significant contribution to cargo security. [ABSTRACT FROM AUTHOR]

The detection of assembled nuclear devices and concealed special nuclear materials (SNM) such as plutonium or uranium in commercial cargo traffic is a major challenge in mitigating the threat of nuclear terrorism. Currently available radiographic and active interrogation systems use ∼1–10 MeV bremsstrahlung photon beams. Although simple to build and operate, bremsstrahlung-based systems deliver high radiation doses to the cargo and to potential stowaways. To eliminate problematic issues of high dose, we are developing a novel technique known as multiple monoenergetic gamma radiography (MMGR). MMGR uses ion-induced nuclear reactions to produce two monoenergetic gammas for dual-energy radiography. This allows us to image the areal density and effective atomic number ( Z eff ) of scanned cargo. We present initial results from the proof-of-concept experiment, which was conducted at the MIT Bates Research and Engineering Center. The purpose of the experiment was to assess the capabilities of MMGR to measure areal density and Z eff of container cargo mockups. The experiment used a 3.0 MeV radiofrequency quadrupole accelerator to create sources of 4.44 MeV and 15.11 MeV gammas from the 11 B(d,n γ ) 12 C reaction in a thick natural boron target; the gammas are detected by an array of NaI(Tl) detectors after transmission through cargo mockups . The measured fluxes of transmitted 4.44 MeV and 15.11 MeV gammas were used to assess the areal density and Z eff . Initial results show that MMGR is capable of discriminating the presence of high- Z materials concealed in up to 30 cm of iron shielding from low- and mid- Z materials present in the cargo mockup. [ABSTRACT FROM AUTHOR]