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Dual energy cargo inspection systems are sensitive to both the area density and the atomic number of an imaged container due to the Z dependence of photon attenuation. The ability to identify cargo contents by their atomic number enables improved detection capabilities of illicit materials. This work introduces a novel method for atomic number reconstruction by minimizing the chi-squared error between measured transparency values and a semiempirical transparency model. This method is tested using two Geant4 Monte Carlo simulated radiographic phantoms, demonstrating the ability to obtain accurate material predictions on noisy input images, even in the presence of shielding. Furthermore, we provide a simple procedure for porting this method to a commercial system, requiring an approximate model of the scanner's beam spectra and detector response, along with only three calibration measurements. [ABSTRACT FROM AUTHOR]

Emerging thorium-based reactor designs and fuel cycles present challenges to traditional non-destructive assay techniques used in international safeguards. Specifically, assaying the masses of 233U and 235U when they are present together in samples with high gamma ray backgrounds is difficult because of similar passive neutron signatures and relatively weak gamma-ray emissions of 233U. The Pacific Northwest National Laboratory (PNNL) and the Massachusetts Institute of Technology (MIT) are developing a portable neutron resonance transmission analysis (pNRTA) system as one potential solution to these challenges. This method provides isotopic concentration data for a sample via neutron time-of-flight (TOF) measurements that exploit epithermal neutron resonance cross-sections. A recently developed pNRTA system uses a commercially available, pulsed deuterium-tritium neutron generator with a ∼2 m flight path and a GS20 lithium glass scintillator detector. This paper describes the prototype pNRTA system design, a refined radiation transport model of the system, preliminary measurements with thorium and uranium sources, and demonstration of a quantitative isotopic estimation algorithm. [ABSTRACT FROM AUTHOR]

To combat the risk of nuclear smuggling, radiography systems are deployed at ports to scan cargo containers for concealed illicit materials. Dual energy radiography systems enable a rough elemental analysis of cargo containers due to the Z -dependence of photon attenuation, allowing for improved material detection. This work studies the capabilities for atomic number discrimination using dual energy MeV systems by considering dual energy { 6 , 4 } MeV , { 10 , 6 } MeV , and { 10 , 4 } MeV bremsstrahlung beams. Results of this analysis show that two different pure materials can sometimes produce identical transparency measurements, leading to a fundamental ambiguity when differentiating between materials of different atomic numbers. Previous literature has observed this property, but the extent of the limitation is poorly understood and the cause of the degeneracy is generally inadequately explained. This non-uniqueness property stems from competition between photoelectric absorption and pair production and is present even in systems with perfect resolution and zero statistical noise. These findings are validated through Monte Carlo transparency simulations. Results of this study show that currently deployed commercial radiographic systems are fundamentally incapable of distinguishing between high- Z nuclear materials and miscellaneous mid- Z cargo contents. • Dual energy cargo scanners are incapable of uniquely characterizing high-Z materials. • This stems from competition between photoelectric absorption and pair production. • Even systems with perfect statistical resolution cannot resolve this degeneracy. • Results are validated through Monte Carlo simulations. [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]

G4NRF (Jordan and Warren, 2007; Vavrek et al., 2018a,b) is a simulation module for modeling nuclear resonance fluorescence (NRF) interactions in the Geant4 framework (Allison et al., 2016). In this work, we validate G4NRF against both absolute and relative measurements of three NRF interactions near 2.2 MeV in 238U and 27Al using the transmission NRF data from the experiments described in Jayson et al. (2018). Agreement between the absolute NRF count rates observed in the data and predicted by extensive Geant4+G4NRF modeling validate the combined Geant4+G4NRF to approximately 15–20% in the 238U NRF transitions and 10% in 27Al, for an average 14% discrepancy across the entire study. The difference between simulation and experiment in relative NRF rates, as expressed as ratios of count rates in various NRF lines, is found at the level of ≲ 3 % , and is statistically identical to zero. Inverting the analysis, approximate values of the absolute level widths and branching ratios for 238U and 27Al are also obtained. [ABSTRACT FROM AUTHOR]

Abstract Nuclear resonance fluorescence (NRF) is a photonuclear interaction that enables highly isotope-specific measurements in both pure and applied physics scenarios. High-accuracy design and analysis of NRF measurements in complex geometries is aided by Monte Carlo simulations of photon physics and transport, motivating Jordan and Warren (2007) to develop the G4NRF codebase for NRF simulation in Geant4. In this work, we enhance the physics accuracy of the G4NRF code and perform improved benchmarking simulations. The NRF cross section calculation in G4NRF, previously a Gaussian approximation, has been replaced with a full numerical integration for improved accuracy in thick-target scenarios. A high-accuracy semi-analytical model of expected NRF count rates in a typical NRF measurement is then constructed and compared against G4NRF simulations for both simple homogeneous and more complex heterogeneous geometries. Agreement between rates predicted by the semi-analytical model and G4NRF simulation is found at a level of ∼ 1 % in simple test cases and ∼ 3 % in more realistic scenarios, improving upon the ∼ 20 % level of the initial benchmarking study and establishing a highly-accurate NRF framework for Geant4. [ABSTRACT FROM AUTHOR]

Abstract With the increasing cost and decreasing availability of 3He, there have been many efforts to find alternative neutron detection materials. Lithium calcium aluminum fluoride (LiCAF) enriched to 95% 6Li doped with europium was evaluated here as a replacement material for 3He. Wafers 0.5 cm thick, consisting of LiCAF crystals in a rubberized matrix, were embedded with wavelength shifting fibers (WSF) and mated to silicon photo-multipliers (SiPMs) to measure the photon response in a flux of neutrons from a DD neutron generator. Excellent discrimination was realized between neutrons and gammas, and both pulse-height discrimination and pulse-shape analysis were explored. A Figure of Merit (FoM) of 1.03 was achieved. By applying pulse-shape analysis, a simple neutron count output was generated by utilizing a low-pass filter to suppress fast pulses from the SiPM output and subsequently applying a threshold to the remaining signal. Custom electronics were built to bias the SiPMs, then amplify, filter, discriminate, and digitize the LiCAF/WSF scintillation photons, resulting in a digital pulse that can easily be counted with any microcontroller or field programmable gate array. A significant advantage of LiCAF is that it can be fabricated into any shape/size (when embedded in a rubberized matrix), and the light output and transparency is sufficient to allow for thicker scintillators which enable detection of both thermal and epithermal neutrons. This work demonstrated that Eu:LiCAF is capable of discriminating gammas from neutrons and is a potential replacement material for 3He, especially for nuclear security applications and neutron spectroscopy. [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]

A portable ten-layer neutron spectrometer has been developed using a novel material, LiCAF. The spectrometer consists of rubber matrix wafers with embedded europium doped lithium calcium aluminum fluoride (Eu:LiCAF) crystals and crosshatched wave shifting fibers (WSF). Layers of high density polyethylene (HDPE) separating each Eu:LiCAF wafer serve as neutron moderating material. Neutrons entering the spectrometer react in the Eu:LiCAF crystal, creating scintillation photons. The photons travel down the WSFs to silicon photomultipliers (SiPMs) that convert the photons to an electrical signal. Custom electronics are used to gather, amplify, process, and readout the signal. A library of several neutron response curves was created in Geant4 and MCNP. The experimental output of the spectrometer was unfolded using a maximum entropy algorithm, MAXED. The spectrometer was tested using two separate DD generators. These two tests resulted in unfolded neutron spectra with average neutron energies of 2.71 and 2.78 MeV and chi-squared values of 0.88 and 1.51 χ 2 /D.O.F. respectively, when fit to a 2.45 MeV monoenergetic energy spectrum. [ABSTRACT FROM AUTHOR]