Detecting contraband using neutrons: Challenges and future directions

/pdf/production-and-applications-of-neutrons-using-particle-3c6q7zliqi.pdf

Production and Applications of Neutrons Using Particle Accelerators

We consider the problem of combining a given set of diagnostic tests into an inspection system to classify items of interest (cases) with maximum accuracy such that the cost of performing the tests does not exceed a given budget constraint. One motivating application is sequencing diagnostic tests for container inspection, where the diagnostic tests may correspond to radiation sensors, document checks, or imaging systems. We consider mixtures of decision trees as inspection systems following the work of Boros et al. (Nav. Res. Logist. 56:404–420, 2009). We establish some properties of efficient inspection systems and characterize the optimal classification of cases, based on some of their test scores. The measure of performance is the fraction of all cases in a specific class of interest, which are classified correctly. We propose a dynamic programming algorithm that constructs more complex policies by iteratively prefixing devices to a subset of policies and thereby enumerating all of the efficient (i.e., undominated) inspection policies in the two dimensional cost-detection space. Our inspection policies may sequence an arbitrary number of tests and are not restricted in the branching factor. Our approach directly solves the bi-criterion optimization problem of maximizing detection and minimizing cost, and thus supports sensitivity analysis over a wide range of budget and detection requirements.

/pdf/optimal-sequential-inspection-policies-52678u0b6i.pdf

Optimal sequential inspection policies

The detection and identification of delayed fission γ -rays emitted between intense bremsstrahlung pulses was investigated as a method to discriminate between fissionable and non-fissionable materials. Photofission and other photonuclear reactions were induced in U 238 , Pb, Fe and Be targets using a pulsed bremsstrahlung beam for several endpoint energies between 8 and 20 MeV . In the 33 ms between irradiating pulses, correlated photon energy and time spectra were collected using a high-purity germanium detector. When non-fissionable materials were irradiated, the high-energy γ -ray yield decreased a couple orders of magnitude within the first ∼ 12 ms after the pulse. In contrast, the high-energy γ -rays emitted from the decay of fission fragments were nearly constant throughout the period between bremsstrahlung pulses. Hence, a basic energy and time discrimination provides a unique high-energy delayed γ -ray signature for fissionable material detection.

The detection of delayed γ-rays between intense bremsstrahlung pulses for discriminating fissionable from non-fissionable materials

The manuscript reviews several selected applications of Activation Analysis that differ from classical laboratory measurements by the fact that they do not require sampling of the analysed object and/or measurements under analytical laboratory conditions. One of great advantages of the Activation Analysis lies in its versatility that allows it to be applied under conditions not available for other analytical techniques.

Non-classical applications of chemical analysis based on nuclear activation

A concept for detecting the presence of special nuclear material (235U or 239Pu) concealed in intermodal cargo containers has been developed, studied, and recent performance results are described. It is based on interrogation with a pulsed beam of 3–7 MeV neutrons that produce fission events and subsequent detection of their β-delayed neutron emission or β-delayed high-energy γ-radiation reveals the presence of fissionable material. Fission product β-delayed γ-rays above 3 MeV are nearly 10 times more abundant than β-delayed neutrons and are distinct from natural radioactivity and from nearly all of the induced activity in a normal cargo. Detector backgrounds and potential interferences with the fission signature radiation have been identified and quantified. Their impact on detection sensitivity is relatively minor and can be addressed readily. Components of a simple laboratory prototype have been assembled, tested with the most challenging cargo threat scenarios, and results compared to computer simulations. Preliminary results will be presented.

The nuclear car wash: A system to detect nuclear weapons in commercial cargo shipments

A large majority of US imports arrive at seaports in maritime cargo containers. The number of containers arriving is nearly 10 million per year, each with a cargo of up to 30 tons of various materials. This provides a vulnerable entry point for the importation of a nuclear weapon or its components by a terrorist group. Passive radiation sensors are being deployed at portals to detect radioactive material and portable instruments are carried by port personnel to augment detection. Those instruments can detect the neutrons and g-rays produced by {sup 240}Pu that is normally present in weapons grade plutonium in cases where cargo overburden is not too great. However, {sup 235}U produces almost no neutron output in its normal radioactive decay and its principal {gamma}-radiation is at 186 keV and is readily attenuated by small amounts of wood or packing materials. Impurities such as {sup 232}U, often present in reactor irradiated material at the 100-200 ppt level, can provide a detectable signal through significant cargo overburden but the wide variations among samples of HEU make this an unreliable means of detecting SNM. High quality radiography may be useful in determining that the majority of containers are clearly free of SNM. However, some containers will lead to ambiguous results from radiography and passive radiation sensing. For these reasons active neutron interrogation is proposed as a means to produce fission and thus greatly amplify the radiation output of fissionable material to facilitate its reliable detection even when well shielded by large cargo overburden. Historically, the fission signature utilized as the unique identifying feature of fissionable materials is the detection of delayed neutrons. However, these neutrons have very low yield {approx} 0.017 per fission in {sup 235}U, and their low energy results in very poor penetration of hydrogenous materials such as fuels, water, wood, or agricultural products. That signature alone does not provide reliable detection in thick cargos. A new signature has been identified and has been developed within the current project for the detection of well shielded SNM. This SNM signature is based on high-energy {beta}-delayed {gamma}-radiation produced by fission products following neutron or photon induced fission. These {gamma}-rays are high enough in energy (E{sub {gamma}} > 3 MeV) to be readily distinguished from any natural background radioactivity since the latter does not extend above 2.6 MeV. Their abundance is nearly a decade greater than delayed neutrons and their short half-lives deliver nearly all of the signature radiation on time scales of one minute or less and thus facilitate rapid scanning. Finally, for this {gamma}-radiation in the 3-6 MeV range attenuation occurs only by Compton scattering and is in the range where minimum attenuation occurs in all materials. Even the thickest cargos of any material attenuate these {gamma}-rays by only a factor of 10-30X so that the signature is readily detected even with the most challenging shield materials. The goals of the current program are to detect significant quantities (much less than IAEA ''significant'' amounts) of well-shielded SNM, and to do so with detection probability P{sub d} {ge} 95% and with false alarm rates P{sub fp} {le} 0.001. It is the goal to meet these requirements in a scan that requires less than one minute to complete and does so without damage to the cargo or to people who may be hidden inside. We intend to meet these requirements even when the cargo overburden is up to {rho}L {le} 150 g/cm{sup 2} of any material ranging from fuels and agricultural products to steel and lead.

/pdf/nuclear-car-wash-status-report-august-2005-54vvmb61hf.pdf

Nuclear car wash status report, August 2005

A Monte Carlo model has been developed for interrogation of fissionable material embedded in thick cargos when high-energy {beta}-delayed {gamma} rays are detected following neutron-induced fission. The model includes the principal structural components of the laboratory, the neutron source and collimator assembly in which it resides, the assembly that represents cargo of given characteristics, a target of highly-enriched uranium (HEU) and large external plastic scintillators for photon detection. The ability of this model to reproduce experimental measurements was tested by comparing simulations with measurements of the number of induced fissions and the number of detected photons when the HUE target was irradiated with 14.25-MeV neutrons in the absence of any cargo and while embedded in assemblies of plywood and iron pipes. The simulations agreed with experimental measurements within a factor of about 2 for irradiation of the bare target and when the areal density of intervening cargo was 33 g cm{sup -2} (wood) and 61 g cm{sup -2} (steel pipes). This suggests that the model can permit exploration of a large range in parameter space with reasonable fidelity.

A Monte Carlo Model for Interrogation of Thick Cargos for Clandestine Fissionable Materials; Tests with 14-MeV Neutrons

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The nuclear car wash: A system to detect nuclear weapons in commercial cargo shipments

Nuclear car wash status report, August 2005

A Monte Carlo Model for Interrogation of Thick Cargos for Clandestine Fissionable Materials; Tests with 14-MeV Neutrons