![]() ![]() 9,10 Elemental identification through photon radiography is limited by the fact that a thick mass of low- Z material may attenuate source photons similarly to a thinner mass of high- Z material. ![]() Sources that span a multitude of photon energies can be used to determine the effective Z ( Z eff) of the material by exploiting differences between either the photoelectric and Compton-scatter cross-sections (at lower energies) or Compton-scatter and pair-production cross-sections (at higher energies). 1 Photon radiography leverages the Z-dependence of the mass attenuation coefficient to localize regions of high- Z or dense material in the transmission image. Radiographic measurements are typically performed with high-energy photons or fast neutrons, though other types of particles, such as muons, are sometimes used. ![]() 2 One measurement technique which is particularly well-suited to the detection of geometric anomalies is transmission radiography. By measuring the type of material, as well as its composition, fissile mass, and geometry, a verification system can detect potential anomalies such as spoofs (false geometries or materials designed to mimic the true SNM core) or diversion of material. There are a number of potential methods by which a host may attempt to deceive inspectors, including replacement, diversion, or dilution of nuclear material. 4 This paper focuses on characterizing the performance of a measurement technique that has a potential for future incorporation into such future zero-knowledge protocols. 7,8 In addition to more complex signature-matching techniques, template-matching methods have been proposed that involve detector arrays which are pre-loaded with geometric or other characteristic data in such a way that only the differential information between the test object and the template is ever recorded by the measurement system. 2,6 For example, with the aid of a 252Cf active-interrogation source, analysis of an array of induced neutron signals in the time and frequency domains has been used to successfully distinguish and identify nuclear weapons components based on reference signatures. 4 Such templates usually consist of a complex radiation signatures that are used to “fingerprint” SNM components. As a result, there is considerable interest the application of zero-knowledge protocols, in which measurements do not record any sensitive information directly, but instead may form part of a differential comparison against a declared standard or template. 5 The political reality associated with such mechanisms thus presents a significant barrier to their adoption and implementation. 2,4 Current information barrier systems can also be expensive to implement and reduce confidence in verification measurement results. ![]() While this problem has traditionally been addressed through engineered information barriers, which prevent the inspector from directly observing the classified information being measured, this approach involves a high degree of complexity and susceptibility to tampering through the use of information trapdoors to falsify results or leak sensitive information. As such, the protection of state secrets represents a significant hurdle that must be cleared for a verification technique to be considered viable. However, the design of a verification system is complicated by the fact that such measurements necessarily involve the collection of highly classified data. The combination of crude imaging and fissionable material detection and quantification in a simple approach may be attractive in certain treaty verification scenarios. We also show an example of detection of material diversion and confirm the presence of fissionable material based on the measurement of high-energy prompt fission neutrons, including estimating the quantity of material from the comparison of measured and predicted fission neutron emission rate. In the experiment, we used monoenergetic neutrons from D(d, n) 3He and T(d, n) 4He reactions and a linear array of liquid scintillation detectors to perform spectroscopic neutron imaging of up to 13.7 kg of highly enriched uranium in a spherical geometry. We experimentally demonstrate a simple method based on monoenergetic fast neutron transmission to realize crude imaging of the geometric configuration of special nuclear material, confirm its fissionable content, and obtain information on its approximate fissile mass. Measurements of the geometric configuration of objects and their material composition are needed for nuclear treaty verification purposes. ![]()
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