Thermal Neutron Imaging in an Active Interrogation Environment

Abstract

Abstract.We have developed a thermal-neutron coded-aperture imager that reveals the locations of hydrogenous materials from which thermal neutrons are being emitted. This imaging detector can be combined with an accelerator to form an active interrogation system in which fast neutrons are produced in a heavy metal target by means of excitation by high energy photons. The photo-induced neutrons can be either prompt or delayed, depending on whether neutronemitting fission products are generated. Provided that there are hydrogenous materials close to the target, some of the photo-induced neutrons slow down and emerge from the surface at thermal energies. These neutrons can be used to create images that show the location and shape of the thermalizing materials. Analysis of the temporal response of the neutron flux provides information about delayed neutrons from induced fission if there are fissionable materials in the target. The combination of imaging and time-of-flight discrimination helps to improve the signal-to-background ratio. It is also possible to interrogate the target with neutrons, for example using a D-T generator. In this case, an image can be obtained from hydrogenous material in a target without the presence of heavy metal. In addition, if fissionable material is present in the target, probing with fast neutrons can stimulate delayed neutrons from fission, and the imager can detect and locate the object of interest, using appropriate time gating. Operation of this sensitive detection equipment in the vicinity of an accelerator presents a number of challenges, because the accelerator emits electromagnetic interference as well as stray ionizing radiation, which can mask the signals of interest.

Uncovering Special Nuclear Materials by Low-energy Nuclear Reaction Imaging

“…Passive detection falls short when materials of interest are shielded, requiring an external source of radiation in order to create or amplify the characteristic signatures from the material, a method referred to as active interrogation 4 . Two well-known challenges are associated with traditional bremsstrahlung-based active interrogation of shielded nuclear material: high radiation doses to cargo 5 and vulnerability of radiation detectors to harsh radiation environments caused by the external beam 6 . The physics of bremsstrahlung-based imaging is well understood and has been demonstrated in practice 7 , including detection methods based on nuclear resonance fluorescence 8 9 .…”