Thermal Neutron Imaging in an Active Interrogation Environment

摘要

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