Thermal neutron backscatter imaging

Abstract

Objects of various shapes, with some appreciable hydrogen content, were exposed to fast neutrons from a pulsed: D-T generator, resulting in a partially-moderated spectrum of backscattered neutrons. The thermal component of the backscatter was used to form images of the objects by means of a coded aperture thermal neutron imaging system. Timing signals; from the neutron generator were used to gate the detection system so as to record only events consistent with thermal neutrons traveling the distance between the target and the detector. It was shown that this time-of-flight method provided a, significant improvement in image contrast compared to counting all events detected by the position-sensitive 3Hr: proportional chamber used in the imager. The technique may have application in the detection and shape-determination 0.f land mines, particularly non-metallic types.

Thermal neutron imaging in an active interrogation environment

“…Coded aperture imaging of thermal neutron sources has previously been proposed for a number of applications, including warhead counting for treaty verification [1], detecting nuclear smuggling [2], explosives detection [3,4], monitoring stored spent nuclear fuel [5]. In earlier testing of our coded aperture camera, which uses a 2-dimensional He-3 positionsensitive wire chamber and a Cd mask, we used isotopic sources (such as 252 Cf or 241 Am-Be) which continuously emitted fast neutrons that were then thermalized in paraffin.…”

“…We also showed that the imager could be operated in a gated mode in combination with a D-T neutron generator to produce images of hydrogenous materials (simulating explosives). In that case [3], the time gate for the data acquisition was chosen to reduce the response of the He-3 chamber to neutrons with energies above the 1-eV absorption threshold of the Cd mask. Such epithermal neutrons travel about 1.4 meters in 100 s or less, and can be detected by 3 He with an efficiency of a few percent.…”

“…In that case [3], the time gate for the data acquisition was chosen to reduce the response of the He-3 chamber to neutrons with energies above the 1-eV absorption threshold of the Cd mask. Such epithermal neutrons travel about 1.4 meters in 100 s or less, and can be detected by 3 He with an efficiency of a few percent. More recent work [6] has made use of pulsed electron accelerators with tungsten targets that produce intense beams of bremsstrahlung photons with energies up to 20 MeV.…”

“…The fissionable material can be distinguished from other heavy metal by adjusting the data acquisition time gate. This type of measurement has previously been carried out using non-imaging PhotoNuclear Detectors (PNDs) consisting of 3 He tubes partially moderated with polyethylene and shielded from external thermal neutrons with Cd sheet around the moderator. The PND is therefore not sensitive to thermal neutrons, and is sometimes operated in combination with a bare 3 He tube that is insensitive to epithermal and fast neutrons.…”

“…This type of measurement has previously been carried out using non-imaging PhotoNuclear Detectors (PNDs) consisting of 3 He tubes partially moderated with polyethylene and shielded from external thermal neutrons with Cd sheet around the moderator. The PND is therefore not sensitive to thermal neutrons, and is sometimes operated in combination with a bare 3 He tube that is insensitive to epithermal and fast neutrons. The thermal neutron imager counts neutrons with temporal and energy distributions very similar to the bare tube, but in addition provides directional information and automatic background subtraction.…”