The most frequently reported neutron imagers are neutron scatter cameras. These imagers use several detectors in conjunction with digitising electronics and nanosecond-level timing electronics to image neutron fields. These systems generally have a large detection volume and therefore high efficiency and sensitivity where good discrimination is employed, though this is partially offset by a reduction in efficiency due to the fact that double scatters are required for operation. These systems have been shown to detect fission and special nuclear material (SNM) sources at large stand-off distances. The drawbacks of scatter cameras is that they are very large, complex and expensive and often produce image artefacts in the image solution. The design goal of these systems overall is to have some level of portability and to rapidly accumulate data to demonstrate presence of a single source and its approximate location. They are therefore ideal for measurements where time is critical such as in nuclear security applications.
Brookhaven National Laboratory large-area fast neutron directional detector
The large-area fast neutron directional detector at Brookhaven National Laboratory uses 1m-long plastic scintillator paddles, each coupled with 2 photomultiplier tubes to detect fast neutrons over a large area giving good efficiency [67] [68] [69]. Fast neutrons interact via proton recoil reactions and gamma rays deposit energy via Compton scattering. All pulses are digitised to 1 ns timing resolution, which are processed to determine information about detected events. The location of the interaction in the paddle is estimated by the relative amplitudes of the pulses in each detector. Discrimination of neutrons against muons or
gamma rays is performed by time-of-flight between the paddles. The angle of scatter is estimated from the energy deposited in the first paddle and the time-of-flight to the second paddle. The image is then formed from back-projections of these angle cones as demonstrated in Fig. 2.15a. This system has been demonstrated to image a252Cf source of emission rate 2
×108neutrons s−1at distances of up to 255 m demonstrating good sensitivity. The field of view of this system is 90°×90 °.
(a) Schematic of the imager operation [67]
(b) Photograph of imager mounted in a truck [68]
(c) Output neutron images of a 252Cf source at 1m central or displaced in X or Y by 50 cm as indicated [68]
Figure 2.15 The large-area fast neutron directional detector at Brookhaven National Labora- tory
Fast Neutron Imaging Telescope
The fast neutron imaging telescope has been reported in the literature for use in measuring solar neutrons and for SNM detection [70] [71] [72]. This imager has been developed through collaboration between the University of Bern, the University of New Hampshire, Texas A and M University and the University of Glasgow. This imager uses 12 bars of NE-213A liquid scintillator, giving good efficiency, to detect neutrons (0.5 MeV neutron minimum), and uses time-of-flight as well as PSD to discriminate neutrons. In very close similarity with the Brookhaven National Laboratory large-area fast neutron directional detector, the location of each detection is calculated from relative signal from each bars 2 PMTs and the image is formed from cone projections. The resolution in the z axis for event detection was demonstrated to be 0.85 cm at single sigma level. This system is modular and was used with 3 detector elements to image a weapons grade plutonium source placed 1m from the detector at Pacific Northwest National Laboratory. The image was accumulated over several days and is shown in Fig. 2.16c; artefacts appearing in this image are expected to be a a result of using only three tubes in the modular system. The estimated angular resolution of the system is 5° and energy spectra can be reconstructed offline at an energy resolution estimated at 20%. The probe is reasonably compact and the system could be adapted for portability. The system has a 360° field of view in azimuth and a good range in elevation.
(a) Schematic of the system with 12 elements and cone projections (b) Photograph of the system with 3 detector elements
(c) Image of weapons grade plutonium source placed 1m from the detector. The image was accumulated over several days with the 3 element system
Figure 2.16 The fast neutron imaging telescope (FNIT) developed through collaboration between the University of Bern, the University of New Hampshire, Texas A and M University and the University of Glasgow [73].
Sandia National Laboratory neutron scatter camera
The neutron scatter camera at Sandia National laboratory is reported in several documents [74] [75] [76] [77] [78] [79] including two patents [80] [81].
(a) Photograph of the scatter camera without housing[76]
(b) Photograph of the scatter camera mounted in a truck[78]
(c) Image of252Cf source at a distance of 30m from the imager [75]
(d) Image of two252Cf sources [76]
Figure 2.17 The neutron scatter camera developed at Sandia National laboratory.
This system is estimated at a technology readiness level of 6 and is offered for licensing opportunities. This camera has been investigated for applications including warhead counting and SNM detection for homeland security applications. The imager comprises a modular array of EJ-309 liquid scintillation detectors, giving good efficiency, coupled with digitisers to perform pulse-shape discrimination. A photograph of the system is given in Fig. 2.17a
without housing and is shown mounted in a vehicle in Fig. 2.17b. Measurements of the deposited energy are used with the cell positions to generate back-projections of radiation paths. These are accumulated to form the image with an achievable resolution of 5°. Energy spectra can be collected sufficiently to distinguish241Am/Be from252Cf in separate images. An image solution of a252Cf source (designed to emulate 8 kg plutonium) at a distance of 30 m is shown in Fig. 2.17c, demonstrating good sensitivity. There, the data collection time was 2 hours. The source position is visible but some image artefacts are visible. The field of view of this detector is approximately 60 °. An image of two252Cf sources is also shown in Fig. 2.17d.
University of Michigan neutron scatter camera
There have been many reports on the neutron scatter camera at the University of Michigan [82] [83] [84] [85] [86] [87] [88]. The system is outlined in Fig. 2.18.
(a) Photograph of the neutron scatter camera [89]
(b) Schematic of gamma-ray and neutron scattering in image contributions [89]
This imager uses three planes of detectors to image combined gamma-ray and neutron fields, giving good efficiency. A photograph of the system and its 48 detectors is shown in Fig. 2.18a. A schematic of possible interactions is shown in Fig. 2.18b; note that planes 1 and 2 comprise EJ-309 liquid scintillation detectors and are used for imaging the neutron field. The third plane comprises NaI(Tl) and is used for gamma-ray-related imaging and measurements only.
(a) Output neutron image of a single localised252Cf source emission rate 2×105neutrons s−1imaged for 1 hour at a distance of 2.5 m[89]
(b) Output neutron image of two MOX canisters (approx. 1 kg each) seperated by 30° imaged for 2 hours at a distance of 2.5 m [83]
Figure 2.19 Output images from the University of Michigan neutron scatter camera. An incoming neutron must scatter in plane 1 and plane 2 in order to contribute to the image data. The pulses from the detectors are digitised and pass through two filters: pulse- shape discrimination and time correlation. The former selects neutron events through the pulse shape, the latter selects neutron events based on the travel time between the detector
planes (gamma rays travel at light speed, neutrons much slower). Events passing through these filters are included in the data set and the solid angle of origin is determined from the geometry of the detectors in each double scatter event. The location of the sources is found by comparison of the many solid angle regions with various methods. This system processes scatters from both directions and therefore gives a near 4π field of view. An image of a252Cf
source, emission rate 2×105neutrons s−1, imaged for 1 hour at a distance of 2.5 m is shown in Fig. 2.19a. The neutron energy spectrum can also be calculated with this approach, using the measured deposited energy and scattering angle. This system has been demonstrated to image plutonium and mixed-oxide (MOX) fuel materials; an image of two MOX canisters separated by 30° are shown in Fig. 2.19b.
University of New Hampshire scatter camera, NSPECT
The University of New Hampshire have developed a scatter camera [90] [91], shown in Fig. 2.20 which is a scaled down more portable version of the University of Michigan neutron scatter camera. Due to the smaller detectors the efficiency is reduced in comparison. The angular resolution of the system is 12 °. This camera has also been demonstrated to measure SNM including plutonium and depleted uranium. Images of a252Cf source moderated by a 15 cm radius sphere were also produced, as well as images from large stand-off distances.
(a) The system, comprising 3 detection layers
(b) The system mounted in an SUV
(c) Neutron hotspot (left) and gamma ray hotspot (right) overlaid on an optical image