Coded aperture imagers

Coded aperture imagers operate in a similar way to pin-hole cameras, using a shielding material as a response modulator to block out a large proportion of the incident radiation, allowing only some trajectories to pass through. Rather than having a single transmission region, as with pin-hole cameras, there are many which greatly improves the signal-to-noise ratio though the image reconstruction is more complex. A sample of a binary coded-aperture mask is shown in Fig. 2.21. The result is that a pattern emerges on the detection medium, a superposition of many pin-hole transmissions which can be related back to the radiation field.

Figure 2.21 Example of a binary coded-aperture mask for coded-aperture imaging. Black and white pixels represent opaque and transmission regions respectively.

Coded aperture imagers discussed in the literature have a range of capabilities depending on the design goals which have been extremely varied, from stand-off source location to counting warheads in nuclear weapons.

Oak Ridge National Laboratory, Sandia National Laboratories and Idaho National Laboratory coded aperture neutron imager

Coded aperture imaging with fast-neutron detection has been investigated in the United States with a joint effort between Oak Ridge National Laboratory, Sandia National Laboratories and Idaho National Laboratory [92] [93]. This system is shown in Fig. 2.22a. A plane of custom-built pixelated EJ-309 liquid scintillator cells (coupled with digitisers for PSD) are located behind a coded mask made of polyethylene 2.22b. The material under scrutiny is placed in front of the mask which blocks out neutrons along some paths, and not through others where holes in the mask are present. The pattern of neutron flux at the cells consisting of transmission or shadow is dependent on the distribution of the neutron sources in front

of the mask. PMTs coupled to groups of cells read the distribution of scintillations due to neutron interactions which is used along with the known mask geometry to calculate the source distribution in front of the imager. Gamma rays are filtered out by digitisation of the pulses and applying a pulse-shape discrimination algorithm. This system has been used to demonstrate effective imaging of complex neutron fields produced by five sources, see Figs. 2.22c and 2.22d with resolution of at least 10 cm (the closest spacing of the sources).

(a) Photograph of system (b) Photograph of mask

(c) Layout of five252Cf sources each with emission rate of 4×104 neu- trons s−1imaged at a distance of 1.11 m

(d) Output neutron image of five 252_{Cf sources after 1 hour exposure}

Figure 2.22 The fast-neutron coded aperture imager developed by national laboratories in the United States [92].

This system has high efficiency due to the large detection volume and clearly has a good image resolution. This system requires digitising electronics to apply neutron-gamma discrimination but does not require coincidence filters, making this approach simpler than for the neutrons scatter camera. The drawbacks of this system is that it is large and non-portable

and has a limited field of view. The system also has a focus length and therefore requires the source to be placed in a known region of space. This imager was designed to have a high resolution suitable for identifying nuclear warheads for treaty verification purposes.

CLYC RadCam™

Figure 2.23 Photographs and annotations of the RadCam-2 combined neutron and gamma-ray imaging system [94].

RMD have recently developed a coded-aperture imager, shown in Fig. 2.23, based on the RadCam™design, using a small amount of the scintillator Cs2LiYCl6:Ce [95] [94]. This

detector interacts via recoil scattering to detect fast neutrons and also allows the detection of thermal neutrons via the6Li(n,α)t channel; these events are separated using pulse-shape

discrimination. The coded aperture mask is composed of tungsten and cadmium, the latter used to shield thermal neutrons. The system has a limited field of view but has been motorised to permit rotation of the probe. The system has an outer probe radius of approximately 65 cm and can be considered lightweight and portable. Output images are shown in Fig. 2.24. Research has shown that this system performs best when imaging fast neutrons below 2 MeV, as many image artefacts are otherwise produced, e.g. when imaging241Am/Be. Although the addition of thermal-neutron imaging can be considered an advantage in a general sense of capability, this may not always be the case in practice. Neutrons have undergone many scatters through the thermalisation process; these fields therefore will have lost much of their directionality, reducing the efficacy of thermal neutron imaging at determining the precise source location. Secondly, there are many situations which involve both very high thermal-neutron backgrounds and nuclear fuel materials. Where the fast-neutron imaging of fuel materials is the goal, the addition of a thermal neutron background to the gamma-ray background would add additional complications when discriminating fast-neutron fields. Contributions from35Cl(n,p) reactions also add thermal neutron points in the PSD plot which

extend into further regions of the PSD distribution than the6Li(n,α)t, making fast-neutron

discrimination increasingly difficult.

(a) Image of a 39µCi252Cf source

at 30 cm imaged for 2 days

(b) Image of a 340 mCi241Am/Be source at 30 cm imaged for 2 days

Figure 2.24 Output images of fast-neutron fields produced by RadCam-2 overlaid on optical images [94].

Sandia National Laboratory time-encoded imager

More recently a system has been described which uses time encoding to image fast-neutron fields [96]. This system, developed by Sandia National Laboratory, works analogously to a coded aperture system, including using a coded mask. Most systems require an array of detectors to read the flux distribution in space, this system requires only two EJ-309 scintillation cells. The mask is moved with time; the known orientation of the mask at a given time in conjunction with the number of neutron detection events at the detectors is compiled into the data set following neutron-gamma discrimination. The data set is used with the known mask parameters (pattern and shape) to determine the distribution of fast- neutron emitters in the local environment. This system requires digitisation and pulse-shape discrimination, though the remaining mechanical requirements are reasonably simplistic. A photograph of this system is shown in Fig.2.25a.

This system overcomes the issues of low efficiency of multi-scatter systems and the high cost and complexity of conventional coded aperture imaging systems. This system has a reasonable field of view, although cannot image directly above or below the cylindrical axis. The system is reasonably compact and portable, however the resolution will be traded off against the size of the mask. This means that smaller masks of this type will be more compact but with a lower resolution. This property is related to the fact that the collimator must be a few cm thick to effectively shield fast neutrons, and the mask cells must therefore be of an

(a) Annotated photograph of the system [96] (b) Image of a ring of californium sources each 35µCi imaged

for 3 days at 2m distance [96]

Figure 2.25 The time encoded fast-neutron imager developed at Sandia National Laboratory.

approximately similar size. The system has been used to image complex fields such as those produced by a ring of252Cf sources shown in Fig.2.25b. This image shows that resolution is better than 12°. There are some artefacts in the images and imaging time is over one day.