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The Raytheon Trimodal Imager (TMI) uses coded aperture and Compton imaging technologies as well as the nonimaging shadow technology to locate an SNM or radiological threat in the presence of background. The heart of the TMI is two arrays of NaI crystals. The front array serves as both a coded aperture and the first scatterer for Compton imaging. It is made of 35 5x5x2" crystals with specially designed low profile PMTs. The back array is made of 30 2.5x3x24" position-sensitive crystals which are read out at both ends. These crystals are specially treated to provide the required position resolution at the best possible energy resolution. Both arrays of detectors are supported by aluminum superstructures. These have been efficiently designed to allow a wide field of view and to provide adequate support to the crystals to permit use of the TMI as a vehicle-mounted, field-deployable system. Each PMT has a locally mounted high-voltage supply that is remotely controlled. Each detector is connected to a dedicated FPGA which performs automated gain alignment and energy calibration, event timing and diagnostic health checking. Data are streamed, eventby- event, from each of the 65 detector FPGAs to one master FPGA. The master FPGA acts both as a synchronization clock, and as an event sorting unit. Event sorting involves stamping events as singles or as coincidences, based on the approximately instantaneous detector hit pattern. Coincidence determination by the master FPGA provides a pre-sorting for the events that will ultimately be used in the Compton imaging and coded aperture imaging algorithms. All data acquisition electronics have been custom designed for the TMI.

The Raytheon Trimodal Imager (TMI) uses coded aperture and Compton imaging technologies as well as the nonimaging shadow technology to locate an SNM or radiological threat in the presence of background. The coded aperture imaging is useful for locating and identifying radiological threats as these threats generally emit lower energy gammas whereas the Compton imaging is useful for SNM threats as in addition to low energy gammas which can be shielded, SNM threats emit higher energy gammas as well. The shadow imaging technology utilizes the structure of the instrument and its vehicle as shadow masks for the individual detectors which shadow changes as the vehicle moves through the environment. Before a radioactive source comes into the fields of view of the imagers it will appear as a shadow cast on the individual detectors themselves. This gives the operator advanced notice that the instrument is approaching something that is radiological and on which side of the vehicle it is located. The two nuclear images will be fused into a combined nuclear image along with isotope ID. This combined image will be further fused with a real-time image of the locale where the vehicle is passing. A satellite image of the locale will also be made available. This instrument is being developed for the Standoff Radiation Detection System (SORDS) program being conducted by Domestic Nuclear Detection Office (DNDO) of the Department of Homeland Security (DHS).

Fast Neutron Resonance Radiography (NRR) has been devised as an elemental imaging method, with applications such as contraband detection and mineral analysis. In the NRR method, a 2-D elemental mapping of hydrogen, carbon, nitrogen, oxygen and the sum of other elements is obtained from fast neutron radiographic images taken at different neutron energies chosen to cover the resonance cross section features of one or more elements. Images are formed using a lens-coupled plastic scintillator-CCD combination. In preliminary experiments, we have produced NRR images of various simulants using a variable energy neutron beam based on the Li(p,n)Be reaction and a variable energy proton beam. In order to overcome practical limitations to this method, we have studied NRR imaging using the D-D reaction at a fixed incident D energy and scanning through various neutron energies by using the angular variation in neutron energy. The object-detector assembly rotates around the neutron source and different energy (2-6 MeV) neutrons can be obtained at different angles from a D-D neutron source. The radiographic image provides a 2-D mapping of the sum of elemental contents (weighted by the attenuation coefficients). Transmission measurements taken at different neutron energies (angles) form a set of linear equations, which can then be solved to map individual elemental contents.

Coded aperture techniques based on a cyclic difference set uniformly redundant array (URA) can increase sensitivity of an imaging system without degrading the spatial resolution. In this paper, we discuss the pattern design and present its application for diagnostic nuclear medicine imaging with experimental results. Point-like, planar, and three- dimensional 140 keV gamma-ray sources are used in our experiments. We have experimentally demonstrated a three- dimensional coded aperture technique for nuclear medicine imaging and have compared it with conventional collimator systems.

We have developed a neutron detection system for accelerator based neutron radiography and tomography based on a combination of scintillation screen and large aperture optics combined with a cooled CCD camera. The system is capable of detecting single neutron events and can therefore be considered as a possible detector for neutron scattering as well as conventional imaging. The system has a resolution of 0.1 mm or 1242 by 1152 pixels. The limit of image size is set by the light output of the scintillator, the light collection of the optical system, the size of the CCD and the desired signal to noise ratio. The lower limit on neutron flux is determined by the dark current of the chip. Equations for these limits have been derived and can be used to predict and optimize performance. The scintillation light output per incident neutron is large enough to permit the use of lens coupled systems with their increased flexibility and ease of implementation. The system can approach a quantum limited noise level, depending on the particular geometry used. For our current system, based on the use of NE 426 scintillator, 3 a 1242 by 1152 pixel EEV CCD operating at -50C, and using a 100 mm focal length, F/0.9 lens, the maximum size for the imaging screen is 0.5 m, and the lower limit for flux is 1 n/pixel/s based on this size screen and a typical dark current of 10 e/pixel/s. We are now investigating a new type of imaging technology based on large amorphous silicon sensor arrays being developed by Xerox and others. A typical device is 200 by 250 mm with a pixel size of 127 micrometers and the entire array with all electronics is in a 400 mm by 37 mm package. Major advantages of this device are the high light coupling between scintillator screen and the sensor as well as the more compact nature of such an array, since no lens systems is required and, potentially, a much lower cost. Currently, the noise performance is worse than that of CCDs, largely due to the current electronic readout but should be adequate to produce quantum limited images if electronics can be improved.

Low energy neutrons are easily thermalized and are useful in a number of applications including radiography and neutron scattering. A consideration of the early literature and recent experimental results suggests that the reaction 9 Be(d,n) 10 B can provide a useful source of neutrons in the region of zero to a few hundred KeV that fulfill this requirement. The characteristics of the reaction are describe din this context and some of the limitations on intensity are considered. The advantages of a low energy source in terms of thermalization have been investigated using a MOnte Carlo simulation and it is proposed that a high current deuteron accelerator with an energy in the region of 1.1 to 1.5 MeV, used with a beryllium target provides a neutron source that merits serious consideration.

Transportable accelerator based sources are the only means by which large structures such as aircraft or industrial components can be inspected practically using neutron radiography. Since such sources are generally considerably lower in flux than reactors, the entire system must be designed for high imaging efficiency. We describe an imaging system for accelerator based thermal neutron tomography and radiography based on the use of a radiofrequency quadrupole (RFQ) using a Be target. Fast neutrons, produced through the reaction 9 Be 4 (d,n) 10 B 5 , are moderated with typical thermalized neutron fluence rates of 10 3 to 10 4 n/cm 2 /s, depending on the collimation. The camera system consists of a 1242 by 1152 pixel CCD cooled to -50 degrees C viewing a 20 by 20 cm NE-426 scintillator through an F/0.9 100 mm focal length lends. Our experiments show that neutron statistical noise and not the performance of the CCD system is the dominant limitation in the camera. We have used the system for imaging corrosion in lap joints of KC-135 aircraft and have been able to quantify corrosion within the joint.

Neutron imaging has been shown to be an excellent imaging tool for many nondestructive evaluation applications. Significantly improved contrast over x-ray images is possible for materials commonly found in engineering assemblies. The major limitations have been the neutron source and detection. A low cost, position sensitive neutron tomography detector system has been designed and built based on an electro-optical detector system using a LiF- ZnS scintillator screen and a cooled charge coupled device. This detector system can be used for neutron radiography as well as two and three-dimensional neutron tomography. Calculated performance of the system predicted near-quantum efficiency for position sensitive neutron detection. Experimental data was recently taken using this system at McClellan Air Force Base, Air Logistic Center, Sacramento, CA. With increased availability of low cost neutron sources and advanced image processing, neutron tomography will become an increasingly important nondestructive imaging method.