Large area (> m2) position-sensitive readout of scintillators is important for passive/active gamma and neutron imaging for counter-terrorism applications. The goal of the LAMPA project is to provide a novel, affordable, large-area photodetector (8” x 8”) by replacing the conventional dynodes of photomultiplier tubes (PMTs) with electron multiplier microstructure boards (MSBs) that can be produced using industrial manufacturing techniques. The square, planar format of the LAMPA assemblies enables tiling of multiple units to support large area applications. The LAMPA performance objectives include comparable gain, noise, timing, and energy resolution relative to conventional PMTs, as well as spatial resolution in the few mm range. The current LAMPA prototype is a stack of 8” x 8” MSBs made commercially by chemical etching of a molybdenum substrate and coated with hydrogen-terminated boron-doped diamond for high secondary emission yield (SEY). The layers of MSBs are electrically isolated using ceramic standoffs. Field-shaping grids are located between adjacent boards to achieve good transmission of electrons from one board to the next. The spacing between layers and the design of the microstructure pattern and grids were guided by simulations performed using an electro-optics code. A position sensitive anode board at the back of the stack of MSBs provides 2-D readout. This presentation discusses the trade studies performed in the design of the MSBs, the measurements of SEY from various electro-emissive materials, the electro-optics simulations conducted, the design of the 2-D readout, and the mechanical aspects of the LAMPA design, in order to achieve a gain of > 104 in an 8-stage stack of MSBs, suitable for use with various scintillators when coupled to an appropriate photocathode.
Daniel Wakeford, H. R. Andrews, E. T. H. Clifford, Liqian Li, Nick Bray, Darren Locklin, Michael Hynes, Maurice Toolin, Bernard Harris, John McElroy, Mark Wallace, Richard Lanza
KEYWORDS: Sensors, Simulation of CCA and DLA aggregates, Crystals, Detector arrays, Field programmable gate arrays, Electronics, Calibration, Coded apertures, Coded aperture imaging, Imaging systems
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.
Michael Hynes, Maurice Toolin, Bernard Harris, John McElroy, Mark Wallace, Larry Schultz, Mark Galassi, Andrew Hoover, Michal Mocko, David Palmer, Shawn Tornga, Daniel Wakeford, H. Andrews, E. Clifford, Liqian Li, Nick Bray, Darren Locklin, Richard Lanza, Berthold Horn, David Wehe
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).
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