Michael V. Hynes

The Raytheon-SORDS trimodal imager

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).

The SORDS trimodal imager detector arrays

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.

Cherenkov counters for the detection of gamma rays from active interrogation

Water-based Cherenkov counters are considered for remote detection of special nuclear materials with aid of active interrogation. We designed and manufactured a detector capable of gamma ray detection and demonstrated particle energy discrimination ability. Background reduction techniques based on energy threshold and photomultiplier tube multiplicity were implemented in particle detection. Both approaches resulted in a significant suppression of low-level background.

Design and Fabrication of Cherenkov Counters for the Detection of SNM

The need for large‐size detectors for long‐range active interrogation (AI) detection of SNM has generated interest in water‐based detector technologies. Water Cherenkov Detectors (WCD) were selected for this research because of their transportability, scalability, and an inherent energy threshold. The detector design and analysis was completed using the Geant4 toolkit. It was demonstrated both computationally and experimentally that it is possible to use WCD to detect and characterize gamma rays. Absolute efficiency of the detector (with no energy cuts applied) was determined to be around 30% for a 60Co source.

Noble Gas Excimer Detectors for Security and Safeguards Applications

Noble gas excimer detectors are a technology that is common in particle physics research and less common in applications for security and international safeguards. These detectors offer the capability to detect gammas with an energy resolution similar to NaI and to detect neutrons with good energy resolution as well. Depending on the noble gas selected and whether or not it is in a gaseous or liquid state, the sensitivity to gammas and neutrons can be tuned according to the needs of the application. All of this flexibility can be available at a significant cost saving over alternative technologies. This paper will review this detector technology and its applicability to security and safeguards.

Optimizing Inspection Parameters for Long Stand‐Off Detection of SNM

Detection of special nuclear material (SNM) at extended ranges (>100 m) through the utilization of high energy (>20 MeV) bremsstrahlung photons requires optimizing the structure and interrelation of irradiation (beam-on) and detection (counting) periods. Conventional inspection schemes at lower energies and smaller distances primarily operate by pulsing an accelerator at frequencies of 0.1-1 kHz while collecting emitted radiation from the target under inspection for the few milliseconds in between pulses. Simulation and experimental results for long stand-off scenarios with source photons >20 MeV, however, indicate that two primary phenomena--(1) induced photoneutrons in proximity to the accelerator and (2) beam induced activation of air and soil--preclude the use of conventional inspection schemes. By considering the time structure and magnitude of the beam-induced photon and neutron backgrounds, signals of interest from the target, and natural backgrounds, inspection schemes have been developed to maximize signal to noise ratios (SNR). Analysis of the data indicates that the highest SNR values are found with short (2-5 s) irradiations followed by a 1-2 s period of collecting emitted neutron and photon signatures.