Tech Briefs

A stand-off detection system for maritime environments enables remote detection of nuclear materials.

The SuperMISTI detection system is a hybrid detection, identification, and imaging system for sources of gamma-ray radiation at stand-off distances. The system is based on the Mobile Imaging and Spectroscopic Threat Identification (MISTI) system designed for the Department of Homeland Security. The SuperMISTI system uses the high-resolution spectra of high-purity germanium (HPGe) detectors to detect and identify gamma-ray sources as well as coded aperture technology, and lower-cost NaI detectors to image and localize the detected sources. The system utilizes a modular design to allow the detection/identification and the imaging/ localization portions to be used separately or together, depending on the situation.

Figure 1. The SuperMISTI detection/identification subsystem pallet-mounted HPGe array.
Each subsystem is housed in a separate 20-foot refrigerated ISO container that provides humidity and temperature control, and each subsystem can run independently as well as together. Power for each subsystem is provided by an individual clip-on diesel generator capable of providing approximately 5 days of continuous operation. Each subsystem is equipped with a pair of global positioning system (GPS) receivers to determine location, speed, and orientation. External optical cameras provide digital photos of the area to be investigated/imaged. Electronics including iseg high-voltage supplies, mesytec shaping amplifiers, peak-sensing VME digitizers, and onboard servers are mounted in standard electronics racks that are shock-mounted inside each ISO container. External Ethernet ports installed on the containers allow the system to be controlled externally via laptop or remotely via wireless communication.

The SuperMISTI detection/identification subsystem utilizes a pallet-mounted array of 24 HPGe detectors individually shielded from above, below, and the sides by 0.5" lead plates. Each dewar has a cryogenic solenoid valve and a temperature sensor on the exhaust to monitor when the dewar is full. The valves and sensors are plate-mounted atop the palletized array. Separate sensors monitor the pressure on the input lines. The SuperMISTI imaging/localization subsystem consists of 78 NaI detectors mounted into an array on one side of the ISO container. The entire array is shielded from above, below, and the sides by 1" of lead; in addition, a 1" lead flooring provides further shielding from background radiation originating from below, and an additional layer of 1" lead on the back wall provides shielding from the rear.

When operated as a single detection system, the two subsystems are deployed together and connected via Ethernet. The system is started via a simple run GUI (graphical user interface) that records a unique run number, start and stop times, and a user-input run description. Real-time system monitoring is accomplished via a Web GUI that displays an overhead map/satellite image of the area with the truck location and, if applicable, detected source location overlaid.

Figure 2. The complete SuperMISTI system on a 60-ft barge. The water ballast tanks used to maintain a level barge are shown on the right.
Source detection and identification are performed with the summed spectra from the HPGe detectors. Summed spectra are produced for integration times ranging from 1 to 120 s. Once a source has been detected and identified, this information is used to determine a ROI in the NaI spectra; the width of this ROI is based on an assumed 8% resolution at 662 keV. The counts in this ROI are used to produce a coded image of the source. A single coded image merely indicates a source direction; however, multiple images taken while in motion allow the system to determine the three-dimensional (3D) location of the source.

In a demonstration, both Super-MISTI subsystems were deployed on a 60-foot barge that was pushed by a tug at speeds of up to 6 kn and at stand-off distances of up to 300 m. The results of this exercise were very close to the expected performance and clearly demonstrate the utility of the SuperMISTI system. All sources that were deployed in the demonstration were successfully detected, identified, and localized at operationally relevant distances ranging up to hundreds of meters.

Since the completion of the demonstration, the number of HPGe detectors in the detection/identification subsystem has been increased from 24 to 48, a modification that significantly enhances the gamma detection/identification capabilities of the system. Further performance enhancements planned for the future include the use of a large-area BF3 detector array to increase neutron detection capabilities and the implementation of better localization algorithms for neutron sources.

This work was done by Anthony L. Hutcheson, Bernard F. Phlips, Eric A. Wulf, Lee J. Mitchell, and W. Neil Johnson of the Naval Research Laboratory; and Byron E. Leas of SRA International. NRL-0062