Design for Stand-off Radiation Detector System Using Compton Scattering

1. Design for Stand-off Radiation Detector System Using Compton Scattering D. Stuenkel, Ph.D R. Wood, Ph.D, PE Trinity Engineering Associates, Inc.

2. Abstract A preliminary design for stand-off radiation detection system has been developed to detect and locate gamma-ray sources at a distance. The electronically collimated detector system uses large area inorganic scintillator crystals, that when operated as a Compton camera provides the reasonably high efficiency and a large field of view. The proposed system would use a combination of higher resolution lanthanum bromide crystals and lower resolution crystals, such as thallium-activated sodium iodide in a stacked array along with multi-mode analysis, including Compton imaging, to detect and locate gamma emitting sources. While inorganic scintillators have poorer timing characteristics and poorer energy resolution than solid state detectors, they can be fabricated in large volumes and a fraction of the cost of comparably sized solid state detectors. Additionally, scintillation detectors are rugged and do not require liquid nitrogen or mechanical coolers to operate. The large volumes of these lower energy resolution crystals provide higher efficiencies to detect sources than smaller high resolution detectors, while the incorporation of higher energy resolution detectors provide an enhanced capability to locate sources after their detection. Preliminary modeling of the system indicates that it can detect and locate a 1 millicurie Cs-137 source at a distance of 100 meters.

3. Introduction A preliminary design for stand-off radiation detection system has been developed to detect and locate gamma-ray sources at a distance. The electronically collimated detector system uses large area inorganic scintillator crystals, that when operated as a Compton camera provides the reasonably high efficiency and a large field of view. The proposed system would use a combination of higher resolution lanthanum bromide crystals and lower resolution crystals, such as thallium-activated sodium iodide in a stacked array along with multi-mode analysis, including Compton imaging, to detect and locate gamma emitting sources.

4. System Design Goals The goals of the proposed system include: • Efficient enough to detect a 1 millicurie Cs-137 at 100 meters (150 counts per second) • Energy range of 100-2,800 keV • Energy resolution of ~2% at 662 keV • Field of view > 6 radians • Angular resolution <0.03 radians • Source localization of ±1 meter at 100 meters • Size < 8m3 • Weight <400 kilograms • Cost < $3,000,000

5. Description of Proposed System • The electronically collimated detector system uses large area planar inorganic scintillator crystals, that when operated as a Compton camera provides the reasonably high efficiency and a large field of view. • The proposed system would use a combination of higher resolution lanthanum bromide crystals and lower resolution crystals, such as thallium-activated sodium iodide in a stacked array along with multi-mode analysis, including Compton imaging, to detect and locate gamma emitting sources.

6. Electronically Collimated Systems Advantages over mechanically collimated systems • Greater sensitivity • Larger field of view • Requires little or no shielding – less weight Disadvantages • More complicated signal processing and electronics • Likely greater power requirements

7. Inorganic Scintillators Advantages • Available in large volumes and at a fraction of the cost of comparably sized solid state detectors • Rugged and do not require liquid nitrogen or mechanical coolers to operate Disadvantages • Poorer timing characteristics than solid state detectors • Poorer energy resolution than solid state detectors

8. Selection of Scintillators Because of the expense and availability of these newly discovered scintillators, the proposed system will use a combination of higher resolution lanthanum bromide crystals and lower resolution crystals. • The large volumes of these lower energy resolution crystals provide higher efficiencies to detect sources than smaller high resolution detectors • The smaller volume higher energy resolution detectors provide an enhanced capability to locate sources after their detection.

9. Proposed Detector System

10. Lanthanum Bromide – LaBr3(Ce) Advantages • Very good energy resolution (~3% at 662 keV) • Fast scintillator (good for timing) Disadvantages • Expensive compared to other inorganic scintillators • Not available in large sizes

11. Sodium Iodide – NaI(Tl) Advantages • Inexpensive compared to solid state detectors or other recently discovered inorganic scintillators • Available in large sizes Disadvantages • Poorer energy resolution compared to solid state detectors or other recently discovered inorganic scintillators(~7% at 662 keV) • Slow scintillator

12. Multiple Data Analysis Modes The proposed system works by measuring the amount of light produced when a gamma ray interacts in one or more of the scintillator crystals. The proposed system will operate in a number of data analysis modes, including: • Singles Mode • Single Compton Scattering Mode • Double Compton Scattering Mode • Differences Mode

13. Singles Mode In Singles Mode, the signals from all detectors are summed together to generate a single pulse height spectrum, or alternatively pulse height acquired for individual crystals are added together. The latter method is necessary if different detector materials are used. The singles mode does not provide any information about the location of the source of the gamma rays, but is a measure of the intensity of the photon flux produced by the source. The intensity of the flux, measured by count rate of events that fall with a given region of interest (ROI) may provide some qualitative information of the location of the source, particularly if singles count rate is measured at several different locations.

14. Compton Scattering

15. “Anger Camera” Position Detectors The energy and position photon interactions in each scintillator are determined using an array of photomultiplier tubes, operated like an uncollimated Anger camera. • The energy is determined by summing the light outputs from all tubes. • The X and Y positions are determined by the relative light output of different tubes. • Each NaI(Tl) crystal viewed by a 5 x 5 array of 2.5 inch diameter PMTs • Each LaBr3(Ce) crystal viewed by a 5 x 5 array of 2.5 inch diameter PMTs

16. Single Compton Scattering Mode • The system detects coincident events in two detectors where the energy deposited in the two detectors falls with the region of interest (ROI) about the full energy peak. • Knowing the locations and the Compton scattering and photoelectric absorption, along with the energy of the recoil electron, it is possible to calculate the photon scattering angle. • The direction of the incident photon can not be uniquely determined. Instead, the photon scattering defines a cone where the path of the incident photon lies somewhere on the surface of a cone.

17. Double Compton Scattering Mode • In the Double Compton Scattering Mode, coincident signals from three individual crystals that add up to an energy within the ROI (i.e. 662 keV peak) will be analyzed to determine the direction of the incident photon. • The single and double scattering modes are particularly suitable for photons in the range of several hundred keV to several MeV because Compton scattering is the dominant interaction mechanism over much of the energy of interest for low to medium Z materials.

18. Differences Mode In the Differences Mode, the differences between count rates between pairs of detectors will used to estimate the general direction of a source of radiation. When the field of view of one detector is blocked by another detector, if the intrinsic efficiency of the detectors is sufficiently large enough, the count rate in the detector “upwind” of the radiation field will be significantly greater than that that of the detector “downwind”. The proposed system should be able to identify the general direction of the source of radiation. This mode of analysis may be particularly useful for photon energies at which photoelectric absorption or pair production are the dominant interaction.

19. Preliminary Design Results • Energy resolutions at 662 keV of 7.5% and 3%, respectively, for NaI(Tl) and LaBr3(Ce) • Field of view of the proposed system would be nearly 2p radians (360°) in the plane of travel and greater than p or radians (180°) in the plane perpendicular to the plane of travel • Predicted to detect approximately 56 counts per second from of a 1 mCi Cs-137 at a distance of 100 m in the 662 keV region of interest, including at least 14 coincident counts per second • Volume of approximately of 2 m3 • Weight of approximately 1,000 kg • Cost is approximately $2,700,000

20. Questions or comments?