MOBILE, HYBRID COMPTON/CODED APERTURE IMAGING FOR DETECTION, IDENTIFICATION AND LOCALIZATION OF GAMMA-RAY SOURCES AT STAND-OFF DISTANCES

The Stand-off radiation detection system (SORDS) program is an advanced technology demonstration (ATD) project through the Domestic Nuclear Detection Office (DNDO) with the goal of detection, identification and localization of weak radiological sources in the presence of large dynamic backgrounds. The Raytheon-Tri-Modal Imager (TMI) is a mobile truck-based, hybrid gamma-ray spectroscopic and imaging system able to quickly detect, identify and localize, radiation sources at standoff distances through improved sensitivity provided by multiple detection modes while minimizing the false alarm rate. Reconstruction of gamma-ray sources is performed using a combination of gamma-ray spectroscopy and two imaging modalities; coded aperture and Compton scatter imaging. The TMI consists of 35 NaI crystals (5x5x2 in each), arranged in a random coded aperture CA, followed by 30 position sensitive NaI bars (24x2.5x3 in each) called the DA. The CA array acts as both a coded aperture mask and scattering detector for Compton events. The large-area DA array acts as a collection detector for both Compton scattered events and coded aperture events. In this thesis, the implemented spectroscopic, coded aperture, Compton and hybrid imaging algorithms will be described along with their performance. It will be shown that multiple imaging modalities can be fused to improve detection sensitivity over a broader energy range than any mode alone. Since the TMI is a moving system, peripheral data, such as a GPS and INS must also be incorporated. A method of adapting static imaging algorithms to a moving platform has been developed. Also, algorithms were developed in parallel with detector hardware, through the use of extensive simulations performed with the GEANT4. Simulations have been well validated against measured data. Results of image reconstruction algorithms at various speeds and distances will be presented as well as localization capability. Utilizing imaging information will show signal-to-noise gains over spectroscopic algorithms alone

A prototype Compton imager : simulations, measurements and algorithm development

Compton imaging is a gamma ray imaging technique that has possible applications in the nuclear and medical industries. Compton imaging can localize the origin of a scattered gamma ray, to a cone, using a minimum of two position and energy measurements. The projection of many cones will overlap at a common point leading to the actual source position. A prototype Compton imager (PCI) has been constructed at Los Alamos National Laboratory (LANL) that uses a combination of silicon and CsI detectors. A model of the PCI has been simulated and validated, showing good agreement with measured data. The angular resolution of the PCI was measured to be 0.156 radians FWHM. Additionally, a method for determining the source-to-detector distance in the near field has been developed and demonstrated. The algorithm presented has the ability to determine the source-to-detector distance in the near field within 1 cm of the actual distance. While traditional back-projection algorithms are adequate for imaging of single point-like sources they are not sufficient to resolve extended shapes or closely spaced multiple point-like sources. Iterative algorithms may provide the necessary deconvolution. Maximum Likelihood Expectation Maximization (MLEM) is an iterative statistical algorithm that reconstructs the most probable source distribution for a given data set. Normally, MLEM makes computations for each possible combination of energy and position, however this becomes a prohibitively large problem for both analysis time and hardware memory limits, depending on the data set. List Mode MLEM attempts to circumvent the calculation of every combination of energy and position and relies on the probability of the given data being observed. An algorithm using List-Mode MLEM is of interest because the number of calculations required for reconstruction is substantially less than that of other iterative processes but will enable imaging of both point and extended sources. This type of algorithm has been written and successfully applied to both experimental data from the PCI and simulations of the PCI. The algorithm will be demonstrated to improve detection of both point-like and extended sources

A new low-background Compton telescope using LaBr3 scintillator

Gamma-ray astronomy in the MeV range suffers from weak fluxes from sources and high background in the nuclear energy range. The background comes primarily from neutron-induced gamma rays, with the neutrons being produced by cosmic-ray interactions in the Earth\u27s atmosphere, the spacecraft, and the instrument. Compton telescope designs often suppress this background by requiring coincidences in multiple detectors and a narrow time-of-flight (ToF) acceptance window. The COMPTEL experience on the Compton Gamma Ray Observatory shows that a 1.9-ns ToF resolution is insufficiently narrow to achieve the required low background count rate. Furthermore, neutron interactions in the detectors themselves generate an irreducible background. By employing LaBr3 scintillators for the calorimeter, one can take advantage of the unique speed and resolving power of the material to improve the instrument sensitivity and simultaneously enhance its spectroscopic performance and thus its imaging performance. We present a concept for a balloon- or space-borne Compton telescope that employs deuterated liquid in the scattering detector and LaBr3 as a calorimeter and estimate the improvement in sensitivity over past realizations of Compton telescopes. We show initial laboratory test results from a small prototype, including energy and timing resolution. Finally, we describe our plan to fly this prototype on a test balloon flight to directly validate our background predictions and guide the development of a full-scale instrument

A fast scintillator Compton telescope for medium-energy gamma-ray astronomy

The field of medium-energy gamma-ray astronomy urgently needs a new mission to build on the success of the COMPTEL instrument on the Compton Gamma Ray Observatory. This mission must achieve sensitivity significantly greater than that of COMPTEL in order to advance the science of relativistic particle accelerators, nuclear astrophysics, and diffuse backgrounds, and bridge the gap between current and future hard X-ray missions and the high-energy Fermi mission. Such an increase in sensitivity can only come about via a dramatic decrease in the instrumental background. We are currently developing a concept for a low-background Compton telescope that employs modern scintillator technology to achieve this increase in sensitivity. Specifically, by employing LaBr3 scintillators for the calorimeter, one can take advantage of the unique speed and resolving power of this material to improve the instrument sensitivity while simultaneously enhancing its spectroscopic and imaging performance. Also, using deuterated organic scintillator in the scattering detector will reduce internal background from neutron capture. We present calibration results from a laboratory prototype of such an instrument, including time-of-flight, energy, and angular resolution, and compare them to simulation results using a detailed Monte Carlo model. We also describe the balloon payload we have built for a test flight of the instrument in the fall of 2010

Mobile, hybrid Compton/coded aperture imaging for detection, identification and localization of gamma-ray sources at stand-off distances

The Stand-off Radiation Detection System (SORDS) program is an Advanced Technology Demonstration (ATD) project through the Department of Homeland Security’s Domestic Nuclear Detection Office (DNDO) with the goal of detection, identification and localization of weak radiological sources in the presence of large dynamic backgrounds. The Raytheon-SORDS Tri-Modal Imager (TMI) is a mobile truckbased, hybrid gamma-ray spectroscopic and imaging system able to quickly detect, identify and localize, radiation sources at standoff distances through improved sensitivity provided by multiple detection modes while minimizing the false alarm rate. Reconstruction of gamma-ray sources is performed using a combination of gamma-ray spectroscopy and two imaging modalities; coded aperture and Compton scatter imaging. The TMI consists of 35 sodium iodide (NaI) crystals (5x5x2 in3 each), arranged in a random coded aperture mask array (CA), followed by 30 position sensitive NaI iv bars (24x2.5x3 in3 each) called the detection array (DA). The CA array acts as both a coded aperture mask and scattering detector for Compton events. The large-area DA array acts as a collection detector for both Compton scattered events and coded aperture events. In this thesis, the implemented spectroscopic, coded aperture, Compton and hybrid imaging algorithms will be described along with their performance. It will be shown that multiple imaging modalities can be fused to improve detection sensitivity over a broader energy range than any mode alone. Since the TMI is a moving system, peripheral data, such as a Global Positioning System (GPS) and Inertial Navigation System (INS) must also be incorporated. A method of adapting static imaging algorithms to a moving platform has been developed. Also, algorithms were developed in parallel with detector hardware, through the use of extensive simulations performed with the Geometry and Tracking Toolkit v4 (GEANT4). Simulations have been well validated against measured data. Results of image reconstruction algorithms at various speeds and distances will be presented as well as localization capability. Utilizing imaging information will show signal-to-noise gains over spectroscopic algorithms alone

A prototype Compton imager : simulations, measurements and algorithm development

Compton imaging is a gamma ray imaging technique that has possible applications in the nuclear and medical industries. Compton imaging can localize the origin of a scattered gamma ray, to a cone, using a minimum of two position and energy measurements. The projection of many cones will overlap at a common point leading to the actual source position. A prototype Compton imager (PCI) has been constructed at Los Alamos National Laboratory (LANL) that uses a combination of silicon and CsI detectors. A model of the PCI has been simulated and validated, showing good agreement with measured data. The angular resolution of the PCI was measured to be 0.156 radians FWHM. Additionally, a method for determining the source-to-detector distance in the near field has been developed and demonstrated. The algorithm presented has the ability to determine the source-to-detector distance in the near field within 1 cm of the actual distance. While traditional back-projection algorithms are adequate for imaging of single point-like sources they are not sufficient to resolve extended shapes or closely spaced multiple point-like sources. Iterative algorithms may provide the necessary deconvolution. Maximum Likelihood Expectation Maximization (MLEM) is an iterative statistical algorithm that reconstructs the most probable source distribution for a given data set. Normally, MLEM makes computations for each possible combination of energy and position, however this becomes a prohibitively large problem for both analysis time and hardware memory limits, depending on the data set. List Mode MLEM attempts to circumvent the calculation of every combination of energy and position and relies on the probability of the given data being observed. An algorithm using List-Mode MLEM is of interest because the number of calculations required for reconstruction is substantially less than that of other iterative processes but will enable imaging of both point and extended sources. This type of algorithm has been written and successfully applied to both experimental data from the PCI and simulations of the PCI. The algorithm will be demonstrated to improve detection of both point-like and extended sources.Department of EnergyNuclear EngineeringMastersUniversity of New Mexico. Dept. of Chemical and Nuclear EngineeringCooper, GaryHecht, AdamSullivan, Joh

Extended radiation source imaging with a prototype Compton imager

This paper reports results from a prototype Compton imager (PCI) which consists of three planes of silicon pixel detectors as a scattering detector followed by an array of CsI(Tl) crystals as an absorbing detector. The CsI(Tl) array is mounted directly behind the silicon detectors. Simple back-projection algorithms are not sufficient to resolve extended shapes, but iterative algorithms provide the necessary de-convolution. List-mode maximum likelihood expectation maximization (LM-MLEM) is an iterative algorithm that reconstructs the most probable source distribution for a given data set. LM-MLEM attempts to reconstruct an image by finding successive approximations to the true source data. Each data set imaged is different, but the number of iterations required for convergence is typically 10-30 for the PCI. In this paper, reconstructed images of point and extended sources using measured PCI data are presented. Data are corroborated using GEANT4 simulations

A simple algorithm for estimation of source-to-detector distance in Compton imaging

Compton imaging is used to predict the location of gamma-emitting radiation sources. The X and Y coordinates of the source can be obtained using a back-projected image and a two-dimensional peak-finding algorithm. The emphasis of this work is to estimate the source-to-detector distance (Z). The algorithm presented uses the solid angle subtended by the reconstructed image at various source-to-detector distances. This algorithm was validated using both measured data from the prototype Compton imager (PCI) constructed at the Los Alamos National Laboratory and simulated data of the same imager. Results show this method can be applied successfully to estimate Z, and it provides a way of determining Z without prior knowledge of the source location. This method is faster than the methods that employ maximum likelihood method because it is based on simple back projections of Compton scatter data