2308 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 3, JUNE 2013 Experimental Benchmark of Electron Trajectory Reconstruction Algorithm for Advanced Compton Imaging Brian Plimley, Member, IEEE, Daniel Chivers, Amy Coffer, Member, IEEE, and Kai Vetter Abstract—Electron-tracking-based Compton imaging of gamma rays reduces the background level of the backprojected Compton image through the additional measurement of the initial mo- mentum vector of the Compton electron. This reduction in image background has the potential for the detection of weaker sources in a complex background radiation field. Electron-tracking-based Compton imaging was demonstrated recently in solid-state detec- tors through the use of scientific Si charge-coupled devices (CCDs) with excellent position and energy resolution characteristics. In addition, the sensitivity of the electron track reconstruction algorithm has been evaluated extensively on the modeled detector response to Monte-Carlo electron tracks. We have now bench- marked the modeled algorithm sensitivity with our experimentally observed algorithm sensitivity, by measuring CCD electron tracks from a collimated 662 keV gamma-ray source in coincidence with a position-sensitive HPGe detector. For all coincident events the electron momentum vector deduced by the reconstruction algorithm is compared to the electron momentum vector calcu- lated from the measured positions. This measured distribution of angular error of the algorithm agrees well with the angular error distribution calculated from our electron transport and detector models. Index Terms—Charge coupled devices, Compton imaging, gamma-ray cameras, gamma-ray detectors, radiation imaging, semiconductor radiation detectors, silicon radiation detectors, solid state tracking detectors, tracking detectors. I. INTRODUCTION F OR several decades, Compton gamma-ray imaging has been used to localize a radioactive source using interac- tion positions and energies of a Compton-scattered photon [1], [2]. Compton imaging has been used in several fields including astrophysics [3]–[5], homeland security [6], [7], and medical imaging [8], [9]. In recent years semiconductor detectors with excellent position and energy characteristics, including Ge and Si double-sided strip detectors and CdZnTe pixelated detectors, have been utilized for Compton imaging. In general, Compton imaging has the potential to improve detection sensitivity by Manuscript received September 19, 2012; revised January 21, 2013; accepted March 06, 2013. Date of publication May 16, 2013; date of current version June 12, 2013. This work was supported by the U.S. Department of Homeland Secu- rity under Contract #ECCS-1140069. B. Plimley and A. Coffer are with the University of California, Berkeley, CA 94720 USA (e-mail: as.white.as.snow@gmail.com; amycoffer@gmail.com). D. Chivers is with Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: dhchivers@lbl.gov). K. Vetter is with the University of California, Berkeley, CA 94720 USA, and also with Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: kvetter@berkeley.edu). Digital Object Identifier 10.1109/TNS.2013.2254498 Fig. 1. Schematic comparison of conventional Compton imaging (a) and elec- tron-tracking-based Compton imaging (b) for a single event. separating source events from background events; semicon- ductor detectors can provide improved energy and angular resolution over scintillator or gas-based systems and thus higher background rejection and detection sensitivity [4], [10], [11]. One important limiting factor in the detection sensitivity of Compton imaging is the axial symmetry between the first two interactions, which normally limits the knowledge of the source location of a single photon to a cone in the image space, which in turn limits the background rejection and detection sensitivity. The axial symmetry of the measurement can be broken by knowledge of the initial momentum vector of the Compton elec- tron scattered from the first photon interaction. A measurement of this electron direction vector constrains the incident photon direction to a segment of the Compton cone, depending on the uncertainty in the electron momentum vector. The essential im- pact of this measurement for a single photon measurement is illustrated in Fig. 1. In addition, the measurement of the ini- tial electron direction might be used to aid the determination of the gamma interaction sequence in the detection system, by re- jecting certain sequences as unphysical. This electron-tracking Compton imaging has been previously demonstrated in gas-based time projection chambers [5], al- though such devices have low efficiency (0.26% first-interac- tion efficiency at 662 keV) due to the low gas density despite volume, and energy resolution (43% at 31 keV) inferior to semiconductor detectors. The inferior efficiency and resolu- tion result in very small imaging efficiencies and limited angular resolution. Electron-tracking Compton imaging has also been 0018-9499/$31.00 © 2013 IEEE