2012 IEEE Nuclear Science Symposiwn and Medical Imaging Conference Record (NSS/MIC) NS-7 Real-Time Radioactive Source Localization with a Moving Coded-Aperture Detector System at Low Count Rates Sam Huh, Jonathan Maltz, Donald Gunter, Lucian Mihailescu, and Kai Vetter Abstract-We present a feasibility study of real-time radioactive source localization in which the efects of low count rates on source localizations with a moving coded-aperture detector system are addressed. The conventional cross correlation method with the installed binary mask was not reliable enough to flter out background noise at low count rates in this study. To improve the cross-correlation performance, we adopted a new binary mask design method for future work. Of line data processing to mimic on-line data processing was based on multi-CPUs and multi-GPUs (graphics processing units) parallel processing. We also show an iterative list-mode localization method using background-free simulated data. I. INTRODUCTION D etection and localization of radioactive target sources have been of great interest. Monitoring of radioactive sources using mobile detector systems suffers increased uncertainty due to background radiation; even non-moving monitoring systems are vulnerable to background radiation. Therefore understanding of background radiation is one of the keys to the detection of radioactive sources in mobile detector systems. As one of efforts for mobile detector systems, the Naval Research Laboratory launched a mobile imaging and spectroscopic threat identifcation (MIST!) project [1][2][3]. The MIST! system is a hybrid detector system mounted on a truck. The hybrid detector system consists of HPGe detectors and a NaI(TI)-based coded aperture imaging system. The HPGe detctors are used to detect the presence of radioactive sources and the NaI(TI)-based coded aperture imaging system is used to localize detected radioactive sources. The MIST! Manuscript received November 16, 2012. This work was supported in part by the U.S. Department of Homeland Security under Grant Award No. 2008- DN-077-ARI-001-02. Sam. S. huh is a physicist postdoctoral fellow in the Applied Nuclear Science Group at the Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: sshuh@lbl.gov). Jonathan Maltz is an afliate scientist in the Applied Nuclear Science Group at the Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: jsmaltz@lbl.gov). Donald Gunter is an afliate scientist in the Applied Nuclear Science Group at the Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: DLGunter@lbl.gov). Lucian Mihailescu is a staff scientist in the Applied Nuclear Science Group at the Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e mail: LMihailescu@lbl.gov). Kai Vetter is an associate professor of the Department of Nuclear Engineering at the University of California, Berkeley and a staf scientist in the Applied Nuclear Science Group at the Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: kvetter@lbl.gov). system was designed to detect and localize a lmCi Cs-137 source at 100 meters in 20 seconds. Figure I. Illustration of the MISTI truck. This picture was adapted from [ I]. . .. . . . .. . :: • • : •• • • • I.; ii •• : : . . . . . ..... . . . . ; . .. . . ... . . : . : .. : . . • •• •• •• • 1 ••• " .. . . . . � . . .. . . : :: . : � f �." ... � . � (a) passenger side view (b) driver side view Figure 2. Illustration of a binary mask and the NaI(T1) array. These pictures were adapted fom [I]. As shown in Fig.2 (b), the bottoms of the binary mask and the NaI(Tl) detector aray are aligned so that the feld of view (FOY) in the vertical direction is limited to above the horizontal line. Therefore if the incoming gamma rays come fom below the horizontal line, part of the shadow by the binary mask may not be recorded in the lO-by-lO array of NaI(TI) detectors. Coded aperture imaging is based on the shadows made by the mask and incoming x-rays or gamma-rays. However if the count rates are low, comparable to the nwnber of openings in the binary mask, it is statistically diffcult to estimate the origins of the radio-active sources [4]. U.S. Goverment work not protected by U.S. copyright 666