1066 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 2, APRIL 2013

A Prototype Compton Camera Array for Localizationand Identification of Remote Radiation Sources

Y. Kong, H. Brands, T. Glaser, Claus-M. Herbach, L. Hoy, M. Kreuels, M. Küster, G. Pausch, J. Petzoldt,C. Plettner, J. Preston, K. Roemer, F. Scherwinski, N. Teofilov, J. Verity, A. Wolf, R. Lentering, and J. Stein

Abstract—A functional prototype two-plane Compton cameraarray for localization and identification of remote radiationsources, consisting of four PVT and four NaI(Tl) scintillationdetectors with PMT readout, is presented. The large-volume,

mm scintillators provide a broad field of view forscattered photons and facilitate maximum efficiency at moderatecost. Each detector is equipped with a voxelSPEC, a compactelectronic module that provides high voltage for the PMT, signalprocessing, detector stabilization, and an Ethernet communica-tion interface. The voxelSPEC delivers list-mode event data withnanosecond precision timing over non-proprietary Ethernet andmakes a system extension very easy. A software package has beendeveloped for real-time data processing and image reconstruction.Advantages in the hard- and software allow stable, unattendedoperation of the camera array for many days, and provideeasy-to-read information on the radiation source in real time.Measurements with the prototype array have been performedfor a few standard scenarios and geometries to verify the modelpredications obtained by Monte-Carlo simulations. Simulationshave been further performed to explore larger camera arrayswith , , , and

detectors.

Index Terms—Compton camera, nuclide identification, radia-tion detection, source imaging.

I. INTRODUCTION

E FFICIENT and accurate -ray imaging remains anobjective in many applications, particularly including

Homeland Security. Generally, the sensitivity of such animaging system for remote radioactive sources is limited byactive detector size and radiation background. The Comptoncamera technique, which combines background reduction with

Manuscript received June 01, 2012; revised August 20, 2012; acceptedSeptember 28, 2012. Date of publication December 11, 2012; date of currentversion April 10, 2013. This work was supported by the U.S. Defense ThreatReduction Agency (DTRA) under contract HDTRA1-11-C-0002. Approvedfor public release, and distribution is unlimited.Y. Kong, C.-M. Herbach, M. Kreuels, M. Küster, G. Pausch, J. Pet-

zoldt, C. Plettner, K. Roemer, F. Scherwinski, N. Teofilov, A. Wolf, and R.Lentering are with FLIR Radiation GmbH, 42653 Solingen, Germany (e-mail:yong.kong@flir.com; claus.herbach@flir.com; martin.kreuels@flir.com;mathias.kuester@flir.com; guntram.pausch@flir.com; joe.petzoldt@gmx.de;cristina.plettner@flir.com; katja.roemer@flir.com; nikolai.teofilov@flir.com;falko.scherwinski@flir.com; andreas.wolf@flir.com; ralf.lentering@flir.com).H. Brands, T. Glaser, L. Hoy, J. Preston, and J. Verity are with FLIR Radiation

Inc., Oak Ridge, TN 37830 USA (e-mail: hartmut.brands@flir.com; thomas.glaser@flir.com; les.hoy@flir.com; jeff.preston@flir.com; jeffrey.verity@flir.com).J. Stein is with FLIR Radiation GmbH, 42653 Solingen, Germany,

and also with FLIR Radiation Inc., Oak Ridge, TN 37830 USA (e-mail:juergen.stein@flir.com).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TNS.2012.2222665

Fig. 1. Principle scheme of a two-plane Compton camera [4].

the ability of source localization and spectroscopy, has beenconsidered a promising technology in various fields like nuclearmedicine [1], astrophysics [2] and industry [3].The Compton camera usually consists of a scatter detector

where Compton scattering occurs and an absorption detectorwhere the scattered photon is absorbed. Position and energymeasurements in both detectors are used to reconstruct thegamma energy spectrum and the location of the source. Atwo-plane planar Compton camera concept for location andnuclide identification of remote radiation sources has beenproposed in a previous study [4]. As shown in Fig. 1, twoscintillation detector arrays constitute the scatter and the ab-sorption planes. A photon emitted from a remote source hitsthe scatter plane with the initial energy under the originalpath . The incident photon is scattered in the scatterplane by the angle , resulting in an energy deposit of .The scattered photon with the energy isassumed to be completely absorbed within the absorption layerby subsequent interactions. The original path of theincident photon is covered by the mantle of the cone as definedby the axis and the angle , which can beworked out by using the measured energies and , the co-ordinates of the detector positions, and the distance betweenthe two planes. The intersections of different cones provide theinformation on the location of radiation sources. For a realisticsetup, reconstructed image locations are broadened due to realdetector size, finite energy resolution and Doppler broadening.By means of Monte-Carlo simulations the construction of theCompton camera has been optimized to meet the requirementsfor Homeland Security applications.

0018-9499/$31.00 © 2012 IEEE

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TABLE IANGULAR RESOLUTIONS (FWHM, IN DEGREE) FOR DIFFERENT

INCIDENT GAMMA ENERGIES, ESTIMATED FOR PLASTIC-NaI(Tl) ANDCaF Eu NaI Tl SYSTEMS WITH DIFFERENT DETECTOR GRANULARITIES.THE AREAS OF SCATTER AND ABSORPTION PLANES ARE FIXED TO 1 m

In this paper we introduce a large-size, scalable Comptoncamera design based on this concept, and present a functionalprototype camera array consisting of four scatter and four ab-sorber detectors, denoted as the system. Experimentswith the prototype camera allowed us to develop and to test soft-ware algorithms, and to verify the model predictions of instru-mental performance, which should be further applied to explorelarger arrays.

II. SCALABLE TWO-PLANE COMPTON CAMERA SYSTEM

Advanced by a new generation of nuclear electronics calledthe voxelSPEC, a large-size scalable Compton camera designutilizing commercially available scintillation detectors withphotomultiplier (PMT) readout has been developed by FLIRRadiation.According to detailed investigations on the system perfor-

mance of the camera system under design, we decided to useplastic (PVT) or CaF (Eu) scintillator for the scatter plane, andNaI(Tl) detector for the absorption plane. For a low-thresholddetector system, the camera angular resolution is basicallydetermined by the limited energy resolution of the scatterdetectors, which may lead to uncertain scattering angles thatare determined from energy depositions in the scatter and theabsorber planes. Moreover, the influence of detector granularityon the achievable angular resolution has been investigated.For a fixed area of scatter and absorption planes of 1 m , theangular resolutions for plastic-NaI(Tl) and CaF (Eu)-NaI(Tl)systems with different granularities were estimated for differentincident gamma energies (Table I). For gamma energies lowerthan 500 keV, there is only little gain in angular resolution ifthe detector size (edge length of the detector face) is smallerthan 5 cm. Finally, a detector size of 7.6 cm (3 inch) has beenchosen for our purpose, considering a tradeoff between costand performance. The large-volume detectors provide a broadfield of view for scattered photons and facilitate maximumefficiency at moderate cost.Each scintillation detector is coupled with a compact voxel-

SPEC electronics module, distinguished by a very small formfactor, which is directly plugged onto the PMT. A voxelSPECmounted on a 76 mm 76 mm plastic detector is shown inFig. 2. The voxelSPEC comprises a high-voltage supply, an

Fig. 2. voxelSPEC mounted on a 76 mm 76 mm NaI(Tl) detector.

TABLE IIHARDWARE SPECIFICATIONS OF THE VOXELSPEC

Fig. 3. Sample Cs-137 energy spectrum from a mm mm LaBr Cedetector collected by the voxelSPEC with energy range 0–3 MeV.

analog frontend and digitization stage, a digital signal processorbased on an FPGA, a LED pulser and corresponding softwarefor stabilization and temperature compensation [5], [6] of thePMT, and an Ethernet interface for communication and datatransmission. Table II lists basic hardware specifications of thevoxelSPEC, and Fig. 3 shows a sample Cs-137 energy spectrummeasured with a mm mm LaBr Ce detector cou-pled to a voxelSPEC with dynamic range 1:1000. With novelsystem-on-chip technology the voxelSPEC integrates all thecomponents into a compact electronic module, and is able todeliver list-mode event data with nanosecond precision timingover non-proprietary Ethernet [7]. Multiple time-synchronizedvoxelSPEC units can serve as an enabling technology for manycoincident-pulse and time-of-flight based radiation detectionsystems.The general block scheme of a Compton camera based on the

voxelSPEC technology is shown in Fig. 4. The list-mode datastreams delivered from individual voxelSPECs have to be com-bined and analyzed, and finally translated into pictures and mes-sages which can be interpreted by an operator. For such a pur-

1068 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 2, APRIL 2013

Fig. 4. Block scheme of the scalable Compton camera systems based on thevoxelSPEC technology. Scatter (S) and absorber (A) detectors are coupled tovoxelSPEC modules (vS), which deliver time-stamped list-mode data to a PChosting Event Builder software. Ethernet switches featuring Power over Eth-ernet (PoE) and PTPv2 cross-link and synchronize the voxelSPECs (red ar-rows). Ethernet connects the switches with the PCs analyzing the data streams(black arrows).

pose a software package has been developed providing real-timelist-mode data processing and image reconstruction. The timestamps of raw list-mode data are firstly considered to identifycoincident detector hits, which are further analyzed to select“valid” event data for image reconstruction. In the case of cur-rent camera construction the “valid” events are distinguished bya single hit in the scatter detectors in coincidence with a singlehit in the absorber detectors, and by energy depositions andwhich are compatible with Compton scattering in the scatter

plane.The selected data stream is then used for monitoring,

alarming, imaging, identification, and localization. Dependingon additional filters implemented in the software, the incomingcoincident events are evaluated for real-time image recon-struction by Compton back-projection, and by an advancedfast-imaging algorithm that is capable of providing a reasonableestimate of the source position.As shown in Fig. 4, the use of the voxelSPEC technology re-

places the signal and HV cables of usual systems and makes asystem extension very easy. The camera system can be extendedby simply connecting additional detector-voxelSPEC modulesto free channels in the Ethernet switch, and editing the list of de-clared detectors in the system software. No adjustment of timingconditions or calibration factors is necessary.

III. THE PROTOTYPE SYSTEM

The prototype camera array used for performancetests andmodel verifications is shown in Fig. 5. The camera con-sists of four mm plastic (PVT) detectors formingthe scatter plane, and four mm NaI(Tl) detectorsforming the absorption plane. Both the PVT and the NaI(Tl) de-tectors have squared cross-sections and are equipped with pro-totypes of the voxelSPEC electronics. All detectors are mountedin a mechanical frame designed for a larger array, sothat the scatter-absorber inter-planar distance as well as the de-tector positions in the planes could be easily rearranged for avariety of test geometries.

Fig. 5. The functional prototype Compton camera system. Four plasticscatter detectors and four NaI(Tl) absorber detectors equipped with voxelSPECprototypes (housed in the black metal tubes coupled with the PMT) are mountedin a mechanical frame. Overall view (top) and view of the voxelSPEC and thedetector planes (bottom).

A. Experimental Results

The prototype camera array has been used for many measure-ments altogether representing weeks of real data acquisition.The detector hard- and software allow stable, unattended opera-tion of the camera array for many days, and provide easy-to-readinformation on the radiation source in real time.Benchmark measurements have demonstrated a system per-

formance very close to the predictions based on detector charac-terization andmodeling. Fig. 6 shows a screen shot of the systemsoftware showing exemplary results for a 10 Ci Cs-137 source,positioned in 50 cm distance from the scatter plane at el-evation and azimuth with respect to the detector axis, inreal time. In fact, a Cs-137 alarm showed up. Both the back-pro-jection (right) and the fast-localization (middle) image indicatethe source position correctly, but the advantage of the fast-lo-calization algorithm is evident.

B. Comparison With Model Predictions

Measurements with the prototype camera arrayfor common radiation sources have been performed to verifythe model predictions made by Monte-Carlo simulations. Twocomplementary simulation frameworks, the SiST package de-veloped by Herbach et al. [4], and Geant4, a standard simulationtool developed at CERN [8], have been applied for modeling the

prototype array. Geant4 is generally appropriate formodeling of detailed geometries. All the construction elementscould be considered in their real geometries, provided that theCAD-based data are available. Nevertheless, the most complexcomponents like electronics boards or PMT dynodes were mod-eled as volumes homogeneously filled with an arbitrary mediumreflecting the average elemental composition (pseudo medium)

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Fig. 6. Screen shot of the prototype camera software showing liveplots representing an estimate of the source position (middle: fast localizationimage; right: back-projection image) and the corresponding sum energy spec-trum with the Cs-137 ROI highlighted (left). Data were obtained with a Cs-137source located top right in front of the prototype system. The image axes are

and in arbitrary units, where and denote polarand azimuth angle, respectively.

Fig. 7. Drawing of the prototype system model as implemented inGeant4. The voxelSPEC is modeled separately and not shown here.

and the total mass (pseudo density). This simplification signifi-cantly reduces the expense of model construction as well as thecomputing time. Note that the simulations discussed here nei-ther consider the real radiation source construction (encapsula-tion), nor the mechanical frame and other supporting materialsas shown in Fig. 5. Fig. 7 shows the Geant4 model of theprototype camera array.Simulations were performed for a few standard scenarios

and geometries which have been realized in measurements.The comparison with measured data has been focused onabsolute count rates reflecting detection efficiencies, and se-lected differential data, such as energy spectra and scatteringangle distributions, which are sensitive to energy resolutionand detection thresholds. These parameters are critical for theinstrumental performance of the prototype camera array.As examples, the sum energy spectra simulated for Co-57

and Cs-137 sources placed in the front center of the prototypesystem are shown in Fig. 8, together with the measured spectrafor the same source geometry. The comparison demonstratesgood agreement between the simulated spectra and the back-ground-corrected measurements. Absolute “valid” total andfull-peak event rates have been derived from the measuredbackground-corrected sum energy spectra, and compared withthe rates from simulations. Both simulation tools provide

Fig. 8. Sum energy spectra for Co-57 (top) and Cs-137 sources (bottom) po-sitioned in the front center of the prototype system at a distance of 50 cm fromthe scatter plane.Measured spectra without background correction (black), mea-sured background spectra (green), measured spectra with background correction(red), and the simulated spectra (blue) are plotted.

reliable event rate estimates. Fig. 9 presents exemplary Geant4results in form of ratios representing the simulated event rates,divided by the measured event rates, for all “valid” events aswell as for full-peak events generated by a couple of commonradioactive sources. The simulation results are confirmed within10%, except for the full-peak rate of Na-22 exhibiting a largerdeviation. The normalization of simulated data is based on thesource activities calibrated with an identiFINDER instrumentof FLIR, thus reducing the activity uncertainties from 30%according to the source provider to about 15%. On the otherhand, both Fig. 9 and Fig. 10 indicate a slight overestimation offull-peak response by simulation. This is not surprising sincenot all the construction details have been considered in themodel. Those details basically represent additional absorbingor scattering materials which can only reduce the photodetec-tion efficiency but might feed the energy region of scatteredgammas in the measured sum energy spectrum.Exemplary differential data are given in Fig. 10, showing

the scattering angle distributions for Co-57, Ba-133 and Cs-137sources in front center position. The scattering angles have been

1070 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 2, APRIL 2013

Fig. 9. Ratio of simulated to measured total (blue) and full-peak rates (red) of“valid” events for different sources placed in the front center of theprototype array at a distance of 50 cm from the scatter plane.

Fig. 10. Distributions of scattering angles derived for Co-57, Ba-133, andCs-137 sources in central position of the prototype system: Comparison ofSiST simulations with measurements.

calculated event-by-event from the linearization-corrected en-ergy depositions in corresponding scatter and absorber detec-tors. It is observed that the scattering-angle distributions mea-sured extend to lower angles in the case of lowest gamma ener-gies (Co-57) when compared with the curves derived from thesimulated data with SiST. The reason is that the real detectionthreshold in the PVT scatter detectors is slightly lower than thethreshold of 10 keV assumed in the simulations.

IV. TOWARDS LARGER CAMERA ARRAYS

As discussed above, the model predictions on theprototype camera array have been verified by a variety of ex-perimental results. The good agreement between simulation andmeasurements provides confidence that both simulation toolsare appropriate for modeling of larger, advanced camera arrays.

Fig. 11. Background contributions to sum energy spectra simulated for modelcamera arrays, corresponding to normal background level as measured inSolingen. The inter-planar distance is set to 30 cm for all the camera arrays.

Geant4 simulations have been performed for larger cameraarrays consisting of 4 4, 8 8, 10 10, 13 13 and 16 16detectors in scatter and absorption planes, respectively. Usingthe terrestrial background model introduced by Mitchell et al.[9], the background contributions to the sum energy spectrahave been simulated for the model camera arrays and are shownin Fig. 11. The background modeling assumes an outdoor back-ground radiation level of about 90 nSv/h as measured at FLIRRadiation in Solingen, Germany. The simulated backgroundsare of great importance for performance predictions of thecamera arrays. The “valid” total and full-peak event ratessimulated for the model camera arrays are presented in Fig. 12as a function of the array size, translated into the number ofdetectors per plane. The lowest data points correspond to the

prototype array.The overall efficiency of a two-plane Compton camera is

expected to be proportional to the scatter plane area (or thenumber of scatter detectors), times the absorption prob-ability of scattered gammas in the absorber plane which scaleswith the solid angle of the absorber plane as seen by an“average” scatter detector. For large plane distances or small ar-rays, is approximately proportional to the numberof absorber detectors, which leads to a square rule forthe given construction with parallel, congruent planes

. For small plane distances or large arrays, ab-sorber detectors far off a given scatter detector contribute lessto than the absorber detectors close by. Consequently,

evolves slower, and the square rule is invalidated. Thiseffect is enforced by the angular dependence of the Comptoncross section preferring forward scattering. This general trendis nicely reflected in the simulation results (Fig. 12): The simu-lated event rates increase with the array size and scale approx-imately with square of the detector number per plane from 44 to 13 13 array but show a noticeable fall-off for 16

16 array compared to the square rule. Because of “shielding”effect of outer detectors the inner detectors in the array planesexperience a reduced level of natural background, and thereforethe background count rates of the camera array increase slower

KONG et al.: PROTOTYPE COMPTON CAMERA ARRAY FOR LOCALIZATION AND IDENTIFICATION 1071

Fig. 12. Simulated “valid” total (top) and full-peak (bottom) event rates fordifferent sources as a function of the number of detectors per plane. In the sim-ulations a 1 mCi point-like source was placed in the front center of the array ata distance of 10 m from the scatter plane. The inter-plane distance is set to 30cm except for the array (15 cm distance). The total event rates arisingfrom simulated backgrounds are plotted in the top panel, too.

than the square rule. Consequently, larger arrays are also distin-guished by an improved effect-to-background ratio.It is worth noting that even in the largest array considered

(16 16 detectors) the detection limits are not noticeably af-fected by random coincidences. With a coincidence windowof 50 ns as applied in the prototype array, and a typical back-ground count rate of 200 to 400 cps per detector, the expectedrandom coincidence rate of 0.5 kcps is small compared withtrue coincidence rate of 2 kcps caused by the natural back-ground. Random coincidences in the presence of strong radi-ation sources causing a high load per detector could be easily

handled by (temporarily) removing part of the detectors fromthe software-controlled coincidence scheme.

V. CONCLUSION

Based on commercially available scintillation detectors andthe voxelSPEC technology, a large-size, scalable two-planeCompton camera design dedicated to localization and identifi-cation of standoff radiation sources is currently being developedat FLIR Radiation. The use of large-volume scintillation de-tectors provides a broad field of view for scattered photonsand facilitates a maximum efficiency at moderate cost. Withthe advanced voxelSPEC technology it is possible to constructcustomized, scalable camera arrays at minimum expense.A functional prototype camera array has been used

for performance tests and for verification of model predictionsobtained by Monte-Carlo simulations. Benchmark tests havedemonstrated a system performance very close to the predic-tions based on previous detector characterization and modeling.Model predictions for the prototype camera array

have been verified by measurements performed for a fewstandard scenarios and geometries. Good agreements betweensimulation results and measurements suggest that the modelingframework including the detector and material data used arewell suited for modeling of larger, advanced camera arrays.Preliminary modeling results for camera arrays with ,

, , and detectorswere presented.

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