ELSEVIER

1. Introduction

Nuclear Instruments and Methods m Physics Research A 353 (1994) 320-323

A solid-state Compton camera for three-dimensional imagingS.E. King a, *, G.W. Phillips a, P.S . Haskins b , J.E . McKisson b , R.B . Piercey e, R.C. Mania d

a Code 6616, Naval Research Laboratory, Washington, DC 20375, USAn Institute for Space Science and Technology, 1810 N W. 6th St., Gainesville, FL 32609, USASparrow Corporation, P.O. Box 6102, Mississippi State, MA 39762, USA

d Division ofMath and Sciences, Kentucky State University, Frankfort, KY40601, USA

AbstractA Compton camera was constructed using four individual high-purity germanium (HPGe) coaxial detectors in the front

plane and four elements of a 15 element HPGe coaxial array in the back plane . 6°Co, 133Ba, and 137CS sources were used inconfigurations that included single sources in positions covering the intended field-of-view and multiple sources of bothidentical isotopes and different isotopes . Experiments were also conducted with sources in waste-container sized attenuatingmedia. This proof-of-concept experiment was designed to demonstrate non-tomographic three-dimensional imaging for thecharacterization of mixed waste containers . The present analysis demonstrates three-dimensional imaging of multiple sourcesat multiple energies and the imaging of sources within a waste-container sized attenuating medium .

Gamma rays are not strongly refracted by matter andtherefore cannot be focused in the "optical" sense. Onecan, however, utilize Compton scattering of gamma-rayphotons in detector materials to retrieve information on thedirection of the incoming gamma ray. In germanium,Compton scattering is the most probable interaction mech-anism from 0.15 to 8.5 MeV. A Compton camera uses thisscattering process in multiple detectors to reconstruct animage of the source of incident photons. The Comptonequation

tnc 2Elcoi 0=1-

EZ(E1 +E2 )

* Corresponding author

describes the direction of the incident photon to within thesurface of a cone with half angle 0 where tnc 2 is the restmass of an electron . Determination of this cone anglerequires knowledge of the energy transferred in the Comp-ton scattering (E 1 ), and the energy of the scattered gammaray (E2 ) . The cone axis is determined by location of thegamma ray interactions . In the simplest image reconstruc-tion techniques, the cones from many events are superim-posed resulting in a peak which uniquely locates thesource. For near-field sources it is possible to reconstructthree-dimensional images of the source field . The sourceimage position resolution is determined by the accuracy of

0168-9002/94/$07.00 © 1994 Elsevier Science B.V . All rights reservedSSDI0168-9002(94)00764-0

NUCLEARINSTRUMENTS& METHODSIN PHYSICSRESEARCH

SectionA

measurement of the energy deposited in the detectors, bythe active size and positional accuracy of the detectors, bythe number of detector elements and their positions, and bythe reconstruction techniques utilized .

The Compton imaging concept has been used for yearsin astrophysics to image sources with a focal plane atinfinity (i .e ., COMPTEL) [1,21 and has been demonstratedin medical imaging [3] . The chief attractions are a widefield of view, high efficiency (which results from the lackof a collimator), source localization and identification inhigh background environments, and non-tomographic threedimensional imaging of near-field sources .

Most previous work has been based on scintillators .Martin et al . [4] have developed a hybrid NaI/HPGesystem with a ring geometry for the second detector array .HPGe detectors were selected for the current system toimprove the energy resolution compared to scintillatorbased systems. This is particularly important for the imag-ing of multiple radionuclides in a complex field in order toreduce the effects of higher energy multiple scattering onthe image reconstruction of lower energy radioisotopes.For HPGe detectors the predominate image reconstructionlimitation is event localization and not energy resolution.

The present research is part of a program on nonde-structive assay and evaluation of mixed waste containers .The characterization of mixed waste containers requires adetermination of the activity level, form, and isotopiccomposition of radioactive wastes within a container inorder to determine the proper methods of further process-ing or disposal . The present work is based on earlierexperimental work [5] and simulations [6] .

2. Experiment

S.E. King et al./Nucl. Instr. and Meth . in Phys. Res. A 353 (1994) 320-323

HV BIAS SUPPLIESPREAMP POWER

SHAPING AMPLIFIERSTIMING AMPLIFIERS

EVENT SORTINGAND

REAL-TIME DISPLAY

EVENT FILE STORAGE

Fig . 1 . Block diagram of the Compton camera data acquisitionsystem.

Existing detector elements, off-the-shelf electronics, andthe Macintosh-based KMAXTm data acquisition and soft-ware control were used. The camera is composed of twoplanes of detectors. The front (scattering) plane consistedof four individual detector elements . The rear (collector)plane consisted of four elements of a 15 element arrayconnected to a single cryostat . The availability of multi-parameter electronics limited the number of back planeelements in use at any one time to four. A schematic of thecamera system and signal processing are shown in Fig. 1 .The detector elements are p-type coaxial high puritygermanium detectors approximately 4 cm diameter X 4 cmlong and efficiencies at 1332 keV ranging from 6% to 12%relative to a 7.62 X 7.62 cm NaI scintillator . The energyresolution of individual detectors (FWHM) at 1332 keVranged from 1.66 to 2.5 keV. The positions of the detectorelements and the sources were measured to within _+2mm. Some additional uncertainty in position arises fromthe uncertainties in the position of the detector elements intheir cryostats, as reported by the manufacturer, and in thethickness of the dead layer of inactive germanium on theoutside surface of the detectors.

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Each event was processed through two channels: en-ergy and timing . The energy signals were processed througha spectroscopic shaping amplifier and sent to a gated quadADC CAMACmodule for digitization into 8k channels . Amore detailed description of the data acquisition system isgiven in McKisson et al . [7]. The ADC gate signal wasproduced by a hardware coincidence requiring at least onedetector in each detector plane to produce a trigger signalwithin a time window of 60 ns . The coincidence timingwas also digitized . This allowed for further timing restric-tions during the data analysis and discrimination againstaccidental coincidences.

For each event 16 parameters (both energy and timefrom all eight detectors) ware acquired . The data weretransferred via a block move to a Macintosh IIcx runningSparrow's KMAXTm data acquisition software and werewritten to either disk or 8 mm tape . The energy and timingdata were sorted into 16 one-parameter histograms forimmediate display . In addition, the energy data could besorted into two-parameter histograms enabling on-line vi-sualization of the energy correlations between selectedfront and back plane detector pairs. Two-dimensional en-ergy cuts could be selected and the resulting projectionsplotted on the energy axis of either detector .

Fig . 2 . Source orientation for five source experiment. The centerof the front detector plane is (0,0,0). Source strengths are in MBqand the positions in cm .

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3. Data analysis and measurements

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S.E . King et al. /Nucl. lnstn and Meth. in Phys. Res. A353 (1994) 320-323

Since the event data were acquired and stored in listmode, data analysis processing is extensive . Each of thedetectors is first calibrated . The selection of the imagereconstruction event filtering requirements are set . Thepresent analysis event selection criteria required an eventin one and only one front plane and back plane detector,required the sum of the energies from the two detectors tobe equal to the energy of the radioisotope being analyzedto within ±3 keV, and required the event to be within aselected timing window . For multiple source energies theimages were reconstructed separately for each energy .

The reconstruction was performed in spherical co-ordinates with the origin located at the center of the frontdetector plane. The reconstruction resolution for the im-ages shown in this paper are 2.25° in 0 and ¢ and 2 cm in

p. This resolution was only limited by the present softwareand computer capacity . The focus of the present analysis isto demonstrate reconstruction of multiple sources and thereconstruction of a source embedded in an attenuating,scattering medium .

4. Results

Fig. 2 is a schematic of a top view looking down on thepositions of five sources used in one of the multisourceconfigurations . Fig. 3A shows the reconstruction of thetwo 60Co point sources and Fig. 3B shows the reconstruc-tion of the 137CS sources from the configuration in Fig. 2.The reconstruction of the 133Ba did not show a distinctlocalization using the present software and selection crite-

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BFig . 3 . Reconstruction images of two 60Co sources (A) and two 137Cs source (B) from the 5 source configuration shown in Fig. 2. Note theimages are in spherical coordinates and display software reverses the direction of the angle 0.

5. Summary and conclusions

S.E. King et al. /Nucl. Instr. andMeth. in Phys. Res. A 353 (1994) 320-323

79.75

Phi

9570

Fig. 4. Three dimensional volume image of a 155 MBq 137Cs

point source in a water filled 55 gallon drum . See note in Fig . 3 .

ria. Because of the disparity in source intensities, theimage color scale must cover a wide range. As a result, thedisplay image shows streaking along the radial axis . Whenthe image of individual sources is properly scaled thisartifact is reduced. The images clearly show that the 137

Cs

sources can be detected above the scattering background of60Co . These images were reconstructed from two hours ofdata collection .

To demonstrate the capability of the system in a highscattering environment, a 155 MBq (4 .2 mCi) 137CS pointsource was placed in the center of a water filled 55 gallondrum . The center of the drum was positioned 1 m distancefrom the Compton camera front plane. The mass of thewater was 208 kg, the mass of the steel barrel was 26 .8 kgand the minimum attenuation thickness between the sourceand the camera was 28.4 cm of water and 0.16 cm of steel .Fig. 4 is the image reconstruction of a 137Cs source . Theimage was reconstructed using 4190 selected events andrepresents 5.8 hours of counting. Because most of thegamma rays scattered from the water and the steel drumfall outside the event selection criteria, they will have littleimpact on the image reconstruction other than the lostcount rate . The point spread width (FWHM) is 6.6 cmindicating some scatter broadening. The image is betterthan is needed to meet a DoE waste criteria of 20 nCi/g.Based on these results, an acceptable image for a barrel ofthis mass should require less than 10 minutes.

The proof-of-concept demonstration has exhibited thecapability of a Compton camera imaging system for threedimensional imaging of multiple point sources and theimaging of sources within a dense matrix . Previous analy-ses [8,9] demonstrated the image reconstruction of individ-ual point sources and determined the system sensitivity .

The results demonstrated the point source resolution of 0.5to 1.0 cm at source-camera distances ranging from 0.25 to1.0 m.

The next stage of development is twofold. First, moreextensive modeling and simulations of the Compton cam-era imager must be developed in order to aid in theunderstanding of the experimental data and in the designof the next system . Second, the experimental hardware,electronics, data acquisition system, and analysis softwareall need to be upgraded . An increase in the number ofdetector elements would improve spatial resolution anddecrease the relative magnitude of image artifacts . Bettertiming electronics should easily improve the timing resolu-tion to below 10 ns . Reducing the ADC gating time to lessthan 3 p,s will reduce the accidental coincidence rate .Many steps can be taken to achieve a higher throughput bythe digital electronics such as use of a FERA bus, in-creased buffer sizes, and reduction in the quantity of nulldata . Faster workstations would allow an increase in thereconstruction resolution . In addition, the present recon-struction algorithm assumes all scatters occur in a frontplane detector with full absorption in the back plane. Amore complete algorithm would allow for reverse scatter-ing. Finally, the anticipated problem of obscuration of lowintensity sources by higher intensity sources could bereduced by an iterative model-based algorithm to sequen-tially remove sources from the data as they are identified.

Acknowledgement

This work was supported by NRL-DOE InteragencyAgreement No . DE-AI01-93W53022.

References

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[4] J.B . Martin, G.F . Knoll, D.K. Wehe, N. Dogan, V. Jordanov,N. Petrick and M. Singh, IEEE Trans . Nucl. Sci. NS-40(1993) 972.R.B . Piercey, A.G . Weisenberger, J.E. McKisson and C.Girit, IEEE Trans. Nucl . Sci . NS-36 (1989) 887.J.E. McKisson, F. Taeng and P.S. Haskins, Conf. Rec. IEEENucl . Sci Symp. Vol. 1 (1992) 667.J.E. McKisson, P.S . Haskins, R.B. Piercey, G.W. Phillips,S.E . King, R.A . August and R.C . Mania, Conf. Rec. IEEENucl . Sci. Symp. 93CH3374-6 (1994) 534.

[8] P.S . Haskins, J.E. McKisson, G.W . Phillips, S.E . King, R.A.August, R.B . Piercey and R.C . Mania, Conf . Rec. IEEE Nucl .Sci . Symp . 93CH3374-6 (1994) 360.J.E. McKisson, P.S . Haskins, G.W . Phillips, S.E . King, R.A.August . R.B . Pierce y and R.C Mania, IEEE Trans . Nucl . Sci.,to be published .

[5]

[6]

[7]

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