Silicon Photomultiplier (SiPM) detectors are investigated

world-wide as a suitable replacement for the conventional

vacuum based PhotoMultiplier Tube (PMT) and are enabling

applications otherwise not possible with PMT detectors. Progress

in recent years has been substantial with SiPM detectors pushing

the boundaries in energy and time resolution as well as photon

detection efficiency and active surface area. In this paper we

report on the performance of a gamma detector comprising latest

generation SiPM detectors from SensL coupled to novel Cerium

doped Gd3Al2Ga3O12 (GAGG) scintillators from Furukawa,

Japan. Both 3mm�3mm N-on-P and P-on-N SiPM detectors have

been optically coupled to 3mm�3mm�30mm crystals. An energy

resolution (662 keV Cs-137) of 9.4% has been measured for

GAGG crystal coupled to a 3mm�3mm N-on-P SiPM detector

Jackson, Carl

O'Neill, Kevin

Seitz, Bjoern

Stewart, Andrew G.

Wall, Liam

English

Enlighten

     Seitz, B., Stewart, A. G., O'Neill, K., Wall , L., and Jackson, C. (2013) Performance evaluation of novel SiPM for medical imaging applications. In: IEEE Nuclear Science Symposium and Medical Imaging Conference, 27 Oct - 2 Nov 2013, Seoul, South Korea.  Copyright © 2013 IEEE   A copy can be downloaded for personal non-commercial research or study, without prior permission or charge  Content must not be changed in any way or reproduced in any format or medium without the formal permission of the copyright holder(s)   When referring to this work, full bibliographic details must be given    http://eprints.gla.ac.uk/94079/      Deposited on: 11 November 2014                Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Performance Evaluation of novel SiPMfor Medical Imaging ApplicationsBjoern Seitz, Andrew G. Stewart, Kevin O’Neill, Liam Wall, and Carl JacksonAbstract—Silicon Photomultiplier (SiPM) detectors are investi-gated world-wide as a suitable replacement for the conventionalvacuum based PhotoMultiplier Tube (PMT) and are enablingapplications otherwise not possible with PMT detectors. Progressin recent years has been substantial with SiPM detectors pushingthe boundaries in energy and time resolution as well as photondetection efficiency and active surface area. In this paper wereport on the performance of a gamma detector comprising latestgeneration SiPM detectors from SensL coupled to novel Ceriumdoped Gd3Al2Ga3O12 (GAGG) scintillators from Furukawa,Japan. Both 3mm×3mm N-on-P and P-on-N SiPM detectors havebeen optically coupled to 3mm×3mm×30mm crystals. An energyresolution (662 keV Cs-137) of 9.4% has been measured forGAGG crystal coupled to a 3mm×3mm N-on-P SiPM detector.Index Terms—Silicon Photomultiplier, Ce:GAGG, gammaspectroscopy, positron emission tomography, PET, SPECT,LYSO, scintillators, geiger mode photodiodes, scintillation de-tection, silicon.I. INTRODUCTIONS ILICON Photomultiplier detectors provide a compact,single-photon sensitive, scalable photon detection plat-form and have received considerable interest for applicationin medical imaging modalities such as Positron EmissionTomography (PET) and Single Photon Emission ComputedTomography (SPECT) [1], [2], [3], [4]. While the presentgeneration of scintillator crystals, cerium doped lutetiumoxyorthosilate (LSO) and lutetium-yttrium oxyorthosilicate(LYSO), currently used in commercial PET imaging systemshave an emission spectrum well matched to the absorptionspectrum of bialkali based photo-cathodes of photomultipliertubes, the emission spectrum of these scintillators is a poormatch to the spectral response of SiPM detectors with aN-on-P structure. This spectral mismatch has pushed thedevelopment of SiPM detectors with a P-on-N structure as thisresults in a shift of the peak efficiency to shorter wavelengths[5], [6].More recently a new fast, high-luminosity single-crystalcerium doped scintillator, Gd3Al2Ga2O12 (Ce:GAGG), hasbeen grown that has a peak emission wavelength well matchedto the peak sensitivity of N-on-P structure SiPM detectors.B. Seitz and A. G. Stewart are with the Scottish Universities PhysicsAlliance (SUPA), School of Physics and Astronomy, University of Glas-gow, Glasgow, G12 8QQ, UK (e-mail: bjoern.seitz@glasgow.ac.uk or an-drew.stewart@glasgow.ac.uk).K. O’Neill, L. Wall and C. Jackson are with SensL Technologies Ltd,Building 6800, Avenue 6000, Cork Airport Business Park, Cork, Ireland (e-mail: cjackson@sensl.com).Fig. 1. 3mm × 3mm × 30mm Ce:GAGG scintillating crystals fromFurukawa Co. Ltd. Japan. The crystals were polished on all faces and 5 facesare covered with a reflective coating. Also shown is a 3mm×3mm SiPM fromSensL.II. SCINTILLATORS FOR MEDICAL IMAGINGThe need for high quality images, particularly for medicalimaging, places high demand on the scintillator, read-outdetector and electronics. The scintillators require a high photonyield, fast response and decay time, high stopping power,good chemical stability and an absence of intrinsic radiation.In addition the ideal scintillator should have an emissionspectrum closely matched to the spectral response of thedetector, be non-hygroscopic and cheap to manufacture.A. Cerium doped GAGGGrowth of GAGG crystals by the Czochralski method andits scintillation properties were first reported in 2011 [7]. Pre-liminary studies of energy resolution and timing performanceof GAGG crystals readout by a variety of photon detectiontechnologies (PMT, MPPC, PIN diode) show that the crystalhas promise in medical imaging and gamma spectroscopyapplications [8], [9].Figure 1 shows a photograph of the 3mm×3mm×30mmGAGG scintillator crystals together with a packaged3mm×3mm active area SiPM detector. All faces of the crystalshave been polished and 5 of the faces were covered in a whitereflective coating.The crystal has a emission peak in the 520nm to 530nmregion while the light yield has been reported as 33,100photons/Mev [9] and 46,000 photons/Mev [10]. The data sheetfrom Furukawa quotes a light yield of 60,000 photons/MeV[11]. The intrinsic energy resolution of the crystal has beenTABLE ISCINTILLATOR PROPERTIESCe:GAGG Ce:LYSO BGOLight Yield (photons/Mev) 46,000 32,000 9000Emission Peak (nm) 520-530 420 480Density (g/cm3) 6.63 7.1 7.13Decay Time (ns) 90 41 300Intrinsic Energy Resolution (%) 5.2 7.9 12TABLE IISIPM DETECTOR PARAMETERSParameter P-on-N (B series) N-on-P (M series)Microcell Dimensions 35 µm × 35µm 35µm × 35µmNumber of Microcells 4774 4774Fill Factor 64% 64%Breakdown Voltage 24.5V 27VPeak Response 420nm 500nmreported as 5.2% [9]. The main properties of Ce:GAGG aresummarized in table I together with the properties of thescintillators LYSO and the Bismuth Germanate (BGO).B. Silicon Photomultiplier DetectorsSilicon Photomultiplier detectors have been proposed as asuitable replacement for the incumbent vacuum tube technol-ogy currently utilized in medical imaging since their inception.Moreover, the SiPM detector platform is considered essentialfor the development of a new generation of hybrid scannersuch as PET/MRI (Magnetic Resonance Imaging) since theyare immune to the high magnetic fields used in MRI [12].Two different SiPM structures are assessed in this study;an N-on-P (M Series) device and a P-on-N (B Series) device.The engineering sample detectors studied here are equivalentto commercial devices with part numbers MicroFM-30035-SMT and MicroFB-30035-SMT. Both detector types have atotal of 4774 microcells and a fill factor (FF) of 64%. Eachmicrocell has an active area of 35µm×35µm. The detectorparameters are summarized in table II.III. EXPERIMENTAL SETUPThe scintillator crystals and SiPM detectors were alignedusing a mechanical holder which allowed the fast and reliableexchange of both the crystals and the SiPM detectors. Figure2 shows a photograph of one of the GAGG crystals insidethe holder. The detector package fits into the holder recessand aligns the SiPM with the crystal. Radioactive sources areplaced on the opposite face of the cylinder adjacent to thecoated facet of the crystal. The packaged SiPM detectors havean epoxy fill to protect the surface of the silicon and the wirebonds. The crystal was optically coupled to the surface of theepoxy fill using optical grease. Reflections at the interfaceswill, however, reduce the number of photons reaching thesilicon and degrade the energy resolution.The SiPM detector bias voltage was supplied by a Keithley2410 source-measure unit (SMU). The SiPM signal was am-plified using a high bandwidth MiniCircuits amplifier circuitFig. 2. GAGG crystal inside the mechanical holder. The SiPM detectorpackage fits into the recess and aligns the SiPM detector with the uncoatedfacet of the crystal. The holder allows both the crystal and the detector to bequickly and reliably changed.Fig. 3. IV characteristics of P-on-N (solid green line) and N-on-P (bluedashed line) 3mm×3mm SiPM devices. Both IV characteristics were recordedat 20◦C.containing a Gali 55+ chip. The amplifier has a gain of approx-imately ×19. The amplified signal was displayed on a 1GHzLeCroy oscilloscope. Oscilloscope signals were transferred toa PC over GPIB using a C++ program for analysis offline. TheSiPM, crystal, source and MiniCircuits amplifier were housedin a Heraeus Votsch HT 4004 environmental chamber. Thechamber provided a dark, temperature controlled environment.All measurements reported here were conducted at 20◦C. Thechamber temperature was monitored using a thermocoupleplaced in close proximity to the SiPM detector.A. IV CharacteristicsFigure 3 shows the current-voltage (IV) characteristics ofboth the N-on-P (M series) and P-on-N (B series) SiPMdetectors recorded at 20◦C. The IV characteristic was recordedusing the Keithley SMU controlled by a LabView program.Fig. 4. Primary y-axis: Typical spectral response of a P-on-N (solid blue line)and N-on-P (dashed green line) SiPM. The spectral responses were measuredat 2.5V above the breakdown voltage of the device. Secondary y-axis: Typicalemission spectrum of Ce:GAGG.The figure shows that the P-on-N device has a break-downvoltage (VBr) of 24.5V while the N-on-P detector has a break-down voltage of 27V.B. Spectral SensitivityFigure 4 compares the typical spectral sensitivity of the N-on-P (M series) and P-on-N (B series) SiPM devices togetherwith the emission spectrum of the GAGG scintillator. Theemission spectrum of GAGG is taken from [9]. While the P-on-N device has a higher overall peak efficiency, the overlap ofthe spectral response of the N-on-P device with the emissionspectrum of GAGG promises a higher detection efficiency andhence an improved energy resolution.C. Energy ResolutionFigure 5 shows the pulse height distribution from a N-on-P(M series) device optically coupled to a GAGG crystal using a137Cs (662 keV) source. The distribution was recorded at 20◦Cand with a bias voltage of 29V or 2V above the breakdownvoltage. The energy resolution (∆E/E), defined as the FullWidth Half Maximum (FWHM) divided by the mean of thepulse height, was determined by a Gaussian fit of the peak inthe distribution. A fit to the distribution shown in figure 5 givesan energy resolution of 12.9%. The pulse height distributionis uncorrected for the effects of dark rate, crosstalk and after-pulsing. These effects add additional Geiger pulses to anysignal photons and can lead to artificially enhanced valuesfor the detection efficiency and hence the energy resolution.D. Energy Resolution as a function of Detector OverbiasFigure 6 shows energy resolution of both the N-on-P andP-on-N devices as a function of detector overbias. The figureshows that the energy resolution varies linearly with overbiasbelow about 3V above breakdown. Energy resolutions of 9.4%and 11.6% were obtained for N-on-P (M series) and P-on-NFig. 5. Pulse height distribution (137Cs) from an N-on-P SiPM coupledto Ce:GAGG. The SiPM was biased at 2V above the breakdown voltage. AGaussian fit to the gamma peak gives an energy resolution of 12.9%. Themeasurements were recorded at 20oC(B series) SiPM devices at a bias of 3V above their respectivebreakdown voltages.E. Linearity of ResponseThe linearity and dynamic range of an SiPM are determinedby the photon detection efficiency (PDE) and the total numberof microcells [13]. When the number of incident photonsis much less than the total number of microcells the SiPMresponse is linear. To investigate the linearity of the responseof the 3mm×3mm SiPM detectors the pulse height distributionfrom Barium-133 (81 keV and 356 keV) and Americium-241 (60 keV) were also measured using the N-on-P deviceoptically coupled to GAGG. Figure 7 shows the SiPM response(mean pulse height), at a bias of 2V above the breakdownvoltage, as a function of gamma energy for the three isotopes.The data points are fitted with an equation of the form,y = M(1 − exp(−Nx)), where y is the mean pulse height,x is the energy of the gamma photon and N and M areconstants. However, over the range of gamma energies studied,the detector is clearly operating in the linear portion of itsdynamic region.IV. SUMMARY AND CONCLUSIONCerium doped GAGG scintillator crystals have many prop-erties that make them suitable for gamma spectroscopy andmedical imaging applications. A high photon yield and emis-sion peak around 530nm makes the material well suited toreadout by Silicon Photomultiplier detectors. High photonyield is critical for the reduced crystal dimensions of high-resolution PET block detectors [14].In addition to the advancement in scintillating materials,significant progress has also been made in improving andoptimising the performance of SiPM detectors. In this studySiPM detectors with two different structures have been usedto readout GAGG crystals. The SiPM detectors have an activearea of 3mm × 3mm and peak spectral responses of 500nmFig. 6. Energy resolution as a function of detector overbais for N-on-P SiPM(green circles) and P-on-N SiPM (blue squares).0 100 200 300 400 500 600 7000200400600800100012001400Mean Pulse Height (mV)Gamma Energy (keV)←241Am←133Ba←133Ba137Cs→Fig. 7. Linearity of response of a 3mm×3mm×30mm GAGG crystal coupledto an N-on-P SiPM biased at 2V above breakdown voltage.(N-on-P) and 420nm (P-on-N). As expected, the close matchbetween the emission spectra of GAGG and the spectral re-sponse of the N-on-P device leads to a better energy resolutionthan that achieved with the P-on-N device. In addition, bothSiPM devices display excellent linearity in their response upto gamma energies of 662keV.ACKNOWLEDGMENTA G Stewart is supported by the a Scottish UniversitiesPhysics Alliance (SUPA) fellowship.REFERENCES[1] A. G. Stewart, V. Saveliev, S. J. Bellis, D. J. Herbert, P. J. Hughes,J. C. Jackson, “Performance of 1-mm2 Silicon Photomultiplier”, IEEEJ. Quantum. Electron., vol. 44, no. 2, pp. 157-164, Feb 2008[2] A. G. Stewart, L. Wall, J. C. Jackson, “Properties of silicon photoncounting detectors and silicon photomultipliers”, J. Mod. Opt., vol. 56,no. 2-3, pp. 240-252, 2009[3] E. Roncali and S. R. Cherry, “Application of Silicon Photomultipliers toPositron Emission Tomography”, Ann. Biomed. Eng., vol. 39, no. 4, pp.1358-1377, Apr. 2011[4] N. D’Ascenzo, V. Saveliev, “Study of silicon photomultipliers for themedical imaging systems”, Nucl. Instr. Meth. A, vol. 695, pp. 265-267,Dec 2012[5] M. Mazzillo, A. Ronzhin, S. Los, S. Abbisso, D. Sanfilippo, G. Valvo,B. Carbone, A. Piana, G. Fallica, M. Albrow and E. Ramberg, “Electro-Optical Performances of p-on-n and n-on-p Silicon Photomultipliers’,IEEE Trans. Electron Devices, vol. 59, no. 12, Dec. 2012[6] J. Y. Yeom, R. Vinke, N. Pavlov, S. Bellis, L. Wall, K. O’Neill, C. Jackson,C. S. Levin, “Fast Timing Silicon Photomultipliers for ScintillationDetectors”, IEEE Photon. Technol. Lett., vol. 25, no. 14, 2013[7] K. Kamada, T. Yanagida, T. Endo, K. Tsutumi, Y. Usuki, M. Nikl,Y. Fujimoto and A. Yoshikawa, “2-inch size single crystal growth andscintillation properties of new Scintillator; Ce:Gd3Al2Ga3O12”, IEEENuclear Science Symposium Conference Record, 2011[8] J. Y. Yeom, S. Yamamoto, S. E. Derenzo, V. Ch. Spanoudaki, K. Ka-mada, T. Endo, C. S. Levin, “First Performance Results of Ce:GAGGScintillation Crystals with Silicon Photomultipliers”, IEEE Trans. Nucl.Sci., vol. 60, no. 2, pp. 988-992 April 2013[9] J. Iwanowska, L. Swiderski, T. Szczesniak, P. Sibczynski, M. Moszynski,M. Grodzicka, K. Kamada, Y. Usuki, T. Yanagida, A. Yoshikawa,“Performance of cerium-doped Gd3Al2Ga3O12 (GAGG:Ce) scintillatorin gamma-ray spectrometry”, Nucl. Instrum. Meth. A, vol. 712, pp. 34-40,2013[10] K. Kamada, T. Yanagida, J. Pejchal, M. Nikl, T. Endo, K. Tsutsumi,Y. Fujimoto, A. Fukabori, A. Yoshikawa, “Crystal Growth and Scintil-lation Properties of Ce Doped Gd3(Ga, Al)5O12 Single Crystals, IEEETrans. Nucl. Sci., vol. 59, no. 5, 2012[11] [Online] URL: www.furukawa.co.jp/english/[12] D. P. McElroy, V. Saveliev, A. Reznik, J. A. Rowlands, “Evaluation ofsilicon photomultipliers: A promising new detector for MR compatiblePET”, Nucl. Instr. Meth. A, vol. 571, pp. 106-109, 2007[13] A. G. Stewart, E. Greene-O’Sullivan, D. J. Herbert, V. Saveliev, F. Quin-lan, L. Wall, P. J. Hughes, A. Mathewson, J. C. Jackson, “Study of theproperties of new SPM detectors”, Proceedings of The Society of Photo-Optical Instrumentation Engineers (SPIE), vol. 6119, pp. A1190-A1190,2006[14] S. Yamamoto, H. Watabe, Y. Kanai, M. Imaizumi, T. Watabe, E. Shi-mosegawa, J. Hatazawa, “Development of a high-resolution Si-PM-basedgamma camera system”, Phys. Med. Biol., vol. 56, pp. 7555-7567, 2011

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Performance evaluation of novel SiPM for medical imaging applications

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Performance evaluation of novel SiPM for medical imaging applications

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