2008 IEEE Nuclear Science Symposium Conference Record N45-6

Radiation Measurements using Solid-StatePhotomultipliers:

Gammas, Charged Particles, and NeutronsErik Johnson, Christopher Stapels, Jarek Glodo, Sharmistha Mukhopadhyay, Paul Linsay, Kanai Shah, Paul Barton,

David Wehe, Eric Benton, Skip Augustine, Member, IEEE

Abstract- Counting individual scintillation pulses produced byradiation provides excellent sensitivity for dosimeterapplications, however, the light pulse produced by thescintillation material has previously needed the high gain of aPMT to read out the signal. The recent development of solid­state photomultipliers provides the high gain, high-bandwidthperformance needed to implement event counting with thescintillation detector. The CMOS SSPM detector is a compact,rugged, low-power photodetector that is' well suited forscintillation-based applications. The digital representation ofscintillator photon multiplicity provides a convenient basis fordigital radiation measurements. Traditional tissue equivalentscintillation materials provide a direct measure of the energydeposited and the dose equivalent and are a good match forcharged-particle dosimetry, as they typically produce relativelysmall light pulses for gamma ray events. In addition, newscintillation materials are being developed that detect neutronsand can implement pulse shape discrimination to rejectinterfering gamma ray signals on an event-by-event basis. Thiswork presents preliminary evaluations of three types of radiationdetectors constructed from an SSPM. An LYSO scintillatorgamma-ray dosimeter, a charged-particle detector using tissue­equivalent plastic scintillator for space dosimetry, and a neutrondetector made from boron-loaded plastic are demonstrated..

When the SSPM is coupled to an appropriate scintillator,spectroscopy and dosimetry measurements can be achievedacross a wide range of radiation, including gammas, protons,and neutrons. The SSPM detector coupled to a scintillationmaterial can detect individual nuclear events, which gives ithigh sensitivity to the scintillator output. The choice ofscintillator can be targeted to the application, for exampleusing very high sensitivity scintillation materials for gamma­ray spectroscopy or low-cost scintillation detectors forwidespread deployment, as in homeland security applications.The small form factor allows smaller and more flexibleinstruments. These small materials can provide limited energyinformation but can still achieve high dose accuracy.

Another major benefit of the CMOS environment is thepossibility of on-chip integration, or a dosimeter-on-a-chip.We have performed a number of measurements with differentscintillator types and SSPM configurations to test thecapabilities of the SSPM in a radiation measurement device.Within each section, we present some of the obstacles andchallenges that have been addressed.

22NaLYSO

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Fig. 1 Spectrum of gamma-ray events from 22Na on a tiny LYSOscintillator as measured by a 1.5 mm square SSPM with 441 square pixels, 50microns wide.

II. GAMMARAYMEASUREMENTS

An SSPM coupled to the appropriate scintillation material canprovide accurate gamma-ray spectroscopy. Fig. 1demonstrates the utility of the tiny SSPM-based radiationdetector. With only 441 pixels and a 1.5 mm x 1.5 mm x 3.0mm LYSO crystal, the 511-keV peak can be resolved from theCompton background.

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I. INTRODUCTION

A wide variety of applications require photodetectorscapable of measuring fast, but weak, optical pulses. The recentdevelopment of solid-state photomultipliers (SSPMs) providesan alternative to traditional optical detectors in applicationsrequiring speed, sensitivity, low-power and compact size.CMOS SSPMs are arrays of Geiger-mode avalanchephotodiodes manufactured using commercial CMOStechnology, operated in parallel so that the output signal isproportional to the incident light intensity.

Manuscript received November 15, 2008. This work was supported in partby the U.S. Department of Energy grant under Grant No. DOE DE-FG02­05ER84589, by the NASA under grant no. NNJ08JA55C , and by U.S.Department of Defense under Grant No. HDRTA 1-07-C-0045.

Erik Johnson, Christopher Stapels, Shannistha Mukhopadhyay, PaulLinsay, Michael. Squillante, and James Christian are with RadiationMonitoring Devices, of 44 Hunt St. Watertown, MA 02472 (telephone: 617­668-6894, e-mail: ejohnson@rmdinc.com).

P. Barton and D. Wehe are with the Nuclear Engineering and RadiologicalSciences, College of Engineering, The University of Michigan, 3038 PhoenixMemorial Laboratory~ 2300 Bonisteel Blvd~ Ann Arbor, MI 48109, USA.(telephone: (734) 763-1151, e-mail: dkw@umich.edu).

F. L. Augustine is president of Augustine Engineering, Encinitas, CA92024, USA (telephone: 970-491-6206, e-mail: s.augustine@ieee.org).

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Fig. 2 Dose measurement error for different exposure times withcalibration and non-calibration sources. The 40% line is the ANSI target doseaccuracy for personnel dosimeters.

As expected, the lower doses exhibit larger errors due to thestatistical fluctuations in the number of events. Althoughcounting statistics dominates the accuracy limitation at lowdoses, the self activity of the LYSO scintillator material alsocontributes to this error at low doses. The low-dose accuracyperformance, however, exceeds that of a typical TLD badgeby at least two orders ofmagnitude.

A. Wireless spectrometer

We have integrated an SSPM with amplifiers, acomparator ladder with four counters, and a wirelesstransmitter to create a prototype ratemeter unit. The first twoiterations of this design are shown in Fig. 3. These devices arecurrently undergoing calibration to provide reliable dose data.

B. Temperature Dependence

A field-deployable dosimeter must be able to function in avariety of temperatures and harsh environments; thus thedosimeter operation must be insensitive to temperaturevariation. The signal size from avalanche devices fabricatedin silicon is temperature dependant. The temperature affectsthe Geiger mode performance by altering the breakdownvoltage, the junction capacitance, and the Geiger probability.For a device with Nttl pixels, operated in the linear regime, thetemperature dependence can be approximated by the followingequation:

qSSPM =Nttl ·QE(A),PG[VA -VB(T)]·CJ(T),[VA -VB(T)],

where PG is the Geiger probability, CJ is the junctioncapacitance, and (VA- VB) is the excess bias. The breakdownvoltage, Geiger probability, and junction capacitance eachhave an inverse dependence on the temperature, thus thesignal output from the SSPM is proportional to T-3

.

The effects of excess bias variation on the gain of the pulse,and the voltage height of the output can be eliminated usingpixel-level signal conditioning; the only remainingtemperature dependence is the change in the Geigerprobability.

Fig. 3 a) Ratemeter revision 1. The USB wireless interface is shown in thebottom of the top image. The unit detects gamma rays and provides a four­channel spectral representation of the radiation field that is transmittedwirelessly. The Teflon-wrapped crystal in the top image is 1.5 mm on a side.b) Revision 2 of the wireless ratemeter, shown with a 3 mm wide cubicscintillator. The total PCB area of the second revision is about a factor of twosmaller than the first.

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Calculation of accurate radiation dose requires someknowledge of the energy of the radiation. Scintillationmaterials provide a low-cost method to obtain energyinformation and measure the total number of ionizingradiation events. Recording complete spectra of the dose seenby a dosimeter is prohibitive in terms of power consumption,size of the electronics, and processing dead time. For optimalperformance ofdosimeter measurements, we have developed amethod that can accurately determine the dose based on athree-channel representation of the energy spectrum, aspreviously reported[l, 2].

Using an SSPM array consisting of just 100 round, 20 J.1mdiameter pixels coupled to a 1.2 mm x 1.2 mm x 0.2 mmLYSO scintillator, we are able to accurately measure dosesfrom five typical gamma-ray sources down to 1 J.1Sv, as seenin Fig. 2. The human-equivalent dose is calculated using theoutput of the SSPM-based dosimeter. The error in themeasurement is the calculated human dose compared to theexpected dose deposited from the known source strength.

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Fig 6 Typical integrated signal processing schematic, showing thecomparator triggered from the anode of the pixel and controlled by the Vthdiscriminator level. The outputs of each pixel are summed in parallel.

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c. All-digital dosimeter

The next level of integration after pixel-level signalprocessing is to calculate the pulse height right on the chip.The digital pulses created in signal processing lend themselvesto a straightforward calculation of the total pulse height. Wehave designed and fabricated a chip that includes the pixellevel processing and a series of comparators that determine thepixel height and increment one of eight onboard counters. Adesign containing this architecture is currently in fabrication.The total size of the die is less than 10 mm and contains all theessential elements of the dosimeter except the power supplyand the scintillator. Combining these elements produces atiny, compact dosimeter with digital output.

III. NEUTRON MEASUREMENTS

A neutron sensitive detector can be fashioned from anSSPM for dosimetry and homeland security applications. Thelow cost and small form factors of the SSPM, especially withintegrated electronics are also well suited to these cases. Wehave investigated two detector configurations. The firstconfiguration uses a coincidence technique to reject thecontribution from gamma rays. In Fig. 10, boron loadedplastic (BLP) on one SSPM is placed near a similar sizedLYSO crystal on another SSPM. This detector distinguishesneutrons from gamma rays by exploiting a coincidentdiscrimination of a neutron d pulse signature created in bothscintillators. When the neutron is captured by lOB in the plasticscintillator, a helium atom and an excited lithium atom areproduced. The excited lithium emits a 478 keV gamma ray

Fig. 7 Temperature feedback circuit. The circuit contains a sampling diodethat is blocked from incident light. The output of the diode is used to trigger acomparator and is fed into a peak holding circuit. The comparator resets thepeak hold just before the peak hold is set to hold the next value.

The next level of temperature control is to modify the setbias as the breakdown value changes with temperature andmaintain a constant excess bias. Such a process removes theeffect on the Geiger probability and the change in voltageoutput. The chip shown in Fig. 4 contains the temperaturesensing circuit in Fig. 7. This circuit uses a sampling pixeland a peak hold to output a DC voltage that is proportional tothe excess bias. Since the excess bias contains the majortemperature dependency, the temperature dependency isreduced to a linear function. The combination of pixel levelprocessing and constant excess bias removes all temperaturedependencies form the SSPM detector.

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Fig. 5 Preliminary signal conditioned spectrum of SSPM output whenilluminated by a pulsed blue LED. The pulse output height recorded in amulticahnnel analyzer is plotted against the number of counts in a channel.The number of pixels fired in easily resolved in this plot up to 180 pixels.

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The CMOS process allows for easy integration of thecomparator with every pixel in the device. The pulse isdigitized with a comparator whose threshold is set above thenoise. We have realized a design with such a conditionedoutput. The basic circuit structure for each pixel is shown inFig 6. This solution with a constant excess bias removes alltemperature dependences and has been implemented in arecent design of a 3 mm x 3 mm die with four test quadrantsof 400 pixels each. A photograph of the completed chip isshown in Fig. 4. Preliminary testing of the chip has indicatedvery high resolution of individual pixels as shown in Fig. 5.

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Fig. 4 The integrated signal processing SSPM. Each pixel in the upper leftquadrant contains the circuit elements shown in Fig 6. The other quadrantscontain similar designs with slight variations in the comparator style and thequenching operation.

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IV. CHARGED PARTICLES

An SSPM-based radiation detector has been constructedusing a plastic scintillator for detection of high-energy chargedparticles. The particular target application is for spacedosimetry, where protons from the solar particle events are themost important component of the radiation background.Fig.ll shows the prototype space dosimeter, with a 3 mm x 3mm SSPM and PCB including shaping, amplification, andpower supply electronics. A multi-channel analyzer based in amicroprocessor will be connected to the output of the detectorboard to create a compact autonomous unit.

A prototype space dosimeter was assembled and irradiatedin proton beams of varying energy at the NASA SpaceRadiation Laboratory (NSRL) at Brookhaven NationalLaboratory. The output from the SSPM at three proton beamenergies is provided in Fig.12. Further calibration for heavyions and a greater range ofproton energies is planned.

Fig. 10 SSPM Neutron detector prototype with coincidence gammarejection method. On the left, a 3 mm x 3 mm x 6 mm LYSO crystal detectsthe gamma-rays created in the 3 mm Boron-loaded plastic cube that sits on theright. Each sits on a separate 3 x 3 mm2 SSPM.

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Fig.9 Plot of the spectra from a CLYC-SSPM prototype. The peak in theAmBe source line at channel 275 is the response from thermal neutrons. Theline below channel 200 is generated by an 8 mCi 137CS source, whichdemonstrates the clear discrimination of gamma-ray event. Thediscrimination against gamma rays is similar to 3He.

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Fig. 8 Spectrum of events detected in boron loaded plastic (BLP) in thepresence of an AmBe neutron source. The peak: at 93 keV is representative ofdetected neutrons.

New scintillation materials provide for a second SSPM­based neutron detector. A Cs2LiYCI6:Ce (CLYC) [3,4]scintillation crystal coupled directly to the SSPM comprises aseparate neutron detection scheme. The bright flash producedduring a neutron absorption event in the CLYC allowsgamma-ray events to be discriminated by pulse height. InFig.9, the spectrum events from neutron irradiation andgamma-ray irradiation with a high energy gamma-ray sourceare shown. Setting a discriminator at the appropriate channelprovides a very high background gamma-ray rejection ratio.

that is detected by the LYSO portion of the detector segment.In addition, the helium and lithium reaction produces a 93 keYelectron-equivalent light flash in the boron loaded plasticscintillator that can be detected by the second SSPM detector.The coincidence detection of the 93 keV electron equivalentlight flash with the 478 keV gamma ray suppressesbackground gamma-ray events.

The detector configuration serves as a promIsIngreplacement for a 3He tube and it also provides additionalcapabilities for detecting and resolving the energy of fastneutrons. Compact packages with or without remote readoutelectronics can also be adapted with the proposed design. Inaddition, the high-performance, ultra-compact segmentedsolid-state detector is very suitable for high-resolution neutronimaging. The use of a detector stack and can also provide ameans to correct the non-linear light yield of fast neutrons bytracking the energy loss of the neutron as it propagatesthrough a stack of multiple detector segments, which shouldimprove the energy resolution of the detector.

Fig. 8 shows a spectrum of events in the boron loadedplastic when irradiated by an AmBe source. The 93 keYequivalent peak indicates the detection of thermalizedneutrons. The BLP also has some direct sensitivity to fastneutrons and the use different layers of additional moderatorscan provide additional energy discrimination.

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Fig.11 NASA space dosimeter prototype and readout PCB. TIlescintillation material is not shown in this photograph.

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Fig.12 Proton detection by the SSPM-based space dosimeter. A nearlylinear energy response is shown and indicates the successful detection of dosedeposited in tissue-equivalent plastic scintillation material.

V. CONCLUSION

Using a CMOS based platform for generating the lightdetector can provide a basis for radiation measurements inmany distinctly different radiation fields. The small formfactor, low-power, and low cost in bulk of these devicesenables a wide range of applications including dosimetry andmonitoring.

ACKNOWLEDGMENT

The authors wish to thank Michael Sivertz, and AdamRusek for help with the proton measurements at the NSRL.

REFERENCES

[1] Stapels, C., et al. Digital scintillation-based dosimeter-on-a-chip. inNuclear Science Symposium Conference Record. 2007. Hawaii.

[2] Johnson, E.B., et al. Solid-State Photo-Multipliers for DeployableGamma-Ray Dosimeters. in IEEE Conference on Technologies forHomeland Security. 2006. Boston, MA.

[3] C. M. Combes, P. Dorenbos, C.W.E. van Eijk, K. W. Kramer, H. U.Glidel, "Optical and scintillation properties of pure and Ce3+­doped Cs2LiYCl6 and Li3YCl6 : Ce3+ crystals," J. Lumin. 82, pp.299-305, 1999.

[4] 1. Glodo, W. M. Higgins, E. V. D. van Loef, K. Shah, "Scintillationproperties of I inch Cs2LiYCI6:Ce crystals," IEEE Trans. Nucl.Sci. vol. 55, pp. 1206-1209, June 2008.

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