2008 IEEE Nuclear Science Symposium Conference Record

A Compact Neutron Detector Based on the use of aSiPM Detector

Mark Foster and David Ramsden Member, IEEE

N26-3

Abstract-This paper explores the performance characteristcs ofa compact neutron detector based on the use of a silicon photo-multiplier (SiPM). These devices offer the first true solid-statealternative to traditional photomultiplier tubes in that theyprovide similar gain and photon-detection efficiency but with thesmall size and ruggedness of PIN diodes. The performance of adesign based on the use of a 23mm diameter Lil crystal, viewedby a 14x14mm square SiPM, has been modeled and testedexperimentally using fission-neutrons. The designincorporates a 10mm thick HDPE moderator in front of the 3mmcrystal and an acrylic light-guide behind it. This provides theability to detect a 104 nls source at 25cm in 12s with a confidencelevel of 93.5%. The gamma-ray rejection capability of thedetector is excellent, allowing reliable detection of neutrons in aCo60 gamma-ray background equivalent to a count-rateequivalent to 100kcps in a 2x2" NaI crystal. The impact oftemperature changes on the performance of the detector, havealso been explored along with methods to compensate for thesechanges.

1. INTRODUCTION

There is a need for a compact, but efficient, neutrondetector for inclusion within hand-held instruments whose

main task is to identify the nature of radioactive sourcesthrough their gamma-ray spectra. Currently, there is a choicebetween using either a small He3 proportional counter (e.g.LND SKO1226) or a scintillation counter based on the use of aLithium compound. Although BF3 proportional counters arewidely used for the detection ofneutrons they are not aseffective in rejecting any gamma-ray background [1] which isimportant in this application. Of the available lithiumscintillators, lithium iodide is preferable to lithium glassbecause of its higher scintillation efficiency (11000 ph/MeVcompared to 5700 ph/MeV [2,3D, better spectral match toSiPMs and slightly greater sensitivity.

I. DESIGN

A. Light Collection

SiPMs are well suited to compact handheld instruments byvirtue oftheir small size and low voltage requirements. SinceSiPMs are not yet available in sizes greater than 9mm2

, a tiled

Manuscript received November 4, 2008. This work was supported in part bythe UK Department of Trade and IndustryMark Foster is a postgraduate student with the University of Southampton,Southampton, Hampshire, UK, SOl7 IBJ (telephone +442380 592246, e-mail: m.a.foster@phys.soton.ac.uk)David Ramsden is with Symetrica Ltd, 2 Venture Road, SouthamptonScience Park, Southampton SOl6 7NP (Telephone +442380762484 e-mailD.Ramsden@Symemtrica.com)

array was sourced from SensL having sixteen elements and atotal active area of 144mm2 and a physical area of 196mm2

•

This tiled array was coupled to the 23mm circular crystal by anacrylic light guide. A range of lengths and wrappings weresimulated using the optical Monte Carlo package in Genat4and a length of 15mm was chosen to provide good light-collection efficiency (LCE), nominally 45%. A PTFEwrapping over a polished surface provided diffuse reflection toimprove LCE and uniformity by randomizing the scintillationlight.

The acrylic light guide also acts as a moderator and contributesan estimated 20% ofthe total neutron count when the source ison the detectors axis.

B. Moderation

Neutron moderation was provided by high densitypolyethylene (HDPE). Several moderator configurations werestudied by both simulation and experiment and it was foundthat a thickness of 15mm offered the best compromise betweendetecting fast and slow neutrons whilst maintaining a compactinstrument.

The moderator layout shown in Fig. 1. was found to offer thebest sensitivity of the configurations studied and allowed ananalysis of how moderator configuration affects the directionalresponse ofthe system.

SiPM

Aeryllic light guide___-4___ LiI(Eu) crystal in AI housing

HOPE moderator

Fig. I. The detector configuration chosen in this experiment. The "axis" isdefined to be down the page so that the SiPM is behind the crystal.

II.EXPERIMENTAL PROCEDURE

The prototype instrument was exposed to a 1.3 x 104 n/s Cf52

source at 25 cm on axis both with and without a 5cm HDPEshield. Gamma-rays were provided by an 85kBq Co60 sourceplaced at 6 cm on axis. The SiPM array was set to a bias of30

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V, that is, an overvoltage of2 V. Pulses were processed intospectra using an Amptek Pocket MCA for analysis. Based onthese spectra, a pulse height threshold was set in software inorder to measure the count-rates when exposed to neutrons,Co60 gamma-rays and background. These rates were then usedin a ROC curve analysis [4] to determine the integration timeneeded to alarm with a confidence of93.5% and a false alarmprobability in this gamma-ray background of no more than0.1%.

III. PERFORMANCE

A. Sensitivity and Gamma Rejection

As shown in Fig. 2. the thermal neutron peak is well resolvedat 12% FWHM and is well separated from the gamma-raybackground thus leading to excellent gamma-ray rejection.The energy threshold was set at six standard deviations fromthe centroid of the neutron peak to give the best compromisebetween neutron detection efficiency and collecting gamma-ray background counts.

The use ofLiI(Eu) as a neutron scintillator is well establishedso this instrument is a test ofhow well SiPMs are suited to itsread-out. The excellent gamma-ray rejection is due to theSiPMs lack ofdirect gamma-ray interactions. The peak isdistinct and well resolved, implying that the light guide isuniform and efficient, as predicted by the Geant4 simulations.

B. Directional Response

A handheld neutron detector with full directionality, that is,sensitive to neutrons incident from the front only, can assistwith locating a source, but this comes at the expense ofabsolute sensitivity. If sensitivity is a greater concern, then afully uniform response is preferred. With this in mind, theinstrument was tested using a Cf52 source at a distance of25cm and placed at 45° increments from the axis (down thepage in Fig. 1.) to establish where between these two extremesit lies and what can be done to achieve omni-directionality.

TABLED

DIRECTIONAL RESPONSE EXPRESSED AS A PERCENTAGE OF AxiS RESPONSE

400

350

300

250fI2

-= 200==U150

100

50

00 200 400 600

Channel

800

--Cf-252

--Co-60

--Threshold

1000

Angle Response

0° (axis) 100%

45° 95%

90° 94%

135° 79%

180° 78%

Rear incident neutrons are over-moderated in the rearmoderator and light guide and are scattered further from thecrystal and are less likely to be scattered toward it.

Fig. 2. The neutron, gamma-ray and background spectra plotted together toshow the excellent gamma-ray rejection. Note the energy threshold set sixstandard deviations from the thermal neutron peak.

Given a background count-rate of 0.04 cps, which did notincrease when the gamma-ray source was present, integrationtimes and number thresholds were calculated to give adetection confidence of93.5% and a false alarm probability of0.1%:

In conclusion, although this instrument does not have auniform response it demonstrates that the response can betailored using different moderator configurations. There isalso significant room for improvement by making the responsemore uniform, perhaps by embedding the light guide within therear moderator to prevent over-moderation. If a fullydirectional response is required, a neutron absorber such as aboron compound can be introduced.

TABLE INUMBER THRESHOLDS AND INTEGRATION TIMES FOR SHIELDED AND

UNSHIELDED NEUTRON SOURCES

Shielded Source

Unshielded Source

Count-rate

2.4 nls

0.7 nls

NumberThreshold

2 counts

4 counts

Integration Time

2.5 seconds

12.0 seconds

IV. COMPARISON WITH EXISTING NEUTRON COUNTERS

A. He3 Tube ofSimilar Size

In order to establish how this prototype detector compareswith small He3 tubes it was compared to a 76mm x I3mmcylindrical He3 tube having a quoted sensitivity of 9cps/nv(Part no. LND SKOI226). Three moderator configurationsaround the tube were tested, cylindrical moderators ofthickness 8mm, I6mm and 24mm. All designs were testedusing a I.3xl04n/s Cf52 source at 25cm from the centre ofthetube.

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TABLE IIINEUTRON COUNT-RATES FOR SHIELDED AND UNSHIELDED SOURCES AS

MEASURED BY THE PROTOTYPE INSTRUMENT AND A HE3

TUBE

temperature testing, only the range +20 to O°C was explored inorder to establish the magnitude of the effect.

Fig. 4. A series of spectra taken at 20°C, 10°C and O°C, respectively,showing the strength of the temperature variation. Note the 100% change inMCA channel number over the 20°C range.

--Threshold at20°C

--10°C

160

140

120

100fo'2

-= 80=eU

60

40

20

0

0 500 1000

Since this instrument does not require a spectrum to measureneutron count-rates, merely a count-rate above a threshold, asolution with simple electronics is desirable, such as varyingthe SiPM bias to compensate for temperature induced changes.Digitally controllable bias supplies and resistors arecommercially available which can be programmed to vary theSiPM bias supply by 25mV1°C. This means that the neutronpeak can be measured once and an energy threshold set inhardware allowing for simple readout electronics.

Temperature cycling was performed using a programmableenvironmental chamber set to dwell at 20, 10 and O°C for 2hours with the intervening steps taking 15 minutes. The timerequired for the SiPM and crystal to reach equilibrium was 1.5hours. Temperatures were taken every minute by two sensors,one in contact with the SiPM and one outside the can in thechamber.The temperature response of the first prototype was verystrong at 5%IOC using earlier SiPMs. The same SiPM arrayhad a temperature response of 3%OC when coupled to CsI(Tl).Later SiPMs have a much more benign temperature response,lo/oI°C with CsI(TI). The strong movement of the peak allowsthe background count-rate to increase by 75% and gamma-raycount-rate to increase by 210%. The twin peaks around MCAchannel 200 are due to electronic noise from a poorlypositioned digital thermometer.

He) with 8mm HDPE 1.7 nls 0.1 nls

He) with 16mm HDPE 2.3 nls 0.6 nls

He3 with 24mm HDPE 2.9 nls 1.0 nls

LiI(Eu) with 15mmHDPE 2.4 nls 0.7 nls

Shielded source Unshielded source

MCA Channel Nwnber

oo

Fig. 3. A Cf252 neutron and Co60 gamma-ray spectra taken with a SCIONTX16P3 I 10-Li-E2-T-X2, the same as used by G.Pausch in [5]. The Cf252

source is shielded and the detector was without a moderator.

It can be seen that the prototype detector is comparable to aHe3 tube surrounded by 16mm ofHDPE, and is roughly thesame size and weight. Therefore, a LiI(Eu) scintillator viewedby a SiPM is competitive in performance with the benefits ofrequiring no high voltage power supply and being safer tohandle and transport.

B. Li/(Eu) read out by a PIN diode

An alternative to SiPM readout is a conventional PIN diode,offering the same size and low voltage requirement. However,as can be seen in Fig. 3, they are vulnerable to direct gamma-ray interaction in the depletion region ofthe diode, leading topulses comparable to the neutron signal, subverting pulseheight discrimination. Methods have been developed to avoidthis problem based upon pulse shape analysis [5], rejectingpulses based upon their width, but these add complicationespecially when the signal is digitized. It is believed that suchmeasures are not required and can be replaced by a simplefilter.

V. TEMPERATURE RESPONSE AND STABILISATION

A handheld radiation sensor can be expected to operate in therange +50°C to -20°C. Therefore, the temperature response ofthe SiPM-based detector was measured. In the first phase of

VI. ARTIFACTS IN THE NEUTRON SPECTRUM

The neutron spectra contain both a single thermal neutron peakand a continuum and the origin of the latter is of interest.Direct gamma-ray and neutron interactions in the SiPM arevery unlikely due to the thin depletion region. Non-uniformity

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....00

300 --Shielded with

(12

== :!oo= --Unshielded ,,,theU

100 --Unshielded\vithoutf\10derator

0 --Threshold0 500 1000

• Excitement of iodine by inelastic scattering ofneutrons resulting in gamma-ray emission

• Activation of the iodine by neutron absorption

• A competing nuclear reaction Li6 + n a + H2 + n

Gamma-rays from cr52

in the light guide can be discounted based on the distinct andwell-resolved peak. It is likely to be an interaction in thecrystal producing scintillation light.

Several possibilities were considered:

The first and last candidates are unlikely as the continuumdoes not follow the same shape as the gamma-ray continuumand cr52 only produces gamma-rays up to 900keV, lower thanC060

. Iodine decays by a with a cut-offof2.2MeV.It is known that LiI(Eu) is 17% more efficientJ6] whenexcited by an electron than an a-particle so we could expect acontinuum with a cut-offof2.57MeV.

The competing nuclear reaction will produce a continuum as itis a three body reaction. The reaction in question is a fast-neutron reaction and becomes dominant at neutron energiesabove 2.2MeV and has a Q-value of -1.47MeV [71. In thiscase, 1.47MeV of the incident neutron energy is used toseparate the lithium nucleus and so goes into the kineticenergies of the H2 and a-particle. If this is correct then onewould expect the count-rate in the continuum to be greater foran unshielded source than one shielded by 5cm HOPE. TableIV and Fig. 5. show data taken using the Cf52 source at 25cmwith and without the 5cm HOPE shield and 15mm thickmoderator.

TABLE IVPEAK AND CONTINUUM COUNT-RATES WITH VARYING MODERATOR

CONFIGURAnON

MCAChannel

Fig. 5. Three neutron spectra showing the thermal peak and continuum withvarying moderator configurations. Note how the continuum is much strongerwhen the source is unshielded. The small peak shift observed here is believedto be due to temperature variations.

Given that the continuum becomes strongest when nomoderating material is present, especially when the source isunshielded, the reaction is indeed due to fast neutrons. In thetotally unmoderated case, the 4.8MeV peak is still present dueto neutrons moderated in the light guide scattering back intothe crystal.

The continuum extends at least up to the thermal neutron peakat 4.8MeV, too high for from iodine. Also, neutronactivation is more likely for thermal neutrons and so thecontinuum should get stronger when more moderation isintroduced which is observed to not be the case.

In conclusion, the continuum is most likely to be due to fastneutrons scattering off lithium nuclei causing them to decay ina three body reaction.

Shielded Sourcewith ModeratedDetector

UnshieldedSource withModeratedDetector

UnshieldedSource withUnmoderatedDetector

Peak

11.5 n/s

7.6 nls

1.3 nls

Continuum

3.0 nls

32.7 n/s

22.6 n/s

VII. CONCLUSION

Development of a compact neutron detector capable ofdetecting a 1.3 x 10 4 n/s source at 25cm within 12 seconds hasbeen successful through the use of a LiI(Eu) scintillator and aSiPM. Finally, the combination of LiI(Eu) and a SiPM hasproven to be competitive with the established He3 tubes.SiPMs are a viable alternative to PIN diodes in compactneutron detectors and are better suited to scaling and do notrequire pulse shape discrimination to achieve gamma-rayrejection.

As a measure ofhow useful this instrument can be in practice,it was compared against the ONOO (Domestic NuclearDetection Office) specification for a handheld radiationdetector. They specify that the neutron detecting componentmust be able to detect a 4 x 104 n/s cr52 source at a distanceof 25cm in less than 30 seconds with and without a 5cm HDPEshield. Given that the detector prototype has been testedagainst a 1.3 x 104 n/s source its performance using the

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stronger source specified has been predicted by scaling thecount rates above background and is shown here in Table V.

TABLE VPREDICTED PERFORMANCE OF THE DETECTOR USING THE DNDO SPECIFIED

SOURCE

Shielded Source

UnshieldedSource

Count-rate

7.3 nls

2.2 nls

NumberThreshold

1 count

2 counts

IntegrationTime

0.7 seconds

2.3 seconds

Therefore, even though this design served as a proof-of-concept and can be greatly improved, it is sufficient to meetthe DNDO specifications.

ACKNOWLEDGMENT

MAF would like to thank Symetrica Ltd and SensL for theirsupport throughout this work and Matthew Dallimore for theuse ofFig. 3.

REFERENCES

[1] G. Knoll, Radiation Detection and Measurement, 3rd ed, Wiley, 2000,pp.514

[2] LiI(Eu) datasheet from Scionix[3] Lithium glass datasheet from Saint Gobain Crystals available online[4] Egan,1. P.: 1975, Signal Detection Theory and ROC Analysis, Series in

Cognitition and Perception. New York: Academic Press.[5] G. Pausch and 1. Stein, "Application of 6LiI(Eu) Scintillators With

Photodiode Readout for Neutron Counting in Mixed Gamma-NeutronFields," IEEE Trans. Nucl. Sci, vol. 55, no. 3,june 2008

[6] G. Knoll, Radiation Detection and Measurement, 3rd ed, Wiley, 2000,pp.517

[7] G. Knoll, Radiation Detection and Measurement, 3rd ed, Wiley, 2000,pp.546

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