N26-3
2008 IEEE Nuclear Science Symposium Conference Record
A Compact Neutron Detector Based on the use of a
SiPM Detector
Mark Foster and David Ramsden Member, IEEE
Abstract-This paper explores the performance characteristcs of
a compact neutron detector based on the use of a silicon photomultiplier (SiPM). These devices offer the first true solid-state
alternative to traditional photomultiplier tubes in that they
provide similar gain and photon-detection efficiency but with the
small size and ruggedness of PIN diodes. The performance of a
design based on the use of a 23mm diameter Lil crystal, viewed
by a 14x14mm square SiPM, has been modeled and tested
experimentally using fission-neutrons. The design
incorporates a 10mm thick HDPE moderator in front of the 3mm
crystal and an acrylic light-guide behind it. This provides the
ability to detect a 104 nls source at 25cm in 12s with a confidence
level of 93.5%. The gamma-ray rejection capability of the
detector is excellent, allowing reliable detection of neutrons in a
Co60 gamma-ray background equivalent to a count-rate
equivalent to 100kcps in a 2x2" NaI crystal. The impact of
temperature changes on the performance of the detector, have
also been explored along with methods to compensate for these
changes.
1. INTRODUCTION
T
here is a need for a compact, but efficient, neutron
detector for inclusion within hand-held instruments whose
main task is to identify the nature of radioactive sources
through their gamma-ray spectra. Currently, there is a choice
between using either a small He 3 proportional counter (e.g.
LND SKO 1226) or a scintillation counter based on the use of a
Lithium compound. Although BF 3 proportional counters are
widely used for the detection of neutrons they are not as
effective in rejecting any gamma-ray background [1] which is
important in this application. Of the available lithium
scintillators, lithium iodide is preferable to lithium glass
because of its higher scintillation efficiency (11000 ph/MeV
compared to 5700 ph/MeV [2,3 D, better spectral match to
SiPMs and slightly greater sensitivity.
array was sourced from SensL having sixteen elements and a
2
total active area of 144mm2 and a physical area of 196mm •
This tiled array was coupled to the 23mm circular crystal by an
acrylic light guide. A range of lengths and wrappings were
simulated using the optical Monte Carlo package in Genat4
and a length of 15mm was chosen to provide good lightcollection efficiency (LCE), nominally 45%. A PTFE
wrapping over a polished surface provided diffuse reflection to
improve LCE and uniformity by randomizing the scintillation
light.
The acrylic light guide also acts as a moderator and contributes
an estimated 20% of the total neutron count when the source is
on the detectors axis.
B. Moderation
Neutron moderation was provided by high density
polyethylene (HDPE). Several moderator configurations were
studied by both simulation and experiment and it was found
that a thickness of 15mm offered the best compromise between
detecting fast and slow neutrons whilst maintaining a compact
instrument.
The moderator layout shown in Fig. 1. was found to offer the
best sensitivity of the configurations studied and allowed an
analysis of how moderator configuration affects the directional
response of the system.
SiPM
Aeryllic light guide
_ _ _- 4 _ _ _
I. DESIGN
LiI(Eu) crystal in AI housing
HOPE moderator
A. Light Collection
SiPMs are well suited to compact handheld instruments by
virtue of their small size and low voltage requirements. Since
SiPMs are not yet available in sizes greater than 9mm2 , a tiled
Manuscript received November 4, 2008. This work was supported in part by
the UK Department of Trade and Industry
Mark Foster is a postgraduate student with the University of Southampton,
Southampton, Hampshire, UK, SOl7 IBJ (telephone +442380 592246, email: [email protected])
David Ramsden is with Symetrica Ltd, 2 Venture Road, Southampton
Science Park, Southampton SOl6 7NP (Telephone +442380762484 e-mail
[email protected])
978-1-4244-2715-4/08/$25.00 ©2008 IEEE
Fig. I. The detector configuration chosen in this experiment. The "axis" is
defined 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 10 4 n/s Cf 52
source at 25 cm on axis both with and without a 5cm HDPE
shield. Gamma-rays were provided by an 85kBq Co 60 source
placed at 6 cm on axis. The SiPM array was set to a bias of 30
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V, that is, an overvoltage of2 V. Pulses were processed into
spectra using an Amptek Pocket MCA for analysis. Based on
these spectra, a pulse height threshold was set in software in
order to measure the count-rates when exposed to neutrons,
Co 60 gamma-rays and background. These rates were then used
in a ROC curve analysis [4] to determine the integration time
needed to alarm with a confidence of93.5% and a false alarm
probability in this gamma-ray background of no more than
0.1%.
III.
PERFORMANCE
A. Sensitivity and Gamma Rejection
As shown in Fig. 2. the thermal neutron peak is well resolved
at 12% FWHM and is well separated from the gamma-ray
background thus leading to excellent gamma-ray rejection.
The energy threshold was set at six standard deviations from
the centroid of the neutron peak to give the best compromise
between neutron detection efficiency and collecting gammaray background counts.
The use of LiI(Eu) as a neutron scintillator is well established
so this instrument is a test of how well SiPMs are suited to its
read-out. The excellent gamma-ray rejection is due to the
SiPMs lack of direct gamma-ray interactions. The peak is
distinct and well resolved, implying that the light guide is
uniform 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 assist
with locating a source, but this comes at the expense of
absolute sensitivity. If sensitivity is a greater concern, then a
fully uniform response is preferred. With this in mind, the
52
instrument was tested using a Cf source at a distance of
25cm and placed at 45° increments from the axis (down the
page in Fig. 1.) to establish where between these two extremes
it lies and what can be done to achieve omni-directionality.
TABLED
DIRECTIONAL RESPONSE EXPRESSED AS A PERCENTAGE OF AxiS RESPONSE
Angle
400
350
Response
0° (axis)
100%
45°
95%
90°
94%
135°
79%
180°
78%
300
250
fI2
-==
--Cf-252
200
U=
150
--Co-60
--Threshold
100
50
0
0
200
400
600
800
Rear incident neutrons are over-moderated in the rear
moderator and light guide and are scattered further from the
crystal and are less likely to be scattered toward it.
1000
Channel
Fig. 2. The neutron, gamma-ray and background spectra plotted together to
show the excellent gamma-ray rejection. Note the energy threshold set six
standard deviations from the thermal neutron peak.
Given a background count-rate of 0.04 cps, which did not
increase when the gamma-ray source was present, integration
times and number thresholds were calculated to give a
detection confidence of93.5% and a false alarm probability of
0.1%:
TABLE
In conclusion, although this instrument does not have a
uniform response it demonstrates that the response can be
tailored using different moderator configurations. There is
also significant room for improvement by making the response
more uniform, perhaps by embedding the light guide within the
rear moderator to prevent over-moderation. If a fully
directional response is required, a neutron absorber such as a
boron compound can be introduced.
IV. COMPARISON WITH EXISTING NEUTRON COUNTERS
I
NUMBER THRESHOLDS AND INTEGRATION TIMES FOR SHIELDED AND
A. He 3 Tube ofSimilar Size
UNSHIELDED NEUTRON SOURCES
Count-rate
Number
Threshold
Integration Time
Shielded Source
2.4 nls
2 counts
2.5 seconds
Unshielded Source
0.7 nls
4 counts
12.0 seconds
In order to establish how this prototype detector compares
3
with small He tubes it was compared to a 76mm x I3mm
3
cylindrical He tube having a quoted sensitivity of 9cps/nv
(Part no. LND SKOI226). Three moderator configurations
around the tube were tested, cylindrical moderators of
thickness 8mm, I6mm and 24mm. All designs were tested
52
using a I.3xl0 4n/s Cf source at 25cm from the centre of the
tube.
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TABLE
III
temperature testing, only the range +20 to O°C was explored in
order to establish the magnitude of the effect.
NEUTRON COUNT-RATES FOR SHIELDED AND UNSHIELDED SOURCES AS
3
MEASURED BY THE PROTOTYPE INSTRUMENT AND A HE TUBE
Shielded source
Unshielded source
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 15mm
HDPE
2.4 nls
0.7 nls
Temperature cycling was performed using a programmable
environmental chamber set to dwell at 20, 10 and O°C for 2
hours with the intervening steps taking 15 minutes. The time
required for the SiPM and crystal to reach equilibrium was 1.5
hours. Temperatures were taken every minute by two sensors,
one in contact with the SiPM and one outside the can in the
chamber.
The temperature response of the first prototype was very
strong at 5%IO C using earlier SiPMs. The same SiPM array
had 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 allows
the background count-rate to increase by 75% and gamma-ray
count-rate to increase by 210%. The twin peaks around MCA
channel 200 are due to electronic noise from a poorly
positioned digital thermometer.
It can be seen that the prototype detector is comparable to a
He 3 tube surrounded by 16mm ofHDPE, and is roughly the
same size and weight. Therefore, a LiI(Eu) scintillator viewed
by a SiPM is competitive in performance with the benefits of
requiring no high voltage power supply and being safer to
handle and transport.
160
B. Li/(Eu) read out by a PIN diode
140
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 gammaray interaction in the depletion region of the diode, leading to
pulses comparable to the neutron signal, subverting pulse
height discrimination. Methods have been developed to avoid
this problem based upon pulse shape analysis [5], rejecting
pulses based upon their width, but these add complication
especially when the signal is digitized. It is believed that such
measures are not required and can be replaced by a simple
filter.
120
100
--10°C
fo'2
-==
e
80
U
60
--Threshold at
40
20°C
20
0
0
500
1000
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 in
MCA channel number over the 20°C range.
o
o
Since this instrument does not require a spectrum to measure
neutron count-rates, merely a count-rate above a threshold, a
solution with simple electronics is desirable, such as varying
the SiPM bias to compensate for temperature induced changes.
Digitally controllable bias supplies and resistors are
commercially available which can be programmed to vary the
SiPM bias supply by 25mV1°C. This means that the neutron
peak can be measured once and an energy threshold set in
hardware allowing for simple readout electronics.
MCA Channel Nwnber
Fig. 3. A Cf252 neutron and Co 60 gamma-ray spectra taken with a SCIONTX
16P3 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.
VI.
V. TEMPERATURE RESPONSE AND STABILISATION
A handheld radiation sensor can be expected to operate in the
range +50°C to -20°C. Therefore, the temperature response of
the SiPM-based detector was measured. In the first phase of
ARTIFACTS IN THE NEUTRON SPECTRUM
The neutron spectra contain both a single thermal neutron peak
and a continuum and the origin of the latter is of interest.
Direct gamma-ray and neutron interactions in the SiPM are
very unlikely due to the thin depletion region. Non-uniformity
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....00
in the light guide can be discounted based on the distinct and
well-resolved peak. It is likely to be an interaction in the
crystal producing scintillation light.
300
- - Shielded with
Several possibilities were considered:
(12
= :!oo
==e
•
Excitement of iodine by inelastic scattering of
neutrons resulting in gamma-ray emission
•
Activation of the iodine by neutron absorption
•
A competing nuclear reaction Li + n 6
Gamma-rays from
cr
- - Unshielded ,,,th
U
100
- - Unshielded
\vithout
f\10derator
2
a +H +n
0
52
0
The first and last candidates are unlikely as the continuum
does not follow the same shape as the gamma-ray continuum
52
only produces gamma-rays up to 900keV, lower than
and
60
C0 . Iodine decays by a with a cut-offof2.2MeV.
It is known that LiI(Eu) is 17% more efficientJ6] when
excited by an electron than an a-particle so we could expect a
continuum with a cut-offof2.57MeV.
The competing nuclear reaction will produce a continuum as it
is a three body reaction. The reaction in question is a fastneutron reaction and becomes dominant at neutron energies
above 2.2MeV and has a Q-value of -1.47MeV [71. In this
case, 1.47MeV of the incident neutron energy is used to
separate the lithium nucleus and so goes into the kinetic
energies of the H 2 and a-particle. If this is correct then one
would expect the count-rate in the continuum to be greater for
an unshielded source than one shielded by 5cm HOPE. Table
IV and Fig. 5. show data taken using the Cf 52 source at 25cm
with and without the 5cm HOPE shield and 15mm thick
moderator.
cr
TABLE IV
PEAK AND CONTINUUM COUNT-RATES WITH VARYING MODERATOR
--Threshold
500
1000
MCAChannel
Fig. 5. Three neutron spectra showing the thermal peak and continuum with
varying moderator configurations. Note how the continuum is much stronger
when the source is unshielded. The small peak shift observed here is believed
to be due to temperature variations.
Given that the continuum becomes strongest when no
moderating material is present, especially when the source is
unshielded, the reaction is indeed due to fast neutrons. In the
totally unmoderated case, the 4.8MeV peak is still present due
to neutrons moderated in the light guide scattering back into
the crystal.
The continuum extends at least up to the thermal neutron peak
at 4.8MeV, too high for from iodine. Also, neutron
activation is more likely for thermal neutrons and so the
continuum should get stronger when more moderation is
introduced which is observed to not be the case.
In conclusion, the continuum is most likely to be due to fast
neutrons scattering off lithium nuclei causing them to decay in
a three body reaction.
CONFIGURAnON
Peak
Shielded Source
with Moderated
Detector
11.5 n/s
3.0 nls
Unshielded
Source with
Moderated
Detector
7.6 nls
32.7 n/s
1.3 nls
22.6 n/s
Unshielded
Source with
Unmoderated
Detector
VII.
Continuum
CONCLUSION
Development of a compact neutron detector capable of
detecting a 1.3 x 10 4 n/s source at 25cm within 12 seconds has
been successful through the use of a LiI(Eu) scintillator and a
SiPM. Finally, the combination of LiI(Eu) and a SiPM has
proven to be competitive with the established He 3 tubes.
SiPMs are a viable alternative to PIN diodes in compact
neutron detectors and are better suited to scaling and do not
require pulse shape discrimination to achieve gamma-ray
rejection.
As a measure of how useful this instrument can be in practice,
it was compared against the ONOO (Domestic Nuclear
Detection Office) specification for a handheld radiation
detector. They specify that the neutron detecting component
52
source at a distance
must be able to detect a 4 x 10 4 n/s
of 25cm in less than 30 seconds with and without a 5cm HDPE
shield. Given that the detector prototype has been tested
against a 1.3 x 10 4 n/s source its performance using the
cr
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stronger source specified has been predicted by scaling the
count rates above background and is shown here in Table V.
TABLE V
PREDICTED PERFORMANCE OF THE DETECTOR USING THE DNDO SPECIFIED
SOURCE
Countrate
Number
Threshold
Integration
Time
Shielded Source
7.3 nls
1 count
0.7 seconds
Unshielded
Source
2.2 nls
2 counts
2.3 seconds
Therefore, even though this design served as a proof-ofconcept and can be greatly improved, it is sufficient to meet
the DNDO specifications.
ACKNOWLEDGMENT
MAF would like to thank Symetrica Ltd and SensL for their
support throughout this work and Matthew Dallimore for the
use of Fig. 3.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
G. Knoll, Radiation Detection and Measurement, 3rd ed, Wiley, 2000,
pp.514
LiI(Eu) datasheet from Scionix
Lithium glass datasheet from Saint Gobain Crystals available online
Egan,1. P.: 1975, Signal Detection Theory and ROC Analysis, Series in
Cognitition and Perception. New York: Academic Press.
G. Pausch and 1. Stein, "Application of 6LiI(Eu) Scintillators With
Photodiode Readout for Neutron Counting in Mixed Gamma-Neutron
Fields," IEEE Trans. Nucl. Sci, vol. 55, no. 3,june 2008
G. Knoll, Radiation Detection and Measurement, 3rd ed, Wiley, 2000,
pp.517
G. Knoll, Radiation Detection and Measurement, 3rd ed, Wiley, 2000,
pp.546
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