ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 551 (2005) 448–457
www.elsevier.com/locate/nima

Development of a wide-range paired scintillator with opticalﬁber neutron monitor for BNCT irradiation ﬁeld study
Masayori Ishikawaa,, Hiroaki Kumadab, Kazuyoshi Yamamotob,
Junichi Kanekoc, Gerard Benguad, Hironobu Unesakid, Yoshinori Sakuraid,
Kenichi Tanakae, Toshiso KosakofaDepartment of Nuclear Engineering and Management, Graduate school of Engineering, The University of Tokyo, Japanb
Japan Atomic Energy Research Institute, Japanc
Graduate school of Engineering, Hokkaido University, Japand
Research Reactor Institute, Kyoto University, Japane
Research Institute for Radiation Biology and Medicine, Hiroshima University, Japanf
Department of Nuclear Professional School, Graduate school of Engineering, The University of Tokyo, Japan
Received 24 May 2005; received in revised form 14 June 2005; accepted 15 June 2005
Available online 6 July 2005

Abstract
A wide range thermal neutron detector was developed based on the Scintillator with Optical Fiber (SOF) detector
which has been previously used for thermal neutron monitoring during boron neutron capture therapy irradiation.
With this new detector system we intended to address the issues of real-time thermal neutron ﬂux measurement and thesimultaneous measurement of a wide range of thermal neutron ﬂux in a BNCT irradiation ﬁeld which were difﬁcult toimplement with the gold wire activation method.
The dynamic range of linearity of the SOF detector was expanded by using a plastic scintillator with a rapid decay
time. On the other hand, the contribution of gamma rays and fast neutrons in the measured signals were compensatedfrom those obtained by a pair of SOF detectors, one with a 6LiF neutron converter and the other without. The
discrimination level for the measured signals was also optimized to further reduce the contribution of gamma rays andfast neutrons signals. A non-paralyzable system model was applied to correct for the dead-time in the detector
system.
A good agreement between the thermal neutron ﬂux measured by the gold wire activation method and the paired
SOF detector system was observed. However, measurements which would normally take a few days to perform with theCorresponding author. Tel.: +81 3 5841 2988; fax: +81 3 5841 2928, +81 3 3815 8540.

E-mail address: masayori@n.t.u-tokyo.ac.jp (M. Ishikawa).
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2005.06.044

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449

gold wire activation method were obtained in just about 15 min using the SOF detector system. We also conﬁrmed the
dynamic range of linearity for the SOF detector system to be in the order of magnitude of 104.
r 2005 Elsevier B.V. All rights reserved.
PACS: 87.66.a; 87.53 Pb; 87.53.Qc
Keywords: SOF detector; Ultra-miniature detector; Thermal neutron monitor; Phantom experiments; BNCT

1. Introduction
There are many detectors used for thermal
neutron monitoring such as the 10BF3 or 3He
gas counter, ﬁssion chambers and the 6Li-doped
photodiode detector [1]. Recently, a new type ofdetector using a scintillator coupled to an opticalﬁber was developed for the same purpose [2].
Among these types of detectors, the Scintillator
with Optical Fiber (SOF) detector has been used
for monitoring thermal neutrons during boron
neutron capture therapy (BNCT) irradiations inJapan [3]. The detector probe of the SOF detector
system is smaller than the other neutron monitors
which makes it more useful for thermal neutron
monitoring in narrow spaces.
The SOF type detector was originally developedby Mori [1]. Mori’s detector used a mixed powderof 6LiF and ZnS(Ag) as scintillator. In thisdetector system the scintillation signals wereconverted to electric pulses via a photo-multipliertube and then the discriminated pulses werecounted to eliminate gamma-ray signals. Mori’s
detector system worked well except for one
limitation which was the dependence of themeasured gamma-ray contribution on the amountof incident gamma rays.
In the neutron irradiation ﬁeld for BNCT,
thermal neutron ﬂux could vary from 106 toalmost 1010 n/cm2/s. The gold wire activation
method has been routinely used to measured
thermal neutron ﬂux distribution in a BNCT
irradiation ﬁeld but real-time measurements using
this method are difﬁcult to implement. Moreover,simultaneous measurement of such a wide range ofthermal neutron ﬂux with activation method is noteasy to carry out.
In this paper, we will report an upgrade of the
SOF type detector by using a plastic scintillator. A

plastic scintillator has a relatively rapid decay time(in the order of a few nanoseconds) which is
expected to expand the detector’s dynamic rangeof linearity.

2. Detector characteristics
2.1. Detector composition
Shown in Fig 1 is a schematic illustration of the
SOF detector system. The SOF detector consists ofa small amount of plastic scintillator, a plastic
optical ﬁber, a photo-multiplier tube, a charge
pre-ampliﬁer, a discriminator and a counter. In
this research, we used a BC490 plastic scintillator
manufactured by Bicron Ltd. The BC490 ispartially polymerized and hardened with a catalyst
and this makes it possible to be tightly connectedto the tip a plastic optical ﬁber (Mitsubishi Rayon
MH4001, 1 mm-diameter optical ﬁber with
2.2 mm-diameter polyethylene shielding). A small
amount of LiF powder (enriched 95% 6Li) is thenpainted over the plastic scintillator. The reactions
between 6Li nuclei and thermal neutrons emit
charged particles (alpha and triton) which
produce scintillation photons in the plastic scintillator. The photon signals are relayed through theoptical ﬁber onto the Photon Counting Unit
(Hamamatsu H7155) and then converted into
30 ns-width TTL pulses. The counting unit ismade up of a photo-multiplier tube, charge preampliﬁer and discriminator. The pulse counts aresent to a personal computer via a universal serialbus (USB) connection. By setting an adequate
discrimination level, a small or negligible gammaray contribution could be achieved since a plastic
scintillator has a small cross-section for photoelectric reactions.

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M. Ishikawa et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 448–457

Fig. 1. Schematic diagram of SOF detector system. The SOF detector made up of a small plastic scintillator with
optical ﬁber, a photon counting unit and data acquisition system connected to personal computer via USB.

2.2. Performance test
To evaluate our SOF detector, a linearity test
was performed using the JRR-4 Medical irradiation facility at the Japan Atomic Energy ResearchInstitute (JAERI). Fig. 2 shows the experimental
setup for the linearity performance test. The
detector probe was placed in an 18.6 cm f  24 cm
water-ﬁlled phantom whose walls were made froma 3 mm thick polymethylmetacrylate (PMMA) toenhance thermal neutron ﬂux. The dynamic rangeof linearity for the SOF detector was investigatedby varying the reactor power. The thermal neutron
ﬂux at the point of measurement ranges from2  104 to 3  109 n/cm2/s depending on the reactor
power which can be set at 20 W to 3.5 MW.
Gamma-ray ﬂux was also found to be dependent
on the reactor power; however, the variation in the
gamma ﬂux with respect to the reactor power was

6

LiF, a plastic

not linear because of the strong gamma rays
coming from the reactor core.
Fig. 3 shows the measured count rate as afunction of the reactor power at TNB-1 irradiation
mode (see Table 1). With the reference point set at
20 kW, non-linearity was observed under 2 kW
and over 200 kW. The non-linearity below 2 kW
could be attributed to gamma rays and/or fastneutrons because the measured count rate by the
SOF detector was larger relatively to the expected
count rate at this range of the reactor power.
On the other hand, the non-linearity over
200 kW could be due to count-drop which may
occur at a high counting rate because the measured
count rate by the SOF detector was smaller
relative to what is expected at this range of thereactor power. As a result, the dynamic rangeof linearity was limited in the order of magnitudeof 102.

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Fig. 2. Experimental setup at JRR-4 medical irradiation facility The detector probe was placed in an 18.6 cm +  24 cm water-ﬁlled
phantom whose walls was made from a 3 mm thick polymethylmetacrylate (PMMA) to enhance thermal neutron ﬂux.

3. Upgrading SOF detector
3.1. Compensation of gamma-ray and fast-neutron
contributions
A compensation method for the SOF detector
for gamma rays was previously reported by
Ishikawa [3]. Fig. 4 shows the pulse heightspectrum in a neutron irradiation ﬁeld taken by
a BC490 plastic scintillator with and without a
6
LiF neutron converter. The solid line is the
spectrum measured by the BC490 with 6LiF,
which includes thermal neutron, gamma-ray
and fast neutron signals. It is sometimes
difﬁcult to completely eliminate the gammaray or fast neutron contribution in the spectrum even if the discrimination level is set highenough.
The dashed line in Fig. 4 gives the measured
spectrum by the BC490 without 6LiF which
includes gamma-ray and fast-neutron signals.

Fig. 3. The measured count rate as a function of the reactor
power. Non-linearity was observed under 2 kW and over
200 kW.

The gamma-ray and fast neutron contribution
can be delineated and compensated from thedifference between the signals obtained from the

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Table 1
Characteristics of JRR-4 medical irradiation facility (collimator size: 15 cm, free beam)
Irradiation
mode

ENB
TNB-1
TNB-2

D2O thickness
(cm)

8
12
33

Cadmium
ﬁlter

On
Off
Off

Neutron ﬂux (n/cm2/s)
Thermal0.53 eV

Epithermal
0.53–10 keVFast
10 keV

3.6  108
2.0  1096.5  1082.2  1099.0  1083.2  107

9.5  107
2.7  107
6.2  105

Cadmium
ratio

Gamma
ray (Gy/h)

1.15
2.5
13.5

0.19
0.24
0.47

to the difﬁculty in their construction. Thisdifference led to a different efﬁciency for eachdetector. Other possible differences between thepaired SOF detectors could be due to thetransmission loss of optical ﬁber, gain of photomultiplier tubes and slightly different discrimination level. In order to account for the difference inthe relative efﬁciencies of the detectors, correction
factors for the measured counts by each detector
were introduced. The measured counts for the
scintillator with and without 6LiF were assumed to
be expressible in terms of Eqs. (1) and (2),respectively:
Fig. 4. The pulse height spectrum of a BC490 plastic scintillator
with and without 6LiF neutron converter in a neutron
irradiation ﬁeld. The solid line is the spectrum measured by
the BC490 with 6LiF, and the dashed line is that of without
6
LiF.

scintillator without 6LiF and the signals from thescintillator with 6LiF.
3.2. Theory of compensation
Fig. 5 shows a schematic diagram of the paired
SOF detector system. The signal processing of thepaired SOF detector is exactly the same as that ofa single SOF detector. As shown in Fig. 5, thepaired SOF detector is composed of two BC490
scintillators, one with 6LiF and the other without
6
LiF. Both scintillators, with and without 6LiF,
were connected to a paired plastic optical ﬁber
(MH4002, Mitsubishi Rayon) and coated with areﬂector paint (BC620, Bicron Ltd.). The sizes ofthe scintillators were not perfectly identical owing

C þ ¼ Rnþ F n þ Rgþ F g þ Rfþ F f(1)

C  ¼ Rg F g þ Rf F f .

(2)

In Eq. (1), C+ represents the measured counts ofthe scintillator with 6LiF. This is given here as thesum of the corresponding products between
the particle ﬂuence (i.e, F n , F g , and F f ) and thedetector response factor (i.e., Rn , Rg , and Rf ) for
thermal neutrons, gamma rays and fast neutrons.
In Eq. (2), C  , Rg , and Rf are the measured
counts, response factor for gamma rays andfast neutrons, respectively, for the scintillator
without 6LiF.
The response factors Rgþ and Rg wereobtained from the measured counts of both
detectors when only a gamma ray ﬁeld was usedfollowing Eqs. (3) and (4). Here, C 0þ and C 0 are
the measured gamma ray counts for the detectors
with and without the neutron converter, respectively. These correction factors were determined from measurements using an intense pure

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Fig. 5. Schematic diagram of the paired SOF detector system. The signal processing of the paired SOF detector is exactly same as thatof a single SOF detector. The paired probe consists of BC490 plastic scintillator with and without 6LiF.

gamma-ray source such as 137Cs:
C 0þ ¼ Rgþ F 0g

The expression for the neutron ﬂux Fn can then be
deduced from Eq. (5) and is given by
(3)
Fn ¼

C 0 ¼ Rg F 0g .

(6)

(4)

The response ratio Rgþ =Rg can be expressed interms of C 0þ and C 0 from Eqs. (3) and (4).
The response factors Rf+ and Rf should
be proportional to Rgþ and Rg , respectively,
since the detector responses were only inﬂuenced
by the scintillator volume. Accordingly, theresponse ratio Rfþ =Rf should be equal toRgþ =Rg . From these four equations, we obtain
C+ in the form
C0
RgþC  ¼ Rnþ F n þ þC.
C þ ¼ Rnþ F n þ
Rg
C 0

C þ  ðC 0þ =C 0 ÞC .
Rnþ

3.3. Optimization of discrimination level
If the discrimination level is high enough, thecontribution of gamma rays and fast neutrons will
be small. However, the amount of thermal neutron
signals will also be small because many thermal
neutron signals are discarded. Therefore, we tried
optimizing the discrimination level by consideringcounting statistics. We rewrite Eq. (6) in the form
Fn ¼

(5)

C þ  ðRgþ =Rg ÞC 
Rgþ
1
¼
Cþ 
C.
Rnþ
Rnþ
Rnþ Rg

(7)

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The standard deviation of the evaluated thermal
neutron ﬂux sF n is given as follows.
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
s


2 ﬃ
Rgþ
1 2 2
sF n ¼sC þ þ
s2C 
RnþRnþ Rgpﬃﬃﬃﬃﬃﬃﬃ sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
Rgþ 2 C 
Cþ
1þ
.
ð8Þ
¼
Rnþ
Rg C þHere, sC þ and sC  are the standard deviations of
the measured count of the scintillators with andwithout the neutron converter, respectively. The
relative standard deviation is given by
sF n =F n
1¼ pﬃﬃﬃﬃﬃﬃﬃ
Cþsﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ,
Rgþ 2 C Rgþ C 1.
1þRg C þRg C þ3.4. Confirmation of compensation
As pointed out in the previous section, the SOF
detector system requires the conversion coefﬁcient
Rn+ and the Rgþ =Rg ratio to evaluate thermal
neutron ﬂux. An Rgþ =Rg ratio of 0.9556
was obtained from a 137Cs gamma-ray source(62 TBq) while a conversion coefﬁcient Rn+ of
2.072  103 was obtained from the thermal neutron irradiation ﬁeld at JRR-4.
A linearity test for the paired SOF detector was
performed at JRR-4 with the same conditions for asingle SOF detector experiment. Fig. 7 shows theresult of the linearity performance test with thepaired SOF detector. From Fig. 7, the compensation effect was observed below 2 kW, and thelinearity for the paired SOF detector was guaranteed from 200 W to 200 kW.

ð9ÞFig. 6 shows the relative standard deviation as afunction of the discrimination level under the same
condition as in Fig. 4. From Fig. 6, it was foundthat the relative standard deviation is minimized ataround 300 channels. However, the discriminationlevel was set to 500 channels for the succeeding
experiments because the minimum point appeared
to shift slightly higher as the gamma-ray contribution increases.

Fig. 6. The relative standard deviation as a function of thediscrimination level. The relative standard deviation is minimized at around 300 channels.

3.5. Dead-time correction
Generally, dead-time correction for the pulse
counting method is applied when a high counting
rate is used. This correction can be carried out
using a paralyzable system model or non-paralyzable system model. For our detector system deadtime can occur while discriminating the output ofthe photo-multiplier and during the counting ofFig. 7. The result of linearity performance test with paired SOF
detector. The compensation effect was observed under 2 kW,
and the linearity for the paired SOF detector was guaranteedfrom 200 W to 200 kW.

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455

the TTL pulses from the photon counting unit.
These processes appear to obey a non-paralyzable
behavior; thus, a non-paralyzable system model
was adapted for the dead-time correction in the
SOF detector system.
The true count rate for the non-paralyzable
system model is expressed byn¼

m.
1  mt(10)Here m stands for the observed count rate and t
for the dead time. To estimate the dead time of the
SOF detector system, a Light Emitting Diode(LED) with peak wavelength similar to a plastic
scintillator (420 nm) was used as a signal source inplace of the scintillation signal. The LED was
driven by a 300 MHz arbitrary waveform generator (Wavetek Model 302) which generated lightpulses similar to a real scintillation pulse. The deadtime of the SOF system was evaluated by changing
the generated pulse rate. Fig. 8 shows the pulse
counting rate as a function of generated pulse rate.
The solid circles show the raw counting rate of the
SOF detector. By ﬁtting this data into Eq. (10), thedead time of this system was evaluated to be
t ¼ 23:62 ns. The dashed line in Fig. 8 represents
the pulse rates with missed counts as a result of adead-time t ¼ 23:62 ns. The open circles in Fig. 8

Fig. 9. The linearity experiment data corrected with nonparalyzable system model. A good linearity was observed up to
3.5 MW.

are the corrected pulse rates computed based on
Eq. (10). These data are in good agreement withthe real pulse rate of 5  106 pulse per second(pps). For the probe used in this research which
has a conversion coefﬁcient of 2.072  103, thedead-time correction can be used for neutron
ﬂuxes of up to 1010 n/cm2/s.
Fig. 9 shows the data for the linearity experimentcorrected with the non-paralyzable system model.
The data sets were completely the same as in Fig. 7.
A good linearity was observed up to 3.5 MW in
Fig. 9. Accordingly, the SOF system has a linearity
range from 2  105 to 3  109 n/cm2/s whichindicates that the dynamic range of linearity was
guaranteed in the order of magnitude of 104.

4. Application for evaluation of neutron ﬁeld

Fig. 8. The pulse counting rate as a function of generated pulse
rate. Solid circle and open circle indicate the raw counting rateof SOF detector and the corrected value with non-paralyzable
system model, respectively. The dashed line represents thefunction of the pulse rate with non-paralyzable system model.

In order to verify the feasibility of using
the SOF detector with modiﬁcations mentioned
earlier, thermal neutron measurements were
performed at the JRR-4 irradiation facility.
Our benchmark for the evaluated characteristics
of JRR-4 irradiation facility were those reported
by Yamamoto [4] for the thermal neutron
distribution along with center axis of waterﬁlled phantom using the gold wire activation
method. For consistency of set-up, we used

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the same water-ﬁlled phantom used by Yamamotoin this research. Since gamma-ray ﬂux distribution
is not proportional to thermal neutron ﬂux
distribution, it was possible to perform measurements that would conﬁrm the effectivenessof the correction method for gamma ray. Additionally, we were able to check the dynamic
range of linearity of the SOF by varying thethermal neutron ﬂux range from 106 to 5  109 n/
cm2/s by using alternating irradiation mode(see Table 1).
The thermal neutron ﬂux measurement was
performed at 5 mm depth-increments from thesurface of the water phantom up to a depthof 53 mm. At depths greater than 53 mm, measurements were done at 10 mm depth-increments.
When the measured thermal neutron ﬂux reachedmore than 108 n/cm2/s, only 10-second measurements were enough to satisfy acceptable statisticalaccuracy, i.e. 0.5% standard deviation. However, at depths where the ﬂux went below
108 n/cm2/s measurement times were ﬂexibly adjusted in order to achieve the same statisticalaccuracy.
Fig. 10 shows the measured thermal neutron
ﬂux distribution by the paired SOF detector andthe gold wire method. Note that the neutron ﬂux
measured by the gold wire method were corrected
in reference to the measured value by the SOF

detector at 38 mm because the rector power was
expected to ﬂuctuate up to a maximum of 5% on
different days of measurement. From Fig. 10, it isfound that the measured value by SOF detector isin good agreement with the gold wire measurement. For the gold wire method, it will take a few
days to measure the thermal neutron ﬂux from 106
to 109; this is inclusive of the irradiation time,
waiting time for decay and measurement time. In
contrast, it took only 15 min to obtain almost samedata using the SOF detector. Moreover, thelinearity of the SOF detector was conﬁrmed in
the range of 106–5  109 n/cm2/s using appropriate
correction methods.

5. Conclusion
In order to upgrade the SOF detector system,
we tried compensating for the gamma-ray andfast-neutron contribution by using a paired
SOF detector, one with a neutron converter
and the other without. The paired SOF was
effective at low thermal neutron ﬂux. Moreover,in order to correct for count-drop at high counting
rate, the non-paralyzable system model wasapplied to the paired SOF detector. As a result,the dynamic range of linearity was conﬁrmed
in the order of magnitude of 104. For this system,a dynamic range of linearity up to an orderof magnitude of 105 should be achievable.
We conﬁrmed that this upgraded SOF detector
is useful for the evaluation of BNCT irradiation
ﬁeld.

Acknowledgement
This research is supported by a Grant-in-Aid
for Young Scientists (A) (#16689022) by the
Ministry of Education, Culture, Sports, Scienceand Technology.
Fig. 10. The measured thermal neutron ﬂux distribution by thepaired SOF detector and the gold wire method. The values
measured by the gold wire method were corrected in reference
to the measured value by the SOF detector at 38 mm.

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