Plasma and Fusion Research: Regular Articles

Volume 11, 2405020 (2016)

Development of Fast-Neutron Directional Detector for Fusion
Neutron Profile Monitor at LHD∗)
Eiji TAKADA, Akihiro FUJISAKI, Naoki NAKADA, Mitsutaka ISOBE1,2) , Kunihiro OGAWA1,2) ,Takeo NISHITANI1) and Hideki TOMITA3)
National Institute of Technology, Toyama College, Toyama 939-8630, Japan
1)
National Institute for Fusion Science, Toki 509-5292, Japan
2)
SOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan
3)
Nagoya University, Nagoya 464-8603, Japan(Received 22 November 2015 / Accepted 23 January 2016)
Fast-neutron directional detectors using scintillating optical fibers have been adopted for neutron emission
profile measurement in magnetic confinement fusion devices. Because of their benefits of rapid response anddirectional properties to reduce restrictions on shielding, they are anticipated for installation in the Large Helical
Device (LHD) of the National Institute for Fusion Science (NIFS). As described herein, the system design has
been re-examined based on the results of recent experiments and simulations. Results show that fast neutrons
with high energy of 14 MeV are attenuated by a self-shielding eﬀect of scintillating fiber. Moreover, it has
been demonstrated that the scintillation photons are attenuated in several centimeters before reaching the photondetector. The control of the photon transmission eﬃciency is important to enhance the directional propertyof the system. Along with reduced eﬀects of the background gamma rays, adopting a thin scintillating fiber iseﬀective. Further evaluation should be conducted to design a detector system considering the detecting eﬃciency,
directional property, and gamma ray eﬀect for application to an actual radiation field of the LHD.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: neutron measurement, fast neutron directional detector, scintillating optical fiber, large helical device(LHD), design
DOI: 10.1585/pfr.11.2405020

1. Introduction
Energetic particle confinement performance has beenassessed through neutron emission profile measurements
taken in magnetically confined experimental fusion devices. Scintillation detectors with multichannel collimators have been installed in many experimental fusion devices such as the Tokamak Fusion Test Reactor [1], theJoint European Torus [2], and JT-60 Upgrade [3]. One
system comprises scintillating optical fiber, which can operate as a directional fast neutron detector. The system has
been expected to reduce requirements for collimation performance. At the Large Helical Device (LHD) of National
Institute for Fusion Science (NIFS), operation with deuterium fuel is scheduled to be conducted in several years.
The scintillating fiber-based system is being planned for
installation as a neutron profile monitor for those experiments on the basement level of the LHD [4] (Fig. 1). The
scintillating fiber-based system was developed more than20 years ago, where the scintillating fiber diameter was
1 mm and its length was 10 cm [5, 6]. Since then, thesystem design has not been reconsidered. In this study,
the authors evaluated the eﬀects of the design parameters
author’s e-mail: takada@nc-toyama.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

Fig. 1 Schematic view of the neutron profile monitor for LHD[1].

and tried to optimize it for 14 MeV neutron measurement.
From the study, self-shielding of 14 MeV neutron by thescintillating fiber itself was revealed as a source of degra-2405020-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 11, 2405020 (2016)

Fig. 2 Structure of fast neutron directional detector.

dation of angular resolution of the system. Some scintillation photons were regarded as transmitted and reflected
at the walls of the holes into which the scintillating fibers
were inserted. Because of this phenomenon, the numberof the photons decreases with longer transmitting distance.
Considering these eﬀects, the system design has been reconsidered for LHD deuterium experiments.

Fig. 3 Energy deposition inside the scintillating fiber simulated
by PHITS.

2. Fast Neutron Directional Detector
Based on Scintillating Fiber
The system comprises several scintillating fibers inserted in shielding material, by which recoiled protons areprevented from entering another scintillating fiber. Thescintillating fibers are connected to a photomultiplier tube(PMT) to convert scintillation photons into electrical signals. Because the recoil protons are generated to a forward direction with greater probability, the energy deposition is greater when the neutrons are incident with smaller
value of θn in Fig. 2, which results in a larger output pulse
from the PMT. Therefore, by setting a proper discrimination level for the output pulse height, events with small θncan be measured selectively. In the system installed in JT60U, the diameter of the scintillating fiber was 1 mm. Its
length was 10 cm [5, 6].

3. Experimental and Simulation
To evaluate the properties of the previously developed
measurement system, we conducted simulations with theMonte Carlo Simulation Code PHITS [7]. Experiments
were conducted at the Fast Neutronics Source (FNS) of
the Japan Atomic Energy Agency (JAEA) [8, 9]. Figure 3
shows the simulated energy deposition inside the scintillating fiber with neutron incident angles of 0 deg. and70 deg. Figure 4 shows measured pulse height distributions
at equal incident angles. Results are shown for scintillatingfibers with lengths of 10 cm.
In the simulated and experimentally obtained results,
one can observe the edge at the highest energy region.
However, the edge shapes in the experiments were broader
than those in the simulation. In Fig. 5, for 5 cm-long scintillating fibers, the relation between the neutron incident

Fig. 4 Measured pulse height distributions by scintillating fiber
detectors, where the diameter was set to 1 mm and thelengths were 10 cm.

angle and the numbers of events with energy deposition of
more than 13 MeV on the neutron incident angle is shown.
In the same figure, counts measured at the edges are also
shown. Both the simulated and measured numbers werenormalized by those with the incident angle of 0 deg. It isapparent that the directional property is worse in the experiments than in the simulation.

4. Discussion and Detector Redesign
The diﬀerences between simulation and experimentprobably resulted from the following eﬀects: (1) gamma
rays in the experimental room, (2) self-shielding of theneutrons by the scintillating fiber itself, and (3) decreaseof the scintillation photons while they transmit through thescintillating fiber. We conducted additional simulations toascertain ways to reduce these eﬀects.
(1) Eﬀects of background gamma rays
Simulations with secondary gamma rays revealed nosignificant eﬀect on the results. Therefore, the background2405020-2

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Fig. 5 Angular distribution of the events by simulations andexperiments, where the values have been normalized tothose with the neutron incident angle of 0 deg.

gamma rays in the experimental room are expected to havebeen a main factor aﬀecting the pulse height distribution.
More scintillation photons are known to be generated in
the interaction with electrons than with protons. By theresultant phenomenon, the signal pulse height generated
by a gamma ray will be greater than that generated by aneutron having the same energy [10].
To find some means to reduce the gamma ray effect, we conducted simulations for the scintillating fiberwith 0.5-mm-diameter. In the simulations, the energies ofgamma rays and neutrons were set to 14 MeV. We recognize that the energy of 14 MeV for gamma rays is unrealistic in the fusion experimental environment. However, weused the value because the energy distribution of gamma
rays in LHD is unknown. For a 0.5-mm-diameter scintillating fiber, for 14 MeV gamma rays, the calculated number of events with energy deposition of more than 4 MeV
becomes 0.036 times smaller than that for 1-mm-diameter
scintillating fiber, whereas the ratio is 0.086 for 14 MeV
neutrons. These ratios are less than 0.25, which is calculated from the change of the sensing volume. That is true
because we set the energy thresholds, as described in section 3, to clarify the proportional property. In the case ofthin fiber, to generate pulses over the threshold, the permissible solid angle becomes smaller, which causes the lowereﬃciencies. Moreover, because the stopping power of thefiber is lower for electrons than for protons, the restriction for the solid angle for electrons is stricter than thatfor protons, which is the cause of the lower sensing eﬃciency for gammas. These results suggest that adopting
0.5-mm-diameter scintillating fiber can reduce the background gamma ray eﬀects.
(2) Self-shielding of the neutrons by the scintillating fiber
itselfTo evaluate the self-shielding eﬀect of neutrons by thescintillating fiber, PHITS simulation was conducted for thesetup depicted in Fig. 6. In the 14 MeV neutron beams

Fig. 6 Setup for the PHITS simulation to evaluate the selfshielding eﬀect by the scintillating optical fiber.

Fig. 7 Calculated distribution of the recoiled protons in a scintillating fiber, where 14 MeV neutrons enter into it as shownin Fig. 6.

with 12 cm width, a scintillating optical fiber was set at
two angles: θf = 0 deg. and 90 deg. Figure 7 shows thedistribution of the recoiled protons in a scintillating fiber.
Apparently, when θf = 0 deg, the number of recoil protonsdecreases as the depth from the neutron incident positionincreases. Up to 3 cm depth, the number decreases to 0.8
times of the value at the surface. At the deeper position, it
decreases gradually and becomes almost 0.4 times at 10 cm
depth. Therefore, the scintillation photons are generated
mainly at a position far from the PMT.
(3) Decrease of the scintillation photons while they transmit through scintillating fiber
In the measurement system, scintillating fibers wereinput into the holes made through the shielding material asaluminum. As presented in Fig. 8, to evaluate the decreaseof the scintillation photons in transmission process, experiments were conducted using a 137 Cs gamma ray source bychanging the gamma ray incident position on the scintillating fiber. In the experiments, scintillating fibers with 1mm-diameter and 10 cm length were used. Figure 9 shows

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Fig. 8 Experimental setup to evaluate the attenuation of photons
with changing the gamma ray incident position.
Fig. 10 Measured counts over a threshold (500 ch) with change ofthe gamma ray incident position. Data were normalized
to the value at the distance of 0.5 cm.

(a) Without an AR layer(b) With an AR layer
Fig. 9 Measured pulse height distributions when 137 Cs gamma
rays were irradiated on to some points along the scintillating fiber.

the measured pulse height distributions for incident positions of 0.5 cm, 5.5 cm, and 9.5 cm from the PMT. In
Fig. 9 (a), the results without an anti-reflection (AR) layeron the scintillating fiber surface are shown, whereas the results in Fig. 9 (b) are results with the AR layer. Figure 9
clarifies that, when without the AR layer, the counts decreased extremely rapidly with increasing distance. However, with the AR layer, although the counts are small at allpositions, the change of the distribution is small. In Fig. 10,
the measured counts over 500 ch are depicted, where thecounts have been normalized by the value at a distance of0.5 cm. We conclude that attenuation of the scintillation
photons has been suppressed by the AR layer.
To elucidate the transmission process, the scintillation
photons must be divided into two groups: (1) photons emitted to the directions which meet the total reflection condition of the scintillating fiber and (2) photons which do notcompletely reflect at the core/cladding interface and which
are reflected at the wall boundary of the holes in which
the scintillating fibers are set. Our experiments show thatthe photons of the latter group decreased rapidly in thefirst 3 cm. Subsequently, they become almost constant.
The change of the collection eﬃciency aﬀects the pulse
height of the output which degrades the directional property. When preparing the AR layer on the scintillating fiber
surface, the photons in group (2) decreased more rapidly
before becoming constant.
From these results, the following modification of parameters is expected to enhance the directional property ofthe detector. First, the scintillating fiber diameter must be
less than 1 mm to reduce the gamma ray eﬀect. Regarding the length, the results implied that, two methods arepossible to maintain the collection eﬃciency of the photons as uniform as possible: (A) increasing the reflection
eﬃciency of the wall surface to the greatest extent possible and setting the scintillating fiber length as short as
possible, less than 1.5 cm, or (B) preparing an AR layeron the scintillating fiber surface and setting their length as
longer, more than 3 cm. In case (a), we use all the scintillation photons. In case (b), only the photons which meet thetotal reflection condition of the scintillating fiber is used.
Smaller diameter and shorter length are expected to decrease the measuring eﬃciency of the system. Therefore,
it will be necessary to increase the number of scintillatingfibers. Additional evaluation is expected to be necessary todesign the detector system considering the detection eﬃciency, directional property, and gamma ray eﬀect to apply
it to the real radiation field of the LHD.

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5. Summary
We have studied the design parameters which determine the properties of scintillating fiber-based fast neutron
detector that was developed and installed in some fusion
experimental devices. In redesigning the parameters, wemust consider self-shielding of the neutrons by the scintillating fiber itself. Moreover, the wall condition of the holes
into which the scintillating fibers are inserted is extremely
important because the scintillation photons are attenuatedby being reflected there. The strategy of setting an ARlayer outside the scintillating fiber or not engenders two
designs: short length without an AR layer or long lengthwith an AR layer. The gamma ray eﬀect on the angularresolution of the system can also be reduced by selecting
a smaller scintillating fiber diameter. Considering thesepresent results, the actual design of the total detection sys-tem should be determined based on the estimated neutronand gamma ray fluxes at the detector position in the LHD.
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