Neutron-Induced Signal on the Single Crystal
CVD Diamond-Based Neutral Particle Analyzer
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言語: English
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公開日: 2021-12-14
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キーワード (En):作成者: KAMIO, Shuji, Fujiwara, Yutaka, OGAWA,
Kunihoro, Kobayashi, Makoto I, Sangaroon, Siriyaporn,
ISOBE, Mitsutaka, SEKI, Ryosuke, NUGA, Hideo,
OSAKABE, Masaki, Matsuyama, Shigeo, Miwa, Misako,
TOYAMA, S.
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http://hdl.handle.net/10655/00012703
This work is licensed under a Creative Commons
Attribution 3.0 International License.

DraftNeutron-Induced Signal on the Single Crystal CVD Diamond-Based Neutral
Particle Analyzer
S. Kamio1,a), Y. Fujiwara1, K. Ogawa1,2, M. I. Kobayashi1, S. Sangaroon1,
M. Isobe1,2, R. Seki1,2, H. Nuga1, M. Osakabe1,2, S. Matsuyama3, M. Miwa3,and S. Toyama3
1

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, JapanThe Graduate University for Advanced Studies SOKENDAI, 322-6 Oroshi-cho, Toki 509-5292, Japan
3
Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
2(Received XXXXX; accepted XXXXX; published online XXXXX)
The diamond-based neutral particle analyzer (DNPA) array composed of single-crystal chemical vapor
deposition (sCVD) diamond detectors was installed on the Large Helical Device (LHD) for measuring thehelically-trapped energetic particles. In the high neutron flux experiment, the unwanted neutron-induced pulse
counting rate should be estimated using the other neutron diagnostics because a diamond detector has
sensitivity to neutron as well as energetic neutral particles. In order to evaluate the quantitative neutroninduced pulse counting rate on DNPA, the response functions of the sCVD diamond detector for monoenergetic neutrons were obtained using accelerator-based D-D and D-7Li neutron sources in Fast Neutron
Laboratory (FNL). As the result of the neutron flux estimation by Monte Carlo N-Particle (MCNP) code at
the NPA position in LHD and the response function obtained in FNL experiment, the counting rate of theneutron-induced signal was predicted to be 2.3 kcps during the source neutron emission rate Sn=1x1015 n/s. In
the LHD experiment, the neutron-induced signals were observed by closing the gate valve during the plasma
discharges. It is found that the counting rates of the neutron-induced signals proportional to Sn reached 1.1
kcps at Sn=1x1015 n/s. As the result of the quantitative estimation of the neutron-induced signals on DNPA
using other neutron measurement, it has become possible to accurately measurement of energetic neutral
particles in the high neutron flux experiment.
Therefore, by measuring the energetic particles in the hard
neutron radiation experiments, comprehensive investigation canbe done together with the neutron measurement. The result of the
NPA is useful because the energy spectrum of the energetic
particles can be obtained.

I. INTRODUCTION
In the future fusion reactor, alpha particles should be wellconfined and used for plasma heating to sustain steady-state
without external heating. Therefore, it is very important to study
the confinement physics of energetic particles because theenergetic particles can cause magnetohydrodynamic (MHD)
instabilities which transport the energetic particles, anddeteriorate the confinement of the energetic particles. In order toinvestigate the behavior of the energetic particles in theexperimental devices for fusion research, neutral particle analyzer(NPA) has been developed [1]. The energy spectrum of theenergetic particles which charge exchanged inside the plasma canbe observed by NPA along the line of sight. silicon-diodedetector is widely used because of the good energy resolution anda size small enough to make an array [2-6]. However, highneutron irradiation causes a permanent damage on thesemiconductor products, such as silicon-diode-based NPA. Since
the silicon-diode detector cannot be used in burning plasma,diamond detector which has sufficient neutron radiation hardness
has been developed [7,8]. On the other hand, in the experimentusing deuterium gas, the neutron emission produced by the D-D
fusion reaction is also used for investigating the behavior of theenergetic particles because the neutron emission rate depends on
the spatial and energy distribution functions of energetic particles.
a)

In the Large Helical Device (LHD), confinement of theenergetic particles in a helical ripple is studied for understanding
the confinement physics in three-dimensional magnetic
configuration because the transport of the helically trappedenergetic particles is one of the important issues for the helicalconfinement system. In LHD, the deuterium plasma experimenthas been started in FY 2016, and the neutron diagnostics are
installed for the study of the energetic particles by D-D fusion
reaction [9,10]. In order to measure the energetic particles in thehard neutron radiation experiment, single-crystal chemical vapor
deposition (sCVD) diamond-based NPA (DNPA) was developed
for measuring the helically trapped energetic particles in LHD[11]. The diamond was adopted because of its high radiation
hardness. Using the DNPA, energy spectra of the energetic
particles were observed with high energy resolution in the highneutron emission experiments. By measuring in the high neutron
emission experiment, DNPA can be used together with theneutron diagnostics [12,13] as well as the other NPAs [14,15] andspectroscopic measurements [16,17]. DNPA also detects theneutron-induced signals during the discharge in the deuterium
experiment. However, the quantitative neutron-induced signals
on the DNPA have not been evaluated yet. As the diagnostic for

Author to whom correspondence should be addressed: kamio@nifs.ac.jp.

1Draftthe energetic neutral particle, the neutron-induced signals shouldbe estimated from the neutron measurement.

demonstrated in Fast Neutron Laboratory (FNL) [20,21] at
Tohoku University. Figure 2 shows the schematic view of the
FNL experiments. The accelerated deuteron DC beam by the
Dynamitron accelerator is delivered to the target with the energyof Ebeam. The deuterium gas or the solid lithium was used as atarget. The neutrons produced by D(d,n)3He or by 7Li(d,n)8Be
reactions were detected at the sCVD diamond detector with theenergy of En+Ebeamcos En of 2.45 MeV with D(d,n)3He reaction
was used for evaluating the neutron-induced signals in D-D
fusion plasma experiment, and En of 15.0 MeV with 7Li(d,n)8Be
reaction was used for measuring the neutrons by DNPA.

In this paper, we show the results of the neutron-induced
signals on the sCVD diamond detectors for NPA. The DNPA
system configuration is shown in the Section II. The Section III
shows the observed energy spectra by the neutron irradiation
experiments in Fast Neutron Laboratory (FNL). Quantitativeevaluation of neutron-induced signal on diamond detector wasobserved in FNL. In Section IV, the results in the LHD
experiments are shown. The neutron-induced signal on DNPA
was measured experimentally. Also, the typical experimental
results are shown with the neutron-induced signal estimation.
Section V is summary.

II. DNPA SYSTEM CONFIGURATIONThe schematic view of the circuit of the DNPA is shown in
Fig. 1. The sCVD diamond B12 [18], made by CIVIDEC, is used
for the detector of the DNPA [11]. The sCVD diamond is 4.5 mm
x 4.5 mm with 0.5 mm thickness, and the electrode is 4.0 mm x
4.0 mm with 100 nm thickness made of titanium. In the case forthe plasma diagnostics in fusion research, one of the target beamenergies is 40-80 keV which is the typical energy of the positive
ion source neutral beam injection (NBI). In order to measure the40-80 keV energy signals, the electrodes were customized to thethinner titanium from the gold electrodes, and the preamplifier
2001A and the shaping amplifier 2026X which are made by
CANBERRA were used to achieve high multiplication. Typicalmagnification at the amplifier is 750 times, and the shaping timeis 0.2 s. The corresponding energy of the thermal noise was lessthan approximately 30 keV. Therefore, the typical energy rangeof the DNPA is 30-600 keV. For acquiring energy calibration
data, 241Am alpha source was used. The digitizer WE7562 forpulse height analysis made by YOKOGAWA is used forcounting the signals. The pulse length of the signal isapproximately 1 s in the counting time window of 10 ms.
Therefore, in order to avoid the pile-up, DNPA should be usedwith the counting rate of less than 100 kcps. The counting loss
rate due to the dead-time of the detector [19] is 10% of the true
count under 100 kcps.

Fig. 2 Schematic view of the FNL experiments.
The deuterium gas target was used for measuring theneutron counting rate by 2.45 MeV D-D fusion reaction. The
detector was installed at l=35 mm and =20 deg. 20 m cables forconnecting the amplifiers were used for reproducing theexperimental conditions. Figure 3 shows the energy distributionsof the DNPA counting rate during the neutron irradiation
experiment. The irradiated neutron energy was 3.57-5.52 MeV
with the acceleration voltage of 2.1-3.0 MV. The neutron
irradiation time was 20 minutes for each energy. The observedcounting rate divided by the neutron flux rate was approximately
4.6x10-4 count/n with the energy range of 50-600 keV. A
characteristic peak was observed at 200-300 keV. This peak isthe elastic scattering of the neutrons on 12C nucleus. The
amplitude and energy of the peak were changed by changing theirradiated neutron energy. The irradiation neutron energy
dependency on the DNPA counting rate was not clear in thisenergy range. The calculation of elastic scattering inside thediamond due to the neutron irradiation by PHITS code [22] also
shows the similar result. Therefore, we consider the energy
spectrum of DNPA due to the neutron irradiation with the energyof 2.45 MW is almost same with the spectra shown in Fig. 3.

Fig. 1 The schematic view of the circuit of the DNPA.

III. NEUTRON IRRADIATION IN FNLIn order to measure the quantitative neutron-induced signal
on DNPA, fast neutron irradiation experiments have been2

DraftFig. 3 The energy distributions of the DNPA counting rate of theneutron irradiation experiments using D-D neutron source.
Figure 4 shows the results of the experiment with D-7Li
neutron source, measured with the wide energy range. By
decreasing the magnification of the amplifier to 50 times, theerror by the thermal noise was decreased. The irradiated neutronenergies were 15.89 MeV, 16.99 MeV, and 17.68 MeV with theacceleration voltage of 1.5 MV, 2.4 MV, and 3.0 MV,
respectively. As the result of the neutron irradiation,
characteristic energy distributions were observed. The peaks of
12C(n,)9Be and 12C(n,n’)3 could be identified by DNPA.

Fig. 5 The lines of sight of DNPA together with the plasma flux
surface and the ICRF resonance layers in the LHD poloidal crosssection.
The diamond detectors were installed at the basement of the
LHD torus hall. Three meter straight pipes were used to extend
the vacuum chamber from LHD lower side port. In order toreduce the neutron effect, the diamond detectors are locatedunder the floor slab through 1.5 m heavy concrete with 5 cm
inner diameter pipes. The detectors are located at approximately
7.8 m lower from the magnetic axis of the torus plasma. The
neutron flux was calculated by Monte Carlo N-Particles transport(MCNP) code [23,24]. The neutron flux at the position of thedetectors were evaluated approximately 5x106 n/cm2/s when theneutron source emission rate is 1x1015 n/s [25]. The neutron
source emission rate Sn is measured by the neutron flux monitor[26]. Considering the estimatated counting rate 4.6x10-4 count/nwith the energy range of 50-600 keV in the FNL experiment in
Fig. 3, the predicted counting rate by Sn is approximately 2.3 kcps
during the LHD experiment with Sn=1x1015 n/s.

Fig. 4 The energy distributions of the DNPA counting rate of theneutron irradiation experiments using D-7Li neutron source.

IV. EXPERIMENTS IN LHD
In LHD, DNPA was developed for measuring the energetic
particles which were accelerated by ion cyclotron range of
frequency (ICRF) wave or injected by perpendicular-neutralbeam (p-NB) injectors. The DNPA measurement lines of sightwith the LHD poloidal cross-section are shown in Fig. 5. Thelines of sight of Ch1 and of Ch7 are across the lower and upper
helical ripple, and line of sight of Ch4 is across the magnetic axis.
Because the direction of the measurement lines of sight areperpendicular to the magnetic field lines, Ch1 and Ch7 are
expected to observe the helical ripple trapping particles. The
DNPA counting rate is expected to be high during the p-NB
injection. On the other hand, neutron emission rate is high during
the tangential-neutral beam (t-NB) injection because the injectionenergy of the t-NBs is higher than the injection energy of p-NBs.
The typical injection energy of t-NB (NB #1, #2 and #3) is 130180 keV, and the typical injection energy of p-NB (NB #4 and#5) is 60-80 keV.

Figure 6 shows the experimental result of the neutroninduced signal measurement. During the discharges with the highneutron emission rate, DNPA measured the signals other than theneutral particles by closing gate valve. The gate valve which ismade of stainless steel prevents passing through energetic neutral
particles. 49 discharges with high Sn rate of up to 2.5x1015 n/s
were used for neutron-induced signal measurement. As shown in
Fig. 6 (a), the energy integrated neutron signals were clearly
proportional to Sn. The counting rate coefficients of three DNPA
channels were almost the same. As the result of the linear fitting,the DNPA average counting rate for Sn was 1.1 kcps when
Sn=1015 n/s. Observed counting rate 1.1 kcps was close to theprediction i.e. 2.3 kcps. This result also indicates that thecalculation of the neutron flux rate at the basement by MCNP of
5x106 n/cm2/s at Sn=1015 n/s was reasonable. The energy
distributions of each channel are shown in Fig. 6 (b). A peak ofneutron-induced signal shape similar to the result in FNL

3

Draftexperiment shown in Fig. 3 was observed at around 300 keV.
These results can be used for estimation of the counting rate of
the neutron-induced signal during the neutral particle
measurement.

of Ch7 was higher than that of Ch1. This count of Ch7 may be
the accelerated particles at the ICRF resonance layers on the Ch7line of sight shown in Fig. 5. The estimated neutron-induced
signal of this timing was negligible.

Fig. 6 (a) The DNPA integrated counting rates of 50-600 keV
during the discharges with the gate valve closed. (b) The energy
distributions of each channel. The counting rates are normalizedby 1015 neutrons.

Fig. 7 Time evolutions of (a) the injection powers for plasma
start-up, sustaining and heating, (b) the electron density and thetemperatures of ion and electron, (c) the total neutron emissionrate, and (d)-(f) the counting rates of the DNPA each channel at50-300 keV (red) and the estimated neutron-induced signal(yellow).

Typical experimental results measuring the energetic neutral
particles in LHD are shown in Figs. 7 and 8. Fig. 7 (a) shows thetime evolutions of injection powers, Fig. 7 (b) shows the plasma
density and the temperatures, and Fig. 7 (c) shows the total
neutron emission rate Sn. In this discharge, p-NB and t-NB areused as the energetic particle source, and ICRF heating is used toaccelerate ions. The highest Sn is 1.1x1015 n/s at the timing of tNB injection. Figs. 7 (d)-(f) are the energy integrated counting
rates of the DNPA (red) and the estimated neutron-induced signal(yellow) of each channel. The highest counting rate was less than
30 kcps at the timing of p-NB injection. The p-NB modulation
pulses, which were injected with the energy of 59 keV, can beclearly confirmed on DNPA Ch1 and Ch7. On the other hand, thecounting rate of Ch4 was lower than the counting rate of Ch1 andof Ch7. This difference indicates that the p-NB particles weretrapped in a helical ripple. In the case of Ch4, the counting rate
was close to the estimation of the counting rate of neutroninduced signal. The total counting rate of Ch1 was higher thanthe counting rate of Ch7 during the p-NB injection. However,during the timing of ICRF injection of 5.3-6.3 s, the counting rate

The energy distributions of the discharge are shown in Fig. 8.
Red lines are the total count during 3-7 seconds shown in Fig. 7.
Black lines are thermal noise estimated before the plasma start-up
in the same discharge. Yellow lines are the estimated spectra of
the neutron-induced signal using the energy distributions of Fig.
6 (b). The counting rate of the neutral particles were much higher
than the neutron-induced signal at the lower energy region.
However, the neutral counting rate was almost the same with theneutron-induced signal at more than 100 keV, 70 keV, and 90
keV for Ch1, Ch4, and Ch7, respectively. The estimation of theneutron noise was very important for discussing the confinementof energetic particles in the high energy region.
The higher energy components than the p-NB energy of 59
keV are considered to be the scattered t-NB particles which were
injected with the energies of 153 keV (NB #1), 170 keV (NB #2),

4

Draftemission rate of 1015 n/s, which is the highest neutron emission
rate in LHD experiments.

and 170 keV (NB #3), or the accelerated particles by ICRF
heating. We will clarify the confinement physics generated by pNB injection and/or ICRF heating using DNPA by changing theexperimental conditions in the LHD.

DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.

ACKNOWLEDGMENTS
The authors wish to thank the support of the LHD
experiment group in performing the experiments. This work is
supported by the NINS program for cross-disciplinary study(Grant No. 0131190), the NIFS Collaboration Research Program(KOAH037, KOAA001), and the NIFS Grants (ULRR006,
ULRR035, and ULRR703).
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Fig. 8 Energy spectra of the total count (red), the estimatedneutron-induced signal (yellow), and the thermal noise (black) of
each channel.

V. SUMMARYThe NPA array using sCVD diamond detectors was developed in
the LHD deuterium experiments for measuring the helicallytrapped energetic particles. In order to measure the quantitative
neutron-induced signals on the DNPA, mono-energetic neutron
was irradiated to the sCVD diamond detector in FNL. As theresult of the FNL experiments and the neutron flux estimation by
MCNP, the neutron-induced signal was estimated to be 2.4 kcps
during plasma discharges with the total neutron emission rate
Sn=1015 n/s in LHD. This counting rate is low enough to measure
the neutral particles without the pile-up due to the neutroninduced signal. The characteristic peak was observed at around200 keV, and the peak was shifted by changing the energy ofirradiated neutron. The characteristic energy spectra of fastneutron signals were also observed by diamond detectors withwide energy range in FNL experiment. In LHD, the neutroninduced signals were measured by closing the gate valve during
the plasma discharges. The amount of the signals due to theneutron result in a linear relation with the neutron emission. The
counting rate of the neutron-induced signal was 1.1 kcps when
Sn=1015 n/s. This counting rate is a similar value to the prediction.
Using the results of the neutron measurement, the neutroninduced signal was estimated quantitatively during theexperiment for measuring the energetic neutral particles. The
counting rate due to neutrons were estimated to be much lower
than the signal of the neutral particles even during the neutron

5

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