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
キーワード (Ja):
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.

Neutron-induced signal on the single
crystal chemical vapor deposition diamondbased neutral particle analyzer
Cite as: Rev. Sci. Instrum. 91, 113304 (2020); https://doi.org/10.1063/5.0020460
Submitted: 01 July 2020 • Accepted: 29 October 2020 • Published Online: 12 November 2020
S. Kamio,

Y. Fujiwara,

K. Ogawa, et al.

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Rev. Sci. Instrum. 91, 113304 (2020); https://doi.org/10.1063/5.0020460© 2020 Author(s).

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Neutron-induced signal on the single crystal
chemical vapor deposition diamond-based
neutral particle analyzer
Cite as: Rev. Sci. Instrum. 91, 113304 (2020); doi: 10.1063/5.0020460
Submitted: 1 July 2020 • Accepted: 29 October 2020 •Published Online: 12 November 2020
S. Kamio,1,a)Y. Fujiwara,1K. Ogawa,1,2
M. I. Kobayashi,1 S. Sangaroon,1,3
M. Isobe,1,2
1,2
1
1,2
4
4
R. Seki,H. Nuga, M. Osakabe,
S. Matsuyama,
M. Miwa, and S. Toyama4
AFFILIATIONS
1National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan

2

The Graduate University for Advanced Studies SOKENDAI, 322-6 Oroshi-cho, Toki 509-5292, Japan3

Mahasarakham University, 20-41 Kamriang, Kantharawichai, Mahasarakham 44150, Thailand

4

Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japana)

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

ABSTRACTA 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 the helically trapped energetic particles. In high neutron flux experiments, theunwanted neutron-induced pulse counting rate should be estimated using the neutron diagnostics because a diamond detector is sensitive
to neutrons as well as energetic neutral particles. In order to evaluate the quantitative neutron-induced pulse counting rate on the DNPA,
the response functions of the sCVD diamond detector for mono-energetic neutrons were obtained using accelerator-based D–D and D–7 Li
neutron sources in Fast Neutron Laboratory (FNL). As a result of the neutron flux estimation by the Monte Carlo N-Particle code at the NPA
position in the LHD and the response function obtained in the FNL experiment, the counting rate of the neutron-induced signal was predicted
to be 1.1 kcps for the source neutron emission rate of Sn = 1 × 1015 n/s. In the LHD experiment, the neutron-induced signals were observedby 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 = 1 × 1015 n/s. As a result of the quantitative estimation of the neutron-induced signals on the DNPA using otherneutron measurements, it has become possible to accurately measure energetic neutral particles in the high neutron flux experiment.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0020460., s

I. INTRODUCTION
In the future fusion reactor, alpha particles should be well confined 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 the energetic particles
can cause magnetohydrodynamic (MHD) instabilities that transport
the energetic particles, deteriorating their confinement. In order toinvestigate the behavior of the energetic particles in the experimental devices for fusion research, a neutral particle analyzer (NPA) has
been developed.1 Some of the energetic particles inside the plasma
undergo charge exchange with the neutral particles and are lost

Rev. Sci. Instrum. 91, 113304 (2020); doi: 10.1063/5.0020460
Published under license by AIP Publishingfrom the magnetic confinement. The energy spectrum of the lostenergetic particles can be observed with the NPA along the line ofsight. A silicon-diode detector is widely used because of the goodenergy resolution and a size small enough to make an array.2–6 However, high neutron irradiation likely causes permanent damage tothe semiconductor. Since the silicon-diode detector cannot be usedin burning plasma, a diamond detector, which has sufficient neutron radiation hardness, has been developed.7–9 On the other hand,in the experiment using deuterium gas, the neutron emission produced by the D–D fusion reaction is also used for investigating the
behavior of the energetic particles because the neutron emission ratedepends on the spatial and energy distribution functions of energetic

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particles. Therefore, by measuring the energetic particles in the hard
neutron radiation experiments, comprehensive investigation can bedone together with the neutron measurement. The result of the NPA
is useful because the energy spectrum of the energetic particles canbe obtained.
In the Large Helical Device (LHD), confinement of the energetic particles in a helical ripple is studied for understanding theconfinement physics in a three-dimensional magnetic configuration
because the transport of the helically trapped energetic particles is
one of the important issues for the helical confinement system. In the
LHD, the deuterium plasma experiment was started in 2016, and theneutron diagnostics were installed for the study of the energetic particles by D–D fusion reaction.10,11 In order to measure the energetic
particles in the hard neutron radiation experiment, a single-crystal
chemical vapor deposition (sCVD) diamond-based NPA (DNPA)
was developed for measuring the helically trapped energetic particles in the LHD.12 Diamond was adopted because of its high radiation hardness. Actually, the sCVD diamond detectors have beenused as the neutron diagnostic for the hard neutron radiation experiments.13,14 Using the DNPA, energy spectra of the energetic particles were observed with high energy resolution in high neutron
emission experiments. By measuring in the high neutron emission
experiment, the DNPA can be used together with the neutron diagnostics15,16 as well as with the other NPAs17,18 and spectroscopic
measurements.19,20 By measuring with other energetic particle diagnostics, a better understanding of fast particle energy and spatial distributions is expected. The DNPA also detects the neutron-induced
signals during the discharge in the deuterium experiment. However, the quantitative neutron-induced signals on the DNPA havenot been evaluated yet. As the diagnostic for the energetic neutral
particle, the neutron-induced signals should be estimated from theneutron measurement.
In this paper, we show the results of the neutron-induced signals on the sCVD diamond detectors for the NPA. The DNPA
system configuration is shown in Sec. II. Section III shows theobserved energy spectra by the neutron irradiation experiments
in Fast Neutron Laboratory (FNL). Quantitative evaluation of theneutron-induced signal on the diamond detector was observedin FNL. In Sec. IV, the results in the LHD experiments areshown. The neutron-induced signal on the DNPA was measured
experimentally. In addition, the typical experimental results areshown with the neutron-induced signal estimation. Section V is thesummary.
II. DNPA SYSTEM CONFIGURATIONThe schematic view of the circuit of the DNPA is shown in
Fig. 1. The sCVD diamond B1221 made by CIVIDEC is used as thedetector of the DNPA.12 The sCVD diamond has 4.5 × 4.5 mm2
area and 0.5 mm thickness, while the electrode has 4.0 × 4.0 mm2
area and 100 nm thickness, made of titanium. The bias voltage is
400 V. In the case of plasma diagnostics in fusion research, the typical energy of the positive ion source neutral beam injection (NBI)
is in the range of 40 keV–80 keV. In order to measure the 40 keV–80 keV energy signals, the preamplifier 2001A and the shaping
amplifier 2026× made by CANBERRA were used to achieve highmultiplication. The typical gain of the amplifier is 750 for the plasmameasurement. The corresponding energy of the thermal noise was

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

less than ∼30 keV. Therefore, the typical energy range of the DNPA
is 30 keV–600 keV for the plasma measurement. In order to perform
calibration, an 241 Am alpha source was used. The gain of the amplifier is 75 for the calibration. The digitizer WE7562 for pulse height
analysis made by YOKOGAWA is used for counting the signals. The
detector measures the pulse height in the range of 0 V–10 V. The
maximum energy of the DNPA measurement range corresponds to
10 V. The pulse length of the signal is ∼1 μs in the counting time window of 10 ms. Therefore, in order to avoid the pile-up, the DNPA
should be used with the counting rate of less than 100 kcps. The
counting loss rate due to the dead-time of the detector22 is 10% of
the true count under 100 kcps.
III. NEUTRON IRRADIATION IN FNLIn order to quantitatively measure the neutron-induced signal on the DNPA, fast neutron irradiation experiments have been
carried out in Fast Neutron Laboratory (FNL)23,24 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 energy Ebeam . Deuterium gas
or solid lithium was used as the target. The neutrons produced byD(d,n)3 He or by 7 Li(d,n)8 Be reactions arrive at the sCVD diamonddetector with the energy of En + Ebeam cos α. En of 2.45 MeV withD(d,n)3 He reaction was used for evaluating the neutron-induced
signals in the D–D fusion plasma experiment, and En of 15.0 MeV
with 7 Li(d,n)8 Be reaction was used for measuring the neutrons with
the DNPA.
The deuterium gas target was used for measuring the neutron
counting rate by 2.45 MeV D–D fusion reaction. In the case of using
the gas target, the energy loss of the accelerated deuteron at the
6.4 μs Harbor foil should be considered. The detector was installed
at l = 35 mm and α = 20○ . 20 m cables for connecting the

FIG. 2. Schematic view of the FNL experiments.

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edge of 12 C(n,α)9 Be and 12 C(n,n′ )3α could be identified by the
DNPA.
IV. EXPERIMENTS IN LHD

FIG. 3. The pulse height spectrum of the DNPA counting rate of the neutron
irradiation experiments using the D–D neutron source.

amplifiers were used for reproducing the experimental conditions.
Figure 3 shows the energy distributions of the DNPA counting rateduring the neutron irradiation experiment. The irradiated neutronenergy was 3.57 MeV–5.52 MeV with the acceleration voltage of
2.1 MV–3.0 MV. Here, the beam energies Ebeam attenuated by the
Harbor foil become 1.34 MeV–2.43 MeV when the acceleration voltages are 2.1 MV–3.0 MV. The neutron irradiation time was 20 min
for each energy. The observed count divided by the neutron fluencewas ∼4.8 × 10−4 count/(n/cm2 ) with the energy range of 50 keV–
600 keV. A characteristic peak was observed at 200 keV–300 keV.
Although the reason for the appearance of the peak is not clear,
the calculation of elastic scattering inside the diamond due to theneutron irradiation by the PHITS code25 shows a similar result inthis energy range. Therefore, we expect the energy spectrum of the
DNPA due to 2.45 MeV neutrons to be comparable to the one shownin Fig. 3.
Figure 4 shows the results of the experiment with the D–7 Li
neutron source measured with the wide energy range. In order
to measure the high energy neutrons, the gain of the amplifier isdecreased to 50. The irradiated neutron energies were 15.89 MeV,
16.99 MeV, and 17.68 MeV with the acceleration voltage of 1.5 MV,
2.4 MV, and 3.0 MV, respectively. As a result of neutron irradiation, characteristic energy distributions were observed. The

FIG. 4. The pulse height spectrum of the DNPA counting rate of the neutron irradiation experiments with the irradiation energy of 15.89 MeV, 16.99 MeV, and17.68 MeV using the D–7 Li neutron source.

Rev. Sci. Instrum. 91, 113304 (2020); doi: 10.1063/5.0020460
Published under license by AIP Publishing

In the LHD, the DNPA was developed for measuring the energetic particles that were accelerated by ion cyclotron range of frequency (ICRF) waves or injected by perpendicular-neutral beam (pNB) injectors. The DNPA measurement lines of sight with the LHD
poloidal cross section are shown in Fig. 5. The lines of sight of Ch1
and of Ch7 are across the lower and upper helical ripples, and theline of sight of Ch4 is across the magnetic axis. Because the directionof the measurement lines of sight is perpendicular 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 highduring the p-NB injection. On the other hand, the neutron emission rate is high during the tangential-neutral beam (t-NB) injection
because the injection energy of the t-NBs is higher than the injectionenergy of p-NBs. The typical injection energy of t-NB (NB Nos. 1–3)
is 130 keV–180 keV, and the typical injection energy of p-NB (NB
Nos. 4 and 5) is 60 keV–80 keV.
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 the LHD lower side port. In order toreduce the neutron effect, the diamond detectors are located under
the floor slab through 1.5 m heavy concrete with 5 cm inner diameter pipes. The detectors are located at ∼7.8 m below the magnetic axisof the torus plasma. The neutron flux was calculated by the Monte
Carlo N-Particle (MCNP) transport code.26,27 The neutron flux atthe position of the detectors was evaluated as 2.4 × 106 n/cm2 /swhen the neutron source emission rate is 1 × 1015 n/s.28 The neutron
source emission rate Sn is measured with the neutron flux monitor.29
Considering the estimated sensitivity of the diamond detector to
the neutron of 4.8 × 10−4 count/(n/cm2 ) with the energy range ofFIG. 5. The lines of sight of the DNPA together with the plasma flux surface andthe ICRF resonance layers in the LHD poloidal cross section.

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50 keV–600 keV in the FNL experiment in Fig. 3, the counting rate
predicted by using Sn is 1.1 kcps during the LHD experiment with
Sn = 1 × 1015 n/s.
Figure 6 shows the experimental result of the neutron-induced
signal measurement. During the discharges with the high neutron
emission rate, the DNPA measured signals other than the neutral
particles by closing the gate valve. The gate valve, which is made ofstainless steel, prevents neutral particles from passing through. 49
discharges with a high Sn rate of up to 2.5 × 1015 n/s were used
for neutron-induced signal measurement. In this experiment, thesignals are not only neutron-induced signals but also the signals
induced by the gamma rays. These signals cannot be identified with
the DNPA. However, estimation of the signals other than the energetic neutral particles is important for measuring energetic neutral
particles. As shown in Fig. 6(a), the energy integrated neutron signals were clearly proportional to Sn . The counting rate coefficientsof three DNPA channels were almost the same. As a result of linear
fitting, the DNPA average counting rate for Sn was 1.1 kcps when
Sn = 1015 n/s. The observed counting rate of 1.1 kcps was consistent
with the prediction. This result also indicates that the calculation of
the neutron flux rate at the basement by MCNP of 2.4 × 106 n/cm2 /s
at Sn = 1015 n/s was reasonable. The energy distributions of eachchannel are shown in Fig. 6(b). A peak of the neutron-induced signal
shaped similar to the result in the FNL experiment 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. The quantitative neutron-induced

FIG. 6. (a) The DNPA integrated counting rates of 50 keV–600 keV during the discharges with the gate valve closed. (b) The pulse height spectrum of each channel.
The counting rates are normalized to 1015 neutrons.

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signal can be estimated by multiplying Sn to the results shown in
Fig. 6 during the experiment.
Typical experimental results obtained by measuring the energetic neutral particles in the LHD are shown in Figs. 7 and 8.
Figure 7(a) shows the time evolutions of injection powers, Fig. 7(b)shows the plasma density and temperatures, and Fig. 7(c) shows thetotal neutron emission rate Sn . In this discharge, p-NB and t-NB
are used as the energetic particle sources, and ICRF heating is used
to accelerate ions. The highest Sn is 1.1 × 1015 n/s at the timing of
t-NB injection. Figures 7(d)–7(f) show the energy integrated counting rates of the DNPA (red lines) and the estimated neutron-induced
signal (green lines) of each channel. The difference between the total
signal and the estimation of the neutron-induced signal is the signalof the energetic neutral particles. The highest counting rate was lessthan 30 kcps at the timing of p-NB injection. The p-NB modulation
pulses, which were injected with the energy of 59 keV, can be clearly
confirmed on DNPA Ch1 and Ch7. On the other hand, the counting rate of Ch4 was lower than the counting rate of Ch1 and of Ch7.
This difference indicates that the p-NB particles were trapped in ahelical ripple, and the energetic neutral particles were successfully
observed with the DNPA. In the case of Ch4, the counting rate wasclose to the estimation of the counting rate of the neutron-induced
signal. The total counting rate of Ch1 was higher than the counting
rate of Ch7 during the p-NB injection. However, during the timingof ICRF injection of 5.3 s–6.3 s, the counting rate of Ch7 was higher
than that of Ch1. This count of Ch7 may be the accelerated particles

FIG. 7. Time evolutions of (a) the injection powers for plasma start-up, sustaining,and heating, (b) the electron density and the temperatures of the ion and electron,(c) the total neutron emission rate, and (d)–(f) the counting rates of each DNPA
channel at 50 keV–300 keV (red lines) and the estimated neutron-induced signal(green lines).

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discharges with the total neutron emission rate of Sn = 1015 n/s inthe LHD. This counting rate is low enough to measure the neutral
particles in the high neutron emission experiments. The characteristic peak was observed at around 200 keV, and the peak was
shifted by changing the energy of irradiated neutrons. The characteristic energy spectra of fast neutron signals were also observedwith diamond detectors with a wide energy range in the FNL experiment. In the LHD, the neutron-induced signals were measured byclosing the gate valve during the plasma discharges. The amountof signal due to the neutron results in a linear relation with theneutron emission. The counting rate of the neutron-induced signal
was 1.1 kcps when Sn = 1015 n/s. This counting rate is the same as
the predicted one. Using the results of the neutron measurement,
the neutron-induced signal was estimated quantitatively during theexperiment for measuring the energetic neutral particles. The counting rate due to neutrons was estimated to be much lower than thesignal of the neutral particles even during the neutron emission
rate of 1015 n/s, which is the highest neutron emission rate in LHD
experiments.
FIG. 8. The pulse height spectrum of the total count (red lines), the estimatedneutron-induced signal (green lines), and the thermal noise (black lines) of eachchannel.

at the ICRF resonance layers on the Ch7 line of sight shown in Fig. 5.
The estimated neutron-induced signal of this timing was negligible.
As shown in Figs. 7(d)–7(f), the difference between the lines of sightclearly appeared for DNPA Ch1, Ch4, and Ch7.
The energy distributions of the discharge are shown in Fig. 8.
Red lines are the total count during 3 s–7 s shown in Fig. 7. Blacklines are the thermal noise estimated before the plasma start-up
in the same discharge. Green lines are the spectra of the neutroninduced signal estimated using the energy distributions of Fig. 6(b).
The counting rate of the neutral particles was much higher than theneutron-induced signal at the lower energy region. However, theneutral counting rate was almost the same with the neutron-induced
signal at more than 100 keV, 70 keV, and 90 keV for Ch1, Ch4,and Ch7, respectively. The estimation of the neutron noise was very
important for discussing the confinement of energetic particles in
the high energy region.
The energy components higher than the p-NB energy of 59 keV
are considered to be the scattered t-NB particles, which were injectedwith the energies of 153 keV (NB No. 1), 170 keV (NB No. 2), and170 keV (NB No. 3), or the particles accelerated by ICRF heating.
We will clarify the confinement physics generated by p-NB injection
and/or ICRF heating using the DNPA by changing the experimental
conditions in the LHD.
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 neutrons
were irradiated to the sCVD diamond detector in FNL. As a result of
the FNL experiments and the neutron flux estimation by MCNP, theneutron-induced signal was estimated to be 1.1 kcps during plasma

Rev. Sci. Instrum. 91, 113304 (2020); doi: 10.1063/5.0020460
Published under license by AIP Publishing

ACKNOWLEDGMENTS
The authors wish to thank the support of the LHD experimentgroup in performing the experiments. This work was supportedby the NINS Program for Cross-Disciplinary Study (Grant No.
0131190), the NIFS Collaboration Research Program (Grant Nos.
KOAH037 and KOAA001), and the NIFS (Grant Nos. ULRR006,
ULRR035, and ULRR703).
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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