Plasma and Fusion Research: Regular Articles

Volume 14, 3405162 (2019)

Measurement of Thermal and Epithermal Neutron Flux
Distribution in the Torus Hall of LHD using Activation Method inthe First Deuterium Experiment Campaign∗)
Tomoyo TANAKA, Makoto KOBAYASHI1,2) , Sachiko YOSHIHASHI, Akira URITANI,
Kenichi WATANABE, Atsushi YAMAZAKI, Takeo NISHITANI1) , Kunihiro OGAWA1,2)
and Mitsutaka ISOBE1,2)
1)

Nagoya University, Nagoya 464-8603, Japan
National Institute for Fusion Science, National Institutes of Natural Sciences, Toki 509-5292, Japan
2)
SOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan(Received 10 January 2019 / Accepted 19 September 2019)

In the Large Helical Device (LHD) of the National Institute for Fusion Science, a deuterium plasma experiment was performed in March 2017. Neutrons with an energy of 2.45 MeV are generated by d (d, n) 3 Hereactions. The evaluation of this neutron flux in the torus hall, as well as within the LHD vessel, is very important
for the decommissioning of the LHD in the future and the radiation shielding for workers and components aroundthe LHD. In this study, we evaluate the spatial distribution of thermal and epithermal neutron flux on the floor
level of the LHD torus hall by measuring the neutron activation of gold foils. The flux information obtained inthe experiment can be used for the evaluation of radio-activation and the shielding design.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: LHD, neutronics, activation foil, IP, HPGe
DOI: 10.1585/pfr.14.3405162

1. Introduction
At the National Institute for Fusion Science, deuterium plasma experiments at the Large Helical Device(LHD) are performed in order to obtain fundamental datafor the design of nuclear fusion power plants. In commercial fusion reactor, neutrons generated by the nuclear fusion reaction will be utilized for thermal energy in fusion
power generation. On the other hand, neutrons activate devices within fusion reactors that are composed of several
materials. Therefore, it is important for the developmentof nuclear fusion reactors to understand the behavior ofneutrons for radiation protection and for future decommissioning measures.
The first deuterium plasma experiment was performed
from March to July of 2017 at the LHD [1]. Neutrons
with an energy of 2.45 MeV are generated by the d (d,n) 3 He reaction. These fast neutrons are scattered by thesurrounding structures of the LHD, are decelerated, andthus become thermal and epithermal neutrons. As a result, a large number of radioactive isotopes are producedin the torus hall. Many radioactive isotopes have specificdecay processes and decay rates and long half-life; therefore, it is necessary to predict the radioactive isotopes thatare produced in the torus hall of the LHD for its decomauthor’s e-mail: s-yoshihashi@energy.nagoya-u.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

missioning. A three-dimensional Monte Carlo simulation
is typically used to predict the neutron behavior, and theGeneral Monte Carlo N-Particle Transport Code version 6(MCNP6) has been applied for neutronics of LHD [2, 3].
In order to confirm the validity of the simulation, experimental evaluation is necessary. The first measurement of
the thermal neutron flux distribution on the floor level ofthe LHD torus hall was carried out using indium foils [4].
However, the neutron population in the LHD torus hall are
included not only thermal neutrons but also epithermal andfast neutrons. The neutron flux of each energy range needs
to be evaluated because the radioactive isotopes generate
depending on the specific neutron energy.
In this study, we measured the spatial distribution ofneutron flux on the floor level of the LHD torus hall by
the neutron activation of gold foils. In order to evaluate the thermal and epithermal neutron distributions separately, gold (Au) foils were covered with cadmium, which
acted as a thermal neutron absorber. We put gold foils withand without cadmium covers on the floor level of the torus
hall. After the gold foil atoms were activated by the 197 Au(n, γ) 198 Au reaction, the gamma rays emitted by gold foils
were measured by a high purity germanium (HPGe) detector and an imaging plate (IP). The flux obtained in theexperiment will be an important benchmark in the safety
evaluation of deuterium experiments in the LHD.

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c 2019 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 14, 3405162 (2019)

2. Experimental Setup
2.1

Activation foil

The stable isotope 197 Au, whose natural abundance is100%, was selected for the activation foil in this work.
197
Au produces the radioactive isotope 198 Au (half-life ofabout 2.7 days [5]) by a neutron capture reaction, (n, γ) reaction, whose cross section is large for the thermal neutron.
The radioactivity of the activation foil does not directly indicate the neutron fluence at the position of the foil
because the radioactive isotopes in the activation foil decay
during further neutron irradiation. The quantities of the radioactive isotopes produced during neutron irradiation canbe calculated simply by the following equation:ln2 (A)dN (A)= N (S ) σφ −N .
dt
T 1/2

Fig. 1 Component layout of the LHD torus hall.

Here, N (A) and N (S ) are the quantities of radioactive
and stable isotopes, respectively, φ presents the neutron
flux [cm−2 s−1 ], and T 1/2 is the half-life [s]. The reaction
cross section, σ [cm2 ] is dependent on the neutron energy.
Here, N (A) is zero before the neutron irradiation. The halflife of 198 Au is discribed above. Note that φ is diﬀerent
in each deuterium plasma operation and that N (A) is determined by the integration of the diﬀerent neutron fields. On
the other hand, the position of the site of neutron generation and the energy (2.45 MeV) are almost the same in alldeuterium plasma operations.
The dimensions of the gold foils used for the measurement were 0.5 × 0.5 mm2 with a thickness of 0.1 mm. In
addition, in order to measure the epithermal neutron flux,
cadmium covers with a thickness of 1 mm were prepared
and acted as thermal neutron absorbers.

2.2

Components in the LHD torus hall

The equipment layout in the torus hall of the LHD
is shown in Fig. 1. The torus hall dimensions are W75,
L45, H40 m3 , and the walls of the torus hall are composedof plain concrete with a thickness of 2 m. The LHD isequipped with many systems for plasma heating, plasma
diagnosis, and other purposes. In addition to the spatial
neutron detection, the neutron yield also was measured byneutron flux monitors a 235 U fission chamber, 10 B countersand 3 He counters, positioned above the vicinity of LHD
central axis of the large external port [6]. Unfortunately,
the 10 B counters and 3 He counters were saturated with highneutron flux, so the neutron generation rates measured bythese counters are not used in this paper.
Borated polyethylene blocks are used in the LHD as
neutron shielding. Polyethylene can eﬀectively decelerate
fast neutrons to thermal neutrons due to their light constituent atoms. The boron in the polyethylene blocks canalso absorb thermal neutrons, due to the large cross sectionof the 10 B (n, α) 7 Li reaction. These borated polyethylene
blocks contain 10wt% boron. A number of electrical components within the torus hall were covered with borated
polyethylene blocks in order to suppress any influence orFig. 2 Layout of polyethylene blocks under the LHD floor.

activation from the neutrons. As shown in Fig. 2, the borated polyethylene blocks were placed in a disk-like shapewith an inner diameter of 2.3 m, an outer radius of 6.9 m,
and a thickness of 5 cm on the floor underneath the LHD.
However, under the west side of the LHD, an unborated
polyethylene block was placed instead.

2.3 Neutron irradiation and gamma ray
analysis
As shown in Fig. 3, the prepared gold foils wereplaced on the floor of the torus hall and left for about two
weeks during the deuterium experiment. Around 500 shots
were carried out, and the total neutron generation measured by the 235 U fission chamber was around 1.0 × 1016
neutrons.
The gamma-rays emitted by 198 Au were measured using two analytical methods: HPGe detector and IP. TheHPGe detector is capable of identifying and evaluating theexact amounts of radioactive isotopes by gamma-ray energy spectroscopy and is widely used in the field of radi-3405162-2

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Fig. 4 An example of the gamma-ray spectrum of gold foil measured by the HPGe detector.

Fig. 3 Foil position on the floor of the LHD.

ation management. However, as this method usually canmeasure only one sample at a time, a long measuring time
would be required if all the foils were to be analyzed thisway. Therefore, we used an IP to measure many samples
at a time. IP is a sheet-type analytical method that accumulates energy transferred from radiation. IP can release accumulated energy as photostimulated luminescence (PSL)
by laser injection. As IP usually has the ability to measurea large area, it is applied to the two-dimensional visualization of radiation sources. The samples were stuck onto
Mylar film, with a thickness of 100 µm, and then placedon the IP. After that, IP measurement was performed toall samples. A small number of samples were removed
from the Mylar film and measured by the HPGe detector
for gamma-ray spectroscopy. Details of these analyses aredescribed in Ref. [7].

3. Results and Discussion
3.1

Neutron flux measurement

An example of the gamma-ray spectrum emitted by
the gold foil measured by the HPGe detector is shown inFig. 4. A dominant 412 keV peak from 198 Au can be seen.
In order to confirm the relationship between the neutron flux calculated by the 412 keV gamma-ray count by
the HPGe detector and the PSL measured by the IP, thetwo measurement results were compared, for foils placedin three places in Fig. 5. It was confirmed the linearity between the neutron flux calculated from the HPGe detector
and radiation intensity measured with IP. Although the IP
detected both gamma rays and beta rays, the released PSL
was predominantly 412 keV gamma rays.

3.2

Neutron flux distribution

Figure 6 shows (a) the thermal neutron flux distribution and (b) the epithermal neutron flux distribution on thefloor level of the LHD torus hall. Here, φ indicates theneutron flux [cm−2 source−1 ] from single neutron generated in the total plasma volume of the LHD vacuum vessel. The thermal neutron flux distribution was obtained bysubtracting the radioactivity of the cadmium-covered gold
foils from that of those not covered with cadmium. The ep-Fig. 5 The relationship the neutron flux calculated using theHPGe detector and radiation intensity measured by theIP at three positions near the LHD.

ithermal neutron flux distribution was found by the resultof gold foils covered with cadmium. These figures werecreated using the Generic Mapping Tool version 5.4.3 [8].
Although the distribution is blurred because of the distance
between samples, an overall trend in the neutron distribution can be seen. The thermal neutron flux was relatively
high in the vicinity of the LHD and is a similar result to thatobtained when using the 155 In (n, g) 116 m In reaction [4].
Figure 6 (a) shows that a higher thermal neutron flux wasobserved in the west side of the LHD, which shows the distribution of thermal neutrons well. The maximum thermal
neutron flux in Fig. 6 (a) is 4.04 × 10−8 cm−2 source−1 .
On the other hand, it was observed that the epithermal
neutron flux was relatively high in the center of the LHD,
2.76 × 10−8 cm−2 source−1 and that is uniformly decreased
outwards to 2.54 × 10−10 cm−2 source−1 .

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Volume 14, 3405162 (2019)
surement position. In other words, although fast neutrons
cannot be measured in this experiment, it is possible toknow how much they have been moderated by the LHD
components. In Fig. 7, the cadmium ratio was low near theLHD and increases at greater distances from the center ofthe LHD. This implies that the fast neutrons were eﬀectively decelerated by the borated polyethylene block under the floor. In addition, the cadmium ratio was relatively
large at the place where the unborated polyethylene block
was located, under the west side of the LHD. This confirms that, since boron was not present in the block, the fastneutrons were not eﬀectively converted into thermal neutrons. In addition, it shows the influence of the polyethylene block installed near the area of large cadmium ratio.
These measurement results show the eﬀect of the moderation by the components around the LHD well and can beexpected to be useful for the future shielding design.

(a) thermal neutron flux distribution

4. Conclusion(b) epithermal neutron flux distribution
Fig. 6 6 The neutron flux distributions Φ on the floor level ofthe LHD torus hall (a) thermal neutrons measured by Au
foils, (b) epithermal neutrons measured by Au foils with
Cd filter.

The thermal and epithermal neutron fluxes on the floor
level of the LHD torus hall were experimentally evaluatedin order to estimate the neutron field generated by deuterium plasma operation in the LHD.
Gold activation foils were used, and the radioactivity of these foils was evaluated with IP and HPGe detector. By using gold foils covered with or without cadmium, we could obtain spatial distributions of the thermal and epithermal neutron flux. A relatively high thermal
neutron flux was observed on the west side of the LHD,
whereas a significant epithermal neutron flux was not observed around the outside of the LHD. The diﬀerence inthose flux distributions is likely caused by the combinationof polyethylene blocks and boron that acted as a fast neutron decelerator and thermal neutron absorber. The neutron
flux distribution evaluated in this work will be used for theevaluation of radioactive waste in the torus hall and neutron shielding designs.

Acknowledgment
Fig. 7 The distribution of the cadmium ratio of the thermal neutron flux on the floor level of the LHD torus hall.

Figure 7 shows the distribution of the cadmium ratioon the floor level of the LHD torus hall. The cadmium ratio
is defined by the following equation.
Cadmium ratio =ΦAu.
ΦAu+CdHere, φAu and φAu+Cd represent the neutron flux givenby the gold foil and the cadmium-covered gold foil, respectively. The fast neutrons generated in the LHD are moderated to thermal or epithermal neutrons by the components
within the LHD torus hall. The cadmium ratio shows towhat extent the thermal neutrons dominate at each mea-This work was supported by the LHD project budget(ULGG801 and ULAA027). T. Tanaka is pleased to acknowledge the LHD Experiment Group for assistance inactivation foil measurement.
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