富山大学研究推進機構水素同位体科学研究センター研究報告 38：1－8，2018.

論文
β線誘起 X 線計測法およびモンテカルロシミュレーションによる大型ヘリカル装置 LHD で使用された黒鉛タイル中のトリチウム深さ方向分布の非破壊測定
Lee Sun Eui1, 波多野雄治 2, 原 正憲 2, 増崎 貴 3
1

富山大学理工学教育部〒930-8555 富山市五福 3190
2

富山大学研究推進機構水素同位体科学研究センター〒930-8555 富山市五福 3190
3

核融合科学研究所〒 509-5292 土岐市下石町 322-6

Non-destructive depth profiling of tritium in graphite tiles
from Large Helical Device using β-ray induced X-ray
spectrometry and Monte Carlo simulation
Sun Eui Lee1, Yuji Hatano2, Masanori Hara2, Suguru Masuzaki3
1

Graduate School of Science and Engineering for Education, University of Toyama,Gofuku 3190, Toyama 930-8555, Japan
23

Hydrogen Isotope Research Center, Organization for Promotion of Research,
University of Toyama, Gofuku 3190, Toyama 930-8555, Japan

National Institute for Fusion Science, Oroshi-cho 322-6, Toki 509-5292, Japan(Received August 31, 2019 ; Accepted October 31, 2019)

Abstract
A special experimental setup was designed and constructed to perform simultaneous
multipoint measurements of β-ray induced X-ray spectra for non-destructive depth
profiling of tritium in plasma-facing tiles retrieved from fusion devices. The setup
consists of compact silicon drift detector and an airtight acrylic chamber. The plasmafacing graphite tiles used in the deuterium experiment of Large Helical Device (LHD) in
National Institute for Fusion Science (NIFS) were examined. The obtained X-ray spectra
were simulated using Monte Carlo simulation tool kit Geant4. It has been found thattritium penetrated beyond the carbon deposition layer.

1β線誘起 X 線計測法およびモンテカルロシミュレーションによる大型ヘリカル装置 LHD で使用された黒鉛タイル中のトリチウム深さ方向分布の非破壊測定

1. Introduction
Accurate evaluation of tritium (T) retention in plasma-facing materials (PFMs) offusion reactors is required for evaluation of T inventory in a vacuum system, and safe andeconomical disposal of PFMs after services in reactors. However, the escape depth of β rays emitted by T is small (several μm or less) due to low kinetic energy (≤18.6 keV), andhence measurement of T retention in the bulk of materials is difficult. To solve thisproblem, β-ray induced X-ray spectrometry (BIXS) was developed [1,2]; far larger escapedepths of X-rays than β-rays allow to get the information about T located in deeper region
by detecting bremsstrahlung and characteristic X-rays induced by β-rays. Analysis of Xray spectra by taking account of generation and attenuation of X-rays in materials
provides a depth profile of T non-destructively.
Tritium distributions in plasma-facing tiles in fusion devices are strongly
inhomogeneous, and the variation of the T concentration can be significant even in asingle tile [3,4]. Therefore, it is preferable to perform simultaneous measurements ofseveral different points even for a single tile because BIXS analysis takes relatively longtime (e.g., several days) in most cases. However, simultaneous multipoint analyses are
difficult with a germanium (Ge) detector commonly used for BIXS analysis. This isbecause Ge detector must be cooled to liquid nitrogen temperature during measurements
to minimize noise and the size of liquid nitrogen tank is larger than that of the tiles.
In this study, a special setup for multipoint analyses was constructed using a compact
silicon drift detector (SDD) cooled by a Peltier element. The main component of the setup
is an airtight acrylic chamber capable of accommodating a whole plasma-facing tile andinstallation of three SDDs. By using this set up, the plasma-facing graphite tiles used indeuterium (D) experiment of Large Helical Device (LHD) [5,6] in National Institute forFusion Science (NIFS) were examined. The obtained spectra were compared with theresults of simulation using Monte Carlo simulation tool kit Geant4 [7-9] for evaluation of
depth profile of T.
2. Experimental procedures
Two graphite tiles, 9RD16-1 and 9RD16-2, retrieved from LHD divertor [10] afterthe 1st deuterium experimental campaign were analyzed. These tiles were from the innerside divertor at the center level. The position of sample tiles can be seen in Fig. 1. LHD

2

Lee Sun Eui・波多野 雄治・原 正憲・増崎 貴

is heliotron-type plasma machine and, because of its superconducting coils in heliotron
configuration, LHD has an advantage of steady-state plasma operation [5,6]. The divertor
tiles of LHD are composed of isotropic graphite (IG-430U, Toyo Tanso Co., Japan), while
the inner surface of vacuum chamber is covered by type 316 stainless steel. Because thesetiles were used in the D experiment, they were exposed to a small amount of T producedby DD fusion reactions: 21D+ 21D → 31T � 11� � ��������.

The detector used was the SDD with 8 μm (0.3 mil) thick Be window (X-123SDD,AMPTEK Inc., USA [11]). This detector iscooled by Peltier element installed in thecasing, and no coolant tank is necessary.
Fig. 2 shows schematic diagram of theBIXS measurement setup (a) and its photo(b). In most cases, BIXS measurements areperformed under Ar gas atmosphere toevaluate the surface T concentration from theintensity of Ar(Kα) X-rays. As mentioned
above, an acrylic airtight chamber which is
the main component of this setup issufficiently large to accommodate a whole
divertor tile; the inner diameter is 305 mm

Fig.1 Photo of inner divertor region ofLarge Helical Device (LHD) in
National Institute of Fusion Science(NIFS). The circled tiles are thoseanalyzed in this study; the tile in rightside is 9RD16-1, and one in left side is
9RD16-2.

Fig. 2 (a) Schematic diagram of experimental set up and (b) photo of chamber withsample tile and silicon drift detector (SDD).

3β線誘起 X 線計測法およびモンテカルロシミュレーションによる大型ヘリカル装置 LHD で使用された黒鉛タイル中のトリチウム深さ方向分布の非破壊測定

and height is 160 mm. The chamber is equipped with two valves to replace air inside thechamber with Ar gas. The top of the chamber is covered with an acrylic lid having threeo-ring-sealed adaptors for mounting SDDs. A divertor tile is place on a height adjustablestage, and the distance between tile surface and detector window is adjusted to be
typically 5 mm.
The measurements of the divertor tiles were performed in a radiation-controlled areain NIFS. After installing the tile at the proper height, the measurements were performed
under Ar gas atmosphere for 5 days for the tile 9RD16-1 and 3 days for the tile 9RD162, respectively.
Prior to the BIXS measurements, 2-dimensional distributions of T at the tile surfaces
were examined by β-ray measurements using an imaging plate (IP) technique. Fig. 3
shows 2-dimensional distributions of photo-stimulated luminescence (PSL) from IPs.
Blue-square contrasts in Fig. 3 (a) and(b)areareas

coveredbytile

identification labels. The T distribution
at the surface of the tile 9RD16-1 wasalmost uniform, and the X-ray spectrum
was acquired from the central part ofthe tile. On the other hand, thedistribution of T at the surface of the tile
9RD16-2 was not uniform. Therefore,
the SDD was placed at the position withthe highest T surface concentration.
The black circles in Fig. 3 indicate theplaces where SDDs were located for
BIXS measurement.

Fig. 3 Two-dimensional images of photostimulated luminescence (PSL) intensity ofsample tiles 9RD16-1 (a) and 9RD16-2 (b).
Blue-square contrasts observed in the tiles
are regions covered by tile identification
labels. The silicon drift detector was placedat the position indicated by a black circle oneach sample.

Penetration depth of T was evaluated from BIXS spectra using Monte Carlosimulation toolkit Geant4 [12, 13]. Rӧllig et al. [14] simulated BIXS spectra with four
different physics packages of Geant4 and reported PENELOPE physics [15] provided thebest agreement with observed X-ray spectra. Therefore, Geant4 with PENELOPE physics
model was used in this study.

4Lee Sun Eui・波多野 雄治・原 正憲・増崎 貴

3. Results and discussion
The X-ray spectra of the LHD
tiles are shown in Fig. 4. The peak at
2.96 keV is Ar(Kα) characteristic Xrays. The transmittance of X-rays
below 0.6 keV is negligible for theSDD Be window [11]. Therefore, thecharacteristic X-rays of carbon (0.277keV) induced in the sample tiles is not
visible. There are broad spectra ofbremsstrahlung in the energy range ofFig. 4 X-ray spectra of (a) 9RD16-1 and (b)9RD16-2 obtained by BIXS measurements.

1–9 keV, and strong absorption ofbremsstrahlung by Ar is observed at 3.2 keV. These characteristics are similar to thoseobserved in X-ray spectra of carbon tiles retrieved from Joint European Torus (JET)
examined using a Ge detector (see Fig. 3 in [2]). Therefore, it was concluded that thepresent setup provides accurate BIXS spectra.
The intensity of Ar(Kα) characteristic X-rays shows the information on the amountof T at/near surface (< 0.5 μm), whereas the intensity of bremsstrahlung gives the T
content in the bulk up to 1 mm [16]. The Ar(Kα) X-ray intensity of the tile 9RD16-1 wasorders-of-magnitude higher than that of the tile 9RD16-2, though these two tiles werelocated in adjacent positions in LHD. These spectra indicate 9RD16-1 contained more T
on the surface than 9RD16-2 tile and it is consistent with the IP result shown in Fig. 3.
As previously explained, T is produced by the DD fusion reactions in LHD. There are
two mechanisms of an accumulation of T in the divertor tiles. A part of T produced by theDD reactions is thermalized in the LHD plasma and deposited onto tile surfaces withcarbon accommodated in plasma by sputtering erosion of carbon tiles. The thickness ofthe deposition layer at the measured region on the tile 9RD16-1 was around 1 μm. Other
part of T is implanted to bulk with high energy (~1.01 MeV). The range of 1.01 MeV Tin carbon was evaluated to be 8 μm at normal incidence using SRIM program [17].
The detailed analysis of the spectrum for the tile 9RD16-2 was impossible due topoor signal to noise ratio caused by low T retention. Nevertheless, the depth profile of Tin the tile 9RD16-1 was evaluated by simulating the X-ray spectrum using Geant4 on the5

β線誘起 X 線計測法およびモンテカルロシミュレーションによる大型ヘリカル装置 LHD で使用された黒鉛タイル中のトリチウム深さ方向分布の非破壊測定

assumption that T was distributed up to depth of 1, 4, 6 and 8 μm from the surface.
Because the range of 1.01 MeV T in carbon was evaluated to be 8 μm at normal incidence,deeper penetration of T than 8 μm
inthesimulations. Fig. 5 shows theobserved for the tile 9RD16-1 andthatobtainedbyGeant4

simulations. The X-ray peaks at

0.25

X-ray intensity (cpm)comparison of the X-ray spectrum

absorption of bremsstrahlung byAr, were well reproduced in theX-ray intensity (cpm)

and50

0.10

00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1635

0.25

Measured (left axis)
4 μm Geant4 (right axis)

3025

0.20

20
0.15
150.1010simulated spectra. The relative

0.05

5intensity of bremsstrahlung to

0.00

00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Energy (keV)

Ar(Kα) X-rays was low in the0.30

depth of T was assumed to be 1
and 4 μm, as shown in Fig. 5 (a)and(b).

Nevertheless,good

agreement between observed and

X-ray intensity (cpm)simulations in which penetration

0.25

0.00

penetration

120

80
60

0.10

assuming

140

100

0.15

0.05

deeperMeasured (left axis)
6 μm Geant4 (right axis)

0.20

simulated spectra was obtained by

40
20
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

depth (6 μm in Fig 5 (c)). In theEnergy (keV)case of penetration depth of 8 μm,Fig. 5 Comparison of measured X-ray spectra forthe tile 9RD16-1 with spectra simulated using
Geant4 by assuming penetration depth of T is
1 μm (a), 4 μm (b), 6 μm (c) and 8 μm (d).

the intensity of bremsstrahlung ina high energy region (> 6 keV) in

60.15

0.30

itself. The main characteristic
X-rays100

Energy (keV)rays of silicon induced in the SDD

Ar(Kα)

1 μm Geant4 (right axis)

0.20

0.00

X-ray spectra are characteristic X-intense

Measured (left axis)

0.05

1.74 keV shown in simulated

points of observed spectrum, i.e.

150

0.30

X-ray intensity (arb. unit)

considered

X-ray intensity (arb. unit)not

X-ray intensity (arb. unit)wasLee Sun Eui・波多野 雄治・原 正憲・増崎 貴

simulated spectrum was higher
shown in Fig. 5 (d). These0.25

observations suggest that thepenetration depth of T was around
6 μm. This value of penetration
depth was larger than the abovementioned thickness of deposition
layer. It appears that a significant
part of T retained in the tile
9RD16-1 was implanted in the tileat high energy (~MeV).

Measured (left axis)
8 μm Geant4 (right axis)

120

X-ray intensity (arb. unit)

0.30

X-ray intensity (cpm)

than that in observed spectra, as

100
0.20

80

0.15

60

0.10

40

0.05

20

0.00

00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Energy (keV)

Fig. 5 (cont’d) Comparison of measured X-ray
spectra for the tile 9RD16-1 with spectra
simulated using Geant4 by assuming penetration
depth of T is 1 μm (a), 4 μm (b), 6 μm (c) and8 μm (d).

4. Conclusions
The measurements of β-ray induced X-ray spectra for divertor tiles retrieved fromLHD were successfully performed using a newly-designed compact setup capable ofmultipoint analysis. The BIXS measurements showed that the T concentration in the tile
9RD16-1 was orders-of-magnitude higher than that in the tile 9RD16-2, though these tiles
were in adjacent positions in LHD. The X-ray spectrum provided by Geant4 simulation
based on the assumption that T penetrated up to the depth of 6 μm showed good agreementwith measured BIXS spectrum. This value of penetration depth was larger than thethickness of carbon deposition layers formed at this position during operation of LHD.
These observations suggested that a significant amount of T was distributed beyond thedeposition layer due to implantation with high energy.
References[1]

M. Matsuyama, S. Ueda, K. Watanabe, In situ observation of tritium interactions
with Pd and Zr by β-ray induced X-ray spectrometry, Fusion Eng. Des., 49-50 (2000)
885-891.

[2]

M. Matsuyama et al., Non-destructive tritium measurements of Mk IIA divertor tileby BIXS, J. Nucl. Mater., 313-316 (2003) 491-495.

7β線誘起 X 線計測法およびモンテカルロシミュレーションによる大型ヘリカル装置 LHD で使用された黒鉛タイル中のトリチウム深さ方向分布の非破壊測定[3]

T. Tanabe, V. Phillips, Tritium detection in plasma facing component by imaging
plate technique, Fusion Eng. Des., 54 (2001) 147-149.

[4]

Y. Hatano, A. Widdowson, N. Bekris et al., 2D tritium distribution on tungsten tiles
used in JET ITER-like wall project, J. Nucl. Mater., 463 (2015) 966-969.

[5]

M. Fujiwara, O. Motojima, Y. Hamada et al., Overview of LHD (Large HelicalDevice) project, J. Plasma Fusion Res., 1 (1998) 57-61.

[6]

M. Fujiwara, K. Kawahata, N. Ohyabu et al., Overview of LHD experiments, Nucl.
Fusion, 41 (2001) 1355.

[7]

S. Agostinelli, J. Allision, K. Amako, et. al., GEANT4 – a simulation toolkit, Nucl.
Instrum. Methods Phys. Res. A, 506 (2003) 250-303.

[8]

J. Allison, K. Amako, J. Apostolakis et al., Geant4 developments and applications,
IEEE Trans. Nucl. Sci. 53 (2006) 270-278.

[9]

J. Allison, K. Amako, J. Apostolakis et al., Recent developments in Geant4, Nucl.
Instrum. Meth. Phys. Res. A, 835 (2016) 186-225.

[10] N. Ohyabu, T. Watanabe, H. Ji et al., The Large Helical Device (LHD) helicaldivertor, Nucl. Fusion, 34 (1994) 387.
[11] http://amptek.com/products/c-series-low-energy-X-ray-windows/.
[12] M. Hara, S. Abe, M. Matsuyama et. al., Design of a tritium gas cell for beta-ray
induced X-ray spectrometry using Monte Carlo simulation, Fusion Eng. Des., 119(2017) 12-16.
[13] M. Hara R. Uchikawa, Y. Hatano et. al., Monte Carlo simulation of tritium beta-ray
induced X-ray spectrum in various gases, Fusion Eng. Des., 131 (2018) 125-129.
[14] M. Rӧllig, T. Aso, M. Hara, F. Priester, Galet – Bechmark of a Geant4 based
application for the simulation and design of Beta Induced X-ray Spectrometry
systems, Fusion. Eng. Des., 143 (2019) 91-98.
[15] J. Baró, J. Sempau, J. M. Fernández-Varea, F. Salvat, “PENELOPE: An algorithmfor Monte Carlo simulation of the penetration and energy loss of electrons andpositrons in matter”, Nucl. Instrum. Meth. B, 100 (1995) 31.
[16] Y. Torikai, M. Matsuyama, N. Bekris et al., Tritium distribution in JET Mark IIA
type diverter tiles analysed by BIXS, J. Nucl. Mater., 337-339 (2005) 575-579.
[17] J. F. Ziegler, http://srim/org.

8