富山大学水素同位体科学研究センター研究報告 21: 27-37, 2001.

論 文
He予照射タングステンにイオン注入したトリチウムの熱的挙動
松山政夫、吉田勝彦、渡辺国昭
富山大学水素同位体科学研究センター〒９３０−８５５５ 富山市五福３１９０

岩切宏友、吉田直亮
九州大学応用力学研究所〒８１６−８５８０ 福岡県春日市春日公園６−１

Thermal Behavior of Tritium Implanted in Tungsten Pre-Irradiated with He
Masao MATSUYAMA, Katsuhiko YOSHIDA, Kunikai WATANABE
Hydrogen Isotope Research Center, Toyama Univ.,
Gofuku 3190, Toyama 930-8555, Japan
and
Hirotomo IWAKIRI, Naoaki YOSHIDA
Research Institute for Applied Mechanics, Kyushu Univ.,
Kasuga, Fukuoka 816-8580, Japan
Dec. 20,(Received
March
2002;
Accepted
2002)(Received
March
31,31,
2002;
Accepted
December20, 2002)

ABSTRACT
Thermal behavior of tritium implanted in tungsten pre-irradiated with a given fluence of helium was
examined by β-ray-induced X-ray spectrometry (BIXS) under a condition of isochronal heating. To estimatechanges in tritium distribution caused by the heating, Ar(Kα) and W(Mα) X-rays were mainly utilized asprobes of tritium detection. The intensity of the former X-rays linearly decreased with increase in heating
temperature, while that of the latter X-rays was almost constant up to 600 K and dramatically decreased
above this temperature, indicating existence of strong trap sites. Such thermal behavior on X-ray intensity
coincides with the understanding of thermal desortption spectra of deuterium. It was thought, therefore, thatstrong trap sites of tritium were due to the effects of bubbles generated by the helium pre-irradiation, andthe trapping energy of tritium was estimated to be about 1.2 eV. Moreover, change in depth profiles oftritium with heating was evaluated by analyzing the observed X-ray spectra by means of computer simulation, and changes in the concentration of tritium retained in surface layers were examined.

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27

松山政夫・吉田勝彦・渡辺国昭・岩切宏友・吉田直亮

1. Introduction
Tungsten is one of promising candidates for divertor materials of the International Thermonuclear Experimental Reactor (ITER), because it has advantageous features such as a high melting
point, a low vapor pressure and a low sputtering erosion yield. From this viewpoint, a number ofstudies have been reported for irradiation effects of hydrogen, deuterium, helium and those mixtureson retention and trapping of hydrogen isotopes in tungsten. Recent activities in evaluation of high-Zmaterials focusing on tungsten and its alloy were reviewed in detail by Yoshida [1]. In addition, forexample, Iwakiri et al. [2] have reported about the effects of helium pre-irradiation on the trappingof deuterium implanted into tungsten by using a technique of thermal desorption spectrometry.
However, few reports on thermal behavior of tritium for tungsten still appear.
The β-ray-induced X-ray spectrometry (BIXS), which has been recently proposed by
Matsuyama et al. [3], is of high potentiality for the measurements of tritium retained in surface
layers and/or bulk of a material. In the present measuring technique, "surface layers" means thepenetration depth of β-rays in a material, and "the bulk" means a far deeper region than the penetration depth. In comparison with other tritium measuring techniques, BIXS has some advantageous
points: e.g., nondestructive and quantitative measurements are possible and tritium amounts insurface layers and bulk of a material can be independently measured.
As an example of practical application of this technique, the present method was employed
to estimate diffusion behavior of tritium at room temperature in a tungsten plate [4]. The tungsten
plate implanted tritium ions was measured for a long duration at room temperature in the argon
atmosphere. Characteristic X-rays of W(Lα) and W(Mα) as well as Ar(Kα) X-rays were observed,
and it was seen that intensities of those X-rays clearly changed with time even at room temperature
because of the diffusion of tritium from surface layers into the bulk of tungsten. However, when atungsten plate was irradiated with a given amount of helium ions at room temperature prior to theimplantation of tritium, changes in the X-ray intensities with time were negligibly small, indicatingthat most of the implanted tritium ions are strongly trapped in surface layers of tungsten.
From this view point, in this study, thermal behavior of tritium implanted in tungsten
pre-irradiated with helium was examined under condition of isochronal heating by using the BIXS.
Changes in the X-ray spectra with heating are shown and effects of helium pre-irradiation fortritium retention will be discussed.

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28

Ｈｅ前照射タングステン材料にイオン照射したトリチウムの熱的挙動

2. Experimental
2. 1. Materials
Polycrystalline tungsten plate fabricated by the powder metallurgy was used as a sample.
The purity and the size of a tungsten plate were 99.95% and 10x10x0.02 mm3, respectively. A given
amount of helium ions was irradiated the tungsten plate at room temperature before tritium implantation. Energy and fluence of helium ions were 8 keV and 5x1021 ions/m2, respectively. Shape of theimplantation spot was ellipse of 5x3.5 mm. It is reported that helium irradiated under the present
conditions distributes from a near surface to 80 nm [2].
Tritium used for an ion implantation was diluted with deuterium, and the concentration oftritium was about 0.8%. The purity of deuterium was 99.6% and that of argon used for X-ray
measurements was 99.999%.

2. 2. Implantation device of tritium ions
The polycrystalline tungsten plate was irradiated with tritium ions by using the speciallydesigned tritium implantation device. Main components of the tritium implantation device consistedof a tritium storage-supply-recovery part, an implantation part equipped with a conventional ion
gun, a high vacuum system and a sample

W-platetransfer system. The detailed construction

He-irrad.
(5x3.5mm)

and specifications of the tritium implantation

T-irrad.

device is described elsewhere [5].

(6mmφ)

Figure 1 shows the detailed drawing

DT-ionsof a sample holder used for tritium implan-

Shieldingplate

He-irrad.

tation. The sample was sandwiched between

Graphite platea thin molybdenum plate attached with an
Mo-platealmel-chromel thermocouple and a shielding

W-sample

plate having four holes of 6 mmφ in diame-

Mica plate

Thermocouple

ter. The molybdenum plate was used to

Supporting plate

Heaterensure uniform thermal transmission from a
Fig. 1. A bird's-eye view of the sample holder used for tritium

heater, which was made of W-Re alloy, andimplantation.

‑ 3 ‑

29

松山政夫・吉田勝彦・渡辺国昭・岩切宏友・吉田直亮

the heater was fixed with a thin mica foil beneath the molybdenum plate. To estimate the tritium
fluence, a thin graphite plate was also placed on the molybdenum plate as a standard material. Thisis due to a reason that a quantitative measuring technique by BIXS has been already established forgraphite sample [6]. To measure ion current during the implantation of tritium, the sample holder
was electrically disconnected from the ground level using an insulating material. In addition, thesample was biased +18 V to prevent secondary electron emission due to the ion implantation inorder to the measurements of a net ion current.
2. 3. Measuring device of β-ray-induced X-rays
High pure Ge detector, which was purchased from CANBERRA, was used to measure
X-rays induced by β-rays. Since major energy distribution of the X-rays is considerably lower than
18.6 keV of the maximum energy of β-rays, a window of the X-ray detector was made of a speciallydesigned beryllium plate (8 µm) to gain effective penetration efficiency of low energy photons. In
addition, to lower background level due to natural radioactivity, both detector and sample were
sandwiched with two lead bricks of 5 cm in thickness.

2. 4. Experimental procedures
After the helium-irradiated sample was installed in the implantation device, it was evacuated for about one week at room temperature and then degassed at 673 K in vacuum. After the
residual pressure reached below 10-5 Pa, the sample was cooled down in vacuum to implant tritium
ions. Tritium implantation was carried out under the following conditions: the implantation
temperature was set at room temperature. In addition, acceleration energy and implantation time oftritium ions were kept at 1 keV and 15 min, respectively. During the implantation, the total pressurein the implantation device was kept constant at 4 Pa.
After the tritium implantation, the implantation device was evacuated for a week to lower
the release of tritium from the walls of the implantation device when taking out the sample. Just
before taking out the sample, the implantation device was finally decontaminated by using a
conventional tritium removal system to ensure safety. After this decontamination, the sample wastransferred from the implantation device to the front of an X-ray detector, and the measurement ofan X-ray spectrum was started. During the X-ray measurements, both the sample and the X-ray
detector were placed in an argon atmosphere and the distance between them was kept at 5 mm. The
flow rate of Ar was controlled at 40 cm3/min.
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30

Ｈｅ前照射タングステン材料にイオン照射したトリチウムの熱的挙動

To examine retention behavior of tritium at elevated temperatures, the sample implanted
tritium was isochronally heated for 30 min at the temperature range from 373 to 973 K by using
another conventional vacuum device. An X-ray spectrum was measured at room temperature after
vacuum heating at a given temperature. Most X-ray spectra were measured for 12 hours, and thoseused for analysis of depth profile were measured for above 60 hours.

3. Results and discussion
3.1. Attenuation of β-rays and characteristic X-rays in tungsten
Three characteristic X-ray peaks were observed for the tungsten sample implanted tritium
as shown later: one is the characteristic X-rays (Ar(Kα)) of argon, and the others (W(Lα) andW(Mα)) are those of tungsten. The former X-rays can be induced by tritium atoms distributed
within the average escape depth of the β-rays. The depth in tungsten was estimated to be 50 nm or
smaller. In this paper, this depth is denoted below as "surface layers". On the other hand, the latter
X-rays can be mainly induced by β-rays emitted in bulk of tungsten. Energies of characteristic
X-rays for W(Lα) and W(Mα) peaks are 8.40 and 1.77 keV, respectively. Intensity of the W(Mα)
X-rays will give the information about tritium concentration in shallow layers in comparison with

2. It is clear from the figure that an intensity of
the W(Mα) X-rays gives information withrespect to tritium concentration within a range
from the surface to about 1 µm. This range isdenoted below as "sub-surface layers". Although
information for tritium distribution in a far
deeper region than the sub-surface layers can beobtained from W(Lα) X-ray intensity, it was notfavorable for the present study because of aconsiderably weak intensity. Namely, tritium
behavior in the surface and/or sub-surface layersof tungsten can be evaluated by tracking changes

‑ 5 ‑

31

1.0

Material: W

0.8

0.6

Attenuation of characteristic X-rays and β-rays, R / -and W(Mα) X-rays in tungsten are shown in Fig.

Attenuation of characteristic X-rays and β-rays, R / -the W(Lα) X-rays. Attenuation curves of β-rays
1.0

Mα (1.77 keV)
0.8

0.6

0.4

β -rays

0.2

0.0
0.00

0.02

0.04

0.06

0.08

0.10

Depth from the surface, d / µm

0.4Mα (1.77 keV)

0.2

0.0
0.0

β -rays

0.5

1.0

1.5

2.0

Depth from the surface, d / µm

Fig. 2. Attenuation curves for β-rays and W(Mα) in tungsten.

松山政夫・吉田勝彦・渡辺国昭・岩切宏友・吉田直亮

2.0

0.3

[A]

Ar(Kα)

0.05[C][573K][723K]

W(Mα)

0.1

0.0
0

1.0

2

4

6

8

10

-1

-1

0.2

-1

-1

0.2

Counting rate, N / counts min(∆E)

1.5Counting rate, N / counts min(∆E)-1Counting rate, N / counts min(∆E)

-1

-1

0.04

Counting rate, N / counts min (∆E)
-1

0.3

[B]

[298K]

0.1

0.0
0

12

2

4

Energy, E / keV

6

8

10

0.03

0.02

0.01

0.00
0

12

2

Energy, E / keV

4

6

8

10

12

Energy, E / keV

0.5

W(Lα)

0.0

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

12

Energy, E / keV
Fig. 3. Example of the X-ray spectra observed after heating at 573 and 723 K for 30 minutes in vacuum.

in each X-ray intensity of Ar(Kα) and W(Mα).
12

Region I

3.2. Changes in the intensity and shape of an

Region II

Region III

10

X-ray spectrum with heating in vacuum[A]
Ar(Kα)

observed after heating at 573 and 723 K for 30
minutes in vacuum. Intensities of the characteristic and bremsstrahlung X-rays decreased withincrease in heating temperature. However, theshape of bremsstrahlung X-rays little changed as
seen from the inset in each spectrum, indicatingthat the depth profile of tritium in the bulk is very
similar to each other. These indicate that a part ofthe implanted tritium was released from theIntensity of characterisitc x-rays, I x / counts min

Figure 3 shows the X-ray spectra

-1

8
6
4
2
0
8

[B]

6

W(Mα)

4

2

0
300

tungsten, but not diffused into the bulk.

400

500

600

700

800

900

Heating temperature, T / K

The changes in intensities of Ar(Kα) andW(Mα) X-rays with a heating temperature are

‑ 6 ‑

32

Fig. 4. Changes in intensities of Ar(Kα) and W(Mα)
X-rays by heating.

1000

Ｈｅ前照射タングステン材料にイオン照射したトリチウムの熱的挙動

illustrated in Fig. 4. It is clear that change in the Ar(Kα) intensity with a temperature was quite
different from that in the (Mα) intensity, indicating that the thermal behavior of tritium trapped in
the surface and sub-surface layers is different. For convenience sake, changes in the X-ray intensity
were divided into three regions, basing on the change in the W(Mα) X-ray intensity: namely,
Region I is the range from room temperature to 600K, Region II is 600 to 800 K, and Region III isabove 800 K. The intensity of Ar(Kα) X-rays slightly increased at 373 K, but it decreased linearlywith increase in heating temperature up to 773 K and above that temperature a slow decrease
appeared. If only tritium retained in the surface layers are released by heating, it seems that thedesorption rate of tritium will increase with a heating temperature and the intensity of Ar(Kα)
X-rays will drastically decrease as a function of temperature. However, the X-ray intensity corresponding to the amount of tritium resided in the surface layers did not decrease in such a manner.
Namely, it is suggested that tritium in the surface layers could be continuously supplied from thebulk. This may be a reason why the linear
12

observed. On the other hand, the intensity ofW(Mα) X-rays decreased very slightly up to600 K, and most of tritium in sub-surface-1(Region I)
I(W(Mα) / counts min

Characterisitc x-ray intensity

decrease of tritium in surface layers was(Region III)(Region II)

10
8

[A]

He pre-irradiation:
21

2x10 m-2

6
4
2

layers disappeared in the Region II. From
m s

-1

0
-2

both intensity changes in Ar(Kα) and W(Mα)[B]

He pre-irradiation:
21

2x10 mDesorption rate, R / 10

17

8

X-rays, it can be understood that at least there
are two or more kinds of the trapping sites forthe implanted tritium.
A technique of thermal desorption-2

D2 irradiation:

6

21

1x10 m

-2

4
2

-2

m s

-1

0

thermalbehavior

of

Desorption rate, R / 10

estimatehydrogenisotopes implanted into materials. Figure 5[B] and [C] show the deuterium desorption

ions/m2.

Detailed

Without He-irradiation

D2 irradiation:

6

21

2

1x10 /m

4
2

400

600

800

Temperature, T / K

with and without helium pre-irradiation. The

1x1021[C]

0spectra obtained by TDS for tungsten samples

deuterium fluence for those samples

817

spectrometry (TDS) is frequently used towasFig. 5. Comparison of the change in W(Mα) intensity withthermal desorption spectra of deuterium.

experimental

‑ 7 ‑

33

1000

松山政夫・吉田勝彦・渡辺国昭・岩切宏友・吉田直亮

conditions for TDS were described elsewhere [2]. To compare with the change in W(Mα) X-ray
intensity, Fig. 4 [B] is also shown in Fig. 5 [A]. The desorption spectrum of deuterium for no
helium pre-irradiation sample formed a broad peak in the Region I, but the pre-irradiation of helium
newly produced the desorption peak in the Region II in addition to the Region I. However, when thedose of deuterium in this sample was lowered to a fluence of 5x1020 ions/m2, only a single desorption peak appeared in the Region II. These suggest that the decrease of W(Mα) X-ray intensity in
the Region II is due to the desorption of tritium trapped at the sites induced newly by helium
pre-irradiation. It is clear, therefore, that the pre-irradiation of helium plays an important role forformation of new trapping sites of hydrogen isotopes.
According to the observation of a helium pre-irradiation sample by a transmission electron
microscope, production of a large number of small helium bubbles were observed under the present
helium pre-irradiation conditions. In addition, it is known that the helium bubbles formed intungsten are almost stable if a heating temperature is below ca. 1000 K. This was confirmed by the
result that no annealing effect was observed for intensity and shape of an X-ray spectrum though thesecond tritium implantation was carried out under the same implantation conditions after heating at
973 K in the first experiment. Namely, it can be seen that the present decreasing behavior of aW(Mα) X-ray intensity will be strongly related to the effects of helium bubbles on the formation of
new trapping sites.
To estimate apparent activation energy of the reduction rate of tritium in sub-surface layers,
the reduction rate of the W(Mα) X-ray intensity was plotted against the reciprocal temperature asshown in Fig. 6. I was assumed that the1

reduction rate obeys the second law

10kinetics with respect to a tritium amount in
0

10

I(T2) describe the W(Mα) X-ray intensity
at respective heating temperatures. The
apparent activation energy was estimated

1/I(T1) - 1/I(T2)

sub-surface layers. In this figure, I(T1) and

-1

10

-2

10

to be 1.21 eV from the slope of a line. It isreported by Franzen et al. [7] that the trap-3

10

with deuterium. Further examinations are

1.3

1.4

1.5

1.6
-1

-3

1.7
-1

Reciprocal Temp., T / 10 K

energy of ion-induced trap sites was 1.25
eV for single crystal tungsten irradiated

1.2

Fig. 6. Reduction rate of the W(Mα) X-ray intensity as a functionof heating temperature.

‑ 8 ‑

34

1.8

Ｈｅ前照射タングステン材料にイオン照射したトリチウムの熱的挙動

required to clarify the desorption mechanism

0.25

Sample: W(He)
Heating temp.: 723 K

and kinetic data of tritium retained in
0.20

3

-1

Counting rate, N / counts min (∆E)

-1

irradiation-induced trap sites.

4

3.3. Simulation of the observed X-ray
spectra and determination of a tritium depth
profileAs shown in Fig. 4,

the different

decreasing behavior in characteristic X-ray
intensities of Ar(Kα) and W(Mα) X-rays
appeared

duringaisochronal

0.15

2
0.10

1
0.05

heating,
0.00

indicating that the thermal behavior of tritium

0

2

4

6

8

10

12

14

16

18

0

Energy, E / keV

retained in the surface layers is different fromthat of tritium trapped in sub-surface layers.

Fig. 7. An example of comparison of the observed spectrum

Namely, it is of an important item to estimate

with the simulation one. The open circles and the solid line
show the observed and the simulated spectrum, respectively.

the changes in a depth profile of tritium intungsten with heating. A tritium depth profile
can be estimated by computational simulation

Sample: W(He)

1.0

Relative tritium concentration, C / -of an observed X-ray spectrum as reported so
far [4]. Parameter in the computational simulation is only a tritium depth profile in thematerials. The detailed analysis procedures aredescribed elsewhere [8]. An example ofcomparison of the observed spectrum with thesimulated one is shown in Fig. 7. It was seenthat both spectra agreed well, indicating thatthe observed X-ray spectrum can be numerically reproduced well by assuming a given

0.8

0.6

0.4

0.2

0.0

depth profile of tritium. The tritium depth: kept at 298K
: heated at 573K
: heated at 723K

0

1

2

3

Depth, d / µm

profiles determined by such computational
Fig. 8. Tritium depth profiles determined at given temperatures bysimulation are shown in Fig. 8.

computational simulation.

It can be seen from Fig. 8 that about a

‑ 9 ‑

35

松山政夫・吉田勝彦・渡辺国昭・岩切宏友・吉田直亮

half of tritium ions injected in tungsten at room

Table 1 Temperature dependence of tritium amounts retained

temperature was trapped within 0.1 µm from thein the surface and sub-surface layers of tungsten determinedfrom simulation analyses of the observed X-ray spectra. An

surface and the others diffused into a depth of aunit of cpm describes the counts/min, and the numericalfew µm: two kinds of tritium distributions werevalues in parentheses are relative amounts of tritium.

suggested from the computational simulation. It

298K

573K

723K

8.69

5.17

1.29

cpm

cpm

cpm

(1.00)(0.59)(0.15)

8.1(1.00)

4.3(0.53)

1.1(0.14)

6.75

6.27

0.94

cpm

cpm

cpm

(1.00)(0.93)(0.14)

Simula.

88.8(1.00)

86.2(0.97)

12.6(0.15)(Surface) / (Total)

0.083

0.048

0.078

is thought that one of reasons for appearance ofsuch profiles is due to the difference of areas
between helium and tritium ion irradiation.

Obs.
SurfacelayersSimula.

Because the tritium irradiation area was about
two times greater than that of theheliumpre-irradiation area as shown in Fig. 1. , The

Subsurface
layersformer distribution, therefore, shows effects of
the helium pre-irradiation, and the latter distribu-

Obs.

tion is due to the diffusion of tritium into thehelium-unirradiated area. For heating at 573 K, only a little decrease in the tritium amount in
sub-surface layers appeared, while for heating at 723 K gave rise to a significant decrease without
diffusion of tritium into the bulk. This agrees well changes in a W(Mα) X-ray intensity as shown in
Fig. 3.
Tritium amounts in the surface and sub-surface layers are summarized in Table 1, whichwere calculated from the observed spectra shown in Fig. 3 and the depth profiles shown in Fig. 8. In
this table, the observed values describe intensities of Ar(Kα) and W(Mα) X-rays, while the simulation shows the values calculated from area of a depth profile. The numerical values in parentheses
represent relative tritium amount in each temperature. Changes in the relative tritium amounts
evaluated from the observed X-ray intensity were almost the same as those from the simulation,
indicating that the tritium depth profiles illustrated in Fig. 8 correspond well to practical ones. In
addition, it was seen that the amount of tritium in surface layers was below 10% in all the remaining
tritium.

4. Conclusions
Thermal behavior of tritium implanted in a tungsten sample which was irradiated in‑ 10 ‑36

Ｈｅ前照射タングステン材料にイオン照射したトリチウムの熱的挙動

advance with energetic helium ions was examined by utilizing the β-ray-induced X-ray
spectrometry (BIXS). Ar(Kα) and W(Mα) X-rays were mainly utilized as probes of tritium detection: the former X-ray intensity gives information about tritium amount in surface layers, whereas
the latter one gives information about the amount and distribution of tritium in sub-surface layers,
taking into account of attenuation of X-rays in tungsten.
Quite different behavior appeared in changes of both X-ray intensities with heating. Intensity of the Ar(Kα) X-rays linearly decreased with a heating temperature, while that of W(Mα)
X-rays was almost constant up to 600 K and above this temperature drastically decreased. Namely,
it was suggested that there are two and more kinds of trapping sites. In addition, changes in thelatter X-ray intensity by heating agreed quite well with appearance of the second desorption peak in
the thermal desorption spectrum of deuterium. It was suggested that creation of the strong trap sites
are due to the effects of He-bubbles formed by the helium pre-irradiation, and the trapping energyof those sites was estimated to be about 1.21 eV. Change in depth profiles of tritium with heating
was evaluated by analyzing the observed X-ray spectra by means of computer simulation. From thesimulation analyses, it was seen that the strong trap sites distributed up to a region of 0.1 µm
beneath the surface and that about half of the implanted tritium ions were trapped in this region.

References
1. N. Yoshida, J. Nucl. Mater., 266-269 (1999) 197.
2. H. Iwakiri, K. Morishita, and N. Yoshida, J. Nucl. Mater., in press.
3. M. Matsuyama, S. Ueda, and K. Watanabe, Fusion Eng. Design, 49-50 (2000) 885.
4. M. Matsuyama, T. Murai, K. Yoshida, K. Watanabe, H. Iwakiri, and N. Yoshida, J. Nucl. Mater.,in press.
5. M. Matsuyama, S. Ueda, T. Ogawa, T. Uda and K. Watanabe, Ann. Rept. Hydrogen Isotope
Research Center, Toyama University, 18 (1998) 69.
6. M. Matsuyama, T. Murai and K. Watanabe, Fusion Sci. Technol., 41 (2002) 505.
7. P. Franzen, C. G.-Rosales, H. Plank and V. Kh. Alimov, J. Nucl. Mater., 241-243 (1997) 1082.
8. M. Matsuyama, K. Watanabe and K. Hasegawa, Fusion Eng. Design, 39-40 (1998) 929.

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