富山大学水素同位体科学研究センター研究報告 24 : 1-8, 2004.

論 文

ホウ素薄膜からのトリチウム脱離挙動に対するヘリウムの予照射効果
松山政夫1)、塚田 真理1)、奥野 健二2)、大矢 恭久3)相良 明男4)、野田 信明4)、渡辺 国昭1)

1) 富山大学水素同位体科学研究センター、〒930-8555
2) 静岡大学理学部放射化学研究施設、〒422-8592

富山市五福 3190

静岡市大谷 836

3) 東京大学アイソトープ総合センター、〒113-0032

東京都文京区弥生 2-11-164) 核融合科学研究所、〒509-5292 土岐市下石町 322-6

Effects of Helium Pre-Irradiation on Tritium Release fromThin Boron Film Deposited on Stainless Steel type 316
M. Matsuyama1), M. Tsukada1), K. Okuno2), Y. Oya3)
A. Sagara4), N. Noda4) and K. Watanabe1)
1) Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930-8555, Japan
2) Radiochemistry Research Laboratory, Faculty of Science, Shizuoka University, Ohya 836,
Shizuoka 422-8529, Japan
3) Radioisotope Center, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
4) National Institute for Fusion Science, Oroshi-cho 322-6, Toki 509-5292, Japan(Received June 30, 2005; Accepted November 28, 2005)
Abstract
The effects of helium pre-irradiation on trapping and release behavior of tritium implanted
into thin boron films deposited on a plate of stainless steel type S316 were examined by anisothermal heating method and β-ray-induced spectrometry (BIXS). BIXS results showed thatthe helium pre-irradiation caused an increase in the trapping amount of surface tritium.
Isothermal heating of the samples was carried out at temperatures in the range of 473 to 723K,
and the decreasing rate of surface tritium was measured by tracking changes in the intensityof X-rays induced by β-rays. It was found that the decreasing rate of residual tritium obeyed
first order reaction kinetics, indicating that the helium pre-irradiation has no influence on thetritium release kinetics. Apparent activation energy of the decreasing rate was determined to１

松山政夫・塚田真理・奥野健二・大矢恭久・相良明男・野田信明・渡辺国昭

be 0.3 eV from temperature dependence of the decreasing rate constant, which was about
two-times larger than that measured without pre-irradiation of helium.
1. Introduction
It is well known that the plasma facing materials (PFMs) play an important role forcontrol of impurities as well as fuel particles in the reactor core of a magnetic fusion device. A
coating technique of the first wall surfaces with low-Z materials such as lithium, beryllium
and boron is widely employed to reduce the oxygen partial pressure and the high-Z metallic
impurities in the plasma. Effectiveness of boron coating for remarkable improvement ofplasma parameters has been reported so far [1-3], but dynamic behavior of tritium in boron
materials has not been clarified yet. In addition, energetic helium particles produced by thethermonuclear reactions impinge on the surfaces of PFMs along with fuel particles. Therefore,
it is indispensable to understand the effects of helium irradiation on trapping and release
behavior of tritium.
In the previous study [4], we examined the release and diffusion behavior of tritium ions
irradiated thin boron films, which were deposited on a stainless steel plate. The examinations
were carried out by isochronal and isothermal heating methods. As a result, it was found thatmost of the desorbed tritium was in an oxidized form and the residual amount of tritium in thesurface layers decreased exponentially with time in the isothermal heating to 723 K. It wassuggested, therefore, that the decreasing rate obeys first order reaction kinetics and therate-determining step is the diffusion of tritium in boron. Furthermore, the Arrhenius plot of
the decreasing rate constants showed a good linear fit and from this relation apparent
activation energy of the diffusion of tritium was determined to be 0.17 eV.
In this study, effects of helium pre-irradiation on tritium retention in a thin boron film
deposited on stainless steel plates were examined using β-ray-induced X-ray spectrometry(BIXS) and isothermal heating.
2. Experimental
2.1. Materials
Prior to coating of a boron film, plates of stainless steel type 316 (described as SS316),
which were used as a model substrate, were finally polished on buff after polishing on a seriesof polishing papers, and then rinsed with water and acetone. A thin boron film was preparedby RF-assisted DC glow discharge using a mixture of helium and decaborane gas (95:5), andit was deposited on a plate of SS316 (described as B/SS316). Size of the SS316 plate was
15x15x0.5 mm3, and thickness of the boron film was about 0.1 µm. Purity of the boron films
prepared was about 90% and the details were described elsewhere [4].
２ホウ素薄膜からのトリチウム脱離挙動に対するヘリウムの予照射効果

Tritium gas used for ion irradiation was diluted with deuterium, and the concentration oftritium was about 0.7 %. Purity of helium used for pre-irradiation was 99.99995%, and themain impurities were nitrogen and oxygen.
2.2. Preparation of the irradiation samples
Tritium ions were irradiated using the tritium irradiation device that was speciallydesigned for the irradiation of hydrogen isotopes. The pre-irradiation of helium ions were also
conducted with the same device. The detailed structure of the irradiation device was described
elsewhere [5]. At first, a B/SS316 sample was fixed on an irradiation stage in the irradiation
device. Then the sample was evacuated at ambient temperature for two days and subsequently
degassed to below 10-5 Pa at 673K. After degassing, an ion irradiation was carried out. Twokinds of irradiation samples were prepared in this study: namely, one was irradiated with only
tritium ions, and the other was irradiated with helium ions prior to tritium irradiation.
Irradiations of tritium and helium ions were carried out at 1 keV for 15 min at room
temperature. During an irradiation, total pressures of tritium and helium gases were kept
constant at 4 and 1.3 Pa, respectively, and the ion current in each irradiation was recorded toestimate the fluence of each ion. Average ion currents of tritium and helium were 1.0 and 0.3µA, respectively. After the irradiation with tritium ions, the irradiated sample was evacuatedfor about one week to avoid the severe release of tritium species from the irradiation chamber
walls when the sample is taken out of the irradiation device.
2.3. Isothermal heating of the irradiation samples
The irradiation samples were subjected to
examination of isothermal heating. It was carried out
using a separate vacuum device as shown in Fig. 1. An
irradiation sample was hung by using a cage made of afine metallic wire, and then evacuation to below 10-5 Pa
was conducted at room temperature. The sample set in
the cage was initially separated from the heating partsurrounded by a furnace, and it was brought down after
the heating part reached at a given temperature. The
sample was heated for 30 min, and the released gases
were evacuated during heating at a constanttemperature. After a given time, the sample was again
lifted, and it was cooled down to the room temperature.
After this, the sample was detached from the vacuum
device, and subjected to measurements of an X-ray
spectrum induced by β-rays in the argon atmosphere.
３

Fig. 1. Schematic drawing of the device
used for isothermal heating of samples.

松山政夫・塚田真理・奥野健二・大矢恭久・相良明男・野田信明・渡辺国昭

The sample was again set in the vacuum device after the measurements and it was heatedunder the same conditions. Namely, heating of an irradiation sample and measurements of an
X-ray spectrum were repeated. The isothermal heating was carried out at temperatures in therange of 473 to 723K.
2.4. Measurements of an X-ray spectrum induced by β-rays
A high purity Ge detector was applied to measure the X-ray spectra induced by β-rays in
the argon atmosphere. Argon was used as working gas to evaluate the residual amount oftritium in surface layers of the B/SS316 samples irradiated with tritium. The flow rate ofargon was kept at 40 cm3/min. All of the X-ray spectra were measured at room temperature
after heating under the given conditions of temperature and time.
3. Results and discussion
Small angle X-ray diffraction analyses wereapplied to examine a crystal structure of the thinboron film deposited on a SS316 plate. Angle ofincident X-rays was kept at one degree. The observed
diffraction pattern for B/SS316 is shown in Fig. 2
along with that for the SS316 plate. Quite similar
diffraction pattern appeared in both X-ray diffraction
analyses, and no characteristic diffraction peaks forboron crystals were observed. That is, all thediffraction peaks observed were attributable to thestructure of stainless steel. This suggests that thestructure of the boron films prepared under thepresent conditions is a state of amorphous.
Figure 3 shows the comparison of the observed
X-ray spectra with and without helium pre-irradiation.
Several intense and sharp peaks appeared in bothFig. 2. XRD patterns for SS316(upper) andboron-coated SS316(lower) samples

spectra, and they were ascribed to the Ar(Kα),
Cr(Kα), Fe(Kα) and Ni(Kα) lines as shown in the figure. The appearance of Ar(Kα) line isdue to a reason that the tritium irradiated samples were measured in an argon atmosphere. The
intensity of Ar(Kα) line corresponds to the amount of tritium trapped in the surface layers of atritium-irradiated B/SS316 sample. The Ar(Kα) intensity in the case of He pre-irradiation was
about 1.5 times greater than that of no helium pre-irradiation [4]. This increase was beyond
the limitation of experimental error. It is considered, therefore, that the increase is due to
production of a variety of defects such as dislocation and vacancy by He pre-irradiation. A
４ホウ素薄膜からのトリチウム脱離挙動に対するヘリウムの予照射効果

of β-rays emitted in the boron
films can strike the surface of astainless steel substrate, because
thickness of the boron films was
about 0.1 µm. This is a reason why
three characteristic X-ray peaks
ascribed to Cr, Fe and Ni atoms
appeared in the B/SS316 sample.
Figure4shows
thecomparison of X-ray spectra
observed before and after heating

Without He pre-irradiaiton
Counting rate, N / counts min-1 (∆E)-1

10

10Ar(Kα)

8

8

6

6

4

4
Cr(Kα)22

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

With He pre-irradiation

Ni(Kα)

0

0

4 6 8 10 12 14 16 0
Energy of X-rays, E / keV

2

2

0
4 6 8 10 12 14 16 18
Energy of X-rays, E / keV

Fig. 3. X-ray spectra observed for B/SS316 samples with andwithout He pre-irradiation.

2.0

0.5

No heating(623K, 5hr)
0.4

1.5
0.3
1.0
0.2
0.5
0.1

0.0

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

range of β-rays in boron isestimated to be about 1 µm,
assuming that the range is closed tothat in carbon. Namely, a half part12

12Counting rate, N / counts min-1 (∆E)-1

great increase, ca. 5 times, wasobserved
fortungstenpre-irradiated with helium ions, but
the difference in effect of theincrease was mainly due to thefluence and energy of helium ions[6].
Characteristic X-ray peaks
ascribed
totheconstituent
elements of stainless steel wasobserved for B/SS316 samples
though their surfaces are coatedwith the thin boron film. Maximum

0.0
0

2

4

6

8 10 12 14 16 18

2

4

6

8 10 12 14 16 18

Energy of X-rays, E / keV

Fig. 4. Effects of heat treatment on the shape and intensity of
X-ray spectrum.

at 623 K for 5 hours. The intensity of X-rays decreased to 25% of the initial intensity byheating, but shape of both X-ray spectra was quite similar as a whole. This indicates that mostof tritium desorbed from the boron film by heating and the diffusion of tritium into thesubstrate is negligibly small. If considerable amount of tritium diffused into the substrate byheating at high temperatures, the shape of bremsstrahlung X-ray spectrum should change as
the maximum shifts to the higher energy side and intensities of characteristic X-rays of Cr and
Fe become larger. Namely, it is suggested that the interface between the boron film and thesubstrate acted as a diffusion barrier of tritium.
Such diffusion behavior of tritium at higher temperatures can be also understood fromchanges in intensity ratios of Cr(Kα) and Fe(Kα) to Ar(Kα) line with heating time. If the５松山政夫・塚田真理・奥野健二・大矢恭久・相良明男・野田信明・渡辺国昭

0.15

Intensity ratio, RCr/Ar / -surface tritium atoms diffuse into thestainless steel, one can observe an increase
in intensity ratios of them. Figures 5 and 6
show changes in the intensity ratio of X-rays
with heating time at all the examination
temperature. It can be seen from both figuresthat the intensity ratios of X-rays areconstant
withinexperimental
error,
indicating that the irradiated tritium did not
diffuse into the substrate of stainless steel
during heating. This agrees with theprevious result [4].
To examine the release behavior oftritium at room temperature, the intensity of: 473K(1): 473K(2): 543K: 623K: 723K

0.10

ICr / IAr

0.05

0.00

0

1

2

3

4

5

6

Heating time, t / hr

Fig. 5. Change in intensity ratio of characteristic
X-rays of Cr to those of Ar.

0.15

Intensity ratio, RFe/Ar / -IFe / IAran X-ray spectrum was tracked for a long
period prior to the isothermal heating of the
0.10
tritium-irradiated B/SS316 samples. A
gradual decrease in the intensity was
0.05
observed. Most of the desorbed tritium: 473K(1): 473K(2): 543K
species was in an oxidized form and the: 623K: 723K
elemental form was quite small.
0.00
0
1
2
3
4
5
6
Figure 7 shows the changes in the
Heating time, t / hr
residual amount of tritium with isothermal
heating for the helium pre-irradiated Fig. 6. Change in intensity ratio of characteristic
X-rays of Fe to those of Ar.
B/SS316 samples at elevated temperatures.
Each symbol in the figure describes the X-ray intensity normalized by setting the initial

Normalized X-ray intensity of Ar(Kα), I / -intensity of Ar(Kα) line in each sample tounity, and the solid lines were drawn by asimilar exponential function as shown in theprevious result [4]. In this figure, two-times
examinations were carried out at 473 K, andreproducibility of the decreasing curve wasobserved although initial intensities ofAr(Kα) line for both samples were different
from each other. As seen clearly from thefigure, decreasing rate of X-ray intensity
increased with an increase in temperature,indicating that release rate of tritium trapped

1.0

0.8

473K(1)
0.6

543K
0.4

623K

0.2

723K
0.0
0

1

2

3

4

5

6

Heating time, t / hr

Fig. 7. Changes in the X-ray intensity of Ar withheating time.

６ホウ素薄膜からのトリチウム脱離挙動に対するヘリウムの予照射効果

in the surface layers was accelerated by heating because the residual amount of tritium

Decreasing rate constant, k / hr-1

corresponds to the intensity of an Ar(Kα) line.
The time-course of the tritium reduction could be reproduced by the following
exponential function:
N(t) – N(∞) ={N0 - N(∞)}exp(-kt),
where N(t) represents the residual amount of tritium at t, N0 the initial amount of tritium,
N(∞) the final amount of tritium remaining in the surface layers of the sample, and k thedecreasing rate constant. N(∞) was dependent on the heating temperature. It is suggested thatthe decreasing rate of residual tritium obeys first order reaction kinetics. It was seen, therefore,that the helium pre-irradiation has no influence on the tritium release kinetics.
Figure 8 shows the tempera- ture dependence of decreasing rate constants determinedfrom Fig. 7 along with the previous
results measured without helium
B / SS316
pre- irradiation for the same
B/SS316 sample. A good linear
relation
appearedinthe100
temperature range from 473 to 723
K, and apparent activation energy
was determined to be 0.30 eV from: without He pre-irrad.
the ramp of the line. The present: with He pre-irrad.
activation energy was about two
10-1
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
times larger than the previous
result (0.17 eV) [4]. It is
Reciprocal temp., T-1 / 10-3 K-1
considered that the increase in theFig. 8. Temperature dependence of the decreasing rate constantactivation energy is due tofor the residual amount of tritium.
formation of strong trapping sites
such as a B-T terminal bond by helium pre-irradiation.
4. Summary
Plasma facing materials (PFMs) play an important role for control of impurities as well
as fuel particles. Low-Z materials such as lithium, beryllium, boron and carbon are promisingmaterials as PFMs. These materials are exposed to energetic helium particles as well as fuelparticles. From this viewpoint, effects of helium pre-irradiation on trapping and release
behavior of tritium were examined under the condition of isothermal heating. Thin boron
films were prepared by plasma chemical vapor deposition on a substrate of stainless steel type316, and those were used as samples.
The amount of tritium retained in surface layers was measured with β-ray-induced X-ray７

松山政夫・塚田真理・奥野健二・大矢恭久・相良明男・野田信明・渡辺国昭

spectrometry (BIXS). The amount of tritium trapped in thin boron films increased to 1.5 times
by He pre-irradiation. The amount of tritium implanted by ion irradiation decreased with atime of isothermal heating. It was seen from the changes in X-ray spectra that the decreasing
behavior is due to the tritium release from the surface but not diffusion into the stainless steel.
From the changes in the Ar(Kα) line, the decreasing rate of residual tritium obeyed firstorder reaction kinetics, which was the same kinetics as the case of no helium irradiation. It
was seen, therefore, that the helium pre-irradiation has no influence on the tritium releasekinetics. From the temperature dependence of the decreasing rate constant, the apparent
activation energy of tritium release was determined to be 0.30 eV, and it was about two times
larger than the previous result of no helium pre-irradiation.
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