富山大学水素同位体科学研究センター研究報告 23 : 1-7, 2003.

論文
BIXS 法によるトリチウム分圧の測定（Ⅰ）−水素同位体の全圧依存性−松山政夫１）、洲亘２）、鈴木 卓美２）、西正孝２）
１）
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
２）
日本原子力研究所 トリチウム工学研究室〒319-1195 茨城県那珂郡東海村

Measurement of Tritium Partial Pressure by BIXS (I)- Total Pressure Dependence of Hydrogen Isotopes M. Matsuyama1), W.M. Shu2), T. Suzuki2) and M.F. Nishi2)
1)
Hydrogen Isotope Research Center, Toyama University,
Gofuku 3190, Toyama 930-8555, Japan2)
Tritium Engineering Laboratory, Japan Atomic Energy Research Institute,
Tokai-mura, Nka-gun, Ibaraki 319-1195, Japan(Received May 24, 2004; accepted March 15, 2005)
ABSTRACTThe correlation between the intensity of X-rays induced by β-rays of tritium and the totalpressure of hydrogen isotope mixtures was examined in a pressure range from 10-1 to 105 Pa
by using three different tritium gases, i.e., pure T2, D2-1%T2 and H2-1%T2 mixtures. It was
found that linear pressure dependence was obtained up to a few kPa for all of the measuredgases by taking dead time of the X-ray detector into account. Above this pressure, however,downward deviation from the linear relation appeared. It was revealed that the effect ofself-absorption of β-rays on the pressure dependence was not negligibly small, because theratios of the counting rates corrected by dead time to total pressure, i.e., specific counting
rates, could not be reproduced by a simple exponential function using the absorption
coefficients of hydrogen isotopes as variables. The results suggested that an additional factor
such as contribution of an X-ray formation on the surfaces of a measuring cell must be taken
into consideration in changing in the specific counting rates.

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松山

政夫・洲

亘・鈴木

卓美・西

正孝

1. Introduction
From a viewpoint of tritium safety and economy, quantitative measurements of highlevel tritium are indispensable for the tritium plant of a large D-T fusion facility such as ITER
and future fusion devices. For this purpose, a variety of the measuring methods based on
different principles have been developed so far: for example, a small ionization chamber [1],laser Raman spectrometer [2], mass spectrometer [3], and so on. Although these techniques
are promising, there are some problems. The ionization current is influenced by changes inthe total pressure and composition of the measuring gases, the lower detection limit of laser
Raman spectrometer is not sufficient for the required detection limit, and the mass
spectrometer consumes a part of the measuring gas during measurement.
On the other hand, Matsuyama et al. recently proposed β-ray-induced X-ray
spectrometry (BIXS) for measurements of tritium partial pressure in tritium-containing gases[4]. This technique is based on detection of X-rays induced by interactions between materials
and β-rays emitted from gaseous tritium in a specially designed cell. Intensity of the observed
X-rays was proportional to the partial pressure of tritium in the cell, when the total pressure
was lower than about 2 kPa. Above this pressure, the downward deviation from the linear
relation increased with increasing total pressure because of self-absorption of β-rays inmeasuring gases. To apply the present technique in a wide range of tritium partial pressure, itis necessary to examine the pressure dependence of the self-absorption in a pressure region
above a few kPa. In addition, effects of hydrogen isotope species in a measuring gas on the
self-absorption are also important, because of the difference in the ionization energy among
the isotope species.
From these viewpoints, correlation between the intensity of β-ray-induced X-rays and thetotal pressure of hydrogen isotopes has been examined usingA
three different tritium gases i.e., pure tritium,
deuterium-diluted tritium, and hydrogen-diluted tritium.
Au/Be window
Be window

NaI(Tl)Scintillation
Probe

Metal bellows
valve

VCR fitting
Probe guideCu

2. Experimental

Cu

Double sided flange

B

Figure 1 shows a schematic diagram (A) and a photograph(B) of the measuring cell used in this study. The measuring
cell consisted of a double-sided flange, two Conflat flanges(ICF with 70 mm in diameter), a radiation window made ofthin beryllium plate, and a metal bellows valve. All the flanges
were made of stainless steel, and they were connected using
copper gaskets. The inner surface of the radiation window as

2

Fig. 1 Cross-sectional view (A) andphotograph (B) of the measuring cell.

Measurements of Tritium Partial Pressure by BIXS(Ⅰ)

well as flanges was coated with thin gold film inorder to enhance the conversion of β-rays to X-rays
and to impede ad/absorption of gaseous tritium on/in
construction materials of the measuring cell.
Internal volume of the measuring cell was about
35 cm3. One of the two Conflat flanges wasconnected to a metal bellows valve for supply andrecovery of tritium, and the other was used tomeasure X-rays by a conventional NaI(Tl) Fig. 2 Schematic diagram of the device used
for preparation of tritium mixtures.
scintillation probe. Total pressure of hydrogenisotopes in the measuring cell was measured using two capacitance manometers with different
sensitivities.
Fig. 2 shows the preparation device of different tritium gases, which consists of a mixing
tank, U-bed, circulation pump and was connected with the metal bellows valve of themeasuring cell. Both the measuring cell and preparation device except a radiation counter(electronics such as high voltage, scaler, and timer) were installed in a glove box to ensure
safety. The glove box is constructed at Tritium Engineering Laboratory at Japan Atomic
Energy Research Institute.
In the present study three different tritium gases were used: high purity tritium (thetritium concentration was determined as 99.2% by gas chromatography) and about 1% tritium
gases diluted with deuterium or hydrogen. These hydrogen isotope mixtures were prepared bysupplying a given amount of high purity tritium from the uranium bed to the mixing tank. In
the measurements of high purity tritium, it was directly introduced into the measuring cell.
Pressure dependence of the X-ray intensity was measured by reducing stepwise the totalpressure of T2 and mixture gasses from about 100 kPa to 0.1 Pa. Measuring time was varied
depending on the counting rate of X-rays at each pressure.
106

3. Results and discussion
Counting rate, N / counts s-1

105

It has been already reported that a linear
relation between the intensity of X-rays
induced by β-rays and the total pressure oftritium-containing gas appears in a pressure
region below about 2 kPa [5]. The results
obtained by the present examination are
illustrated by three symbols in Fig. 3.
Although three solid lines having a unit ramp

: H2 - T2(1%): D2 - T2(1%)
: T2

104
103
102
101
100
10-1

100

101

102

103

104

105

Total pressure, P / Pa

Fig. 3 Total pressure dependence of the counting rates
for T2, D2-1%T2 and H2-1%T2 mixtures.

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松山

政夫・洲

亘・鈴木

卓美・西

正孝

106

are shown in the figure, they are only eye
guides. As clearly seen, the counting rates forthe high purity tritium were about 100 times
greater than those for H2-T2 and D2-T2
mixtures. This difference is mainly based ona large difference in the partial pressures oftritium among those gases. On the other hand,the counting rates for H2-T2 and D2-T2
mixtures were proportional well to their totalpressures up to about 4 kPa, whereas for thehigh purity tritium lower deviation from thelinear relation appeared above a pressure ofabout 0.5 kPa. There are two plausible
reasons for the downward deviation: one isthe effect of dead time of the present detector,
and the other is the effect of self-absorption: H2 - T2(1%): D2 - T2(1%): T2(Dead Time=3.5 µs)

Counting rate, N / counts s-1

105
104
103
102
101
100
10-1

100

101

102

103

104

105

106

Total pressure, P / Pa

Specific counting rate, NS / counts s-1 Pa-1

Fig. 4 Total pressure dependence of the counting rates
corrected by dead time of the X-ray detector.
102

T2

101

100

H2-1%T2 mixture
D2-1%T2 mixture

10-1

of β-rays in the measuring gases. On theother hand, below 10 counts/s the counting
10
10
10
10
10
10
10
10
rates scattered around the line. It is
Total pressure, P / Pa
Fig. 5 Total pressure dependence of the specific counting
considered that this is mainly due torates for T2, D2-1%T2 and H2-1%T2 mixtures.
significant increase in the background levelcaused by adsorption of tritium oxides on the inner surface of the measuring cell.
-2

-1

0

1

2

3

4

5

The dead time of the present radiation counter was 3.5 µs. When τ is the dead time of thecounting system and N(obs) the observed counting rate, the unconcerned fraction in N(obs)counts is τN(obs). If N(true) is the true counting rate, the number of counts lost is τN(obs)N(true). Therefore,N(true)=N(obs) + τN(obs) N(true) .
−−−−− (1)Figure 4 shows the results calculated by Eq. (1). The dead-time correction appeared above 104
counts/s. Namely, the linearity of the counting rates for the high purity tritium was improvedin a pressure region higher than a few kPa.
During the present examinations, each tritium concentration in three measured gases was
kept constant at a whole pressure region. Specific counting rate (Ns), i.e., the ratio of thecorrected counting rate by Eq. (1) to total pressure, should be constant, if the self-absorptionof β-rays in a measuring gas is negligibly small. To confirm this effect, the specific counting
rates (closed symbols) were plotted against the total pressure as shown in Fig. 5. At a pressure
region above around a few kPa, the specific counting rate for each gas became lower than therespective constant value with increasing total pressure. This indicates clearly that the4Measurements of Tritium Partial Pressure by BIXS(Ⅰ)

self-absorption of β-rays in the gas phase is not negligible.
Supposing that the effect of self-absorption can be represented by a simple exponentialfunction, the specific counting rate can be described by the following equation as a first
approximation:
Ns = a0 {1 – exp (- µ P)} / µ P,
where a0 is the true counting rate per unit----- (2)100

Specific counting rate, NS / counts s-1 Pa-1

H -1%T mixture
pressure, µ the absorption coefficient oftritium β-rays for each gas, and P the totalpressure. Figure 6 compares the observed
10
specific counting rate with the estimatedvalues by Eq. (2) for the H2-1%T2 mixture as
an example (broken line). In this calculation,
the absorption coefficient for pure hydrogen
10
10
10
10
10
10
-1
gas (1.8 cm at 101.3 kPa [Ref]) was
Total pressure, P / Pa
employed, because the tritium concentration Fig. 6 Comparison between the specific counting rates andthose calculated by Eq. (2) for the H2-1%T2 mixture. Broken
was quite low. The broken line can be line: µ=1.8 cm-1, dotted and broken line:µ=5.5 cm-1.
represented by the following equation:
Ns = 0.317{1-exp (-1.8P/101300)}/(1.8P/101300)------(3)Disagreement between the observed specific counting rate and the broken line appeared in apressure region above 1 kPa. Even though the different absorption coefficients were applied to
2

2

-1

-2

1

2

3

4

5

Eq. (3), as shown by the dotted and broken line (µ=5.5 cm-1), they did not agree well in ahigh-pressure region. This indicates that the major change in the specific counting rates does
not solely arise the self-absorption of β-rays in the gas phase.
A certain fraction of β-rays emitted in the measuring cell strikes on the gold surfaces onboth the beryllium window and stainless steel constructing the measuring cell, and thencontributes X-ray emission. A part of such X-rays can penetrate through the beryllium plate.
Namely, it is considered that the observed counting rate include contribution of X-rays
produced at the gold surfaces of stainless steel as well as beryllium. The intensity of theformer X-rays may also increase with increasing total pressure, but a linear increase in the
X-ray intensity can not be expected owing to the self-absorption of β-rays. It is hard toevaluate quantitatively the contribution of such X-rays to the observed counting rate, because
a complex structural factor has to be taken into account.
The solid line in the Fig. 6 shows a fitting curve of the specific counting rate. The
difference (A) between the broken and the solid lines could be described as
A = 0.496 + 0.489exp(-3.84P/101300).
----- (4)Namely, the pressure dependence of the counting rate for H2-1%T2 mixture gas can be writtenby the following equation.

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松山

政夫・洲

亘・鈴木

卓美・西

正孝

(1) H2-1%T2 mixture gas:
Ns=0.317{1-exp(-1.8P/101300)}/(1.8P/101300){0.496+0.489exp(-3.84P/101300)}
In a similar manner, the pressure dependence of specific counting rates for the D2-1%T2
mixture gas and the T2 gas were described by the following equations:(2) D2-1%T2 mixture gas;
Ns=0.248{1-exp(-2.1P/101300)}/(2.1P/101300){0.524+0.471exp(-3.85P/101300)} ,(3) T2 gas;
Ns=29.6{1-exp(-2.2P/101300)}/(2.2P/101300){0.344+0.626exp(-2.67P/101300)} .
The specific counting rates for threehydrogen isotopes were finally reproduced
10well using above equations as shown in Fig.
T
10
7. The first numerical values in the threeequations mentioned above depend on the10
H -1%T mixture
tritium concentration in the measuring gases.
Since the chemical purity of T2 gas
10
D -1%T mixture
determined by gas chromatography was
10
99.2%, those of the H2-1%T2 and D2-1%T2
10
10
10
10
10
Total pressure, P / Pa
mixtures used in this study were evaluated as
7 Comparison between the specific counting rates and1.06 and 0.83%, respectively. In addition, Fig.
the reproducing equations.
the absorption coefficients for pure D2 andT2 gases estimated to be 2.1 and 2.2, respectively.
Specific counting rate, NS / counts s-1 Pa-1

2

2

1

0

-1

2

2

2

2

-2

1

2

3

4

5

Absorption coefficient of β-rays, in general, depends on some factors such as electron
density in a given volume, energy of β-rays, and average excitation energy of each hydrogenisotope species. The average excitation energy includes excitation and ionization energies ofeach hydrogen isotope species. In the present case, the former two factors are the same, while
the last one may be different among H2, D2, and T2 molecules, because there is a little
difference in ionization energy among them, i.e., H2: 15.43, D2: 15.47 and T2: 15.49 eV [6].
4. Conclusions
Total pressure dependence of the X-ray intensities induced by β-rays of three different
tritium gases such as pure T2, D2-1%T2 and H2-1%T2 mixtures was examined. The results
obtained in this study were as follows:(1) Linear relations between the intensity of X-rays induced by β-rays of tritium and the totalpressure of hydrogen isotope mixtures were obtained up to a few kPa for all the measuredgases by taking the dead time of the X-ray counting into account.
(2) Above this pressure, however, downward deviation from the linear relation still appeared

6Measurements of Tritium Partial Pressure by BIXS(Ⅰ)

even though the observed counting rates were corrected by self-absorption of β-rays in gas
phase.
(3) Changes in the observed counting rates with total pressure were finally reproduced bytaking a structural factor in addition to two factors of the dead time and self-absorption
into account.

References(1) M. Matsuyama and K. Watanabe, Fusion Eng. Design, 18 (1991) 91.
(2) S. O’hira, T. Hayashi, H. Nakamura, et al., Nucl. Fusion, 40 (2000) 519.
(3) R.E. Ellefson, W.E. Moddeman and H.F. Dylla, J. Vac. Sci. Technol., 18 (1981) 1062.
(4) M. Matsuyama, H. Nakatani and K. Watanabe, Fusion Eng. Design, 10 (1989) 405.
(5) M. Matsuyama, K. Watanabe and T. Yamazaki, Fusion Technol., 28 (1995) 1045.
(6) P.C. Sours, “Hydrogen Properties for Fusion Energy”, University of California Press,
London, (1986), p. 257.

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