富LL｢大学水素同位体科学研究センター研究報告26 : 77‑81, 2006.

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BIXS法によるトリチウム分圧の測定(Ⅱ)‑混合気体の影響一松山政夫l)､洲亘2)､鈴木卓美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)日本原子力研究開発機構･核融合エネルギー工学研究開発ユニット

トリチウム工学研究グループ〒319‑1195茨城県那珂郡東海村村松4‑49

Measurement of Tritium Partial Pressure by BIXS (II)‑ Effects of Mixture Gases ‑M･ Matsuyamal), W･ M･ Shu2), and T･ Suztlki2)1 ) Hydrogen Isotope Research Center, University ofToyama
Gofuku 3 190, Toyama 930‑8555, Japan2) Tritium Teclmology Group, Division of Fusion Energy TeclmOlogy
JapanAtomic Energy Agency, Ibaraki 31911 195, Japan(Received May ll, 2007; accepted June 29, 2007)■ノAbstracI
The bremsstrahlung X‑ray countlng method is one of the promlSlng methods for in‑situ measurements
or high‑level tritium, and the applicability of this method to a tritium handling system has been
examinedJn this study, pressure dependence or Xィay intensity in a high pressure region above 1 kPa

was mainly examined uslng mixtures ofH2‑, D21, He‑ and Ar‑1%T2. In a low pressure region below IkPa, the X‑ray intensitywas proportional to the partial pressure oftritium in all of the mixtures, while
above that pressure it deviated downward from the proportional tendency. The deviation behavior in

the mixtures ofH2‑, D2‑, He‑1%T2 Was Similar, but that in the mixture ofAr‑1%T2WaS different fromthat in the other three mixtures: namely, the downward deviation in the three mixtures is mainly

caused by the self‑absorption of P‑rays in the hydrogen isotope and helium gases, whereas in themixture of Ar‑1%T2 the emission of characteristic X‑rays from argon atoms plays a major role in thedeviation.

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松山｣政夫･洲亘･鈴木卓莱In order to safely and precisely control tritium in afuel circulation system of thethermonuclear fusion devices, it is indispensable to measure in‑situ the concentration oftritium in circulation gases. For this purpose, it is widely recognized that an ionizationchamber is convenient as a conventional method. However, there are some disadvantageouspolntS: fTor example, since the ionization current depends on the total pressure and thecomposition of a mixture gas containlng tritium, different ionization currents appear even if
the number oftritium atoms in an ionization chamber is the same. h addition to this, a small
quantity of impunty such as helium strongly affects the ionization current, which is so‑called

Jesse's efrTect ll]. Helium is a main product in the D‑Tfusion reactions. This effect is causedby energy transfTer丘･om an excited state of the helium to a hydrogen isotope molecule,causlng it to iomize, since ionization potential ofa hydrogen isotope molecule is less than theexcitation potential of the helium. Because the helium is the main products in the D‑Tfusion
reactions, it is necessary to develop a new measunng method to evaluate precisely the partialpressure of the tritium species in a mixture gas.

Recently, a bremsstrahlung X‑ray counting method has been proposed by Matsuyama et al.
[2‑3], and it isknown as one of the promising methods fわr in‑situ measurements of gaseoustritium. This method is based on utilization of the bremsstrahlung and characteristic X‑raysemitted by the interactions between β‑rays and substances. A schematic drawing of theproposed measunng cell is shown in Fig. 1. The present measunng cell was speciallydesigned and manufactured to measure in‑silu high‑level tritium. The conversion kom tritium
β‑rays to X‑rays mainly takes place on the sur払ce of an Au/Be plate equipped with themeasunng cell as a radiation window.

Inner surfaces of the meahnng cell except two Cu gaskets are coated with a thin gold
film. The X‑rays are mainly generated on the gold surfaces bombarded with P‑rays. A part of
X‑rays generated penetrates through a radiation window material which was made or a thinberyllium disk (150 pm) coated with a thin gold film (described as Au/Be). The penetratedXィays implnge On the head of an X‑ray detector and then those are detected. Namely, thestmcture of the measunng cell is very
simple and the detection processes ofthe X‑rays are not complicated. These
are advantageous polntS Of the present

CO 0 19

method. The X‑rays produced on the場綾解約毎巧
X‑rayDetectorl⊂ここ=こし.J VCRFittin9 lCF

祐桝堺静等

surfTace of the Au/Be disk are major Partof the detected X‑rays.
In the previous

report l4], totaltang 唐｢ Xｲ+ﾂ 8ﾒ(UN汀:mm)pressure dependence or X‑ray Intensity
was examined uslng tritium species

Fig. 1 Cross‑Sectional view of血e measunng cell f♭rsuch as elemental T2 and the mixtures

78

gaseous tritium.

BIXS法によるトリチウム分圧の測定(Ⅱ)
Table 1. Absorption coefrlCient (p) ofor D2‑T2.and H2‑T2. As a result, it was fわund that theP‑rays.and average WIValue

X‑ray Intensity was proportional to the total pressure

for vanous gases.

below a fTew kPa f♭r all of the gases, while above thisGas 中6ﾗ ﾔ鋳 W‑value (eV/ionpair)pressure downward deviation from the linear relation
appeared. quite similar phenomenon was also obseⅣed

H2

繧

36.6±0.3He縒

41.5±0.4Ne途綯

36.2±0.4

N2免ﾂ

34.6±0.3

for the mixture of He‑T2 [3]. The deviation from thelinear relation was attributed to the self‑absorption ofβ‑rays in a gas phase orthe measuring cell. The energy

loss of tritium P‑rays in various gases strongly depends
not only on the total pressure but also on atomic

Ar

number of gaseous species. The absorption coefficients

CH4l516] of P‑rays for several gases are shown along withH20"纈

26.2±0.2

27.3±0.3
唐30.1±0.3

the average W‑values l7] in Table I. It is consideredthat the similar phenomenon to the deviation mentioned above is due to a reason that theabsorptlOn COefrlCient for He is almost equlValent to that f♭r H2･To examine in detail the effects of a mixed gas in the tritium mixtures on thebremsstrahlung X‑ray countlng, a mixture gas of Ar‑T2 Was prepared and total pressure
dependence of the mixture was measured at an ambient temperature･ Tritium concentration inthe preparedmixture was nominally 1 atomic percent (herea氏er described as Ar‑1%T2mixture). Totalpressure of the mixture was measured using a capacitance manometer

equlpped with two sensors of 1.3 kPa and O･13 MPa, and the X‑ray lntenSlty Was measured
using a specially designed Nat(Tl) scintillation counter. The X‑ray intensity was repeatedly
measured

5times fわr 1

minute in each pressure and''一一日ー

the average value was剪

E.:Ar‑1%T2mixturet

calculated. Background level
TS IUnOD/〜.altZJ6u!IunOU

was 443 counts/S, which was

103 102 101

mainly due to the adso叩tlOn

ortritiated water vapor on thesurfTace of the Au/Be disk.
Figure 2 shows the totalpressure dependence of●Xィay intenslty fわr theAト1%T2 mixture. The solidlines in the figure are an eye

100

101

102

103

104

105

Total pressure, P / Paguide. Two kinds of linear
relation were observed

Fig. 2 Pressure dependence or X‑ray intenslty fわr theAr‑1 %T2 mixture.
below/above about 2 kPa. In79

松山政夫･洲付 o ♂ ootlJ. ･鈴木卓美

一一 〇:H2‑1%T2mixture 剪ll劍ﾂﾂﾂ●●●●ー●:He‑1%T2mi 凅ture+:D2‑1%T2mixture Fl 剪LIS )unO3JN+OleJ6U!tunou

TS lun8＼N‑01ELJ6U!一UnOU
1 03 佃 1 0､ 仰0100

ﾈﾂ101?ｩ?｣

102｣

103

101

105

100

101

Total pressure, P / Pa

Fig･ 3 Pressure dependence or X‑ray lntenSlty

for the H21 and D2‑1%T2 mixtures.

102

103

104

105

Total pressure, P / Pa

Fig･ 4 Pressure dependence of X‑ray Intensity

for the Hell %T2mixture.

the lower pressure reglOn, ramp Orthe line was unlty, While it was O･73 in the higher pressure

reglOn･ Namely, such a different pressure dependence was also observed for the mixtures ofH2‑ and D2‑1%T2 and He‑1%T2 aS Shown in Figs･ 3 and 4, respectively･
However, there was a large difference in the higher pressure region between the Ar11%T2
mixture and other mixtures･ Above a pressure or about 2 kPa, a linear relation appeared in theAr‑1%T2 mixture, although the ramp of the line was different from that in the lower pressure
reglOm On the other hand, the convex changes were obseⅣed fわr pure tritium gas and themixtures ofH2‑ and D2‑1%T2 and He‑1%T2 aS Shown in Figs･ 3 and 4･ Namely, this indicatesthat the linear relation obseⅣed in the higher pressure reglOn OfAr‑1%T2 Can not attribute to

only the self‑absorption ofP‑rays in the gas phase. If the linear relation at the higher pressure

region is mainly due to the self‑absorption of P‑rays like othermixtures, a convex deviation
should be observed since the absorption coefficient of P‑rays for argon is seven times greaterthan that for hydrogen and helium as shown in Table I. Therefore, we have to considercontribution of other interactions inthe Ar‑1%T2 mixture.
It is well known that the characteristic X‑rays are emitted by the interactions between

P‑rays and substances･ In the case of argon atoms as a substance, three characteristic X‑rays(Ka1‑21958, Ka2‑2･956 and Kpl,3‑3･190 keV) can be produced by colliding with P‑raysemitted from tritium atoms, because the maximum energy of the β‑rays is 18.6 keV. For

example, the total number of characteristic X‑ray photons produced in the measunng cell is7･68xl06 sl1, when the total pressure of the Ar‑1%T2 mixture is 0.1 MPa. 0ffcourse, thisdepends on the total pressure of the mixture. The conversion coefficient of P‑rays to theX‑rays was assumed to be 3.2xlO4 photons/β‑particle l8]. About lO% of the produced

characteristic X‑rays hit on the surface of the Au/Be disk and penetrate through it. Intensltyof
the characteristic X‑rays is reduced to 15% aRer penetrating through it･ Finally, lxlO5 0f

photons shall enter the detector, which are large enoughin comparison with the80

bremsstrahlung X‑rays. In this calculation, effects of the scattering and absorption Of X‑raysin the measunng cell are not taken into account of the calculation.
It is clear, however, that the intenslty Of characteristic Xィays increases with increaslng apartial pressure of argon. If generation of the characteristic X‑rays could be negligibly small,the obseⅣed X‑ray Intensity should be largely deviated downward. That is, the deviation

based on self‑absorption of P‑rays as shown in Figs. 3 and 4 will be similarly observed. It issuggested, therefore, that the effects of generation of the characteristic X‑rays appeared in themixture of Ar‑1%T2, Whereas the self‑absorption in cases of the mixtures of hydrogenisotopes and helium was mainly caused by excitation and/or iomization of gases without

emission of X‑rays･ Accordingly, it is necessary to correct previously the effects of X‑rayemission by the impunty molecules irthe total pressure in the measunng cell is above about 1kPa. In addition to this,further detailed analyses of the pressure dependence will be required
to clarify the deviation behavior in a pressure reg10n around 2 kPa.

References【1] W.P. Jesse and ∫. Sadauskis, Phys. Rev., 88 (1952) 417.

[2] M･ Matsuyama, H. Nakatani and K. Watanabe, Fusion Eng. Design, 10 (1989) 405.
[3] W･M･ Shu, M. Matsuyama, T. Suzuki and M.F. Nishi, Fusion Eng. Design, 81 (2006) 803.
[4】 M･ Matsuyama, W･M･ Shu, T･ Suzuki and M･F･ Nishi, Am･ Rept. Hydrogen Isotope

Research Center, Toyama University, 23 (2003) 1.

[5] P･S･ Papagiannakopoulos and C･E･ Easterly, Oak Ridgs National Laboratory Report,

ORNL/TM‑6699, (1 979).

[6] LM. Dorfman, Phys. Rev., 95 (1954) 393.
【7】 F･H･ Attix and W.C. Roesch, "Radiation Dosimetry", Academic Press, New York, (1968),

p.320.

[8] MI Matsuyama, T. Murai and K. Watanabe, Fusion Science and Teclm01., 41 (2002) 505.

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