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K. Ashida, K. Ichimura, M. Matsuyama, K. Watanabe
Tritium Research Center, Toyama University, Gofuku 3190, Toyama 930, Japan
Abstract
Thermal release of hydrogen, deuterium and tritium implanted into a pyrolytic graphite was studied by means of mass analyzed thermal desorption spectroscopy along with surface characterization by X-ray photoelectron spectroscopy. Hydrogen (or its isotopes) ions ware implanted into the sample at room temperature with an applied voltage of 5 kV using a conventional ion gun. Subsequently, the sample was heated to 900℃ with various temperature ramps to measure the thermal desorption spectra. The implanted hydrogen (or its isotopes) was predominantly desorbed as H2 and in small amount as CH4. The desorption spectra of H2 changed gradually while repeating the implantation-desorption cycles and became reproducible after the total dose amounting to 1×1019 ion/cm2, Indicating that the virgin graphite is modified due to formation/accumulation of radiation damage. For the modified graphite, three desorption peaks were observed. The first peak is attributed to the desorption of hydrogen atoms trapped on the carbon atoms in the normal graphite lattice. The others correspond to differently trapped hydrogen atoms in the graphite. The desorption of the first peak obeyed the second kinetics with respect to the amount of the implantation, indicating that the rate determining step is the surface association reaction of the hydrogen atoms. The activation energy was estimated as 44 kcal/mol for three hydrogen isotopes. However, the isotope effect appeared on the frequency factor: their ratio was estimated as H2:D2:T2=3:1.5:1.
The desorption of methane obeyed the pseudo-first order kinetics with an activation energy of 38 kcal/mol.Keywords: graphite; thermal desorption; tritium
Osamu Takayasu, Yoshiki Nakano, Toyosaburo Takeuchi
Faculty of Science, Toyama University, Toyama 930, Japan
Abstract
The distribution of tritium on the surface layer of neutron-irradiated LiF was studied by means of autoradiography. The autoradiographs were compared with microphotographs of the surfaces which were chemically etched after the autoradiographs had been taken. Temperature-programmed desorption of tritium and u.v. absorption spectra were studied on the same sample. Many sharp lines appeared in the autoradiograph. Some of these lines are observed in the microphotograph, but some of them appeared only when the surface was etched. It was concluded from these results that tritium accumulates preferentially on step-edges and dislocations of the surface.
Kenji Ichimura, Naoya Inoue, Kuniaki Watanabe, Toyosaburo Takeuchi
Tritium Research Center, Toyama University, 3190 Gofuku, Toyama 930, Japan
Abstract
Nonevaporable getters have wide applicability for developing the tritium
handing techniques for thermonuclear fusion devices.
From this viewpoint, mechanisms of the absorption and desorption of hydrogen isotopes and the isotope
effects were investigated for a Zr-V-Fe alloy (St-707) by means of the mass analyzed thermal desorption
spectroscopy.
It was observed that the absorption rate was proportional to the first power of the pressure,
indicating that the rate limiting step is the dissociative adsorption of hydrogen isotopes on the surface.
The activation was very small, in the order of magnitude of a few tens of calories per mole in a temperature
rang from -196 to 200℃.
The desorption rate was proportional to the amount of absorption, indicating that the rate limiting step
is the associative desorption reaction of hydrogen atoms or ions diffused to the surface from the bulk.
The rate constants for hydrogen and deuterium were determined as
kd(H2) = (5.3+2.6 -1.7)exp[-(28.0±0.7)×103/RT] and
kd(D2) = (5.0+2.7-1.7)exp[-(28.6±0.8)×103/RT]
in [1/Pa 1 s], respectively, where R is in [cal/mol deg].
With regard to tritium, the rate constant was evaluated as
kd(T2) = (5.0+20-4.0)exp[-(29.3±3)×103/RT],
however, the frequency factor will have to be corrected by knowing the relative sensitivity factor
of the mass spectrometer for tritium (T2).
PACS numbers: 68.45. Dq. 82.65. My
K. Ichimura, N. Inoue, K. Watanabe, T. Takeuchi
Tritium Research Center, Toyama University, 3190, Gofuku, Toyama 930, Japan
Abstract
Nonevaporable getters have wide applicability for tritium handing systems.
From this view point, the activation process of the Zr-V-Fe getter (St-707) and absorption/desorption
of D2O on the getter surface were investigated, by means of XPS-SIMS and mass analyzed
thermal desorption spectroscopy.
XPS-SIMS measurements revealed that the getter surface exposed to air was covered
with adsorbed H2O, CO and small amounts of hydrocarbons and that the getter components
are oxidized.
Upon heating of the getter above 500℃, the adsorbed species disappeared from the surface,
partly due to desorption and partly due to migration into the bulk.
Consequently, metallic Zr and V appeared on the surface, whereas Fe disappeared.
The surface composition was evaluated to be 87 at% Zr-13 at% V.
After the activation, water (D2O) was readily absorbed into the getter
at 300℃ in the form of deuterium atoms.
The absorption rate was proportional to the partial pressure of water, indicating that the rate
determining step for the absorption is the dissociation of water molecules on the surface.
The absorption rate constant was 0.009 and 0.24 cc/s/cm2 (net surface area)
[or 1.5 and 39 cc/s/cm2 (projected area)] at 25 and 300℃, respectively.
Only D2 was desorbed from the getter exposed to D2O at 25 and 300℃.
The rate determining step for the desorption is association of deuterium atoms on the surface diffused
from the bulk.
Toshikatsu Mikia, Motoji Ikeyaa, Masao Matsuyamab, Kuniaki Watanabeb
a Technical College, Yamaguchi University, Tokiwadai, Ube 755
b Tritium Research Center, Toyama University, Gofuku, Toyama 930
Abstract
Response of an electret dosimeter for tritium beta-rays has been investigated by examining the charge decrease of polymer electrets by the exposure to tritium-involved air. The electret charge decreases proportionally to the tritium activity and the exposure time with a sensitivity of about 10-18 C/pCi・s, roughly corresponding to the theoretical response. Electret dosimeters can be used for the detection of tritium beta-rays of 10 pCi・h/cm3.
Kenji Ichimura, Masao Matsuyama, Kuniaki Watanabe
Tritium Research Center, Toyama University
Abstract
As a first step toward solving the problem of contamination by tritium
affecting secondary electron multipliers, the material used in a marketed multiplier was studied
in respect of adsorption and desorption behavior, with application of thermal desorption spectroscopy.
It was found that the tritium adsorbed on the material was desorbed in large part
in the form of tritiated water, generated by chemical reaction that combined adsorbed tritium with
excess oxygen present on the material and in small part in the form of tritiated hydrocarbons produced
by exchange reactions taking place between the tritium and hydrocarbons also found adsorbed on the material surface.
It was further indicated that tritium desorption took place from two distinct kinds of adsorption/desorption
sites.
Effective removal from the material was obtained of the adsorbed tritium by heating in inert gas flow
at 500℃ for 6h.
If a longer heating time can be allowed, a lower heating temperature should be suffice for decontamination.
Kenji Ichimuraa, Kuniaki Watanabea, Kazunari Nishizawab, Junji Fujitac
a Tritium Research Center, Toyama University, 3190 Gofuku, Toyama 930, Japan
b Faculty of Engineering, Osaka University, Osaka 565, Japan
c Institute of Plasma Physics, Nagoya University, Nagoya 464, Japan
Abstract
A ceramic secondary electron multiplier (SEM), Ceratron, was used to study
impairment of the SEM performance due to adsorbed tritium, its decontamination, and the applicability
of the SEM to measure tritium pressure.
The background level of the SEM increased significantly, up to its counting limit, due to tritium adsorption.
Heating it to 300℃ in vacuo and/or in the presence of reactive gases such as D2 and CO at
1×10-4 Pa was not effective to decontaminate the SEM, whereas photon irradiation was extremely
powerful for the decontamination.
The tritium (HT) pressure in a range of 1×10-6-1×10-3 Pa could
be measured with no significant impairment of the SEM performance with the aid of photon irradiation.
It is revealed that a particle flux as low as 1 particle/s will be able to measure in the presence of tritium
if suitable photon sources are installed in the systems.
I. Kanesaka, M. Tsuchida, K. Kawai, T. Takeuchi
Faculty of Science, Toyama University, Gofuku, Toyama 930, Japan