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Experiments on deuterium trapping in helium-irradiated copper
Fusion Engineering and Design 81 (2006) 785–789
Experiments on deuterium trapping in helium-irradiated copper I. Takagi ∗ , M. Akiyoshi, N. Matsubara, K. Moritani, H. Moriyama Department of Nuclear Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Received 29 January 2005; received in revised form 3 July 2005; accepted 11 July 2005 Available online 27 December 2005
Abstract Deuterium trapping in helium-irradiated copper was experimentally studied by use of nuclear reaction analysis (NRA). During the irradiation and the NRA, the sample was continuously charged with deuterium by exposure to deuterium plasma and the permeation flux was monitored. Before the irradiation, the deuterium concentration near the plasma-exposure surface was 2.2 × 1022 m−3 at 398 K while it increased by 10,000 times after the irradiation with a dose of 1.5 × 1021 m−2 . From the shape of the depth profile of trapped deuterium and its temperature dependence, it was found that two types of the traps were produced. One was considered to be vacancies and the other probably interstitial loops. These traps were not annihilated even at 575 K. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen trapping; Copper; Tritium inventory; Ion irradiation; Radiation damage; Ion beam analysis
1. Introduction Copper and copper alloys are candidates for high thermal conductivity layers such as coolant pipes in plasma-facing components of fusion reactors [1]. As the plasma-facing components will be heavily irradiated by energetic particles, one needs to know effects of radiation damages on physical and mechanical properties of these materials [2]. Influence on tritium inventory is also an important issue. As copper is an endothermic metal with positive heat of solution for hydrogen, the tritium inventory is expected to be very ∗ Corresponding author. Tel.: +81 75 753 5838; fax: +81 75 753 5845. E-mail address: [email protected] (I. Takagi).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.07.021
low compared with exothermic metals such as vanadium alloys. When irradiated, however, deep potential sites, called traps here, would be produced and they trap hydrogen isotopes to increase the inventory [3,4]. In the present work, depth profiles of deuterium in irradiated copper are observed by nuclear reaction analysis (NRA). Helium is used for irradiation since it introduces a large amount of defects, of which variety has been well investigated. A feature is that a sample membrane is continuously charged with deuterium by exposure of one side of the sample to deuterium plasma. The steady state of deuterium permeation through the sample can be achieved and concentrations of dissolved deuterium and trapped deuterium are kept constant during the NRA. This allows one to observe clear depth profiles and inform about characteristics of the traps.
I. Takagi et al. / Fusion Engineering and Design 81 (2006) 785–789
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In the above experiments, the sample was continuously exposed to the plasma. During the NRA the sample temperature was not changed and the permeation was in the steady state. The latter means that the concentration of deuterium in solution sites did not change with time. The analyzing dose for each run was 2 × 1019 m−2 to minimize irradiation effects.
3. Results and discussion
Fig. 1. Schematic showing of the experimental procedure.
2. Experimental A sample was a copper membrane with thickness of 0.1 mm (Johnson Matthey Plc.). Its purity was 99.999% and impurities were B (1 ppm), Ca (1 ppm), Fe (<1 ppm), Mg (<1 ppm) and Ag (10 ppm). Prior to the experiment, the sample was annealed in a vacuum of 10 Pa at 973 K for 3600 s. The experimental set-up has been described elsewhere [5] and will be explained briefly here. The experimental procedure is shown in Fig. 1. The sample was placed between two vacuum chambers and heated up to 398 K by a lamp. One side of the sample was exposed to deuterium rf-plasma produced in a discharge tube. Conditions of the discharge were a deuterium pressure of 1 Pa in the tube and the rf power of 20 W. No bias was supplied to the plasma or to the sample so atomic deuterium with energy of a few eV impinged to the sample. A quadruple mass analyzer monitored a deuterium permeation flux through the sample. When permeation reached steady-state conditions, an analyzing beam of 1.7-MeV 3 He for the NRA from a 4 MV van de Graaff accelerator of Kyoto University was incident on the plasma-exposure side at 45◦ to the surface normal and protons produced by the reaction D(3 He,p)4 He were detected. The energy spectrum of protons was converted into a deuterium depth profile near the surface [6]. The probe depth was around 1.5 m. Next 0.8-MeV 3 He ions from the same accelerator irradiated the same side. After the irradiation, the NRA was again conducted. This procedure was repeated until the irradiation dose reached to 1.5 × 1021 m−2 .
Before the irradiation, the deuterium depth profile consisted of only a peak at 0-depth as shown by triangles in Fig. 2. It is attributed to absorbed deuterium on the surface [7]. Due to a finite resolution of the detecting system, the profile expanded to positive and negative depths. The deuterium concentration in the bulk was too low to be detected. As the deuterium permeation was limited by the diffusion process [7], the permeation flux J was expressed as J = DC/L, where D is the diffusion coefficient, C the concentration of dissolved deuterium near the plasma-exposure surface and L is the sample thickness. At 398 K, J was 1.5 × 1015 m−2 s−1 from the experiment and D = 6.8 × 10−12 m2 s−1 from Ref. [8], so C was estimated as 2.2 × 1022 m−3 . It was far below the ordinate scale of 1026 m−3 in the figure. After the irradiation of 3 He with the dose of 1.5 × 1021 m−2 , the deuterium concentration increased remarkably to show another peak in the bulk as shown in Fig. 2. The concentration at the peak was about 104
Fig. 2. Depth profiles of deuterium in copper, continuously exposed to deuterium rf-plasma at 398 K, before and after irradiation of 0.8MeV 3 He ions.
I. Takagi et al. / Fusion Engineering and Design 81 (2006) 785–789
Fig. 3. A depth profile of trapped deuterium in copper at 398 K. Distributions of the displaced host atoms and the 3 He ions, estimated by the TRIM code, are also shown.
times as large as C. This indicated that many traps were produced by the irradiation. The depth profile of trapped deuterium, which was obtained by subtraction of the profile before the irradiation from that after the irradiation, is shown in Fig. 3. The concentration of trapped deuterium is nearly constant below 0.5-m depth but it increases to show a peak at 1.0-m depth. Distributions of 3 He ions and atomic displacements, estimated by the TRIM code [9], are also shown. In the estimation, the displacement energy is assumed to be 20 eV and ‘vacancies’ are regarded as the displacements. The distribution of the displacements is similar to the depth profile while that of helium ions is not, especially below 0.5-m depth. Helium could not migrate from its projected position under our experimental conditions [10]. The traps are not related to helium but to the displacements. The atomic displacements cause several kinds of defects such as vacancies, interstitials, clusters and dislocations. Among them, monovacancies and interstitial atoms are recovered at lower temperatures [11]. Vacancy clusters and interstitial loops in heliumimplanted copper have been observed by a transmission electron microscope [12]. These defects would exist in the sample for the present work. The concentration of trapped deuterium below 0.5-m depth is relatively higher than the number of the displacement as shown in Fig. 3. This is attributed to two types of the traps, which will be discussed later. Here, other possible explanations will be considered.
787
Effects of analyzing beam was very small; the total beam dose was 2.6 × 1020 m−2 . It corresponded to only 0.045 dpa at 0.5-m depth whereas the displacement by the irradiation was 0.83 dpa at the same depth. Migration of the traps was implausible; just after the irradiation and after 4 × 104 s, the depth profiles were observed but no changes in the shape were seen. The FWHM (full-width at half maximum) of the peak at 1.0-m depth in the depth profile is 0.48 m. It agrees well with the FWHM for the displacement distribution of 0.47 m when the energy resolution is taken into account. If the traps had migrated, the former should become much larger. These results indicate that the traps are immobile at 398 K. After the irradiation at 398 K, the sample temperature was increased to 575 K, subsequently decreased to 356 K and returned to 398 K. No changes in the depth profiles before and after heating were observed at 398 K, that is, the traps were not annihilated. When the sample temperature was increased, significant change in the shape of the depth profile was observed as shown in Fig. 4. The permeation flux was constant and the concentration of dissolved deuterium, C, was in the steady state at each temperature. Decrease in the height of the peak at the surface was independent of the traps [7]. The peak at 1.0-m depth disappeared and the profile became almost flat. The result indicates two types of the traps; one trap, called a shallow trap here, has a low binding energy and uniformly distributes in the radiation-damaged region. The other trap, called a deep trap here, has a high binding energy and its distribution is similar to that of the displacement.
Fig. 4. Depth profiles of deuterium in copper at different temperatures.
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Fig. 6. Evolution of the averaged concentrations of trapped deuterium, Ct1 and Ct2 , with the irradiation dose. Fig. 5. Temperature dependence of the averaged concentration of trapped deuterium, Ct . Ct1 and Ct2 are values from 0.2 to 0.5 m depth and 0.5 to 1.5 m depth, respectively.
The bulk region is divided into two parts: 0.2–0.5 m and 0.5–1.5 m, as shown in Fig. 4 and the averaged concentration of trapped deuterium, Ct1 and Ct2 , respectively, are plotted in Fig. 5. The numeral symbols indicate the temperature procedure. The two types of the traps would explain the temperature dependence of Ct1 and Ct2 as follows. At lower temperatures below 400 K, deuterium in the shallow traps and in the deep traps mainly contribute to Ct1 and Ct2 , respectively. The deep traps are fully occupied by deuterium due to a high binding energy. When the temperature is slightly increased, deuterium atoms in the deep traps are still remained and those in the shallow traps partly escape due to a low binding energy. So Ct1 and Ct2 decrease by a same amount. At higher temperatures above 400 K, a large amount of deuterium in the deep traps begins to escape and Ct2 decreases rapidly. Finally the deep traps become empty and only shallow traps are partly occupied by deuterium above 450 K. As the result Ct1 and Ct2 show a similar temperature dependence. Dissociation of vacancy–hydrogen complexes occurs around 450 K, which has been observed by positron annihilation measurement [13]. Besenbacher et al. [4] has analyzed behavior of deuterium retention during linear-ramp annealing and determined two types of traps with the binding enthalpies of 0.22 and 0.42 eV. From theoretical consideration, they have assigned the
former to self-interstitials tentatively and the latter to vacancies. From the above discussions, the deep traps are considered to be vacancy clusters and the shallow traps would be interstitial loops. Evolution of the averaged concentrations, Ct1 and Ct2 , at 398 K with the irradiation dose is shown in Fig. 6. The value of Ct1 increases with the dose until it tends to saturation around 1 × 1021 m−2 dose. This indicates saturation of the number of shallow traps since deuterium atoms in the shallow traps would mainly contribute to Ct1 . Saturation of the areal density of the interstitial loops has been observed in heliumimplanted copper by Yasuda et al. [12]. The value of Ct2 increases monotonously with the dose. Considering that a significant amount of deuterium in the shallow traps contribute to Ct2 , the number of the deep traps increases almost linearly with the dose. The maximum concentration of trapped deuterium at 1.5 × 1021 m−2 dose is 3 × 1026 m−3 or 0.36 at.%. There would be left a much area for the deep traps to be produced. In a fusion reactor, the tritium inventory below 400 K may be large since heavy irradiation by neutron produces a large amount of the deep traps. Operating temperatures, however, will be much higher in fact and the shallow traps are dominant to the inventory. Assuming that the experimental conditions and results such the number of the shallow traps and the concentration of dissolved deuterium are directly applied to copper components, the inventory at 575 K is 8 × 1024 m−3 from Fig. 5. It is 80 times as large as the concentration of dissolved tritium, estimated from the permeation flux.
I. Takagi et al. / Fusion Engineering and Design 81 (2006) 785–789
4. Summary The depth profiles of deuterium in helium-irradiated copper were successfully observed using the NRA technique, due to continuous charging the sample with deuterium atoms. The results showed that the two types of the traps, associated with radiation defects, were produced. The deep traps and the shallow traps are considered to be vacancy clusters and probably interstitial loops, respectively. The deep traps would no affect the tritium inventory at elevated temperatures although they are largely produced by irradiation. The shallow traps would be dominant to the inventory, which is not so significant due to restriction of the number of the shallow traps. Further works, however, will be needed to determine the binding energies, to identify the shallow traps and to investigate a way to reduce the tritium inventory since the amount of trapped tritium is expected to be still much higher than that of dissolved tritium even at elevated temperatures.
[2]
Acknowledgement
[9]
This work is supported by a grant-in-aid for scientific research of Japan Society for the Promotion of Science.
[3]
[4]
[5]
[6]
[7]
[8]
[10]
[11] [12]
References [13] [1] K. Ioki, M. Akiba, P. Barabaschi, V. Barabash, S. Chiocchio, W. Daenner, et al., ITER nuclear components, preparing for the
789
construction and R&D results, J. Nucl. Mater. 329–333 (2004) 31–38. J.W. Davis, G.M. Kalinin, Materials properties and design requirements for copper alloys used in ITER, J. Nucl. Mater. 258–263 (1998) 323–328. K.L. Wilson, R.A. Causey, M.I. Baskes, J. Kamperschroer, Hydrogen isotope retention and release from copper, J. Vac. Sci. Technol. A 5 (1987) 2319–2324. F. Besenbacher, B.B. Nielsen, S.M. Myers, Defect trapping of ion-implanted deuterium in copper, J. Appl. Phys. 56 (1984) 3384–3393. I. Takagi, K. Yoshida, K. Shin, K. Higashi, A combined technique of nuclear reaction analysis and plasma-driven permeation for a quantitative study on deuterium trapping, Nucl. Instr. Meth. B 84 (1994) 393–399. D. Dieumegard, D. Dubreuil, G. Amsel, Analysis and depth profiling of deuterium with the D(3 He,p)4 He reaction by detecting the protons at backward angles, Nucl. Instr. Meth. 166 (1979) 431–445. I. Takagi, H. Hashimoto, H. Fujita, K. Higashi, An experimental study on the potential energy diagram for hydrogen isotopes on copper surfaces, Fusion Eng. Des. 41 (1998) 73–78. W. Eichenauer, W. L¨oser, H. Witte, L¨oslichkeit und Diffusionsgeschwindigkeit von Wasserstoff und Deuterium in Einkristallen aus Nickel und Kupfer, Z. Metallkde. 56 (1965) 287–293. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. T. Yamauchi, S. Yamanaka, M. Miyake, Thermal release behavior of helium implanted into copper at high fluences, J. Nucl. Mater. 174 (1990) 53–59. W. Sch¨ule, On the recovery stages in fcc materials, J. Nucl. Mater. 233–237 (1996) 969–973. K. Yasuda, C. Kinoshita, M. Kutsuwada, T. Hirai, Nucleation and growth process of defect clusters in copper during helium ion irradiation, J. Nucl. Mater. 233–237 (1996) 1051–1056. B. Lengeler, S. Mantl, W. Triftshaeuser, Interaction of hydrogen and vacancies in copper investigated by positron annihilation, J. Phys. F 8 (1978) 1691–1698.
Experiments on deuterium trapping in helium-irradiated copper I. Takagi ∗ , M. Akiyoshi, N. Matsubara, K. Moritani, H. Moriyama Department of Nuclear Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Received 29 January 2005; received in revised form 3 July 2005; accepted 11 July 2005 Available online 27 December 2005
Abstract Deuterium trapping in helium-irradiated copper was experimentally studied by use of nuclear reaction analysis (NRA). During the irradiation and the NRA, the sample was continuously charged with deuterium by exposure to deuterium plasma and the permeation flux was monitored. Before the irradiation, the deuterium concentration near the plasma-exposure surface was 2.2 × 1022 m−3 at 398 K while it increased by 10,000 times after the irradiation with a dose of 1.5 × 1021 m−2 . From the shape of the depth profile of trapped deuterium and its temperature dependence, it was found that two types of the traps were produced. One was considered to be vacancies and the other probably interstitial loops. These traps were not annihilated even at 575 K. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen trapping; Copper; Tritium inventory; Ion irradiation; Radiation damage; Ion beam analysis
1. Introduction Copper and copper alloys are candidates for high thermal conductivity layers such as coolant pipes in plasma-facing components of fusion reactors [1]. As the plasma-facing components will be heavily irradiated by energetic particles, one needs to know effects of radiation damages on physical and mechanical properties of these materials [2]. Influence on tritium inventory is also an important issue. As copper is an endothermic metal with positive heat of solution for hydrogen, the tritium inventory is expected to be very ∗ Corresponding author. Tel.: +81 75 753 5838; fax: +81 75 753 5845. E-mail address: [email protected] (I. Takagi).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.07.021
low compared with exothermic metals such as vanadium alloys. When irradiated, however, deep potential sites, called traps here, would be produced and they trap hydrogen isotopes to increase the inventory [3,4]. In the present work, depth profiles of deuterium in irradiated copper are observed by nuclear reaction analysis (NRA). Helium is used for irradiation since it introduces a large amount of defects, of which variety has been well investigated. A feature is that a sample membrane is continuously charged with deuterium by exposure of one side of the sample to deuterium plasma. The steady state of deuterium permeation through the sample can be achieved and concentrations of dissolved deuterium and trapped deuterium are kept constant during the NRA. This allows one to observe clear depth profiles and inform about characteristics of the traps.
I. Takagi et al. / Fusion Engineering and Design 81 (2006) 785–789
786
In the above experiments, the sample was continuously exposed to the plasma. During the NRA the sample temperature was not changed and the permeation was in the steady state. The latter means that the concentration of deuterium in solution sites did not change with time. The analyzing dose for each run was 2 × 1019 m−2 to minimize irradiation effects.
3. Results and discussion
Fig. 1. Schematic showing of the experimental procedure.
2. Experimental A sample was a copper membrane with thickness of 0.1 mm (Johnson Matthey Plc.). Its purity was 99.999% and impurities were B (1 ppm), Ca (1 ppm), Fe (<1 ppm), Mg (<1 ppm) and Ag (10 ppm). Prior to the experiment, the sample was annealed in a vacuum of 10 Pa at 973 K for 3600 s. The experimental set-up has been described elsewhere [5] and will be explained briefly here. The experimental procedure is shown in Fig. 1. The sample was placed between two vacuum chambers and heated up to 398 K by a lamp. One side of the sample was exposed to deuterium rf-plasma produced in a discharge tube. Conditions of the discharge were a deuterium pressure of 1 Pa in the tube and the rf power of 20 W. No bias was supplied to the plasma or to the sample so atomic deuterium with energy of a few eV impinged to the sample. A quadruple mass analyzer monitored a deuterium permeation flux through the sample. When permeation reached steady-state conditions, an analyzing beam of 1.7-MeV 3 He for the NRA from a 4 MV van de Graaff accelerator of Kyoto University was incident on the plasma-exposure side at 45◦ to the surface normal and protons produced by the reaction D(3 He,p)4 He were detected. The energy spectrum of protons was converted into a deuterium depth profile near the surface [6]. The probe depth was around 1.5 m. Next 0.8-MeV 3 He ions from the same accelerator irradiated the same side. After the irradiation, the NRA was again conducted. This procedure was repeated until the irradiation dose reached to 1.5 × 1021 m−2 .
Before the irradiation, the deuterium depth profile consisted of only a peak at 0-depth as shown by triangles in Fig. 2. It is attributed to absorbed deuterium on the surface [7]. Due to a finite resolution of the detecting system, the profile expanded to positive and negative depths. The deuterium concentration in the bulk was too low to be detected. As the deuterium permeation was limited by the diffusion process [7], the permeation flux J was expressed as J = DC/L, where D is the diffusion coefficient, C the concentration of dissolved deuterium near the plasma-exposure surface and L is the sample thickness. At 398 K, J was 1.5 × 1015 m−2 s−1 from the experiment and D = 6.8 × 10−12 m2 s−1 from Ref. [8], so C was estimated as 2.2 × 1022 m−3 . It was far below the ordinate scale of 1026 m−3 in the figure. After the irradiation of 3 He with the dose of 1.5 × 1021 m−2 , the deuterium concentration increased remarkably to show another peak in the bulk as shown in Fig. 2. The concentration at the peak was about 104
Fig. 2. Depth profiles of deuterium in copper, continuously exposed to deuterium rf-plasma at 398 K, before and after irradiation of 0.8MeV 3 He ions.
I. Takagi et al. / Fusion Engineering and Design 81 (2006) 785–789
Fig. 3. A depth profile of trapped deuterium in copper at 398 K. Distributions of the displaced host atoms and the 3 He ions, estimated by the TRIM code, are also shown.
times as large as C. This indicated that many traps were produced by the irradiation. The depth profile of trapped deuterium, which was obtained by subtraction of the profile before the irradiation from that after the irradiation, is shown in Fig. 3. The concentration of trapped deuterium is nearly constant below 0.5-m depth but it increases to show a peak at 1.0-m depth. Distributions of 3 He ions and atomic displacements, estimated by the TRIM code [9], are also shown. In the estimation, the displacement energy is assumed to be 20 eV and ‘vacancies’ are regarded as the displacements. The distribution of the displacements is similar to the depth profile while that of helium ions is not, especially below 0.5-m depth. Helium could not migrate from its projected position under our experimental conditions [10]. The traps are not related to helium but to the displacements. The atomic displacements cause several kinds of defects such as vacancies, interstitials, clusters and dislocations. Among them, monovacancies and interstitial atoms are recovered at lower temperatures [11]. Vacancy clusters and interstitial loops in heliumimplanted copper have been observed by a transmission electron microscope [12]. These defects would exist in the sample for the present work. The concentration of trapped deuterium below 0.5-m depth is relatively higher than the number of the displacement as shown in Fig. 3. This is attributed to two types of the traps, which will be discussed later. Here, other possible explanations will be considered.
787
Effects of analyzing beam was very small; the total beam dose was 2.6 × 1020 m−2 . It corresponded to only 0.045 dpa at 0.5-m depth whereas the displacement by the irradiation was 0.83 dpa at the same depth. Migration of the traps was implausible; just after the irradiation and after 4 × 104 s, the depth profiles were observed but no changes in the shape were seen. The FWHM (full-width at half maximum) of the peak at 1.0-m depth in the depth profile is 0.48 m. It agrees well with the FWHM for the displacement distribution of 0.47 m when the energy resolution is taken into account. If the traps had migrated, the former should become much larger. These results indicate that the traps are immobile at 398 K. After the irradiation at 398 K, the sample temperature was increased to 575 K, subsequently decreased to 356 K and returned to 398 K. No changes in the depth profiles before and after heating were observed at 398 K, that is, the traps were not annihilated. When the sample temperature was increased, significant change in the shape of the depth profile was observed as shown in Fig. 4. The permeation flux was constant and the concentration of dissolved deuterium, C, was in the steady state at each temperature. Decrease in the height of the peak at the surface was independent of the traps [7]. The peak at 1.0-m depth disappeared and the profile became almost flat. The result indicates two types of the traps; one trap, called a shallow trap here, has a low binding energy and uniformly distributes in the radiation-damaged region. The other trap, called a deep trap here, has a high binding energy and its distribution is similar to that of the displacement.
Fig. 4. Depth profiles of deuterium in copper at different temperatures.
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Fig. 6. Evolution of the averaged concentrations of trapped deuterium, Ct1 and Ct2 , with the irradiation dose. Fig. 5. Temperature dependence of the averaged concentration of trapped deuterium, Ct . Ct1 and Ct2 are values from 0.2 to 0.5 m depth and 0.5 to 1.5 m depth, respectively.
The bulk region is divided into two parts: 0.2–0.5 m and 0.5–1.5 m, as shown in Fig. 4 and the averaged concentration of trapped deuterium, Ct1 and Ct2 , respectively, are plotted in Fig. 5. The numeral symbols indicate the temperature procedure. The two types of the traps would explain the temperature dependence of Ct1 and Ct2 as follows. At lower temperatures below 400 K, deuterium in the shallow traps and in the deep traps mainly contribute to Ct1 and Ct2 , respectively. The deep traps are fully occupied by deuterium due to a high binding energy. When the temperature is slightly increased, deuterium atoms in the deep traps are still remained and those in the shallow traps partly escape due to a low binding energy. So Ct1 and Ct2 decrease by a same amount. At higher temperatures above 400 K, a large amount of deuterium in the deep traps begins to escape and Ct2 decreases rapidly. Finally the deep traps become empty and only shallow traps are partly occupied by deuterium above 450 K. As the result Ct1 and Ct2 show a similar temperature dependence. Dissociation of vacancy–hydrogen complexes occurs around 450 K, which has been observed by positron annihilation measurement [13]. Besenbacher et al. [4] has analyzed behavior of deuterium retention during linear-ramp annealing and determined two types of traps with the binding enthalpies of 0.22 and 0.42 eV. From theoretical consideration, they have assigned the
former to self-interstitials tentatively and the latter to vacancies. From the above discussions, the deep traps are considered to be vacancy clusters and the shallow traps would be interstitial loops. Evolution of the averaged concentrations, Ct1 and Ct2 , at 398 K with the irradiation dose is shown in Fig. 6. The value of Ct1 increases with the dose until it tends to saturation around 1 × 1021 m−2 dose. This indicates saturation of the number of shallow traps since deuterium atoms in the shallow traps would mainly contribute to Ct1 . Saturation of the areal density of the interstitial loops has been observed in heliumimplanted copper by Yasuda et al. [12]. The value of Ct2 increases monotonously with the dose. Considering that a significant amount of deuterium in the shallow traps contribute to Ct2 , the number of the deep traps increases almost linearly with the dose. The maximum concentration of trapped deuterium at 1.5 × 1021 m−2 dose is 3 × 1026 m−3 or 0.36 at.%. There would be left a much area for the deep traps to be produced. In a fusion reactor, the tritium inventory below 400 K may be large since heavy irradiation by neutron produces a large amount of the deep traps. Operating temperatures, however, will be much higher in fact and the shallow traps are dominant to the inventory. Assuming that the experimental conditions and results such the number of the shallow traps and the concentration of dissolved deuterium are directly applied to copper components, the inventory at 575 K is 8 × 1024 m−3 from Fig. 5. It is 80 times as large as the concentration of dissolved tritium, estimated from the permeation flux.
I. Takagi et al. / Fusion Engineering and Design 81 (2006) 785–789
4. Summary The depth profiles of deuterium in helium-irradiated copper were successfully observed using the NRA technique, due to continuous charging the sample with deuterium atoms. The results showed that the two types of the traps, associated with radiation defects, were produced. The deep traps and the shallow traps are considered to be vacancy clusters and probably interstitial loops, respectively. The deep traps would no affect the tritium inventory at elevated temperatures although they are largely produced by irradiation. The shallow traps would be dominant to the inventory, which is not so significant due to restriction of the number of the shallow traps. Further works, however, will be needed to determine the binding energies, to identify the shallow traps and to investigate a way to reduce the tritium inventory since the amount of trapped tritium is expected to be still much higher than that of dissolved tritium even at elevated temperatures.
[2]
Acknowledgement
[9]
This work is supported by a grant-in-aid for scientific research of Japan Society for the Promotion of Science.
[3]
[4]
[5]
[6]
[7]
[8]
[10]
[11] [12]
References [13] [1] K. Ioki, M. Akiba, P. Barabaschi, V. Barabash, S. Chiocchio, W. Daenner, et al., ITER nuclear components, preparing for the
789
construction and R&D results, J. Nucl. Mater. 329–333 (2004) 31–38. J.W. Davis, G.M. Kalinin, Materials properties and design requirements for copper alloys used in ITER, J. Nucl. Mater. 258–263 (1998) 323–328. K.L. Wilson, R.A. Causey, M.I. Baskes, J. Kamperschroer, Hydrogen isotope retention and release from copper, J. Vac. Sci. Technol. A 5 (1987) 2319–2324. F. Besenbacher, B.B. Nielsen, S.M. Myers, Defect trapping of ion-implanted deuterium in copper, J. Appl. Phys. 56 (1984) 3384–3393. I. Takagi, K. Yoshida, K. Shin, K. Higashi, A combined technique of nuclear reaction analysis and plasma-driven permeation for a quantitative study on deuterium trapping, Nucl. Instr. Meth. B 84 (1994) 393–399. D. Dieumegard, D. Dubreuil, G. Amsel, Analysis and depth profiling of deuterium with the D(3 He,p)4 He reaction by detecting the protons at backward angles, Nucl. Instr. Meth. 166 (1979) 431–445. I. Takagi, H. Hashimoto, H. Fujita, K. Higashi, An experimental study on the potential energy diagram for hydrogen isotopes on copper surfaces, Fusion Eng. Des. 41 (1998) 73–78. W. Eichenauer, W. L¨oser, H. Witte, L¨oslichkeit und Diffusionsgeschwindigkeit von Wasserstoff und Deuterium in Einkristallen aus Nickel und Kupfer, Z. Metallkde. 56 (1965) 287–293. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. T. Yamauchi, S. Yamanaka, M. Miyake, Thermal release behavior of helium implanted into copper at high fluences, J. Nucl. Mater. 174 (1990) 53–59. W. Sch¨ule, On the recovery stages in fcc materials, J. Nucl. Mater. 233–237 (1996) 969–973. K. Yasuda, C. Kinoshita, M. Kutsuwada, T. Hirai, Nucleation and growth process of defect clusters in copper during helium ion irradiation, J. Nucl. Mater. 233–237 (1996) 1051–1056. B. Lengeler, S. Mantl, W. Triftshaeuser, Interaction of hydrogen and vacancies in copper investigated by positron annihilation, J. Phys. F 8 (1978) 1691–1698.