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Ion beam analysis of helium and its irradiation effect on hydrogen trapping in W single crystals
Nuclear Instruments and Methods in Physics Research B 190 (2002) 652–656 www.elsevier.com/locate/nimb
Ion beam analysis of helium and its irradiation effect on hydrogen trapping in W single crystals S. Nagata *, B. Tsuchiya, T. Sugawara, N. Ohtsu, T. Shikama Institute for Material Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Abstract Retention of He implanted into W single crystals and the He irradiation effects on H behavior were studied by ion beam analysis techniques. During implantation of 4 Heþ with 2–10 keV at 295 K, an accumulation of H started in the He implanted layer when the retained He concentration saturated. For the crystal irradiated by 10 keV Heþ at 820 K, a remarkable increase of H was found in the He saturated layer, after stopping the implantation and cooling down the crystal below 400 K. Though blisters and exfoliation were observed for the surface irradiated at 820 K, less lattice disorder was found in the implanted layer and the thermal release of H occurred at lower temperature, in comparison with the crystal implanted at 295 K. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Bg; 61.72.Ss Keywords: Helium; Ion implantation; Tungsten; Elastic recoil detection analysis; Hydrogen
1. Introduction An interaction between the hydrogen and He has a great importance in materials where both hydrogen isotopes and He exist, such as first wall components in a fusion reactor and fission fuel elements. It is well known that the He irradiation of metals efficiently enhances the retention of hydrogen isotopes in the penetrated region [1]. Possible mechanisms for hydrogen trapping by the He induced damage have been proposed, including an adsorption on the He bubble
*
Corresponding author. Tel.: +81-22-215-2058; fax: +81-22215-2061. E-mail address: [email protected] (S. Nagata).
walls, a stress/strain field around bubbles, and formation of the H2 molecules [2–4]. However, the He bombardment effect on the hydrogen behavior was not fully understood except for some metals. Ion beam analysis techniques have been successfully applied to study the trapping and release of hydrogen in metals and hydrogen transport characteristics quantitatively [4]. Especially for depth profiling of hydrogen isotopes and/or helium in near surface of solids, the elastic recoil detection analysis (ERDA) method has the advantage to be able to measure both light spices simultaneously during the ion implantation. Previously, we demonstrated an enrichment of hydrogen in the hydride forming metals during He ion irradiation [5]. In the present paper, we report the He retention and hydrogen accumulation in
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 2 4 2 - 3
S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
W single crystals studied by the ERDA. The He implanted layer of the crystal was also examined by Rutherford backscatering spectroscopy (RBS) combined with ion channeling experiments to investigate lattice disorder and impurities induced by the He implantation.
2. Experiments Samples used for the present study were W single crystal disks of 8 mm diameter and 0.5 mm thickness. Details of the sample preparation methods were described elsewhere [6]. The He ion implantation and the ion beam analysis were performed in a vacuum system with a base pressure of 3 106 Pa, at the Laboratory for Developmental Research of Advance Materials, the Institute for Materials Research, Tohoku University. The implantation was carried out with incident energies of 2, 5 and 10 keV, with a typical current density of about 1 1018 He/m2 s, at a temperature range between 295 and 860 K. An incident angle of mass analyzed 4 He ions was about 10° from the surface normal, avoiding the channeling direction of the crystal. During and after the He implantation, concentration depth profiles of He and H atoms retained in the surface layer of the W crystal were measured by ERDA. A beam of 4.0 MeV O3þ was incident on the sample at an angle of the 20° with respect to the surface, and recoiled particles were detected at 35° of recoil angle through an Al foil of 4 lm thickness. The depth resolution of the present experiments was estimated to be about 20 nm at the surface of W. The H concentration was estimated using metal hydride of known hydrogen content, and was also checked by a beam of 2.8 MeV 4 He2þ . The number of retained He atoms were calibrated at a small dose where 100% of incident He are assumed to be trapped except for backscattering fraction. Thermal release of the accumulated hydrogen was examined by measuring H profiles after each stage of the isochronal annealing for 600 s in a temperature range between 295 and 800 K. Changes of the surface morphology were investigated by scanning electron microscope (SEM) observation.
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3. Results and discussion Fig. 1 shows typical energy spectra of recoil particles obtained from a W single crystal irradiated with 2 keV He at 295 K. Retained He in the crystal appeared as a broad peak nearly saturated at doses above 3 1020 He/m2 , with a maximum He concentration to W atoms of about 0.3 He/W, though the distribution of the He atoms slightly moved toward a large depth at higher dose. No significant change of the H profile was observed at the beginning of the He implantation. With an increase of the He dose above 3 1021 He/m2 , however, the H profile enlarged and became broad. A similar H accumulation in the He implanted layer was shown for W crystals irradiated by He of 5 and 10 keV, for which energy distribution of H and He were overlapped. Dissolved H atoms in the crystal can be one of the sources of hydrogen retained at the He implanted layer, as seen for Ti, Zr and Ta [5]. On the other hand, the D uptake occurred from the gas phase into solution in iron implanted with He [7]. Diffusivity and solubility of H in W is estimated be very small at room temperature, if those obtained at very high temperature [8] are extrapolated to the lower temperature. In previous work [9] concerning He pre-bombardment effects on retention of post implanted D
Fig. 1. Energy spectra of He and H recoil particles from a W single crystal before (—) and after implanted with 2 keV 4 He at doses of 1:3 1021 He/m2 ( ) and 6:2 1021 He/m2 ( ) at 295 K.
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S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
ions in W, no accumulation of H was found, though the 10 keV He irradiation created trapping sites for D effectively at low doses below 1 1018 He/m2 . It is conjectured that He implantation to a high dose enhanced dissociative adsorption of hydrogen impinging at the irradiated surface from the residual gas. For 820 K implantation with 10 keV He, the efficient accumulation of H was observed in the He implanted depth, when the crystal was cooled down below 400 K, just after stopping He implantation. Fig. 2 shows total amounts of retained He and H in the surface layer of W crystals implanted by 4 Heþ with 10 keV at 295 and 820 K. The prompt H accumulation after cooling down the crystal indicates a large hydrogen uptake rate, which is comparable to the impingement rate of residual gas containing hydrogen, such as H2 and H2 O with partial pressure of about 106 Pa. Fig. 3 shows backscattering spectra for 2 MeV He incident along the h1 1 1i axial direction in W crystals after the irradiation with 10 keV 4 He ions at a dose about 1 1022 He/m2 at 295 and 820 K. A pronounced peak appeared for the crystal irra-
Fig. 2. Total amounts of retained He and H in the surface layer of W crystals implanted by 4 Heþ with 10 keV at 295 K (He: , H: ) and 820 K (He: , H: ). For 820 K implantation, the ERDA measurements were made at temperatures below 400 K, after stopping 4 Heþ implantation.
Fig. 3. Backscattering spectra for 2 MeV 4 He ion beam incident along the h1 1 1i axial direction in W crystals after the implantation with 10 keV 4 Heþ at a dose about 1:2 1022 He/m2 at 295 K ( ) and 820 K ( ), incorporating spectra for random orientation.
diated at 295 K, indicating significant misalignment introduced in the He implanted layer. Less direct scattering yields from the He implanted layer and dechanneling yields from the depth beyond the implanted layer were detected for the crystal implanted at 820 K. The energy dependence of dechanneling parameter was examined for h1 1 1i direction of the crystal. A flat energy dependence on the analyzing beam energy for the crystal irradiated at 820 K may be attributed to the He bubbles which can be ordered in BCC metals at temperatures around 0.2Tm [10], where Tm is the melting temperature. Blisters and exfoliation were observed for the crystal implanted at 820 K as shown in Fig. 4, indicating formation of large interconnected bubbles with relatively low pressure. He bubbles of small sizes with relatively high pressure can be formed at room temperature [11], though no change of the surface morphology was found by SEM observation for the crystal irradiated at 295 K. If the chemisorption-like interaction is a dominant mechanism for trapping H atoms, the thermal release of H is expected to occur at a high temperature corresponding to the relatively high dissociation enthalpy [12] for the H adsorption
S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
Fig. 4. SEM micrographs of the surface of W crystals implanted with 10 keV 4 Heþ to a dose of about 1:5 1022 He/m2 at 295 and 820 K, for (a) and (b), respectively.
at the bubble wall. On the contrary, the thermal release temperature of H in the crystal implanted at 820 K was lower than that in the crystal implanted at 295 K as shown in Fig. 5. These results lead to a conclusion that the trapping of H in the He implanted layer can be related to a stress/strain field around bubbles. The activated surface induced by He irradiation may easily adsorb impurity atoms other than H atoms, resulting in a decrease of the number of the sites where dissociation can occur. This is consistent with the fact that the prompt accumulation of H was not observed during the isochronal annealing experiments; the H retention was slowly recovered within several hours. For the crystal implanted at 295 K, the concentration ratio of He to W estimated from the reduction of the scattering yield in random orientation as shown in Fig. 3, is in good agreement with that obtained from ERDA, whereas the He retention was about the same for 295 and 820 K implantation, the deficiency for the crystal implanted at 820 K was about twice as large as that for 295 K. The large differ-
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Fig. 5. Integrated amounts of H in the near surface layer of W crystals implanted with 10 keV 4 Heþ to a dose of about 1 1022 He/m2 at 295 K ( ) and 820 K ( ) plotted as a function of the annealing temperature. The solid line refers to the result for the crystal irradiated 5 keV D ions to a dose of 1:4 1022 D/m2 without the He implantation [6].
ence is not be reasonably explained by the stopping power which depends on the physical state of implanted He atoms in the W crystal, suggesting a possibility of the presence of impurities other than the He and H in the crystal irradiated at 820 K. Though an attempt was made to check impurities in the He implanted layer by using resonant scattering for carbon and oxygen [13,14], it showed no clear indication of a large concentration of C and O in the He implanted layer of W crystals. We are planning further investigation of impurities in the He implanted surface of the W crystal, including ERDA using incident ions heavier than O.
4. Conclusions The He implanted surface layer of the W single crystal was studied by ion beam analysis techniques. A significant hydrogen accumulation was found in the He retained layer of the crystals during and after He implantation, when the retained He
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S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
concentration reached to a maximum in the surface layer. The large hydrogen uptake rate suggested that the surface irradiated by He ion play a role for enhancing dissociative adsorption of hydrogen from the residual gas. The present results on the thermal release of hydrogen and He implantation induced lattice disorder indicated that hydrogen trapping was attributed to the stress/strain field around the He bubbles created in the implanted layer, whereas the trapped hydrogen was not identified as the form of atoms or molecules.
References [1] S.T. Picraux, J. Bøttiger, N. Rud, Appl. Phys. Lett. 28 (1976) 179.
[2] S.M. Myers, F. Besenbacher, J.K. Nørskov, J. Appl. Phys. 58 (1985) 1841. [3] E. Abramov, D. Eliezer, J. Mater. Sci. 27 (1992) 2595. [4] S.M. Myers, P.M. Richards, W.R. Wampler, F. Besenbacher, J. Nucl. Mater. 165 (1989) 9. [5] S. Nagata, K. Takahiro, S. Yamaguchi, S. Yamamoto, B. Tsuchiya, H. Naramoto, Nucl. Instr. and Meth. B 136–138 (1998) 680. [6] S. Nagata, K. Takahiro, S. Horiike, S. Yamaguchi, J. Nucl. Mater. 266–269 (1999) 1151. [7] W.R. Wampler, J. Appl. Phys. 65 (1989) 4040. [8] R. Frauenfelder, J. Vac. Sci. Technol. 6 (1969) 388. [9] S. Nagata, K. Takahiro, J. Nucl. Mater. 290–293 (2001) 135. [10] P.B. Johnson, D.J. Mazey, J. Nucl. Mater. 218 (1995) 273. [11] H. Iwakiri, K. Yasunaga, K. Morishita, N. Yoshida, J. Nucl. Mater. 283–287 (2000) 1134. [12] H. Eleveld, A. vanVeen, J. Nucl. Mater. 191–194 (1992) 433. [13] E. Rauhara, Nucl. Instr. and Meth. B 12 (1985) 447. [14] J.A. Leavitt et al., Nucl. Instr. and Meth. B 44 (1990) 260.
Ion beam analysis of helium and its irradiation effect on hydrogen trapping in W single crystals S. Nagata *, B. Tsuchiya, T. Sugawara, N. Ohtsu, T. Shikama Institute for Material Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Abstract Retention of He implanted into W single crystals and the He irradiation effects on H behavior were studied by ion beam analysis techniques. During implantation of 4 Heþ with 2–10 keV at 295 K, an accumulation of H started in the He implanted layer when the retained He concentration saturated. For the crystal irradiated by 10 keV Heþ at 820 K, a remarkable increase of H was found in the He saturated layer, after stopping the implantation and cooling down the crystal below 400 K. Though blisters and exfoliation were observed for the surface irradiated at 820 K, less lattice disorder was found in the implanted layer and the thermal release of H occurred at lower temperature, in comparison with the crystal implanted at 295 K. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Bg; 61.72.Ss Keywords: Helium; Ion implantation; Tungsten; Elastic recoil detection analysis; Hydrogen
1. Introduction An interaction between the hydrogen and He has a great importance in materials where both hydrogen isotopes and He exist, such as first wall components in a fusion reactor and fission fuel elements. It is well known that the He irradiation of metals efficiently enhances the retention of hydrogen isotopes in the penetrated region [1]. Possible mechanisms for hydrogen trapping by the He induced damage have been proposed, including an adsorption on the He bubble
*
Corresponding author. Tel.: +81-22-215-2058; fax: +81-22215-2061. E-mail address: [email protected] (S. Nagata).
walls, a stress/strain field around bubbles, and formation of the H2 molecules [2–4]. However, the He bombardment effect on the hydrogen behavior was not fully understood except for some metals. Ion beam analysis techniques have been successfully applied to study the trapping and release of hydrogen in metals and hydrogen transport characteristics quantitatively [4]. Especially for depth profiling of hydrogen isotopes and/or helium in near surface of solids, the elastic recoil detection analysis (ERDA) method has the advantage to be able to measure both light spices simultaneously during the ion implantation. Previously, we demonstrated an enrichment of hydrogen in the hydride forming metals during He ion irradiation [5]. In the present paper, we report the He retention and hydrogen accumulation in
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 2 4 2 - 3
S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
W single crystals studied by the ERDA. The He implanted layer of the crystal was also examined by Rutherford backscatering spectroscopy (RBS) combined with ion channeling experiments to investigate lattice disorder and impurities induced by the He implantation.
2. Experiments Samples used for the present study were W single crystal disks of 8 mm diameter and 0.5 mm thickness. Details of the sample preparation methods were described elsewhere [6]. The He ion implantation and the ion beam analysis were performed in a vacuum system with a base pressure of 3 106 Pa, at the Laboratory for Developmental Research of Advance Materials, the Institute for Materials Research, Tohoku University. The implantation was carried out with incident energies of 2, 5 and 10 keV, with a typical current density of about 1 1018 He/m2 s, at a temperature range between 295 and 860 K. An incident angle of mass analyzed 4 He ions was about 10° from the surface normal, avoiding the channeling direction of the crystal. During and after the He implantation, concentration depth profiles of He and H atoms retained in the surface layer of the W crystal were measured by ERDA. A beam of 4.0 MeV O3þ was incident on the sample at an angle of the 20° with respect to the surface, and recoiled particles were detected at 35° of recoil angle through an Al foil of 4 lm thickness. The depth resolution of the present experiments was estimated to be about 20 nm at the surface of W. The H concentration was estimated using metal hydride of known hydrogen content, and was also checked by a beam of 2.8 MeV 4 He2þ . The number of retained He atoms were calibrated at a small dose where 100% of incident He are assumed to be trapped except for backscattering fraction. Thermal release of the accumulated hydrogen was examined by measuring H profiles after each stage of the isochronal annealing for 600 s in a temperature range between 295 and 800 K. Changes of the surface morphology were investigated by scanning electron microscope (SEM) observation.
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3. Results and discussion Fig. 1 shows typical energy spectra of recoil particles obtained from a W single crystal irradiated with 2 keV He at 295 K. Retained He in the crystal appeared as a broad peak nearly saturated at doses above 3 1020 He/m2 , with a maximum He concentration to W atoms of about 0.3 He/W, though the distribution of the He atoms slightly moved toward a large depth at higher dose. No significant change of the H profile was observed at the beginning of the He implantation. With an increase of the He dose above 3 1021 He/m2 , however, the H profile enlarged and became broad. A similar H accumulation in the He implanted layer was shown for W crystals irradiated by He of 5 and 10 keV, for which energy distribution of H and He were overlapped. Dissolved H atoms in the crystal can be one of the sources of hydrogen retained at the He implanted layer, as seen for Ti, Zr and Ta [5]. On the other hand, the D uptake occurred from the gas phase into solution in iron implanted with He [7]. Diffusivity and solubility of H in W is estimated be very small at room temperature, if those obtained at very high temperature [8] are extrapolated to the lower temperature. In previous work [9] concerning He pre-bombardment effects on retention of post implanted D
Fig. 1. Energy spectra of He and H recoil particles from a W single crystal before (—) and after implanted with 2 keV 4 He at doses of 1:3 1021 He/m2 ( ) and 6:2 1021 He/m2 ( ) at 295 K.
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S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
ions in W, no accumulation of H was found, though the 10 keV He irradiation created trapping sites for D effectively at low doses below 1 1018 He/m2 . It is conjectured that He implantation to a high dose enhanced dissociative adsorption of hydrogen impinging at the irradiated surface from the residual gas. For 820 K implantation with 10 keV He, the efficient accumulation of H was observed in the He implanted depth, when the crystal was cooled down below 400 K, just after stopping He implantation. Fig. 2 shows total amounts of retained He and H in the surface layer of W crystals implanted by 4 Heþ with 10 keV at 295 and 820 K. The prompt H accumulation after cooling down the crystal indicates a large hydrogen uptake rate, which is comparable to the impingement rate of residual gas containing hydrogen, such as H2 and H2 O with partial pressure of about 106 Pa. Fig. 3 shows backscattering spectra for 2 MeV He incident along the h1 1 1i axial direction in W crystals after the irradiation with 10 keV 4 He ions at a dose about 1 1022 He/m2 at 295 and 820 K. A pronounced peak appeared for the crystal irra-
Fig. 2. Total amounts of retained He and H in the surface layer of W crystals implanted by 4 Heþ with 10 keV at 295 K (He: , H: ) and 820 K (He: , H: ). For 820 K implantation, the ERDA measurements were made at temperatures below 400 K, after stopping 4 Heþ implantation.
Fig. 3. Backscattering spectra for 2 MeV 4 He ion beam incident along the h1 1 1i axial direction in W crystals after the implantation with 10 keV 4 Heþ at a dose about 1:2 1022 He/m2 at 295 K ( ) and 820 K ( ), incorporating spectra for random orientation.
diated at 295 K, indicating significant misalignment introduced in the He implanted layer. Less direct scattering yields from the He implanted layer and dechanneling yields from the depth beyond the implanted layer were detected for the crystal implanted at 820 K. The energy dependence of dechanneling parameter was examined for h1 1 1i direction of the crystal. A flat energy dependence on the analyzing beam energy for the crystal irradiated at 820 K may be attributed to the He bubbles which can be ordered in BCC metals at temperatures around 0.2Tm [10], where Tm is the melting temperature. Blisters and exfoliation were observed for the crystal implanted at 820 K as shown in Fig. 4, indicating formation of large interconnected bubbles with relatively low pressure. He bubbles of small sizes with relatively high pressure can be formed at room temperature [11], though no change of the surface morphology was found by SEM observation for the crystal irradiated at 295 K. If the chemisorption-like interaction is a dominant mechanism for trapping H atoms, the thermal release of H is expected to occur at a high temperature corresponding to the relatively high dissociation enthalpy [12] for the H adsorption
S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
Fig. 4. SEM micrographs of the surface of W crystals implanted with 10 keV 4 Heþ to a dose of about 1:5 1022 He/m2 at 295 and 820 K, for (a) and (b), respectively.
at the bubble wall. On the contrary, the thermal release temperature of H in the crystal implanted at 820 K was lower than that in the crystal implanted at 295 K as shown in Fig. 5. These results lead to a conclusion that the trapping of H in the He implanted layer can be related to a stress/strain field around bubbles. The activated surface induced by He irradiation may easily adsorb impurity atoms other than H atoms, resulting in a decrease of the number of the sites where dissociation can occur. This is consistent with the fact that the prompt accumulation of H was not observed during the isochronal annealing experiments; the H retention was slowly recovered within several hours. For the crystal implanted at 295 K, the concentration ratio of He to W estimated from the reduction of the scattering yield in random orientation as shown in Fig. 3, is in good agreement with that obtained from ERDA, whereas the He retention was about the same for 295 and 820 K implantation, the deficiency for the crystal implanted at 820 K was about twice as large as that for 295 K. The large differ-
655
Fig. 5. Integrated amounts of H in the near surface layer of W crystals implanted with 10 keV 4 Heþ to a dose of about 1 1022 He/m2 at 295 K ( ) and 820 K ( ) plotted as a function of the annealing temperature. The solid line refers to the result for the crystal irradiated 5 keV D ions to a dose of 1:4 1022 D/m2 without the He implantation [6].
ence is not be reasonably explained by the stopping power which depends on the physical state of implanted He atoms in the W crystal, suggesting a possibility of the presence of impurities other than the He and H in the crystal irradiated at 820 K. Though an attempt was made to check impurities in the He implanted layer by using resonant scattering for carbon and oxygen [13,14], it showed no clear indication of a large concentration of C and O in the He implanted layer of W crystals. We are planning further investigation of impurities in the He implanted surface of the W crystal, including ERDA using incident ions heavier than O.
4. Conclusions The He implanted surface layer of the W single crystal was studied by ion beam analysis techniques. A significant hydrogen accumulation was found in the He retained layer of the crystals during and after He implantation, when the retained He
656
S. Nagata et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 652–656
concentration reached to a maximum in the surface layer. The large hydrogen uptake rate suggested that the surface irradiated by He ion play a role for enhancing dissociative adsorption of hydrogen from the residual gas. The present results on the thermal release of hydrogen and He implantation induced lattice disorder indicated that hydrogen trapping was attributed to the stress/strain field around the He bubbles created in the implanted layer, whereas the trapped hydrogen was not identified as the form of atoms or molecules.
References [1] S.T. Picraux, J. Bøttiger, N. Rud, Appl. Phys. Lett. 28 (1976) 179.
[2] S.M. Myers, F. Besenbacher, J.K. Nørskov, J. Appl. Phys. 58 (1985) 1841. [3] E. Abramov, D. Eliezer, J. Mater. Sci. 27 (1992) 2595. [4] S.M. Myers, P.M. Richards, W.R. Wampler, F. Besenbacher, J. Nucl. Mater. 165 (1989) 9. [5] S. Nagata, K. Takahiro, S. Yamaguchi, S. Yamamoto, B. Tsuchiya, H. Naramoto, Nucl. Instr. and Meth. B 136–138 (1998) 680. [6] S. Nagata, K. Takahiro, S. Horiike, S. Yamaguchi, J. Nucl. Mater. 266–269 (1999) 1151. [7] W.R. Wampler, J. Appl. Phys. 65 (1989) 4040. [8] R. Frauenfelder, J. Vac. Sci. Technol. 6 (1969) 388. [9] S. Nagata, K. Takahiro, J. Nucl. Mater. 290–293 (2001) 135. [10] P.B. Johnson, D.J. Mazey, J. Nucl. Mater. 218 (1995) 273. [11] H. Iwakiri, K. Yasunaga, K. Morishita, N. Yoshida, J. Nucl. Mater. 283–287 (2000) 1134. [12] H. Eleveld, A. vanVeen, J. Nucl. Mater. 191–194 (1992) 433. [13] E. Rauhara, Nucl. Instr. and Meth. B 12 (1985) 447. [14] J.A. Leavitt et al., Nucl. Instr. and Meth. B 44 (1990) 260.