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Critical helium fluence for hydrogen retention in stainless steel
768
Nuclear Instruments and Methods in Physics Research B33 (1988) 768-771 North-Holland, Amsterdam
CRITICAL HELIUM M. OGAWA, Department
FLUENCE
K. SANEYOSHI
FOR HYDROGEN
RETENTION
IN STAINLESS
STEEL
and T. HARADA
of Energy Sciences, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227, Japan
Hydrogen depth profiles have been measured for stainless steel pre-implanted with 10 keV helium ions to fluences below and above the critical fluence. We observed double peaking in the profile for a fluence of 3.2 X 10” He cm-*, which is above the critical fluence. The double peaking was possibly caused by the onset of blistering.
1. Introduction
ping in stainless steel pre-irradiated with helium ions to fluences below and above the critical fluence F,.
The behavior of hydrogen in materials is one of the important factors for the first-wall problem in fusionenergy technology. The deuterium trapping in metals pre-irradiated with helium ion has been investigated by many authors [l-lo]. Lanford et al. have studied the hydrogen trapping using the ‘H(“N, cyy)‘*C reaction [ll]. The advantage of this nuclear reaction analysis is its high depth resolution compared to the D(3He, P)~H~ reaction. Scherzer et al. have measured the re-emission rate of helium atoms implanted in Nb as a function of helium fluence. They observed an abrupt increase in the reemission rate when the fluence was increased from 2 X 10” to 3 X 10” cm-’ [12]. In this fluence region, they introduced a critical fluence F, and explained it as an onset of blistering. Terreault et al. have measured the helium depth profiles as a function of the helium fluence [13-161. They observed Gaussian-like distributions below the critical fluence and double-peaking distributions above the critical fluence. Besenbacher et al. have studied deuterium trapping in nickel and they have explained that the helium-associated traps are due to small helium bubbles/voids [6-9,171. Phenomena related to the critical fluence have also been observed by Wilson et al. for the deuterium retention in stainless steel [3]. They found a maximum deuterium retention at a helium fluence of 1.7 X 10” cm-*. The existence of a critical helium fluence for hydrogen trapping in stainless steel has been confirmed in our previous work using the lH(“N, clly)‘*C reaction [18]. We observed an abrupt decrease of hydrogen retention when the helium fluence was increased from 2 X 10” to 3 x 10” cmm2. The high depth resolution of the “N reaction analysis is suitable to investigate the dependence of hydrogen depth profiles under various irradiation conditions. In this work we report hydrogen trap0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
2. Experimental Commercially available SUS-316 stainless steel was used as the target material. Target specimens of 16 mm diameter and 1 mm thickness were polished down to a final finish of 0.3 pm. The ion implantations and the depth profile measurements were carried out at room temperature. The experimental method has been described in detail elsewhere [18,19]. The setup used is shown schematically in fig. 1. The helium ions of 10 keV were pre-implanted into the specimens with fluences close to the critical fluence F,. Subsequent to the helium pre-implantation, molecular hydrogen ions of 10 keV were implanted to a fluence of 4 X 10” atoms cmp2. The implantation beam was collimated with an aperture of 12 mm diameter without scanning. The accuracy of the fluence measurement was about +20%. The depth profiling of hydrogen was performed with the ‘H(15N, cyy)‘*C resonance reaction. The “N beam was generated by the Tokyo Institute of Technology 4.75 MV Van de Graaff accelerator. The resonance y-rays of 4.43 MeV were detected with a 7.5 cm x 7.5 cm NaI counter placed 3 cm behind the target. The fluence of “N ions was determined by Rutherford scattering using a Si surface barrier detector. The uncertainty in depth of profile peaks was about + 10 nm because of the small deflection angle of the analyzing magnet used and the counting statistics. We have used Kapton to calibrate the absolute hydrogen concentration. The Kapton foil was obtained from Toray Co. Its chemical composition is (C,,H,, N,O,),. Fig. 2 indicates a hydrogen-concentration dependence on beam dose at a depth of 170 nm. This dependence was measured using a lsN2+ beam of 8.5 pnA current with a beam spot size of 5 mm x 5 mm.
M. Ogawa et al. / Critical helium fluence for hydrogen retention in stainless steel MAGNET +V.d.G
----
DISPLACEMENT
PIG ION SOURCE
Fig. 1. Schematic viewof the experimentalsetup. The half-life dose obtained was 2.6 x 1014 ions, which was six times higher than the value observed by Westerberg [20]. We have used the Monte Carlo program TRIM-86 to calculate the atom displacements and range distributions of helium ions in specimens. This program, written for IBM-PC computers, has been supplied by Ziegler [21].
3. Results Fig. 3a shows the Monte Carlo simulation of 10 keV helium ions in stainless steel with lo5 events. The distribution of atom distribution of the helium ranges is
nearly Gaussian with a peak at 53 nm. Fig. 3b shows the hydrogen depth profile measured with a pre-implantation fluence of 1.5 X 1Or7 He cme2 at 18 h after the hydrogen implantation. The depth of the profile peaks was 60 nm, which was independent of the time after hydrogen implantation. For the pre-implantation case with the 3.2 x 10” He cme2 fluence, which is slightly
1023_
“E -_ u 2 -
0 0
w
Fig. 3. (a) Distributions of atom displacements and ranges for 10 keV helium ions in stainless steel simulated with TRIM-86. The ordinate is scaled for a 1 X 10” He cm-2 fluence. (b) and (c) Hydrogen depth profiles in hydrogen-implanted stainless steel pre-irradiated with 10 keV helium ions to fluences of 1.5 X 10” and 3.2 X 10’7cm-2, respectively.
-
O
_
1.5X1O’7 He/cm
----3.2~10’~
He/cm’
z” 0
U
-
I
6 k I
t I@02
15N IONS [10141 Fig. 2. Hydrogen
. 0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~______ 0 o
concentration in Kapton as a function of “N beam dose. The beam current was 8.5 pnA with a beam spot sizeof5mmXSmm.
0
0
TIME
(houy)
Fig. 4. Hydrogen retention as a function of time after hydrogen implantation in stainless-steel specimens pre-implanted with 10 keV helium ions to fluences of 1.5 X lOI and 3.2 x 10’7cm-2. X. RADIATION
DAMAGE
170
M. Ogawa et al. / Critical he&m fkence
above the critical fluence F,, the profile split into two peaks at depths of 40 and 90 nm, as shown in fig. 3c. The profile shape was independent of the time after hydrogen implantation. Fig. 4 shows the hydrogen retention as a function of the time after hydrogen impl~tation for both helium pre-irradiation cases, where the hydrogen fluence was fixed to 4 x 1017 cmS2. The hydrogen retention was constant for the higher helium fluence. For the lower helium fluence, the hydrogen retention decreased with a decay constant of 5.6 X 10v5 s-l to a constant level, keeping the same peak position and width for the profiles. The initial hydrogen retention extrapolated to zero time was 1.3 X 1Or7cm-*.
4. Discussion For a helium fluence of 1.5 X 10” cmm2, which is below F,, the peak position of the hydrogen depth profiles was about 60 nm (see fig. 3b), which was close to the value of 53 nm for the calculated helium range dist~bution. The hydrogen retention decayed from the initial value of 1.3 X 10” crnw2 to a constant level of 0.7 x 10” cmd2 in about 10 h after the hydrogen implantation. Therefore, two types of hydrogen trapping centers were involved in the lower fluence case. The decay component was due to a type of trapping center with a smaller binding energy and the constant component was caused by another type of trapping center with a larger binding energy. Both types of trapping centers were related to the helium atoms stopped in the specimen because the profiles were similar to the helium atom distribution. For a helium fluence of 3.2 X 1017 cm-‘, which is above F,, double peaking was observed in the hydrogen depth profile, as shown in fig. 3c. The deeper peak was located at a depth of about 90 nm. This depth value exceeded the peak position of the helium range distribution and correspond to the tail of the former distribution. Schemer has explained the critical fluence as the onset of blistering [12], and Kusanagi has observed an increase of cavity size in the region of the critical fluence with a transmission electron microscope [22]. If we assume that the blistering was formed at the peak position of the helium range distribution, fig. 3c indicates that the hydrogen trapping that originated from the blistering was less than the trapping due to the normal helium atom distribution shown in fig. 3b. In other words, when the helium fluence was increased from the level below F, to the level above F,, the excess helium participated in forming the blistering. This blistering caused a dip in the hydrogen trapping distribution at a position where maximum trapping was observed at the fluence below F,. There may also occur a
for hydrogen
retention in stainless steel
dip in the helium distributions after blistering as was observed for the helium-copper system by Terreault [13-161. The shallower peak located at a depth of 40 nm was similar to the distribution of atom displacements that would correspond to hydrogen trapping caused by radiation damage. This peak could also be a shallower tail of the initial helium range distribution free from blistering. Further experiments are necessary to discuss the origin of the shallower peak. In our previous work, we observed flattening instead of double peaking in the hydrogen depth profiles for three different helium fluences above F,. The discrepancy of the double-pe~ng observations between the present and previous works could be due to a poor uniformity of implantation because the implantation beam was not scanned.
5. Conclusion We have observed that the hydrogen trapping centers consist of two components with different binding energies for a helium fluence below F,. For a fluence above F,, we have found double peaking in the hydrogen trapping caused by blistering at the peak position of the helium range distribution.
References [l] K.L. Wilson and M.I. Baskes, J. Nucl. Mater. 76/77 (1978) 291. [2] S.M. Myers, S.T. Picraux and R.E. Stoltz, Appl. Phys. Lett. 37 (1980) 168. [3] K.L. Wilson, A.E. Pontau, L.C. Haggmark, M.I. Baskes, J. Bohdansky and J. Roth, J. Nucl. Mater. 103/104 (1981) 493. [4] S.M. Myers and W.R. Warnpler, J. Nucl. Mater. 111/112 (1982) 579. 15) A.E. Pontau, MI. Baskes, K.L. Wilson, L.G. Haggmark, J. Bohdansky, B.M.U. Schemer and J. Roth, J. Nucl. Mater. 111/112 (1982) 651. [6] F. Besenbacher, J. B&tiger, T. Laursen and W. M&r, J. Nucl. Mater. 93/94 (1980) 617. [7] F. Besenbacher, J. Bottiger and SM. Myers, J. Appl. Phys. 53 (1982) 3536. [8] F. Besenbacher, J. Bettiger and S.M. Myers, J. Appl. Phys. 53 (1982) 3547. [9] S.M. Myers, D.M. Follstaedt, F. Besenbacher and J. Bottiger, J. Appl. Phys. 53 (1982) 8734. [lo] S.M. Myers and D.M. Follstaedt, J. Nucl. Mater. 145-147 (1987) 322. (111 W.A. Lanford, H.P. Trautvetter, J.F. Ziegler and J. Keiler, Appl. Phys. Lett. 28 (1976) 566. [12] B.M.U. Schemer, P. Borgesen, J. Ehrenberg and W. Moller, Nucl. Instr. and Meth. B7/8 (1985) 71.
M. Ogawa et al. / Critical helium fiuence for hydrogen retention in stainless steel [13] B. Terreault, J.G. Martel, R.G. St-Jacques, G. Veilleux, J. L’Ecuyer, C. Brassard, C. Cardinal, L. Deschenes and J.P. Labrie, J. Nucl. Mater. 68 (1977) 334. [14] B. Terreault, G. Abel, J.G. Martel, R.G. St-Jacques, J.P. Labrie and J. L’Ecuyer, J. Nucl. Mater. 76/77 (1978) 249. [15] B. Terreault, R.G. St-Jacques, G. Veilleux, J.G. Martel, J. L’Ecuyer, C. Brassard and C. Cardinal, Can. J. Phys. 56 (1978) 235. [16] B. Terreault, G. Ross, R.G. St-Jacques and G. Veilleux J. Appl. Phys. 51 (1980) 1491. [17] F. Besenbacher, S.M. Myers and J.K. Norskov, Nucl. Instr. and Meth. B7/8 (1985) 55.
771
[18] M. Ogawa, K. Saneyoshi, Y. Takagi, A. Shirota and Y. Suzawa, J. Nucl. Mater. 149 (1987) 247. [19] E. Arai, K. Hayashi, Y. Oguri, K. Sato, M. Ogawa and H. Miyasaka, Nucl. Instr. and Meth. B5 (1984) 58. [20] L. Westerberg, L.E. Svesson, E. Karlsson, M.W. Richardson and K. Lundstrom, Nucl. Instr. and Meth. B9 (1985) 49. [21] J.F. Ziegler, J.P. Biersack and U. Littmark, The stopping and range of ions in solids (Pergamon, New York, 1985). [22] H. Kusanagj, H. Kimura, M. Tokiwai and T. Suzuki, J. Nucl. Mater. 133/134 (1985) 473.
X. RADIATION
DAMAGE
Nuclear Instruments and Methods in Physics Research B33 (1988) 768-771 North-Holland, Amsterdam
CRITICAL HELIUM M. OGAWA, Department
FLUENCE
K. SANEYOSHI
FOR HYDROGEN
RETENTION
IN STAINLESS
STEEL
and T. HARADA
of Energy Sciences, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227, Japan
Hydrogen depth profiles have been measured for stainless steel pre-implanted with 10 keV helium ions to fluences below and above the critical fluence. We observed double peaking in the profile for a fluence of 3.2 X 10” He cm-*, which is above the critical fluence. The double peaking was possibly caused by the onset of blistering.
1. Introduction
ping in stainless steel pre-irradiated with helium ions to fluences below and above the critical fluence F,.
The behavior of hydrogen in materials is one of the important factors for the first-wall problem in fusionenergy technology. The deuterium trapping in metals pre-irradiated with helium ion has been investigated by many authors [l-lo]. Lanford et al. have studied the hydrogen trapping using the ‘H(“N, cyy)‘*C reaction [ll]. The advantage of this nuclear reaction analysis is its high depth resolution compared to the D(3He, P)~H~ reaction. Scherzer et al. have measured the re-emission rate of helium atoms implanted in Nb as a function of helium fluence. They observed an abrupt increase in the reemission rate when the fluence was increased from 2 X 10” to 3 X 10” cm-’ [12]. In this fluence region, they introduced a critical fluence F, and explained it as an onset of blistering. Terreault et al. have measured the helium depth profiles as a function of the helium fluence [13-161. They observed Gaussian-like distributions below the critical fluence and double-peaking distributions above the critical fluence. Besenbacher et al. have studied deuterium trapping in nickel and they have explained that the helium-associated traps are due to small helium bubbles/voids [6-9,171. Phenomena related to the critical fluence have also been observed by Wilson et al. for the deuterium retention in stainless steel [3]. They found a maximum deuterium retention at a helium fluence of 1.7 X 10” cm-*. The existence of a critical helium fluence for hydrogen trapping in stainless steel has been confirmed in our previous work using the lH(“N, clly)‘*C reaction [18]. We observed an abrupt decrease of hydrogen retention when the helium fluence was increased from 2 X 10” to 3 x 10” cmm2. The high depth resolution of the “N reaction analysis is suitable to investigate the dependence of hydrogen depth profiles under various irradiation conditions. In this work we report hydrogen trap0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
2. Experimental Commercially available SUS-316 stainless steel was used as the target material. Target specimens of 16 mm diameter and 1 mm thickness were polished down to a final finish of 0.3 pm. The ion implantations and the depth profile measurements were carried out at room temperature. The experimental method has been described in detail elsewhere [18,19]. The setup used is shown schematically in fig. 1. The helium ions of 10 keV were pre-implanted into the specimens with fluences close to the critical fluence F,. Subsequent to the helium pre-implantation, molecular hydrogen ions of 10 keV were implanted to a fluence of 4 X 10” atoms cmp2. The implantation beam was collimated with an aperture of 12 mm diameter without scanning. The accuracy of the fluence measurement was about +20%. The depth profiling of hydrogen was performed with the ‘H(15N, cyy)‘*C resonance reaction. The “N beam was generated by the Tokyo Institute of Technology 4.75 MV Van de Graaff accelerator. The resonance y-rays of 4.43 MeV were detected with a 7.5 cm x 7.5 cm NaI counter placed 3 cm behind the target. The fluence of “N ions was determined by Rutherford scattering using a Si surface barrier detector. The uncertainty in depth of profile peaks was about + 10 nm because of the small deflection angle of the analyzing magnet used and the counting statistics. We have used Kapton to calibrate the absolute hydrogen concentration. The Kapton foil was obtained from Toray Co. Its chemical composition is (C,,H,, N,O,),. Fig. 2 indicates a hydrogen-concentration dependence on beam dose at a depth of 170 nm. This dependence was measured using a lsN2+ beam of 8.5 pnA current with a beam spot size of 5 mm x 5 mm.
M. Ogawa et al. / Critical helium fluence for hydrogen retention in stainless steel MAGNET +V.d.G
----
DISPLACEMENT
PIG ION SOURCE
Fig. 1. Schematic viewof the experimentalsetup. The half-life dose obtained was 2.6 x 1014 ions, which was six times higher than the value observed by Westerberg [20]. We have used the Monte Carlo program TRIM-86 to calculate the atom displacements and range distributions of helium ions in specimens. This program, written for IBM-PC computers, has been supplied by Ziegler [21].
3. Results Fig. 3a shows the Monte Carlo simulation of 10 keV helium ions in stainless steel with lo5 events. The distribution of atom distribution of the helium ranges is
nearly Gaussian with a peak at 53 nm. Fig. 3b shows the hydrogen depth profile measured with a pre-implantation fluence of 1.5 X 1Or7 He cme2 at 18 h after the hydrogen implantation. The depth of the profile peaks was 60 nm, which was independent of the time after hydrogen implantation. For the pre-implantation case with the 3.2 x 10” He cme2 fluence, which is slightly
1023_
“E -_ u 2 -
0 0
w
Fig. 3. (a) Distributions of atom displacements and ranges for 10 keV helium ions in stainless steel simulated with TRIM-86. The ordinate is scaled for a 1 X 10” He cm-2 fluence. (b) and (c) Hydrogen depth profiles in hydrogen-implanted stainless steel pre-irradiated with 10 keV helium ions to fluences of 1.5 X 10” and 3.2 X 10’7cm-2, respectively.
-
O
_
1.5X1O’7 He/cm
----3.2~10’~
He/cm’
z” 0
U
-
I
6 k I
t I@02
15N IONS [10141 Fig. 2. Hydrogen
. 0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~______ 0 o
concentration in Kapton as a function of “N beam dose. The beam current was 8.5 pnA with a beam spot sizeof5mmXSmm.
0
0
TIME
(houy)
Fig. 4. Hydrogen retention as a function of time after hydrogen implantation in stainless-steel specimens pre-implanted with 10 keV helium ions to fluences of 1.5 X lOI and 3.2 x 10’7cm-2. X. RADIATION
DAMAGE
170
M. Ogawa et al. / Critical he&m fkence
above the critical fluence F,, the profile split into two peaks at depths of 40 and 90 nm, as shown in fig. 3c. The profile shape was independent of the time after hydrogen implantation. Fig. 4 shows the hydrogen retention as a function of the time after hydrogen impl~tation for both helium pre-irradiation cases, where the hydrogen fluence was fixed to 4 x 1017 cmS2. The hydrogen retention was constant for the higher helium fluence. For the lower helium fluence, the hydrogen retention decreased with a decay constant of 5.6 X 10v5 s-l to a constant level, keeping the same peak position and width for the profiles. The initial hydrogen retention extrapolated to zero time was 1.3 X 1Or7cm-*.
4. Discussion For a helium fluence of 1.5 X 10” cmm2, which is below F,, the peak position of the hydrogen depth profiles was about 60 nm (see fig. 3b), which was close to the value of 53 nm for the calculated helium range dist~bution. The hydrogen retention decayed from the initial value of 1.3 X 10” crnw2 to a constant level of 0.7 x 10” cmd2 in about 10 h after the hydrogen implantation. Therefore, two types of hydrogen trapping centers were involved in the lower fluence case. The decay component was due to a type of trapping center with a smaller binding energy and the constant component was caused by another type of trapping center with a larger binding energy. Both types of trapping centers were related to the helium atoms stopped in the specimen because the profiles were similar to the helium atom distribution. For a helium fluence of 3.2 X 1017 cm-‘, which is above F,, double peaking was observed in the hydrogen depth profile, as shown in fig. 3c. The deeper peak was located at a depth of about 90 nm. This depth value exceeded the peak position of the helium range distribution and correspond to the tail of the former distribution. Schemer has explained the critical fluence as the onset of blistering [12], and Kusanagi has observed an increase of cavity size in the region of the critical fluence with a transmission electron microscope [22]. If we assume that the blistering was formed at the peak position of the helium range distribution, fig. 3c indicates that the hydrogen trapping that originated from the blistering was less than the trapping due to the normal helium atom distribution shown in fig. 3b. In other words, when the helium fluence was increased from the level below F, to the level above F,, the excess helium participated in forming the blistering. This blistering caused a dip in the hydrogen trapping distribution at a position where maximum trapping was observed at the fluence below F,. There may also occur a
for hydrogen
retention in stainless steel
dip in the helium distributions after blistering as was observed for the helium-copper system by Terreault [13-161. The shallower peak located at a depth of 40 nm was similar to the distribution of atom displacements that would correspond to hydrogen trapping caused by radiation damage. This peak could also be a shallower tail of the initial helium range distribution free from blistering. Further experiments are necessary to discuss the origin of the shallower peak. In our previous work, we observed flattening instead of double peaking in the hydrogen depth profiles for three different helium fluences above F,. The discrepancy of the double-pe~ng observations between the present and previous works could be due to a poor uniformity of implantation because the implantation beam was not scanned.
5. Conclusion We have observed that the hydrogen trapping centers consist of two components with different binding energies for a helium fluence below F,. For a fluence above F,, we have found double peaking in the hydrogen trapping caused by blistering at the peak position of the helium range distribution.
References [l] K.L. Wilson and M.I. Baskes, J. Nucl. Mater. 76/77 (1978) 291. [2] S.M. Myers, S.T. Picraux and R.E. Stoltz, Appl. Phys. Lett. 37 (1980) 168. [3] K.L. Wilson, A.E. Pontau, L.C. Haggmark, M.I. Baskes, J. Bohdansky and J. Roth, J. Nucl. Mater. 103/104 (1981) 493. [4] S.M. Myers and W.R. Warnpler, J. Nucl. Mater. 111/112 (1982) 579. 15) A.E. Pontau, MI. Baskes, K.L. Wilson, L.G. Haggmark, J. Bohdansky, B.M.U. Schemer and J. Roth, J. Nucl. Mater. 111/112 (1982) 651. [6] F. Besenbacher, J. B&tiger, T. Laursen and W. M&r, J. Nucl. Mater. 93/94 (1980) 617. [7] F. Besenbacher, J. Bottiger and SM. Myers, J. Appl. Phys. 53 (1982) 3536. [8] F. Besenbacher, J. Bettiger and S.M. Myers, J. Appl. Phys. 53 (1982) 3547. [9] S.M. Myers, D.M. Follstaedt, F. Besenbacher and J. Bottiger, J. Appl. Phys. 53 (1982) 8734. [lo] S.M. Myers and D.M. Follstaedt, J. Nucl. Mater. 145-147 (1987) 322. (111 W.A. Lanford, H.P. Trautvetter, J.F. Ziegler and J. Keiler, Appl. Phys. Lett. 28 (1976) 566. [12] B.M.U. Schemer, P. Borgesen, J. Ehrenberg and W. Moller, Nucl. Instr. and Meth. B7/8 (1985) 71.
M. Ogawa et al. / Critical helium fiuence for hydrogen retention in stainless steel [13] B. Terreault, J.G. Martel, R.G. St-Jacques, G. Veilleux, J. L’Ecuyer, C. Brassard, C. Cardinal, L. Deschenes and J.P. Labrie, J. Nucl. Mater. 68 (1977) 334. [14] B. Terreault, G. Abel, J.G. Martel, R.G. St-Jacques, J.P. Labrie and J. L’Ecuyer, J. Nucl. Mater. 76/77 (1978) 249. [15] B. Terreault, R.G. St-Jacques, G. Veilleux, J.G. Martel, J. L’Ecuyer, C. Brassard and C. Cardinal, Can. J. Phys. 56 (1978) 235. [16] B. Terreault, G. Ross, R.G. St-Jacques and G. Veilleux J. Appl. Phys. 51 (1980) 1491. [17] F. Besenbacher, S.M. Myers and J.K. Norskov, Nucl. Instr. and Meth. B7/8 (1985) 55.
771
[18] M. Ogawa, K. Saneyoshi, Y. Takagi, A. Shirota and Y. Suzawa, J. Nucl. Mater. 149 (1987) 247. [19] E. Arai, K. Hayashi, Y. Oguri, K. Sato, M. Ogawa and H. Miyasaka, Nucl. Instr. and Meth. B5 (1984) 58. [20] L. Westerberg, L.E. Svesson, E. Karlsson, M.W. Richardson and K. Lundstrom, Nucl. Instr. and Meth. B9 (1985) 49. [21] J.F. Ziegler, J.P. Biersack and U. Littmark, The stopping and range of ions in solids (Pergamon, New York, 1985). [22] H. Kusanagj, H. Kimura, M. Tokiwai and T. Suzuki, J. Nucl. Mater. 133/134 (1985) 473.
X. RADIATION
DAMAGE