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SIMS depth profiling of implanted helium in pure iron using CsHe+ detection mode
Nuclear Instruments and Methods in Physics Research B 295 (2013) 69–71
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Short Communication
SIMS depth profiling of implanted helium in pure iron using CsHe+ detection mode H. Lefaix-Jeuland a,⇑, S. Moll a, F. Legendre a, F. Jomard b a b
CEA, DEN, Service De Recherches De Métallurgie Physique, F-91191 Gif-Sur-Yvette, France Groupe d’Etude de la Matière Condensée (CNRS and University of Versailles Saint Quentin), 45 avenue des Etats-Unis, 78035 Versailles cedex, France
a r t i c l e
i n f o
Article history: Received 7 May 2012 Received in revised form 19 October 2012 Available online 3 December 2012 Keywords: SIMS Ion implantation Helium Depth profiling
a b s t r a c t Helium distribution in implanted monocrystalline and polycrystalline Fe samples has been measured by secondary ion mass spectrometry (SIMS). The use of Cs+ primary ions in conjunction with the detection of CsHe+ molecular ions was shown to be an efficient method to overcome the very high first ionization potential of helium. The implantation ranges of 60 keV He ions in samples are measured about 220 nm in agreement with projected ranges calculated by TRIM. He concentrations at or above 5 1018 at/cm3 (60 ppm) were measured. This study confirms the paramount interest of SIMS as a direct He depth profiling technique. Ó 2012 Elsevier B.V. All rights reserved.
Reliable He depth profiles are highly desirable for designing the first wall of future fusion reactors. In addition to helium from plasma, He also accumulates in all the structural materials exposed to neutrons due to (n, a) nuclear reaction. Helium migration and trapping may lead to swelling, hardening, drastic creep rupture, blistering and surface exfoliation [1]. To ensure longevity of the structural nuclear materials, it is thus of paramount importance to better understand He diffusion mechanisms as well as to describe gas distribution below the surface. Different analytical techniques were used to determine He depth ranges in solids. The most popular ones such as nuclear reaction analysis (NRA) and elastic recoil method (ERDA) need energetic ion beams [2,3]. They thus imply the use of rather large accelerators. Moreover, the experimentally obtained signals have to be mathematically treated [2,4]. Only very few studies proposed secondary ions mass spectrometry (SIMS) as He depth profiling method. This is mainly due to the really high first ionization potential of helium (25 eV) leading to a very small He+ secondary ion intensity. Nevertheless, some attempts have been made in the past to measure He implantation profiles by SIMS. Wilson et al. reported depth distributions in semiconductor systems such as Si, GaAs, HdCdTe and CdTe under Oþ 2 bombardment. However, the use of a very high Oþ 2 ion current densities (35–40 mA/cm2) resulted in a high sample sputtering rate (>10 nm/s) and consequently deteriorated the accuracy of measurement [5,6]. More recently, Tyagi et al. reported the measurement of He distribution in a metallic alloy Al60Mn40 by monitoring CsHe+ molecular ions under Cs+ primary ion bombardment [7]. They obtained a He profile with reasonable detection ⇑ Corresponding author. Tel.: +33 (0) 1 69 08 43 51; fax: +33 (0) 1 69 08 68 67. E-mail address: [email protected] (H. Lefaix-Jeuland). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.11.003
limits (100 ppm) and low erosion rates (0.5 nm/s). To our knowledge, it is the only SIMS study dealing with helium distribution in metals which, in contrast to semiconductors, may pose problems due to different electronic transfer properties. This letter confirms that He can be profiled by SIMS in metallic systems by using CsHe+ molecular ion detection instead of direct sputtered ions. Semi-quantitative analysis derived from the hereby used implantation conditions are in agreement with expected data from TRIM simulation. Pure iron samples with (1 0 0) monocrystalline or polycrystalline structure, respectively Fe-MC and Fe-PC, were used in this study. All implantations were carried out with IRMA ion implantor at the CSNSM (Orsay–France). Mirror-polished specimens were implanted with 60 keV 4He+ beam at room temperature. The doses for Fe-PC samples were 1 1016 and 1 1017 He/cm2 at a flux of few 1012 He/cm2/s. Fe-MC was implanted only at 1 1016 He/cm2. Polycrystalline samples were implanted at normal incidence whereas monocrystalline ones with an incident angle of 7° to avoid channelling effect. The helium profile in the so-implanted iron samples was measured using a CAMECA IMS 4f Secondary Ions Mass Spectrometer (SIMS) at GEMaC (Meudon-France). A primary Cs+ ion beam (10 keV, 40 nA) was scanned over a (150 150) lm2 surface. The incidence angle between the primary beam and the surface was 46°. The impact energy used was +5500 eV and the sample held in a high positive potential (+4500 V). Moreover all SIMS parameters were adjusted to improve the instrument transmission. The analysed zone was limited to a 30 lm diameter circle centred on the swept area in order to avoid crater edge effects. In the hereby experimental conditions, the mass resolution was low (M/DM = 300) but sufficient to discriminate the interferences between analysed molecular ions and other atomic ions. Several
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H. Lefaix-Jeuland et al. / Nuclear Instruments and Methods in Physics Research B 295 (2013) 69–71
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Fig. 1. Secondary ion yield from 58Fe matrix and 4He ions during SIMS analysis for Fe samples implanted at 60 keV. The same marker was used to plot signals of molecular ions coming from the same specimen, empty markers for Cs58Fe+ and full ones for Cs4He+: triangles for Fe-MC implanted at 1 1016 ions/cm2 fluence; circles for Fe-PC implanted at 1 1016 ions/cm2 fluence and squares for Fe-PC implanted at 1 1017 ions/cm2 fluence.
Fig. 2. Depth distribution of He implanted in Fe samples at 60 keV. Markers were used to plot experimental Cs4He+ signals: circles for Fe-MC implanted at 1 1016 ions/cm2 fluence; triangles for Fe-PC implanted at 1 1016 ions/cm2 fluence and squares for Fe-PC implanted at 1 1017 ions/cm2 fluence. Solid curves were obtained using the TRIM code for both fluences.
depth profiling analyses recording Cs4He+ and Cs58Fe+ molecular ions were performed on Fe samples and the results were quite reproducible. In the following, representative results will be presented for an improved readability. After the analysis, the average crater depth was measured with a profilometer (DEKTAK 8) and related to the analysis time in order to obtain the sputtering rate. From crater depth measurement, we calculated the specimen sputtering rate (0.8 nm/s) and converted signal versus time to signal versus depth. Fig. 1 shows SIMS depth profiles obtained for He-implanted Fe samples. All matrix signals (Cs58Fe+ molecular ions) were equivalent and homogeneous all along the erosion time. Small differences appear at the first sub-surface depths. They could be ascribed to the transient sputtering process in the first atomic layers since no blister nor particular morphology were observed on the surface by SEM analyses. Furthermore, as a preliminary remark, it is worth mentioning that the detected molecular ion intensities are consistent with the implanted He fluences. Quantitative SIMS analyses usually require standard implanted materials from which it is possible to deduce a relative sensitive factor (RSF). Because ion yields depend on the analysed element, the sputtering species, and the sample matrix, separate RSF must be calculated for each using the equation (Eq. (1)):
results are really encouraging considering the extremely difficult He ion detection by SIMS. Moreover, the low sputtering rate hereby obtained ensures a better statistic of ion molecular created than former He depth profiling carried out in semiconductors [5]. From SIMS helium distributions, we determined the maximum of helium concentration Cm and the projected range Rp. The same Cm and Rp values are obtained for Fe-MC and Fe-PC implanted at 1 1016 He/cm2 corresponding respectively to 6 1020 at/cm3 and 220 nm. For Fe-PC implanted at 1 1017 He/cm2, Cm and Rp values are respectively 5.6 1021 at/cm3 and 225 nm. Assuming no appreciable diffusion takes place during or just after implantation, experimental data can be compared to the corresponding profile calculated by the Monte–Carlo range code TRIM. The modelling depth profile is plotted in Fig. 2. The same characteristic Cm and Rp values have been estimated at 6.6 1020 at/cm3 (6.3 1021 at/cm3 for the highest fluence) and 206 nm, respectively. Concerning the general shape of He implantation, TRIM modelling is in perfect accordance with the experimental He profiles at the projected range. Few discrepancies are observed at large depths and widths seem to be slightly larger for experimental depth distribution than in the simulated ones. Although SIMS analyses could contribute to a slight modification of implanted He profile, two hypothesis directly in relation with materials could be advanced. First, this observation could evidence a small interstitial diffusion of helium during implantation (samples were stored in a cryogenic tank avoiding any diffusion between implantation and analyses). The second assumption to explain these small differences is the effect of microstructure. TRIM does not take into account the structure, neither monocrystalline nor polycrystalline. Predicted profiles can thus deviate from observed ones due to unintentional channelling during implantation inducing an enlargement of He distribution in deeper ranges. Finally, one can notice that helium concentrations calculated in implanted polycrystalline samples by using RSF method correspond to expected depth ranges and concentrations for both fluencies. Even if TEM analyses performed on implanted samples evidenced the presence of nanometric bubbles uniformly and randomly distributed in the structure (Fig. 3), we do not observe artefacts such as fluctuations of matrix count rate or He signal due to pressured objects on the contrary of Bailey et al. who underlined possible artefacts in SIMS analysis due to microcavities or platelets [8]. The hereby used implantation fluencies were probably not enough to form so large bubbles that helium trapped in could be
Imatrix fluence t RSF ¼ R t ; ð 0 Ielement dtÞ Z
ð1Þ
with Imatrix the Cs58Fe+ intensity homogeneous with erosion time, Ielement the Cs4He+ intensity to be integrated all along the erosion time, t the maximum erosion time, Z the crater depth at the end of sputtering and fluence the implanted dose. Thus, by setting the integral of the measured depth profile in Fe-MC equal to its expected implanted fluence (i.e. 1 1016 He/cm2), helium concentrations (at/cm3) were deduced from the measured secondary ion intensities (counts/s). The deduced relative sensitive factor (RSF) of 5.2 1022 at/cm3 was thereafter applied to Fe-PC to achieve the He concentration. Fig. 2 shows quantitative SIMS depth profiles obtained for the three implanted samples. The concentration depth profiles demonstrate a dynamic range better than two decades and a detection limit of about 5 1018 He/ cm3 (60 ppm) in the same range than limits obtained by Wilson et al. for He profiling in semiconductors [5], and one order better than results obtained in Al60Mn40 alloys by Tyagi et al. [7]. These
H. Lefaix-Jeuland et al. / Nuclear Instruments and Methods in Physics Research B 295 (2013) 69–71
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served in the present implantation conditions. Compare to the other usual depth profiling methods such as ion beam techniques, SIMS often provides higher sensibility, wide dynamic range and high depth resolution adding to the fact that the impurity profile can be directly obtained without any mathematical treatment. However, although applied to a limited area, this sputtering technique is destructive since material is progressively eroded by the primary ion beam. Ongoing research is carried out on other pure He-implanted metals as well as on annealed systems. The objectives are now to use SIMS, eventually in association with other techniques, to determine the evolution of helium concentration with temperature as well as the involved diffusion mechanism in materials of nuclear interest. Acknowledgements Fig. 3. Helium bubbles (white contrast) observed by TEM in Fe-PC implanted at 1 1016 He/cm2 in under focus condition. Bubble diameter was estimated at about 0.9 nm.
The authors thank Cyril BACHELET from the Centre de Spectrométrie Nucléaire et Spectrométrie de Masse (CSNSM-Orsay) for helium implantation. References
released without being detected. In the next step of the study, experiments will be carried out after annealing treatment inducing formation of large size bubbles and care will be particularly taken for these analyses. To conclude, this study highlighted the possible use of SIMS to depth profile implanted helium in the near-surface region of metallic materials. He-implanted Fe samples have been hereby used as examples of systems of nuclear interest. Helium densities greater than 5 1018 He/cm3 have been measured with a good depth resolution and no artefact due to helium bubbles was ob-
[1] [2] [3] [4] [5] [6] [7] [8]
H. Ullmaier, Radiat. Eff. 78 (1983) 1–10. J. Bøttiger, J. Nucl. Mater. 78 (1978) 161–181. M.B. Lewis, W.R. Allen, Nucl. Instrum. Methods Phys. Res. B 35 (1988) 10–16. M.B. Lewis, Nucl. Instrum. Methods 190 (1981) 605–611. R.G. Wilson, V.R. Delin, C.G. Hopkins, Appl. Phys. Lett. 41 (1982) 929–931. R.G. Wilson, Appl. Phys. Lett. 45 (1984) 107–109. A.K. Tyagi, K.G.M. Nair, K. Krishan, Bull. Mater. Sci. 16 (1993) 45–49. M.J. Bailey, C. Jeynes, B.J. Sealy, R.P. Webb, R.M. Gwilliam, Nucl. Instrum. Methods Phys. Rev. B 268 (2010) 2051–2055.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Short Communication
SIMS depth profiling of implanted helium in pure iron using CsHe+ detection mode H. Lefaix-Jeuland a,⇑, S. Moll a, F. Legendre a, F. Jomard b a b
CEA, DEN, Service De Recherches De Métallurgie Physique, F-91191 Gif-Sur-Yvette, France Groupe d’Etude de la Matière Condensée (CNRS and University of Versailles Saint Quentin), 45 avenue des Etats-Unis, 78035 Versailles cedex, France
a r t i c l e
i n f o
Article history: Received 7 May 2012 Received in revised form 19 October 2012 Available online 3 December 2012 Keywords: SIMS Ion implantation Helium Depth profiling
a b s t r a c t Helium distribution in implanted monocrystalline and polycrystalline Fe samples has been measured by secondary ion mass spectrometry (SIMS). The use of Cs+ primary ions in conjunction with the detection of CsHe+ molecular ions was shown to be an efficient method to overcome the very high first ionization potential of helium. The implantation ranges of 60 keV He ions in samples are measured about 220 nm in agreement with projected ranges calculated by TRIM. He concentrations at or above 5 1018 at/cm3 (60 ppm) were measured. This study confirms the paramount interest of SIMS as a direct He depth profiling technique. Ó 2012 Elsevier B.V. All rights reserved.
Reliable He depth profiles are highly desirable for designing the first wall of future fusion reactors. In addition to helium from plasma, He also accumulates in all the structural materials exposed to neutrons due to (n, a) nuclear reaction. Helium migration and trapping may lead to swelling, hardening, drastic creep rupture, blistering and surface exfoliation [1]. To ensure longevity of the structural nuclear materials, it is thus of paramount importance to better understand He diffusion mechanisms as well as to describe gas distribution below the surface. Different analytical techniques were used to determine He depth ranges in solids. The most popular ones such as nuclear reaction analysis (NRA) and elastic recoil method (ERDA) need energetic ion beams [2,3]. They thus imply the use of rather large accelerators. Moreover, the experimentally obtained signals have to be mathematically treated [2,4]. Only very few studies proposed secondary ions mass spectrometry (SIMS) as He depth profiling method. This is mainly due to the really high first ionization potential of helium (25 eV) leading to a very small He+ secondary ion intensity. Nevertheless, some attempts have been made in the past to measure He implantation profiles by SIMS. Wilson et al. reported depth distributions in semiconductor systems such as Si, GaAs, HdCdTe and CdTe under Oþ 2 bombardment. However, the use of a very high Oþ 2 ion current densities (35–40 mA/cm2) resulted in a high sample sputtering rate (>10 nm/s) and consequently deteriorated the accuracy of measurement [5,6]. More recently, Tyagi et al. reported the measurement of He distribution in a metallic alloy Al60Mn40 by monitoring CsHe+ molecular ions under Cs+ primary ion bombardment [7]. They obtained a He profile with reasonable detection ⇑ Corresponding author. Tel.: +33 (0) 1 69 08 43 51; fax: +33 (0) 1 69 08 68 67. E-mail address: [email protected] (H. Lefaix-Jeuland). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.11.003
limits (100 ppm) and low erosion rates (0.5 nm/s). To our knowledge, it is the only SIMS study dealing with helium distribution in metals which, in contrast to semiconductors, may pose problems due to different electronic transfer properties. This letter confirms that He can be profiled by SIMS in metallic systems by using CsHe+ molecular ion detection instead of direct sputtered ions. Semi-quantitative analysis derived from the hereby used implantation conditions are in agreement with expected data from TRIM simulation. Pure iron samples with (1 0 0) monocrystalline or polycrystalline structure, respectively Fe-MC and Fe-PC, were used in this study. All implantations were carried out with IRMA ion implantor at the CSNSM (Orsay–France). Mirror-polished specimens were implanted with 60 keV 4He+ beam at room temperature. The doses for Fe-PC samples were 1 1016 and 1 1017 He/cm2 at a flux of few 1012 He/cm2/s. Fe-MC was implanted only at 1 1016 He/cm2. Polycrystalline samples were implanted at normal incidence whereas monocrystalline ones with an incident angle of 7° to avoid channelling effect. The helium profile in the so-implanted iron samples was measured using a CAMECA IMS 4f Secondary Ions Mass Spectrometer (SIMS) at GEMaC (Meudon-France). A primary Cs+ ion beam (10 keV, 40 nA) was scanned over a (150 150) lm2 surface. The incidence angle between the primary beam and the surface was 46°. The impact energy used was +5500 eV and the sample held in a high positive potential (+4500 V). Moreover all SIMS parameters were adjusted to improve the instrument transmission. The analysed zone was limited to a 30 lm diameter circle centred on the swept area in order to avoid crater edge effects. In the hereby experimental conditions, the mass resolution was low (M/DM = 300) but sufficient to discriminate the interferences between analysed molecular ions and other atomic ions. Several
70
H. Lefaix-Jeuland et al. / Nuclear Instruments and Methods in Physics Research B 295 (2013) 69–71
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Fig. 1. Secondary ion yield from 58Fe matrix and 4He ions during SIMS analysis for Fe samples implanted at 60 keV. The same marker was used to plot signals of molecular ions coming from the same specimen, empty markers for Cs58Fe+ and full ones for Cs4He+: triangles for Fe-MC implanted at 1 1016 ions/cm2 fluence; circles for Fe-PC implanted at 1 1016 ions/cm2 fluence and squares for Fe-PC implanted at 1 1017 ions/cm2 fluence.
Fig. 2. Depth distribution of He implanted in Fe samples at 60 keV. Markers were used to plot experimental Cs4He+ signals: circles for Fe-MC implanted at 1 1016 ions/cm2 fluence; triangles for Fe-PC implanted at 1 1016 ions/cm2 fluence and squares for Fe-PC implanted at 1 1017 ions/cm2 fluence. Solid curves were obtained using the TRIM code for both fluences.
depth profiling analyses recording Cs4He+ and Cs58Fe+ molecular ions were performed on Fe samples and the results were quite reproducible. In the following, representative results will be presented for an improved readability. After the analysis, the average crater depth was measured with a profilometer (DEKTAK 8) and related to the analysis time in order to obtain the sputtering rate. From crater depth measurement, we calculated the specimen sputtering rate (0.8 nm/s) and converted signal versus time to signal versus depth. Fig. 1 shows SIMS depth profiles obtained for He-implanted Fe samples. All matrix signals (Cs58Fe+ molecular ions) were equivalent and homogeneous all along the erosion time. Small differences appear at the first sub-surface depths. They could be ascribed to the transient sputtering process in the first atomic layers since no blister nor particular morphology were observed on the surface by SEM analyses. Furthermore, as a preliminary remark, it is worth mentioning that the detected molecular ion intensities are consistent with the implanted He fluences. Quantitative SIMS analyses usually require standard implanted materials from which it is possible to deduce a relative sensitive factor (RSF). Because ion yields depend on the analysed element, the sputtering species, and the sample matrix, separate RSF must be calculated for each using the equation (Eq. (1)):
results are really encouraging considering the extremely difficult He ion detection by SIMS. Moreover, the low sputtering rate hereby obtained ensures a better statistic of ion molecular created than former He depth profiling carried out in semiconductors [5]. From SIMS helium distributions, we determined the maximum of helium concentration Cm and the projected range Rp. The same Cm and Rp values are obtained for Fe-MC and Fe-PC implanted at 1 1016 He/cm2 corresponding respectively to 6 1020 at/cm3 and 220 nm. For Fe-PC implanted at 1 1017 He/cm2, Cm and Rp values are respectively 5.6 1021 at/cm3 and 225 nm. Assuming no appreciable diffusion takes place during or just after implantation, experimental data can be compared to the corresponding profile calculated by the Monte–Carlo range code TRIM. The modelling depth profile is plotted in Fig. 2. The same characteristic Cm and Rp values have been estimated at 6.6 1020 at/cm3 (6.3 1021 at/cm3 for the highest fluence) and 206 nm, respectively. Concerning the general shape of He implantation, TRIM modelling is in perfect accordance with the experimental He profiles at the projected range. Few discrepancies are observed at large depths and widths seem to be slightly larger for experimental depth distribution than in the simulated ones. Although SIMS analyses could contribute to a slight modification of implanted He profile, two hypothesis directly in relation with materials could be advanced. First, this observation could evidence a small interstitial diffusion of helium during implantation (samples were stored in a cryogenic tank avoiding any diffusion between implantation and analyses). The second assumption to explain these small differences is the effect of microstructure. TRIM does not take into account the structure, neither monocrystalline nor polycrystalline. Predicted profiles can thus deviate from observed ones due to unintentional channelling during implantation inducing an enlargement of He distribution in deeper ranges. Finally, one can notice that helium concentrations calculated in implanted polycrystalline samples by using RSF method correspond to expected depth ranges and concentrations for both fluencies. Even if TEM analyses performed on implanted samples evidenced the presence of nanometric bubbles uniformly and randomly distributed in the structure (Fig. 3), we do not observe artefacts such as fluctuations of matrix count rate or He signal due to pressured objects on the contrary of Bailey et al. who underlined possible artefacts in SIMS analysis due to microcavities or platelets [8]. The hereby used implantation fluencies were probably not enough to form so large bubbles that helium trapped in could be
Imatrix fluence t RSF ¼ R t ; ð 0 Ielement dtÞ Z
ð1Þ
with Imatrix the Cs58Fe+ intensity homogeneous with erosion time, Ielement the Cs4He+ intensity to be integrated all along the erosion time, t the maximum erosion time, Z the crater depth at the end of sputtering and fluence the implanted dose. Thus, by setting the integral of the measured depth profile in Fe-MC equal to its expected implanted fluence (i.e. 1 1016 He/cm2), helium concentrations (at/cm3) were deduced from the measured secondary ion intensities (counts/s). The deduced relative sensitive factor (RSF) of 5.2 1022 at/cm3 was thereafter applied to Fe-PC to achieve the He concentration. Fig. 2 shows quantitative SIMS depth profiles obtained for the three implanted samples. The concentration depth profiles demonstrate a dynamic range better than two decades and a detection limit of about 5 1018 He/ cm3 (60 ppm) in the same range than limits obtained by Wilson et al. for He profiling in semiconductors [5], and one order better than results obtained in Al60Mn40 alloys by Tyagi et al. [7]. These
H. Lefaix-Jeuland et al. / Nuclear Instruments and Methods in Physics Research B 295 (2013) 69–71
71
served in the present implantation conditions. Compare to the other usual depth profiling methods such as ion beam techniques, SIMS often provides higher sensibility, wide dynamic range and high depth resolution adding to the fact that the impurity profile can be directly obtained without any mathematical treatment. However, although applied to a limited area, this sputtering technique is destructive since material is progressively eroded by the primary ion beam. Ongoing research is carried out on other pure He-implanted metals as well as on annealed systems. The objectives are now to use SIMS, eventually in association with other techniques, to determine the evolution of helium concentration with temperature as well as the involved diffusion mechanism in materials of nuclear interest. Acknowledgements Fig. 3. Helium bubbles (white contrast) observed by TEM in Fe-PC implanted at 1 1016 He/cm2 in under focus condition. Bubble diameter was estimated at about 0.9 nm.
The authors thank Cyril BACHELET from the Centre de Spectrométrie Nucléaire et Spectrométrie de Masse (CSNSM-Orsay) for helium implantation. References
released without being detected. In the next step of the study, experiments will be carried out after annealing treatment inducing formation of large size bubbles and care will be particularly taken for these analyses. To conclude, this study highlighted the possible use of SIMS to depth profile implanted helium in the near-surface region of metallic materials. He-implanted Fe samples have been hereby used as examples of systems of nuclear interest. Helium densities greater than 5 1018 He/cm3 have been measured with a good depth resolution and no artefact due to helium bubbles was ob-
[1] [2] [3] [4] [5] [6] [7] [8]
H. Ullmaier, Radiat. Eff. 78 (1983) 1–10. J. Bøttiger, J. Nucl. Mater. 78 (1978) 161–181. M.B. Lewis, W.R. Allen, Nucl. Instrum. Methods Phys. Res. B 35 (1988) 10–16. M.B. Lewis, Nucl. Instrum. Methods 190 (1981) 605–611. R.G. Wilson, V.R. Delin, C.G. Hopkins, Appl. Phys. Lett. 41 (1982) 929–931. R.G. Wilson, Appl. Phys. Lett. 45 (1984) 107–109. A.K. Tyagi, K.G.M. Nair, K. Krishan, Bull. Mater. Sci. 16 (1993) 45–49. M.J. Bailey, C. Jeynes, B.J. Sealy, R.P. Webb, R.M. Gwilliam, Nucl. Instrum. Methods Phys. Rev. B 268 (2010) 2051–2055.