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Positron and deuteron depth profiling in helium-3-implanted electrum-like alloy
Applied Surface Science 252 (2006) 3252–3255 www.elsevier.com/locate/apsusc
Positron and deuteron depth profiling in helium-3-implanted electrum-like alloy R.I. Grynszpan a,b,*, N. Baclet c, A. Darque c, J.L. Flament d, F. Zielinski d, W. Anwand e, G. Brauer e a
Lab. de Chimie Me´tallurgique des Terres Rares, UPR-CNRS 209, ISCSA, F-94320 Thiais, France b Positive Leptons Spectroscopy Cell, DGA/CEP/CGN/LOT, Department of Defense, F-94114 Arcueil, France c CEA/VA/DRMN/SEMP, F-21120 Is-sur-Tille, France d CEA/DIF/DPTA/SP2A, BP 12, F-91680 Bruye`res-le-Chaˆtel, France e Forschungszentrum Rossendorf, Postfach 510119, 013114 Dresden, Germany Available online 24 October 2005
Abstract In spite of previous extensive studies, the helium behavior in metals still remains an issue in microelectronics as well as in nuclear technology. A gold–silver solid solution (Au60Ag40: synthetic gold-rich electrum) was chosen as a relevant model to study helium irradiation of heavy metals. After helium-3 ion implantation at an energy ranging from 4.2 to 5.6 MeV, nuclear reaction analysis (NRA) based on the 3He(d,p)4He reaction, was performed in order to study the thermal diffusion of helium atoms. At room temperature, NRA data reveal that a single Gaussian can fit the He-distribution, which remains unchanged after annealing at temperatures below 0.45 of the melting point. Slow positron implantation spectroscopy, used to monitor the fluence dependence of induced defects unveils a positron saturation trapping, which occurs for He contents of the order of 50–100 appm, whereas concentrations larger than 500 appm seem to favor an increase in the S-parameter of Doppler broadening. Moreover, at high temperature, NRA results clearly show that helium long range diffusion occurs, though, without following a simple Fick law. # 2005 Elsevier B.V. All rights reserved. PACS: 61.72Ww; 61.80.Jh; 78.70Bj Keywords: Ion-implantation; Gold; Silver; Helium; Positrons; NRA
1. Introduction * Corresponding author. Fax: +33 1 49 78 12 03. E-mail address: [email protected] (R.I. Grynszpan).
Helium ion-implantation is a common procedure to address the issue of radiation damages induced by a-decay, in particular in radioactive materials [1].
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.084
R.I. Grynszpan et al. / Applied Surface Science 252 (2006) 3252–3255
It allows access to high fluence, hence simulating a time contraction in long lasting processes involved in nuclear waste storage, for instance. In order to predict the alteration of in-service properties of such materials, one needs to monitor the production, the subsequent behavior and the interaction of both open-volume defects and helium atoms, and hence to understand the basic mechanisms of the damaging process. In this work, we therefore opted for positron and deuteron probing by implementing slow positron implantation spectroscopy (SPIS) [2] and nuclear reaction analysis (NRA) [3], respectively, the latter technique compelling to resort to irradiation with the light helium-3 isotope. We focused our investigation on an electrum-like solid solution Au– 40 at% Ag (designated hereafter as Au60Ag40). Such a ‘‘model’’ alloy has a low melting point (Tm = 1313 K); hence, it allows an easy access to the range of temperatures encompassing the recovery stages dealing with vacancy migration and annealing. Moreover, its density (15.8 g cm3) also insures that implantation depths lie in the same range as those, which are achievable for instance in actinide alloys.
2. Material and experimental procedures Silver pieces embedded in gold were melted in an induction-furnace. Alloy concentration and homogeneity were checked via PIXE, XRD and electron microprobe. After cold-rolling down to 35 mm, 12 mm diameter discs were cut and annealed at 1120 K for 8 h in vacuum. Samples were subsequently irradiated at the CEADPTA facility with 3He-ions at fluences ranging from 0.17 to 5.2 1015 cm2. Implantation was carried out either by decreasing the energy in steps of 0.2 MeV, from 5.6 to 4.2 (sample series-1) or at a single-energy (5.2 MeV) (series-2). Defect depth profiling by SPIS on sample series-1 could only be achieved via Doppler broadening (DB), owing to the continuous time structure of the positron beam used at FZR. Its incident energy (Ee+) ranges from 0.03 to 35 keV. Other relevant experimental conditions defining conventional DB-parameters are described elsewhere [4,5]. At the DPTA facility, we complemented these measurements with deuteron depth profiling (NRA),
3253
based on the 3He(d,p)4He reaction. Procedures and experimental details are given in ref. [6]. Whereas the sample surface was always directly exposed to the positron beam during DB analyses, a foil of havar (Co-based alloy; density: 8.3 g cm3) was, sometimes inserted in front of the sample, in order to adjust the penetration depth of either helium or deuteron.
3. Results and discussion 3.1. Fluence dependence Room temperature SPIS studies of Au60Ag40 samples irradiated with 300 keV 4He-ions have already been reported [4]. A concentration maximum of vacancy-type defects is detected at about 69% of the ion projected-range (Rp = 610 nm), i.e., off-centered by more than one unit-range of ion-straggling (160 nm) from the vacancy distribution generated by the TRIM code [7]. This ‘‘Rp/2 effect’’ [8] seems to be a rather common feature observed in positron depth profiling. It can be considered as an artifact of singleenergy implantation, and therefore casts some doubts about the relevance of this procedure to model bulk phenomena such as, for instance, self-a-irradiation in actinides [9]. Our present multiple-energy implantation, though with 3He, constitutes a more realistic approach. Moreover, the energy absorber intercalated in the beam, hinders the Rp/2 effect in the Au60Ag40 part of the target. Indeed, according to TRIM calculations [7] (using a displacement energy of 25 eV for all metal atoms), this effect should mostly occur within the havar foil and should not affect more than 0.5 mm of the gold–silver surface layer at 5.6 MeV. Hence, in Fig. 1, which displays the fluence dependence of SPIS depth profiling, it is valid to focus on data collected for the uttermost positron energy, i.e., at hzi 700 nm. The relative change (DS/S) versus helium concentration (derived from the fluence value within a surface layer of 4.5 mm) unveils a saturation behavior at about 100 appm He, which seems to persist up to 500 appm. This is likely due to defect stabilization by helium, as vacancy clusters will host these atoms and favor nucleation and swelling of bubbles. As long as the internal pressure in these
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Fig. 2. Deuteron energy dependence of the number of high-energy protons resulting from the 3He(d,p)4He reaction, for three different samples (series-2) irradiated at the same fluence and annealed at increasing temperatures. Solid line: smoothed curve of average data for as-implanted samples. Fig. 1. S-parameter dependence on positron mean depth hzi (expressed as: hzi (nm) = 2.28 [Ee+ (keV)]1.62, see ref. [5]) for various (multiple-energy) fluences. Inset: DS/S vs. mean concentration of helium [He] measured at 35 keV. Points at 100 appm are extrapolated from previous measurements [4]. Lines (solid or dashed) are only eye guides.
bubble is high enough (>1 GPa) [10], positron trapping may be impeded. Since DS/S more than doubles its value for 1000 appm, we infer that heavier irradiation may produce much larger open-volume defects, which will be more trapping effective for positrons.
single energy a-particles [4], the temperature dependence of the Doppler broadening R2g-parameter [13] (or jDS/DWj, where DS and DW are changes induced by implantation) clearly indicates that a new type of defect is acting above 0.45Tm (Fig. 3). Since low temperature isochronal annealing was ineffective, we resorted to isothermal annealing at higher temperature (1050 K or 0.8Tm) to check for Hediffusion with NRA (Fig. 4). Here again, a single
3.2. Annealing and helium effusion Examples of nuclear reaction analyses of series2 samples, single-energy implanted with 5.1 1015 helium-ions/cm2, are presented in Fig. 2. In the asirradiated state, deuteron excitation curves can be adequately deconvoluted, assuming a single Gaussian distribution for the helium profile [6]. After annealing for 30 min at increasing temperatures below 600 K, the shape of the helium distribution remains practically unchanged, within the measurement precision. Such thermal stability of the helium profile is in good agreement with previous results suggesting that temperature induced diffusion and growth of helium bubbles only start at around 0.45Tm [11,12]. Indeed, after irradiation with multiple or
Fig. 3. Annealing temperature dependence of the R2g-parameter after a-irradiation (100 appm) with multi-energy or single-energy ions (measured at Rp 700 nm). Values are normalized with respect to room temperature data. Straight line fit: R2g = 0.233 + 1.55T/Tm.
R.I. Grynszpan et al. / Applied Surface Science 252 (2006) 3252–3255
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4. Conclusion
Fig. 4. Annealing time dependence (at 0.8Tm) of a single-Gaussian helium profile with initial concentration 1000 appm. D[He]: helium concentration loss. DXc: centroid shift towards the surface. [D(s 2G )]1/2: profile broadening and relevant linear regression (dashed line) corresponding to a square root time scale. Solid lines are eye guides.
Gaussian fit is the best choice to resolve the resulting excitation curves. The Gaussian variance (s2) seems to follow a linear time (t) dependence, which, according to a simple Fick law, is related to the diffusion coefficient D(T) at the given temperature T: Dðs 2G Þ
2
2
¼ sðt; TÞ s ðt ¼ 0; TÞ ¼ DðTÞt
(1)
A 0.8Tm, the above expression of the variance shift yields: D1050 K = (4.2 2) 1010 cm2/s. This is a rather low value, which indicates that helium is hardly diffusing even at such high temperature. However, it is questionable to apply such a simple law to the present NRA results since the Gaussian profile area, representing the total helium concentration, [He], decreases by 30% after 8 h. Indeed, helium effusion takes place, most likely by a bubble-breaking away process. As annealing time increases, the profile becomes more asymmetrical as shown by the shift of the Gaussian centroid. It has already been shown that for larger times (100 h), a two-Gaussian fit is applicable, which accounts for a ‘‘free’’ diffusing part of helium, while another stabilized part remains centered at the initial implantation location [6].
We implemented both the SPIS and the NRA techniques to check for the behavior of open-volume defects and helium diffusion, respectively. Positron probing unveils a two-stage fluence dependence of the defect concentration. On the other hand, NRA shows that helium profiles – adequately resolved by a single Gaussian distribution – are thermally stable below 0.45Tm, as suggested by Doppler broadening. At higher temperature, helium profiles become asymmetrical, suggesting that effusion takes place, but not according to a simple Fick law.
Acknowledgements The authors express their gratitude to FZR and DPTA staffs, and to Dr. V. Lalanne (LCMTR-CNRS) for assistance.
References [1] H. Ullmaier, Rad. Effects 78 (1983) 1. [2] P.G. Coleman (Ed.), Positron Beams & Their Applications, World Scientific, Singapore, 2000, pp. 1–325. [3] G. Amsel, J.P. Nadai, E. Artemare, D. David, E. Girard, Nucl. Instrum. Methods B 92 (1971) 481. [4] T. Thome´, J. Fradin, R.I. Grynszpan, W. Anwand, G. Brauer, Nucl. Instrum. Methods B 178 (2001) 342. [5] S. Saude´, R.I. Grynszpan, W. Anwand, G. Brauer, J. Alloys Compd. 382 (2004) 252. [6] J.L. Flament, F. Zielinski, S. Saude´, R.I. Grynszpan, Nucl. Instrum. Methods B 216 (2004) 161. [7] J.F. Ziegler, et al., http://www.SRIM.org. [8] P.G. Coleman, C.P. Burrows, A.P. Knights, Appl. Phys. Lett. 80 (2002) 947. [9] N. Baclet, M. Dormeval, P. Pochet, J.M. Fournier, F. Wastin, E. Colineau, J. Rebizant, G.H. Lander, J. Nucl. Sci. Technol. 3 (2002) 148. [10] R.A. Hakvoort, A. van Veen, P.E. Mijnarends, H. Schut, Appl. Surf. Sci. 85 (1995) 271. [11] B. Viswanathan, G. Ko¨gel, A. Seeger, Rad. Effects 78 (1983) 231. [12] H.E. Hansen, H. Rajainmaki, R. Talja, M.D. Bentzon, R.M. Nieminen, K. Petersen, J. Phys. F: Met. Phys. 15 (1985) 1. [13] W. Trifstha¨user, Phys. Rev. B 18 (1975) 4634.
Positron and deuteron depth profiling in helium-3-implanted electrum-like alloy R.I. Grynszpan a,b,*, N. Baclet c, A. Darque c, J.L. Flament d, F. Zielinski d, W. Anwand e, G. Brauer e a
Lab. de Chimie Me´tallurgique des Terres Rares, UPR-CNRS 209, ISCSA, F-94320 Thiais, France b Positive Leptons Spectroscopy Cell, DGA/CEP/CGN/LOT, Department of Defense, F-94114 Arcueil, France c CEA/VA/DRMN/SEMP, F-21120 Is-sur-Tille, France d CEA/DIF/DPTA/SP2A, BP 12, F-91680 Bruye`res-le-Chaˆtel, France e Forschungszentrum Rossendorf, Postfach 510119, 013114 Dresden, Germany Available online 24 October 2005
Abstract In spite of previous extensive studies, the helium behavior in metals still remains an issue in microelectronics as well as in nuclear technology. A gold–silver solid solution (Au60Ag40: synthetic gold-rich electrum) was chosen as a relevant model to study helium irradiation of heavy metals. After helium-3 ion implantation at an energy ranging from 4.2 to 5.6 MeV, nuclear reaction analysis (NRA) based on the 3He(d,p)4He reaction, was performed in order to study the thermal diffusion of helium atoms. At room temperature, NRA data reveal that a single Gaussian can fit the He-distribution, which remains unchanged after annealing at temperatures below 0.45 of the melting point. Slow positron implantation spectroscopy, used to monitor the fluence dependence of induced defects unveils a positron saturation trapping, which occurs for He contents of the order of 50–100 appm, whereas concentrations larger than 500 appm seem to favor an increase in the S-parameter of Doppler broadening. Moreover, at high temperature, NRA results clearly show that helium long range diffusion occurs, though, without following a simple Fick law. # 2005 Elsevier B.V. All rights reserved. PACS: 61.72Ww; 61.80.Jh; 78.70Bj Keywords: Ion-implantation; Gold; Silver; Helium; Positrons; NRA
1. Introduction * Corresponding author. Fax: +33 1 49 78 12 03. E-mail address: [email protected] (R.I. Grynszpan).
Helium ion-implantation is a common procedure to address the issue of radiation damages induced by a-decay, in particular in radioactive materials [1].
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.084
R.I. Grynszpan et al. / Applied Surface Science 252 (2006) 3252–3255
It allows access to high fluence, hence simulating a time contraction in long lasting processes involved in nuclear waste storage, for instance. In order to predict the alteration of in-service properties of such materials, one needs to monitor the production, the subsequent behavior and the interaction of both open-volume defects and helium atoms, and hence to understand the basic mechanisms of the damaging process. In this work, we therefore opted for positron and deuteron probing by implementing slow positron implantation spectroscopy (SPIS) [2] and nuclear reaction analysis (NRA) [3], respectively, the latter technique compelling to resort to irradiation with the light helium-3 isotope. We focused our investigation on an electrum-like solid solution Au– 40 at% Ag (designated hereafter as Au60Ag40). Such a ‘‘model’’ alloy has a low melting point (Tm = 1313 K); hence, it allows an easy access to the range of temperatures encompassing the recovery stages dealing with vacancy migration and annealing. Moreover, its density (15.8 g cm3) also insures that implantation depths lie in the same range as those, which are achievable for instance in actinide alloys.
2. Material and experimental procedures Silver pieces embedded in gold were melted in an induction-furnace. Alloy concentration and homogeneity were checked via PIXE, XRD and electron microprobe. After cold-rolling down to 35 mm, 12 mm diameter discs were cut and annealed at 1120 K for 8 h in vacuum. Samples were subsequently irradiated at the CEADPTA facility with 3He-ions at fluences ranging from 0.17 to 5.2 1015 cm2. Implantation was carried out either by decreasing the energy in steps of 0.2 MeV, from 5.6 to 4.2 (sample series-1) or at a single-energy (5.2 MeV) (series-2). Defect depth profiling by SPIS on sample series-1 could only be achieved via Doppler broadening (DB), owing to the continuous time structure of the positron beam used at FZR. Its incident energy (Ee+) ranges from 0.03 to 35 keV. Other relevant experimental conditions defining conventional DB-parameters are described elsewhere [4,5]. At the DPTA facility, we complemented these measurements with deuteron depth profiling (NRA),
3253
based on the 3He(d,p)4He reaction. Procedures and experimental details are given in ref. [6]. Whereas the sample surface was always directly exposed to the positron beam during DB analyses, a foil of havar (Co-based alloy; density: 8.3 g cm3) was, sometimes inserted in front of the sample, in order to adjust the penetration depth of either helium or deuteron.
3. Results and discussion 3.1. Fluence dependence Room temperature SPIS studies of Au60Ag40 samples irradiated with 300 keV 4He-ions have already been reported [4]. A concentration maximum of vacancy-type defects is detected at about 69% of the ion projected-range (Rp = 610 nm), i.e., off-centered by more than one unit-range of ion-straggling (160 nm) from the vacancy distribution generated by the TRIM code [7]. This ‘‘Rp/2 effect’’ [8] seems to be a rather common feature observed in positron depth profiling. It can be considered as an artifact of singleenergy implantation, and therefore casts some doubts about the relevance of this procedure to model bulk phenomena such as, for instance, self-a-irradiation in actinides [9]. Our present multiple-energy implantation, though with 3He, constitutes a more realistic approach. Moreover, the energy absorber intercalated in the beam, hinders the Rp/2 effect in the Au60Ag40 part of the target. Indeed, according to TRIM calculations [7] (using a displacement energy of 25 eV for all metal atoms), this effect should mostly occur within the havar foil and should not affect more than 0.5 mm of the gold–silver surface layer at 5.6 MeV. Hence, in Fig. 1, which displays the fluence dependence of SPIS depth profiling, it is valid to focus on data collected for the uttermost positron energy, i.e., at hzi 700 nm. The relative change (DS/S) versus helium concentration (derived from the fluence value within a surface layer of 4.5 mm) unveils a saturation behavior at about 100 appm He, which seems to persist up to 500 appm. This is likely due to defect stabilization by helium, as vacancy clusters will host these atoms and favor nucleation and swelling of bubbles. As long as the internal pressure in these
3254
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Fig. 2. Deuteron energy dependence of the number of high-energy protons resulting from the 3He(d,p)4He reaction, for three different samples (series-2) irradiated at the same fluence and annealed at increasing temperatures. Solid line: smoothed curve of average data for as-implanted samples. Fig. 1. S-parameter dependence on positron mean depth hzi (expressed as: hzi (nm) = 2.28 [Ee+ (keV)]1.62, see ref. [5]) for various (multiple-energy) fluences. Inset: DS/S vs. mean concentration of helium [He] measured at 35 keV. Points at 100 appm are extrapolated from previous measurements [4]. Lines (solid or dashed) are only eye guides.
bubble is high enough (>1 GPa) [10], positron trapping may be impeded. Since DS/S more than doubles its value for 1000 appm, we infer that heavier irradiation may produce much larger open-volume defects, which will be more trapping effective for positrons.
single energy a-particles [4], the temperature dependence of the Doppler broadening R2g-parameter [13] (or jDS/DWj, where DS and DW are changes induced by implantation) clearly indicates that a new type of defect is acting above 0.45Tm (Fig. 3). Since low temperature isochronal annealing was ineffective, we resorted to isothermal annealing at higher temperature (1050 K or 0.8Tm) to check for Hediffusion with NRA (Fig. 4). Here again, a single
3.2. Annealing and helium effusion Examples of nuclear reaction analyses of series2 samples, single-energy implanted with 5.1 1015 helium-ions/cm2, are presented in Fig. 2. In the asirradiated state, deuteron excitation curves can be adequately deconvoluted, assuming a single Gaussian distribution for the helium profile [6]. After annealing for 30 min at increasing temperatures below 600 K, the shape of the helium distribution remains practically unchanged, within the measurement precision. Such thermal stability of the helium profile is in good agreement with previous results suggesting that temperature induced diffusion and growth of helium bubbles only start at around 0.45Tm [11,12]. Indeed, after irradiation with multiple or
Fig. 3. Annealing temperature dependence of the R2g-parameter after a-irradiation (100 appm) with multi-energy or single-energy ions (measured at Rp 700 nm). Values are normalized with respect to room temperature data. Straight line fit: R2g = 0.233 + 1.55T/Tm.
R.I. Grynszpan et al. / Applied Surface Science 252 (2006) 3252–3255
3255
4. Conclusion
Fig. 4. Annealing time dependence (at 0.8Tm) of a single-Gaussian helium profile with initial concentration 1000 appm. D[He]: helium concentration loss. DXc: centroid shift towards the surface. [D(s 2G )]1/2: profile broadening and relevant linear regression (dashed line) corresponding to a square root time scale. Solid lines are eye guides.
Gaussian fit is the best choice to resolve the resulting excitation curves. The Gaussian variance (s2) seems to follow a linear time (t) dependence, which, according to a simple Fick law, is related to the diffusion coefficient D(T) at the given temperature T: Dðs 2G Þ
2
2
¼ sðt; TÞ s ðt ¼ 0; TÞ ¼ DðTÞt
(1)
A 0.8Tm, the above expression of the variance shift yields: D1050 K = (4.2 2) 1010 cm2/s. This is a rather low value, which indicates that helium is hardly diffusing even at such high temperature. However, it is questionable to apply such a simple law to the present NRA results since the Gaussian profile area, representing the total helium concentration, [He], decreases by 30% after 8 h. Indeed, helium effusion takes place, most likely by a bubble-breaking away process. As annealing time increases, the profile becomes more asymmetrical as shown by the shift of the Gaussian centroid. It has already been shown that for larger times (100 h), a two-Gaussian fit is applicable, which accounts for a ‘‘free’’ diffusing part of helium, while another stabilized part remains centered at the initial implantation location [6].
We implemented both the SPIS and the NRA techniques to check for the behavior of open-volume defects and helium diffusion, respectively. Positron probing unveils a two-stage fluence dependence of the defect concentration. On the other hand, NRA shows that helium profiles – adequately resolved by a single Gaussian distribution – are thermally stable below 0.45Tm, as suggested by Doppler broadening. At higher temperature, helium profiles become asymmetrical, suggesting that effusion takes place, but not according to a simple Fick law.
Acknowledgements The authors express their gratitude to FZR and DPTA staffs, and to Dr. V. Lalanne (LCMTR-CNRS) for assistance.
References [1] H. Ullmaier, Rad. Effects 78 (1983) 1. [2] P.G. Coleman (Ed.), Positron Beams & Their Applications, World Scientific, Singapore, 2000, pp. 1–325. [3] G. Amsel, J.P. Nadai, E. Artemare, D. David, E. Girard, Nucl. Instrum. Methods B 92 (1971) 481. [4] T. Thome´, J. Fradin, R.I. Grynszpan, W. Anwand, G. Brauer, Nucl. Instrum. Methods B 178 (2001) 342. [5] S. Saude´, R.I. Grynszpan, W. Anwand, G. Brauer, J. Alloys Compd. 382 (2004) 252. [6] J.L. Flament, F. Zielinski, S. Saude´, R.I. Grynszpan, Nucl. Instrum. Methods B 216 (2004) 161. [7] J.F. Ziegler, et al., http://www.SRIM.org. [8] P.G. Coleman, C.P. Burrows, A.P. Knights, Appl. Phys. Lett. 80 (2002) 947. [9] N. Baclet, M. Dormeval, P. Pochet, J.M. Fournier, F. Wastin, E. Colineau, J. Rebizant, G.H. Lander, J. Nucl. Sci. Technol. 3 (2002) 148. [10] R.A. Hakvoort, A. van Veen, P.E. Mijnarends, H. Schut, Appl. Surf. Sci. 85 (1995) 271. [11] B. Viswanathan, G. Ko¨gel, A. Seeger, Rad. Effects 78 (1983) 231. [12] H.E. Hansen, H. Rajainmaki, R. Talja, M.D. Bentzon, R.M. Nieminen, K. Petersen, J. Phys. F: Met. Phys. 15 (1985) 1. [13] W. Trifstha¨user, Phys. Rev. B 18 (1975) 4634.