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4He irradiation of zircon, ZrSiO4, using a micro-patterned, Si-based energy filter
Nuclear Inst. and Methods in Physics Research B 443 (2019) 38–42
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4
He irradiation of zircon, ZrSiO4, using a micro-patterned, Si-based energy filter
T
Lutz Nasdalaa, , Shavkat Akhmadalievb, Chutimun Chanmuang N.a, André Zowallac, Constantin Csatoc, Michael Rübc,d ⁎
a
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria Institut für Ionenstrahlphysik und Materialforschung, Helmholtz-Zentrum Dresden-Rossendorf e.V., 01328 Dresden, Germany c mi2-factory GmbH, 07745 Jena, Germany d Fachbereich SciTec, Ernst-Abbe-Hochschule Jena, 07745 Jena, Germany b
ARTICLE INFO
ABSTRACT
Keywords: Radiation damage Helium irradiation Energy filter Focused ion beam Raman spectroscopy
The quantitative evaluation of alpha-particle damage in the mineral zircon, ZrSiO4, using 4He irradiation experiments is difficult because the vast majority of atomic knock-ons in the target are concentrated in a narrow depth range near the ends of the He-ion trajectories. Here we present a new concept to overcome this problem, namely, tailoring the depth profile of damage by means of a micromechanically fabricated “energy filter”. Lamellae of 1.5 μm thickness, prepared from ZrSiO4 using the focused-ion-beam technique, were subjected to irradiation with 8.8 MeV 4He ions. Five irradiations with ion fluences in the range 2.5 × 1015–1 × 1017 cm−2 have resulted in mild to severe damage, as monitored by the broadening and downshift of SiO4-stretching Raman bands. Our results may provide a means for quantifying the contribution of alpha particles to the total selfirradiation damage in zircon.
1. Introduction
bulk damage is significant [22]. Irradiation experiments have revealed that 4He ions, with energies corresponding to those of natural alpha particles, produce structural damage in zircon, predominantly near the ends of the ion trajectories [23,24]. In naturally radiation-damaged zircon, 4He irradiation results in the creation of additional damage, whereas any structural reconstitution of previous damage (which may be caused in some radiation-damaged target materials by the high electronic stopping power of irradiated ions [24–28]) was not observed. Even fully metamict zircon irradiated with 4He ions did not yield any detectable structural changes but remained amorphous [24]. Analytical quantification of the 4He-irradiation damage, however, is difficult because the vast majority of the damage is concentrated in a narrow penetration-depth zone near the end of the He trajectories (Fig. 1a). The application of confocal laser spectroscopy is problematic because the thickness of the intensely damaged zone is close to, or even below, the spatial resolution of modern confocal systems [24,30,31]. Electron-beam techniques would provide sufficient special resolution; however, potential bias is to be considered as the impact of a high-energy electron beam may result in damage annealing in zircon [32,33]. Here, we propose to overcome this problem by the application of a micromechanically fabricated Si filter [34] that affects a significant widening of the Bragg peak of damage, which in turn facilitates the
The actinide-bearing mineral zircon (ZrSiO4; tetragonal space group I41/amd) suffers from structural damage that, after being accumulated over geologic periods of time, may eventually lead to amorphisation [1–4]. The self-irradiation-induced amorphisation of initially crystalline minerals is commonly referred to as the “metamictisation” process [5–7]. This process is driven by alpha-decay events of 238U, 235U and 232 Th, which in the zircon lattice substitute for eight-coordinated Zr4+, and instable daughter nuclei in their decay chains. The irradiation damage comprises (i) scattered defects created by alpha particles (4He cores with energies in the range 3.9–8.8 MeV) [1,3,8], and (ii) damage clusters tens of nm in size, created by recoils of heavy daughter nuclei upon alpha emission [1,9–11]. Quantitative knowledge of self-irradiation effects is, among others, crucial for the interpretation of zircon U–Pb geochronology [12–14] and (U–Th)/He thermochronology [15,16] results, for the understanding of post-growth alteration processes of this mineral [13,17,18], and for the performance assessment of ZrSiO4-based ceramics as potential host materials for the immobilization of nuclear waste [3,5,8,19,20]. Even though the majority of self-irradiation damage in zircon is created by recoil nuclei [3,8,21], the contribution of alpha particles to ⁎
Corresponding author. E-mail address: [email protected] (L. Nasdala).
https://doi.org/10.1016/j.nimb.2019.01.046 Received 13 December 2018; Received in revised form 23 January 2019; Accepted 27 January 2019 0168-583X/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Nuclear Inst. and Methods in Physics Research B 443 (2019) 38–42
L. Nasdala et al.
thickness of the first layer (Si) was varied between 1 and 17 μm, at 0.5 μm steps. The resulting mean displacement distribution in the second layer (ZrSiO4) is shown in Fig. 1a, which visualises nicely the tailoring effect of the energy filter. The sharp Bragg peak that is obtained upon irradiation without Si filter is now turned into a flat plateau. The calculated displacement number within this plateau is about half an order of magnitude lower than that of the initial Bragg peak, but it shows a well predictable level in the depth range 23–30 μm below the ZrSiO4 surface (Fig. 1a). However, the intensely damaged (and, consequently, volume-expanded) target zone is sandwiched between less damaged volume areas; it hence may be affected by significant strain. Therefore, it needs to be considered that results of in situ spectroscopic analyses may be biased by strain. Instead of strain removal by extracting the intensely damaged zone after the irradiation experiment (which might result in uncontrolled sample warming and hence defect annealing), we prefer to do sample preparation before the irradiation experiment and suggest irradiation of a 1.5 μm thick zircon lamella produced using the focused ion beam (FIB) technique (in analogy to [31,37]), through the Si filter and placed behind a ∼26 μm thick zircon window (Fig. 1a). The mechanical preparation of a thin, plane-parallel zircon window, however, is much more difficult than producing a thin metal window. We have therefore repeated SRIM calculations, assuming a three-layer (Si–metal–ZrSiO4) target, for different metals. We found that if a Cu plate of ∼18.5 μm thickness is placed in front of the 1.5 μm thick zircon lamella, the latter is located well within the tailored Bragg peak (Fig. 1b). 3. Experimental Thin lamellae were prepared from a natural, virtually not radiationdamaged, gem quality zircon specimen (#R–5) originating from the Ratanakiri province, Cambodia [37,38]. Plane-parallel lamellae (thickness 1.50 ± 0.05 μm; average sizes 20 μm × 12 μm) were prepared using a FEI Quanta 3D FEG dual beam scanning electron microscope (SEM) equipped with a field-emission Ga source, Pt and C gasinjection systems, and an Omniprobe 100.7 micromanipulator. A detailed description of the FIB lamellae preparation procedure is given elsewhere [37]. Custom-made rolled Cu plates (6 mm diameter) were checked by obtaining secondary electron images in the SEM; five plates with thicknesses of 18.5 ± 0.5 μm were selected. The design of the sample holders, allowing us to irradiate simultaneously three lamellae (individually attached to three Omniprobe lift-out grids), is shown in Fig. 2. The distance between Si filter and Omniprobe grids was set to ∼23 mm. The 4He irradiations were done using the 6 MV Tandetron accelerator of the Helmholtz-Zentrum Dresden-Rossendorf, Germany. The implantation chamber was evacuated to ∼3 × 10−7 bar. Samples were cooled with liquid N2 to avoid uncontrolled heating during irradiation. Five irradiations with 8.8 MeV 4He ions were done. The ion fluences were chosen based on previous He-irradiation results [24] and had approximately uniform factors among them; they were varied between 2.5 × 1015 and 1 × 1017 He cm−2 (Table 1). The beam current was ca. 100–115 nA, resulting in a current density of ca. 200–230 nA cm−2. After inspection and imaging using a high-magnification optical microscope, lamellae were analysed by means of a Horiba LabRAM Evolution spectrometer system (focal length 800 mm). The system was equipped with an Olympus BX-series optical microscope, a 1800 grooves per mm diffraction grating, and a Peltier-cooled, Si-based charge-coupled device detector. Raman spectra were excited with the 632.8 nm emission of a He-Ne laser (5 mW power measured behind the microscope objective). A 100 × objective (numerical aperture 0.9) was used. Wavenumber calibration was done using the Rayleigh line and emission lines of a Kr lamp. The wavenumber accuracy was 0.5 cm−1, and the spectral resolution was 0.8 cm−1. After background correction, band fitting was done assuming pseudo-Voigt band shapes. Measured
Fig. 1. Prediction of the tailoring effect of the Si-based energy filter, based on SRIM [35] calculations. (a) Calculated distribution patterns of displacements as created by 8.8 MeV He ions in ZrSiO4, without and with energy filter. Without energy filter, the Bragg peak of damage is characterised by strong lateral damage gradient. With energy filter, the damage distribution shows a broad plateau. Irradiation of a 1.5 μm thick ZrSiO4 target (grey) behind a 26 μm ZrSiO4 window hence results in fairly uniform damage across the target. (b) Analogous effect of a 18.5 μm Cu window placed in front of the 1.5 μm ZrSiO4 target.
application of laser spectroscopy for damage analysis. 2. Theoretical considerations The depth distribution of He-irradiation damage in ZrSiO4 was predicted by Monte Carlo simulations using the SRIM program [35]. We have chosen a 4He ion energy of 8.8 MeV, which is equivalent to the highest energy of common alpha particles (produced in the 212Po → 208 Pb decay; 232Th decay chain). Displacement threshold energies of 75 eV for Zr, 75 eV for Si and 60 eV for O atoms in ZrSiO4 [36] were used. The SRIM defaults for displacement threshold energies were used for atoms in the Si filter membrane (15 eV) and the Cu window (25 eV). Also, SRIM defaults for binding energies for all atoms were accepted. The target mass densities were set to 4.7 g cm−3 (ZrSiO4), 2.33 g cm−3 (Si) and 8.92 g cm−3 (Cu), respectively. Full damage cascades, that is, including sub-branches of displacements as caused by displaced target atoms, were calculated. For statistical precision, all calculations were done for 10,000 incoming He ions. The Si filter membrane had groove-like topography, consisting of linear spikes with triangular cross section (height 16 μm) atop a supporting layer (thickness 1 μm; more details are given in [34]). To simulate the effect of the Si filter on the depth distribution of displacements in ZrSiO4, multiple SRIM calculations of atomic displacements upon He irradiation into a two-layer target were done. Here, the 39
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Fig. 2. Design of the irradiation experiment. (a) Simplified sketch (not to scale) of the sample holder used to irradiate simultaneously three FIB-prepared lamellae, attached to three Omniprobe grids. (b) Photograph of the three samples through the hole in the Ta cover plate (after removal of the Cu window). Each of the tiny samples (ca. 20 μm × 12 μm size) is attached to the “B” finger of an Omniprobe grid. (c) One-inch sample mount attached to a steel plate inside the implantation chamber, before attachment of the Si-based energy filter.
preparation did not induce significant ion beam damage. After irradiation, the lamellae yielded mildly to strongly broadened Raman bands whose FWHM increase depends on the ion fluence, indicating disturbance of the short-range order due to creation of irradiation damage. The broadening of bands is associated with a downshift of their spectral positions (Table 1; Fig. 3a), with the spectral trend FWHM – Raman shift coinciding with that of Au-irradiated and naturally self-irradiated zircon (Fig. 3b). Note that irradiation with the highest fluence (1 × 1017 He cm−2) has resulted in a ν3(SiO4) FWHM of 20.9 cm−1, which according to [42,43] indicates severe radiation damage with an amorphous fraction of approximately 50 vol%. To compare quantitatively the FWHM increases with their causal ion fluences, the latter were, based on SRIM results, converted into dpa (displacements per lattice atom). For this, fluences were multiplied by the predicted displacement number of 8.8 × 10−3 nm−1 per 4He ion irradiated (Fig. 1b) and divided by the number of 9.23 × 1022 cm−3 lattice atoms (the unit cell of zircon has a volume of ∼0.260 nm3 and contains four formula units = 24 atoms). Calculated values span the range 0.002–0.095 dpa (Table 1). Such FWHM-dpa relationships, after proper calibration that should also include further He ion energies, may open up the opportunity for spectroscopy-based, non-destructive damage estimates in He-irradiated samples. However, Fig. 4 visualises that the relationship of Raman FWHM (present damage) and dpa (predicted damage based on SRIM results) obtained in the present He-irradiation study deviates appreciably from that obtained for Au-irradiated zircon [37]; with the analogous trend for natural zircon that did not have experienced any thermal annealing [40] plotting in between the two former. The different relationships cannot be explained at the present point; we may merely speculate that either calculated dpa values of He-irradiated samples are too high and/
Table 1 Fluences and Raman spectroscopic results for 4He-irradiated lamellae. No.
1 2 3 4 5 6
Fluence [cm−2]
unirradiated 2.5 × 1015 6.5 × 1015 1.6 × 1016 4 × 1016 1 × 1017
Calculated displacements [dpa]
Raman parameters* Raman shift [cm−1]
FWHM [cm−1]
0 0.002 0.006 0.015 0.038 0.095
1007.4 1007.1 1006.6 1005.3 1002.8 999.2
1.7 2.2 2.9 5.3 11.2 20.9
* Quoted for the ν3(SiO4) band.
FWHM (full width at half maximum) values were corrected for instrumental band broadening according to [39]. 4. Results and discussion The 4He irradiation damage in the ZrSiO4 lamellae is estimated from the broadening of the main Raman band [40], which is assigned to the ν3(SiO4) vibration (antisymmetric stretching of SiO4 tetrahedrons; B1g mode [41]). Results are presented in Table 1. All unirradiated lamellae yielded narrow Raman bands that are indistinguishable from that of pure synthetic ZrSiO4. This reconfirms again that (i) the natural sample material used in the present study is virtually not radiation damaged, which is explained by its very low time-integrated alpha dose of 4 × 1014 g−1 [37,38], and (ii) possible “chemical band broadening” due to the presence of trace elements is negligible. Also, the observation of narrow Raman bands from unirradiated lamellae verifies that the FIB 40
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Fig. 4. Plot of the observed broadening of the zircon ν3(SiO4) Raman band against calculated displacements. Au irradiation [37] appears significantly more efficient, compared to He irradiation, in creating spectral changes. FWHM increases obtained from naturally self-irradiated, unannealed zircon [40] lie on a trend that plots in between the two former.
atomic knock-ons, which rely solely on the nuclear stopping power of the irradiated heavy ions, likely underestimate the actual damage indeed. The issue deserves further investigation. Finally, it is worthy of note that our SRIM calculations predict that ∼56% of all irradiated He ions are deposited within a 0.5 μm thick zone (at ∼32.5 μm below the surface). For a high fluence of 1 × 1018 He cm−2, this would result in a local concentration of ∼3 He atoms per unit cell, whose emplacement may contribute to local volume expansion and the build-up of strain, which in turn might bias the analytical quantification of damage. This problem is also diminished by the application of an energy filter, which was initially developed for tailoring implantation profiles [34]. Consequently, in addition to tailoring the damage profile, the use of an energy filter in He-irradiation experiments may also help to avoid the accumulation of too high local concentrations of implanted ions at very high He fluences. Acknowledgements
Fig. 3. (a) Raman spectra (He–Ne 632.8 nm excitation) of the SiO4 stretching region of unirradiated and irradiated ZrSiO4 lamellae. Spectra are adjusted in intensity, and are shown with vertical offset for clarity. Note the increasing band broadening and band downshift with increasing irradiation fluence. (b) Plot of band position against band width of the main B1g-type mode near 1000 cm−1. There is no significant mismatch of spectral parameters, when compared to naturally self-irradiated zircon and Au-irradiated ZrSiO4 [37].
We thank Andreas Wagner and Thomas Rosen (both Universität Wien) for the preparation of the sample holders, and Goodfellow Cambridge Ltd. (Huntingdon, UK) for the production of custom-made Cu discs. Experimental assistance by Gerlinde Habler, Christoph Lenz (both Universität Wien) and the team of the Ion Beam Centre (Helmholtz-Zentrum Dresden-Rossendorf) is gratefully acknowledged. We are indebted to anonymous expert for constructive comments that helped to improve and strengthen the manuscript. Financial support was provided by the Faculty of Geosciences, Geography and Astronomy (Universität Wien) and the Austrian Science Fund (FWF) through project P24448–N19 to L.N.
or calculated dpa values of Au-irradiated samples are too low. The former might be the case if there was some (at least gradual) unrecognised annealing of the He-irradiation damage by the He ions themselves. As mentioned above, “light-ion-assisted annealing” in various minerals and materials has been discussed intensely in the literature [25–29]; however, to the best of our knowledge, never for the mineral zircon. Also, there is no experimental indication for any damage annealing in zircon as caused by He ions [24]. The second possible explanation of the above difference in FWHM–dpa trends might be that dpa values calculated for Au-irradiated (and natural) samples underestimate the present damage. This appears likely as heavy ions cause damage cascades that contain a significantly higher number of defects than directly displaced by ballistic atomic knock-ons (for instance shown by molecular dynamics simulations by [9,10]). Furthermore, it has been shown for zircon [44] and other solids [45–47] that the high electronic stopping power of heavy ions may contribute to the creation of significant amounts of additional damage in the target. Consequently, all dpa estimates based on calculations of
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4
He irradiation of zircon, ZrSiO4, using a micro-patterned, Si-based energy filter
T
Lutz Nasdalaa, , Shavkat Akhmadalievb, Chutimun Chanmuang N.a, André Zowallac, Constantin Csatoc, Michael Rübc,d ⁎
a
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria Institut für Ionenstrahlphysik und Materialforschung, Helmholtz-Zentrum Dresden-Rossendorf e.V., 01328 Dresden, Germany c mi2-factory GmbH, 07745 Jena, Germany d Fachbereich SciTec, Ernst-Abbe-Hochschule Jena, 07745 Jena, Germany b
ARTICLE INFO
ABSTRACT
Keywords: Radiation damage Helium irradiation Energy filter Focused ion beam Raman spectroscopy
The quantitative evaluation of alpha-particle damage in the mineral zircon, ZrSiO4, using 4He irradiation experiments is difficult because the vast majority of atomic knock-ons in the target are concentrated in a narrow depth range near the ends of the He-ion trajectories. Here we present a new concept to overcome this problem, namely, tailoring the depth profile of damage by means of a micromechanically fabricated “energy filter”. Lamellae of 1.5 μm thickness, prepared from ZrSiO4 using the focused-ion-beam technique, were subjected to irradiation with 8.8 MeV 4He ions. Five irradiations with ion fluences in the range 2.5 × 1015–1 × 1017 cm−2 have resulted in mild to severe damage, as monitored by the broadening and downshift of SiO4-stretching Raman bands. Our results may provide a means for quantifying the contribution of alpha particles to the total selfirradiation damage in zircon.
1. Introduction
bulk damage is significant [22]. Irradiation experiments have revealed that 4He ions, with energies corresponding to those of natural alpha particles, produce structural damage in zircon, predominantly near the ends of the ion trajectories [23,24]. In naturally radiation-damaged zircon, 4He irradiation results in the creation of additional damage, whereas any structural reconstitution of previous damage (which may be caused in some radiation-damaged target materials by the high electronic stopping power of irradiated ions [24–28]) was not observed. Even fully metamict zircon irradiated with 4He ions did not yield any detectable structural changes but remained amorphous [24]. Analytical quantification of the 4He-irradiation damage, however, is difficult because the vast majority of the damage is concentrated in a narrow penetration-depth zone near the end of the He trajectories (Fig. 1a). The application of confocal laser spectroscopy is problematic because the thickness of the intensely damaged zone is close to, or even below, the spatial resolution of modern confocal systems [24,30,31]. Electron-beam techniques would provide sufficient special resolution; however, potential bias is to be considered as the impact of a high-energy electron beam may result in damage annealing in zircon [32,33]. Here, we propose to overcome this problem by the application of a micromechanically fabricated Si filter [34] that affects a significant widening of the Bragg peak of damage, which in turn facilitates the
The actinide-bearing mineral zircon (ZrSiO4; tetragonal space group I41/amd) suffers from structural damage that, after being accumulated over geologic periods of time, may eventually lead to amorphisation [1–4]. The self-irradiation-induced amorphisation of initially crystalline minerals is commonly referred to as the “metamictisation” process [5–7]. This process is driven by alpha-decay events of 238U, 235U and 232 Th, which in the zircon lattice substitute for eight-coordinated Zr4+, and instable daughter nuclei in their decay chains. The irradiation damage comprises (i) scattered defects created by alpha particles (4He cores with energies in the range 3.9–8.8 MeV) [1,3,8], and (ii) damage clusters tens of nm in size, created by recoils of heavy daughter nuclei upon alpha emission [1,9–11]. Quantitative knowledge of self-irradiation effects is, among others, crucial for the interpretation of zircon U–Pb geochronology [12–14] and (U–Th)/He thermochronology [15,16] results, for the understanding of post-growth alteration processes of this mineral [13,17,18], and for the performance assessment of ZrSiO4-based ceramics as potential host materials for the immobilization of nuclear waste [3,5,8,19,20]. Even though the majority of self-irradiation damage in zircon is created by recoil nuclei [3,8,21], the contribution of alpha particles to ⁎
Corresponding author. E-mail address: [email protected] (L. Nasdala).
https://doi.org/10.1016/j.nimb.2019.01.046 Received 13 December 2018; Received in revised form 23 January 2019; Accepted 27 January 2019 0168-583X/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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thickness of the first layer (Si) was varied between 1 and 17 μm, at 0.5 μm steps. The resulting mean displacement distribution in the second layer (ZrSiO4) is shown in Fig. 1a, which visualises nicely the tailoring effect of the energy filter. The sharp Bragg peak that is obtained upon irradiation without Si filter is now turned into a flat plateau. The calculated displacement number within this plateau is about half an order of magnitude lower than that of the initial Bragg peak, but it shows a well predictable level in the depth range 23–30 μm below the ZrSiO4 surface (Fig. 1a). However, the intensely damaged (and, consequently, volume-expanded) target zone is sandwiched between less damaged volume areas; it hence may be affected by significant strain. Therefore, it needs to be considered that results of in situ spectroscopic analyses may be biased by strain. Instead of strain removal by extracting the intensely damaged zone after the irradiation experiment (which might result in uncontrolled sample warming and hence defect annealing), we prefer to do sample preparation before the irradiation experiment and suggest irradiation of a 1.5 μm thick zircon lamella produced using the focused ion beam (FIB) technique (in analogy to [31,37]), through the Si filter and placed behind a ∼26 μm thick zircon window (Fig. 1a). The mechanical preparation of a thin, plane-parallel zircon window, however, is much more difficult than producing a thin metal window. We have therefore repeated SRIM calculations, assuming a three-layer (Si–metal–ZrSiO4) target, for different metals. We found that if a Cu plate of ∼18.5 μm thickness is placed in front of the 1.5 μm thick zircon lamella, the latter is located well within the tailored Bragg peak (Fig. 1b). 3. Experimental Thin lamellae were prepared from a natural, virtually not radiationdamaged, gem quality zircon specimen (#R–5) originating from the Ratanakiri province, Cambodia [37,38]. Plane-parallel lamellae (thickness 1.50 ± 0.05 μm; average sizes 20 μm × 12 μm) were prepared using a FEI Quanta 3D FEG dual beam scanning electron microscope (SEM) equipped with a field-emission Ga source, Pt and C gasinjection systems, and an Omniprobe 100.7 micromanipulator. A detailed description of the FIB lamellae preparation procedure is given elsewhere [37]. Custom-made rolled Cu plates (6 mm diameter) were checked by obtaining secondary electron images in the SEM; five plates with thicknesses of 18.5 ± 0.5 μm were selected. The design of the sample holders, allowing us to irradiate simultaneously three lamellae (individually attached to three Omniprobe lift-out grids), is shown in Fig. 2. The distance between Si filter and Omniprobe grids was set to ∼23 mm. The 4He irradiations were done using the 6 MV Tandetron accelerator of the Helmholtz-Zentrum Dresden-Rossendorf, Germany. The implantation chamber was evacuated to ∼3 × 10−7 bar. Samples were cooled with liquid N2 to avoid uncontrolled heating during irradiation. Five irradiations with 8.8 MeV 4He ions were done. The ion fluences were chosen based on previous He-irradiation results [24] and had approximately uniform factors among them; they were varied between 2.5 × 1015 and 1 × 1017 He cm−2 (Table 1). The beam current was ca. 100–115 nA, resulting in a current density of ca. 200–230 nA cm−2. After inspection and imaging using a high-magnification optical microscope, lamellae were analysed by means of a Horiba LabRAM Evolution spectrometer system (focal length 800 mm). The system was equipped with an Olympus BX-series optical microscope, a 1800 grooves per mm diffraction grating, and a Peltier-cooled, Si-based charge-coupled device detector. Raman spectra were excited with the 632.8 nm emission of a He-Ne laser (5 mW power measured behind the microscope objective). A 100 × objective (numerical aperture 0.9) was used. Wavenumber calibration was done using the Rayleigh line and emission lines of a Kr lamp. The wavenumber accuracy was 0.5 cm−1, and the spectral resolution was 0.8 cm−1. After background correction, band fitting was done assuming pseudo-Voigt band shapes. Measured
Fig. 1. Prediction of the tailoring effect of the Si-based energy filter, based on SRIM [35] calculations. (a) Calculated distribution patterns of displacements as created by 8.8 MeV He ions in ZrSiO4, without and with energy filter. Without energy filter, the Bragg peak of damage is characterised by strong lateral damage gradient. With energy filter, the damage distribution shows a broad plateau. Irradiation of a 1.5 μm thick ZrSiO4 target (grey) behind a 26 μm ZrSiO4 window hence results in fairly uniform damage across the target. (b) Analogous effect of a 18.5 μm Cu window placed in front of the 1.5 μm ZrSiO4 target.
application of laser spectroscopy for damage analysis. 2. Theoretical considerations The depth distribution of He-irradiation damage in ZrSiO4 was predicted by Monte Carlo simulations using the SRIM program [35]. We have chosen a 4He ion energy of 8.8 MeV, which is equivalent to the highest energy of common alpha particles (produced in the 212Po → 208 Pb decay; 232Th decay chain). Displacement threshold energies of 75 eV for Zr, 75 eV for Si and 60 eV for O atoms in ZrSiO4 [36] were used. The SRIM defaults for displacement threshold energies were used for atoms in the Si filter membrane (15 eV) and the Cu window (25 eV). Also, SRIM defaults for binding energies for all atoms were accepted. The target mass densities were set to 4.7 g cm−3 (ZrSiO4), 2.33 g cm−3 (Si) and 8.92 g cm−3 (Cu), respectively. Full damage cascades, that is, including sub-branches of displacements as caused by displaced target atoms, were calculated. For statistical precision, all calculations were done for 10,000 incoming He ions. The Si filter membrane had groove-like topography, consisting of linear spikes with triangular cross section (height 16 μm) atop a supporting layer (thickness 1 μm; more details are given in [34]). To simulate the effect of the Si filter on the depth distribution of displacements in ZrSiO4, multiple SRIM calculations of atomic displacements upon He irradiation into a two-layer target were done. Here, the 39
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Fig. 2. Design of the irradiation experiment. (a) Simplified sketch (not to scale) of the sample holder used to irradiate simultaneously three FIB-prepared lamellae, attached to three Omniprobe grids. (b) Photograph of the three samples through the hole in the Ta cover plate (after removal of the Cu window). Each of the tiny samples (ca. 20 μm × 12 μm size) is attached to the “B” finger of an Omniprobe grid. (c) One-inch sample mount attached to a steel plate inside the implantation chamber, before attachment of the Si-based energy filter.
preparation did not induce significant ion beam damage. After irradiation, the lamellae yielded mildly to strongly broadened Raman bands whose FWHM increase depends on the ion fluence, indicating disturbance of the short-range order due to creation of irradiation damage. The broadening of bands is associated with a downshift of their spectral positions (Table 1; Fig. 3a), with the spectral trend FWHM – Raman shift coinciding with that of Au-irradiated and naturally self-irradiated zircon (Fig. 3b). Note that irradiation with the highest fluence (1 × 1017 He cm−2) has resulted in a ν3(SiO4) FWHM of 20.9 cm−1, which according to [42,43] indicates severe radiation damage with an amorphous fraction of approximately 50 vol%. To compare quantitatively the FWHM increases with their causal ion fluences, the latter were, based on SRIM results, converted into dpa (displacements per lattice atom). For this, fluences were multiplied by the predicted displacement number of 8.8 × 10−3 nm−1 per 4He ion irradiated (Fig. 1b) and divided by the number of 9.23 × 1022 cm−3 lattice atoms (the unit cell of zircon has a volume of ∼0.260 nm3 and contains four formula units = 24 atoms). Calculated values span the range 0.002–0.095 dpa (Table 1). Such FWHM-dpa relationships, after proper calibration that should also include further He ion energies, may open up the opportunity for spectroscopy-based, non-destructive damage estimates in He-irradiated samples. However, Fig. 4 visualises that the relationship of Raman FWHM (present damage) and dpa (predicted damage based on SRIM results) obtained in the present He-irradiation study deviates appreciably from that obtained for Au-irradiated zircon [37]; with the analogous trend for natural zircon that did not have experienced any thermal annealing [40] plotting in between the two former. The different relationships cannot be explained at the present point; we may merely speculate that either calculated dpa values of He-irradiated samples are too high and/
Table 1 Fluences and Raman spectroscopic results for 4He-irradiated lamellae. No.
1 2 3 4 5 6
Fluence [cm−2]
unirradiated 2.5 × 1015 6.5 × 1015 1.6 × 1016 4 × 1016 1 × 1017
Calculated displacements [dpa]
Raman parameters* Raman shift [cm−1]
FWHM [cm−1]
0 0.002 0.006 0.015 0.038 0.095
1007.4 1007.1 1006.6 1005.3 1002.8 999.2
1.7 2.2 2.9 5.3 11.2 20.9
* Quoted for the ν3(SiO4) band.
FWHM (full width at half maximum) values were corrected for instrumental band broadening according to [39]. 4. Results and discussion The 4He irradiation damage in the ZrSiO4 lamellae is estimated from the broadening of the main Raman band [40], which is assigned to the ν3(SiO4) vibration (antisymmetric stretching of SiO4 tetrahedrons; B1g mode [41]). Results are presented in Table 1. All unirradiated lamellae yielded narrow Raman bands that are indistinguishable from that of pure synthetic ZrSiO4. This reconfirms again that (i) the natural sample material used in the present study is virtually not radiation damaged, which is explained by its very low time-integrated alpha dose of 4 × 1014 g−1 [37,38], and (ii) possible “chemical band broadening” due to the presence of trace elements is negligible. Also, the observation of narrow Raman bands from unirradiated lamellae verifies that the FIB 40
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Fig. 4. Plot of the observed broadening of the zircon ν3(SiO4) Raman band against calculated displacements. Au irradiation [37] appears significantly more efficient, compared to He irradiation, in creating spectral changes. FWHM increases obtained from naturally self-irradiated, unannealed zircon [40] lie on a trend that plots in between the two former.
atomic knock-ons, which rely solely on the nuclear stopping power of the irradiated heavy ions, likely underestimate the actual damage indeed. The issue deserves further investigation. Finally, it is worthy of note that our SRIM calculations predict that ∼56% of all irradiated He ions are deposited within a 0.5 μm thick zone (at ∼32.5 μm below the surface). For a high fluence of 1 × 1018 He cm−2, this would result in a local concentration of ∼3 He atoms per unit cell, whose emplacement may contribute to local volume expansion and the build-up of strain, which in turn might bias the analytical quantification of damage. This problem is also diminished by the application of an energy filter, which was initially developed for tailoring implantation profiles [34]. Consequently, in addition to tailoring the damage profile, the use of an energy filter in He-irradiation experiments may also help to avoid the accumulation of too high local concentrations of implanted ions at very high He fluences. Acknowledgements
Fig. 3. (a) Raman spectra (He–Ne 632.8 nm excitation) of the SiO4 stretching region of unirradiated and irradiated ZrSiO4 lamellae. Spectra are adjusted in intensity, and are shown with vertical offset for clarity. Note the increasing band broadening and band downshift with increasing irradiation fluence. (b) Plot of band position against band width of the main B1g-type mode near 1000 cm−1. There is no significant mismatch of spectral parameters, when compared to naturally self-irradiated zircon and Au-irradiated ZrSiO4 [37].
We thank Andreas Wagner and Thomas Rosen (both Universität Wien) for the preparation of the sample holders, and Goodfellow Cambridge Ltd. (Huntingdon, UK) for the production of custom-made Cu discs. Experimental assistance by Gerlinde Habler, Christoph Lenz (both Universität Wien) and the team of the Ion Beam Centre (Helmholtz-Zentrum Dresden-Rossendorf) is gratefully acknowledged. We are indebted to anonymous expert for constructive comments that helped to improve and strengthen the manuscript. Financial support was provided by the Faculty of Geosciences, Geography and Astronomy (Universität Wien) and the Austrian Science Fund (FWF) through project P24448–N19 to L.N.
or calculated dpa values of Au-irradiated samples are too low. The former might be the case if there was some (at least gradual) unrecognised annealing of the He-irradiation damage by the He ions themselves. As mentioned above, “light-ion-assisted annealing” in various minerals and materials has been discussed intensely in the literature [25–29]; however, to the best of our knowledge, never for the mineral zircon. Also, there is no experimental indication for any damage annealing in zircon as caused by He ions [24]. The second possible explanation of the above difference in FWHM–dpa trends might be that dpa values calculated for Au-irradiated (and natural) samples underestimate the present damage. This appears likely as heavy ions cause damage cascades that contain a significantly higher number of defects than directly displaced by ballistic atomic knock-ons (for instance shown by molecular dynamics simulations by [9,10]). Furthermore, it has been shown for zircon [44] and other solids [45–47] that the high electronic stopping power of heavy ions may contribute to the creation of significant amounts of additional damage in the target. Consequently, all dpa estimates based on calculations of
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