- Email: [email protected]
Evaluation of radiation-damage halos in quartz by cathodoluminescence as a geochronological tool
ARTICLE IN PRESS
Quaternary Geochronology 3 (2008) 342–345 www.elsevier.com/locate/quageo
Research Paper
Evaluation of radiation-damage halos in quartz by cathodoluminescence as a geochronological tool T. Okumuraa,, H. Nishidoa, S. Toyodab, T. Kanekob, S. Kosugib, Y. Sawadab, K. Komuroc a
Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan b Department of Applied Physics, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan c Institute of Geoscience, University of Tsukuba, Ten-nodai, Ibaraki 305-8571, Japan Received 27 September 2007; accepted 27 January 2008 Available online 5 February 2008
Abstract Detailed quantitative cathodoluminescence (CL) imaging analysis was carried out for radiation-damage halos observed by CL (CL halo) created in natural quartz by implantation of 4 MeV He+ ions. The band of CL halo was approximately 14 mm in width and was constant for any He+ ion dose. The width of the halo is consistent with the theoretical range of 4He ions in quartz. A quantitative response of CL intensity to He+ ion dose was obtained, leading to the application of CL halos to geodosimetrical use. The CL intensity increases exponentially in the luminescent band from the implantation surface to the inside, until it reaches a maximum at 14 mm depth, with a rapid decrease beyond this point. This result is as predicted by Bragg’s law, although we find some differences between the CL intensity and the theoretical stopping power. r 2008 Elsevier Ltd. All rights reserved. Keywords: Cathodoluminescence; Radiation-damage halo; CL halo; Geodosimeter
1. Introduction Cathodoluminescence (CL) is an optical phenomenon that is based on the generation of visible radiation through sample excitation by high-energy electrons. CL occurs as a result of radiative electronic transition, in which an electron transfers from a higher energy state to a lower one. The CL method has been used to determine the impurities of trace elements such as rare earth and transition metal elements and to detect lattice defects. Recently, CL microscopy and spectroscopy have been actively used in petrology and mineralogy (e.g., Pagel et al., 2000; Boggs and Krinsley, 2006). CL in quartz has been extensively applied to research of growth zonation (e.g., Watt et al., 1997), identification of trace elements and their distributions (e.g., Mu¨ller et al., 2002), shock effects by impact events (e.g., Boggs et al., 2001; Gucsik et al., 2003) and provenance studies (e.g., Zinkernagel, 1978; Matter and Ramseyer, 1985). Corresponding author. Tel./fax: +81 86 256 9406.
E-mail address: [email protected] (T. Okumura). 1871-1014/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2008.01.006
Radiation-damage halos in quartz are not observable with a polarized microscope. They can be detected only with a CL microscope and are called CL halos (Smith and Stenstrom, 1965; Owen, 1988; Ramseyer et al., 1988; Meunier et al., 1990; Go¨tze et al., 2001; Botis et al., 2005). Radiation-damage halos caused mainly by a-particles are recognized in mineral grains of quartz and biotite with radioactive uranium- and thorium-bearing mineral inclusions (e.g., Nasdala et al., 2001). Owen (1988) demonstrated that the CL halos in quartz grains from igneous and metamorphic rocks were created by a-radiation. Furthermore, the size of the CL halo was consistent with the predicted range of a-particles from the 238U and 232Th decay series, suggesting that CL halos can be used as a dating tool if the CL contrast can be quantified. Komuro et al. (2002) first observed CL halos in synthetic quartz irradiated experimentally by He+ ions, and revealed that the width of the halos was consistent with the theoretical range estimated by the Bragg–Kleenman rule. CCD analysis of CL halos showed a correlation between CL intensity of the halo and the He+ ion dose, suggesting the application of the CL halo in quartz to geodosimetry.
ARTICLE IN PRESS T. Okumura et al. / Quaternary Geochronology 3 (2008) 342–345
Radiation-damage halos in quartz have been investigated mainly with CL microscopes, which are used for color CL imaging. Spectral measurements and highresolution CL imaging of CL halos have not been quantitatively examined so far, however, because they are micrometers in size and because halos in natural quartz may have a complex cause due to the presence of a mixed radiation field. The purpose of this study is to quantitatively measure the intensities of CL halos due to a-particle radiation from He+ ions implanted experimentally in natural quartz, using a scanning electron microscopecathodoluminescence (SEM-CL) system. We also compare the CL halo dose-response with theoretically simulated halos to illustrate the possible applications of CL halos to radiation dosimetry and geochronology.
SE He+
CL
XX
Y Y
halo
epoxy resin
2. Samples and methods
host material
surface 1200
Y
X
1000 CL intensity (a.u.)
A single crystal of colorless optical grade quartz from Minas Gerais, Brazil, was employed for CL measurements. The sample was cut into slices (10 10 1 mm3) with the square face perpendicular to the c-axis. The slices were polished and finished with a diamond abrasive of 1 mm particle size. He+ ion implantation in the sample was performed in a 3 M-tandem ion accelerator at the Takasaki Research Center of the Japan Atomic Energy Research Institute. To enable a comparison with previous experiments on synthetic quartz (Komuro et al., 2002), the energy of He+ was set at 4 MeV. This corresponds to the energy of a-particles emitted from the decay of 238U (4.18 MeV). The homogeneous He+ ion beam was irradiated perpendicular to the surface of the slice. Implantation was made under four sets of conditions, with He+ ion doses ranging from 1.772 10 5 to 3.544 10 4 C/cm2. For the CL experiments, the sliced samples were cut vertically to the induced surfaces for cross-sectional observation of the CL halos. The cut chips were embedded together in non-luminescent resin for CL measurement under the same conditions, with their cut sections facing the surface of the resin block. After abrasive finishing of the block surface with 1 mm diamond particles, a 20 nm coat of evaporated carbon was applied to the surface to prevent the build-up of electrical charge during the CL operation. CL imaging was carried out on a SEM-CL system comprising a SEM (JEOL JSM-5410LV) with a gratingtype monochromator (Oxford Instruments MonoCL2). CL emission from the sample surface was collected using a retractable parabolic mirror with high collection efficiency and resolved by scanning grating with 1200 grooves/mm. Monochromatic light was measured by the photon counting method using a Hamamatsu R2228 photomultiplier tube (PMT). Details of the construction of the equipment and analytical system are described in Ikenaga et al. (2000). Operation conditions for SEM-CL imaging and SEM imaging were set at an acceleration voltage of 15 kV with a beam current of 1.0 nA. Images were collected from a
343
800 600 400 200 0
−5
0
5
10 15 Distance (µm)
20
25
30
Fig. 1. Secondary electron (a) and SEM-CL (b) images of natural quartz after He+ ion implantation with a dose of 3.544 10 4 C/cm2. Scale bar is 10 mm. (c) change of CL intensity with ion implantation depth from the surface to the inside, as obtained by CL line analysis.
rectangular area, of 38 27 mm in the scanning mode. CL line analysis was carried out under the same conditions as SEM imaging, by measuring 200 points across a distance of 35 mm along the line X–Y in Fig. 1b. 3. Results and discussion CL halos were observed in all samples after He+ ion implantation, whereas no radiation-damage features were detected in the SEM images (Fig. 1a) or from optical observations using a polarized microscope. Fig. 1 shows SEM and SEM-CL images of the sample irradiated by He+ ions with a dose of 3.544 10 4 C/cm2. CL halos were recognized in the SEM-CL images as a luminescent band with a width of about 14 mm parallel to the irradiated surface (Fig. 1b). This width for the CL halo is consistent with the theoretical range of a-particles resulting from disintegration of 238U in quartz, as calculated by Owen
ARTICLE IN PRESS 344
T. Okumura et al. / Quaternary Geochronology 3 (2008) 342–345
(1988), and it agrees closely with that observed previously in synthetic quartz after He+ ion implantation under the same condition using a CL microscope, LUMINOSCOPE (Komuro et al., 2002). The monochromatic CL intensity of quartz is weak and easily changed during electron beam irradiation (Ramseyer and Mullis, 1990). Komuro et al. (2002) also reported an alteration in the CL color of experimentally He+ ionimplanted quartz during observation with a CL microscope. CL line analysis in the panchromatic CL mode was carried out along the line X–Y shown on the SEM-CL image (Fig. 1b) at 200 points (spread over a distance of 35 mm) at 1 point/s. The intensity of CL emission within the halo band gradually increased from the implantation surface to a depth of 14 mm (Fig. 1c). Regardless of the He+ ion dose, this feature of CL halos was almost the same for all samples, whereas the brightness of the CL halo emission increased with He+ ion dose. An increase of He+ ion dose leads to an enhancement of the overall intensity of the CL halo area, similar to the results obtained from CL imaging. The distribution pattern of CL intensity (shown in Fig. 1c) has a similar appearance to Bragg’s curve, which relates specific energy loss to specific ionization along the track of a charged particle (see discussion below). This pattern of CL intensity is consistent with that previously obtained in a study of synthetic quartz, using a CCD camera and a CL microscope (Komuro et al., 2002). The He+ ion dose can be related to the integrated CL intensity (Fig. 2) by subtracting the measured intensity of the host material from that of the CL halo area (the shaded area in Fig. 1c). The integrated CL intensity increases with dose density, indicating that the CL intensity is a function of the population of lattice defects created by He+ ion implantation. Komuro et al. (2002) also reported that the red component of CL halos (detected by color CCD analysis) was enhanced by an increase in dose density. Here we show that the panchromatic analysis of CL halo
Fig. 2. The relationship between integrated CL intensity of the halo area (obtained by CL line analysis) and dose density.
intensity could be used to investigate the dosimetry of natural radiation and, hence, for dating using the radiation damage in quartz. As CL in quartz corresponds to lattice defects created by radiation (Botis et al., 2005), its intensity should be a function of the amount of energy deposited by radiation, which is related to stopping power. The distribution of stopping power in quartz for 4 MeV 4He ions was obtained by theoretical calculations and is shown in Fig. 3a and b, together with the distribution of measured CL intensities obtained by the present experiments. The pattern of CL distributions is similar to ‘‘Bragg’s peak’’ derived from theoretical considerations. The stopping power peaks at 12 mm from the surface and is zero at a depth of 12.7 mm, while the CL intensity peaks at depths of 13–14 mm, decreasing to zero at 16 mm depth. Such a discrepancy between theory and practice might be caused by the irradiation conditions, because the true dose depends on the position of the sample relative to the radiation source and on secondary radiation from the collision of He+ ions with the sample holder. Furthermore, the effective area
Fig. 3. The relationship between CL intensities (obtained by CL line analysis) and the stopping power for 4 MeV 4He ions (derived from theoretical calculations) for the quartz samples after He+ ion implantation at 4 MeV with dose densities of 3.544 10 4 C/cm2 (a) and 5.907 10 5 C/cm2 (b).
ARTICLE IN PRESS T. Okumura et al. / Quaternary Geochronology 3 (2008) 342–345
excited by the electron beam should be approximately 2 mm in diameter, which is several times larger than the spot size of the electron beam (o0.5 mm). Hence, measurement at steps of o0.2 mm for CL line analysis may be influenced by the capture of afterglow from a previously irradiated spot. This could result in a broadening of the peak shape and a shift in the peak position of the CL intensity distribution, relative to Bragg’s peak. The vertical scales in Fig. 3a and b have been adjusted so that the stopping power and CL intensity coincide for depths of 1–10 mm in Fig. 3a. The latter plot indicates that these two quantities are proportional to each other for stopping power in the range 350–650 MeV/mm. At the high end of this range of stopping power, the number of lattice defects created per unit of radiation energy decreases, due to the higher probability of recombination: this explains the CL intensity being lower than that for the stopping power at depths of between 10 and 12 mm in Fig. 3a. In Fig. 3b, however, the tendency is opposite: that is, the CL intensity is higher than the corresponding value of the stopping power for depths greater than 10 mm. These observations indicate that the relationships among these factors (stopping power, concentration of lattice defects, and CL intensity) are complex. Further studies, including CL spectrum and electron spin resonance (ESR) analyses, are needed to clarify these problems. 4. Conclusions Radiation-damage halos in natural quartz created by He+ ion implantation were quantitatively measured using a high-resolution CL imaging system. The band of the CL halo was about 14 mm in width, which is roughly consistent with the theoretical range. We confirmed the results of our previous study (using a CCD camera with low spatial resolution) that the width of the CL halo was constant for any He+ ion dose but that the intensity of the halo was enhanced by increasing the dose. The dose response of the integrated CL intensity of the halo was quantitatively evaluated by CL line analysis. In this way, high-resolution CL analysis of CL halos could prove useful for geodosimetry of natural radiation. A detailed study of the distribution of CL intensity with depth in the luminescent band showed that the intensity increases exponentially from the implantation surface to the interior, reaching a maximum at 14 mm depth and decreasing rapidly beyond that point. This pattern is consistent qualitatively with the curve predicted by Bragg’s law, although there are some differences between the measured CL intensities and theoretical calculations of the stopping power in quartz for 4 MeV 4He ions. Acknowledgments The authors are grateful to Kiyotaka Ninagawa, Okayama University of Science, for valuable advice in relation to SEM-CL analysis. We thank Arnold Gucsik,
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Max Plank Institute for Chemistry, for improving the manuscript. This study was supported by the InterUniversity Program for the Joint Use of the Japan Atomic Energy Research Institute (JAERI) Facilities and by the public fellowship of the Japan Nuclear Cycle Development Institute (JNC). Editorial handling by: R. Grun References Boggs Jr., S., Krinsley, D., 2006. Application of Cathodoluminescence Imaging to the Study of Sedimentary Rocks. Cambridge University Press, Cambridge, p. 165. Boggs Jr., S., Krinsley, D.H., Goles, G.G., Setedolali, A., Dypvik, H., 2001. Identification of shocked quartz by scanning cathodoluminescence imaging. Meteoritics and Planetary Science 36, 783–791. Botis, S., Nokhrin, S.M., Pan, Y., Xu, Y., Bonli, T., 2005. Natural radiation-induced damage in quartz. I. Correlations between cathodoluminescence colors and paramagnetic defects. The Canadian Mineralogist 43, 1565–1580. Go¨tze, J., Plo¨tze, M., Habermann, D., 2001. Origin, spectral characteristics and practical applications of the cathodoluminescence (CL) of quartz—a review. Mineralogy and Petrology 71, 225–250. Gucsik, A., Koeberl, C., Brandsta¨tter, F., Libowitzky, E., Reimold, W.U., 2003. Scanning electron microscopy, cathodoluminescence, and Raman spectroscopy of experimentally shock-metamorphosed quartzite. Meteoritics and Planetary Science 38, 1187–1197. Ikenaga, M., Nishido, H., Ninagawa, K., 2000. Perfomance and analytical conditions of cathodoluminescence scanning electron microscope (CLSEM). The Bulletin of Research Institute of Natural Sciences, Okayama University of Science 26, 61–75 (in Japanese with English abstract). Komuro, K., Horikawa, Y., Toyoda, S., 2002. Development of radiationdamage halos in low-quartz: cathodoluminescence measurement after He+ ion implantation. Mineralogy and Petrology 76, 261–266. Matter, A., Ramseyer, K., 1985. Cathodoluminescence microscopy as a tool for provenance studies of sandstones. In: Zuffa, G.G. (Ed.), Provenance of Arenites. D. Reidel Publishing Co., Boston, pp. 191–211. Meunier, J.D., Sellier, E., Pagel, M., 1990. Radiation-damage rims in quartz from uranium-bearing sandstones. Journal of Sedimentary Research 60, 53–58. Mu¨ller, A., Lennox, P., Trzebski, R., 2002. Cathodoluminescence and micro-structural evidence for crystallization and deformation processes of granites in the Eastern Lachlan Fold belt (SE Australia). Contributions to Mineralogy and Petrology 143, 510–524. Nasdala, L., Wenzel, M., Andrut, M., Wirth, R., Blaum, P., 2001. The nature of radiohaloes in biotite: experimental studies and modeling. American Mineralogist 86, 498–512. Owen, M.R., 1988. Radiation-damage halos in quartz. Geology 16, 529–532. Pagel, M., Barbin, V., Blanc, P., Ohnenstetter, D., 2000. Cathodoluminescence in Geosciences. Springer, Berlin, Heidelberg, New York, p. 514. Ramseyer, K., Baumann, J., Matter, A., Mullis, J., 1988. Cathodoluminescence colours of alpha-quartz. Mineralogical Magazine 52, 669–677. Ramseyer, K., Mullis, J., 1990. Factors influencing short-lived blue cathodoluminescence of alpha-quartz. American Mineralogist 75, 791–800. Smith, J.V., Stenstrom, R.C., 1965. Electron-excited luminescence as a petrologic tool. Journal of Geology 73, 627–635. Watt, G.R., Wright, P., Galloway, S., McLean, C., 1997. Cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts. Geochimica et Cosmochimica Acta 61, 4337–4348. Zinkernagel, U., 1978. Cathodoluminescence of quartz and its application to sandstone petrology. Contributions to Sedimentology 8, 1–69.
Quaternary Geochronology 3 (2008) 342–345 www.elsevier.com/locate/quageo
Research Paper
Evaluation of radiation-damage halos in quartz by cathodoluminescence as a geochronological tool T. Okumuraa,, H. Nishidoa, S. Toyodab, T. Kanekob, S. Kosugib, Y. Sawadab, K. Komuroc a
Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan b Department of Applied Physics, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan c Institute of Geoscience, University of Tsukuba, Ten-nodai, Ibaraki 305-8571, Japan Received 27 September 2007; accepted 27 January 2008 Available online 5 February 2008
Abstract Detailed quantitative cathodoluminescence (CL) imaging analysis was carried out for radiation-damage halos observed by CL (CL halo) created in natural quartz by implantation of 4 MeV He+ ions. The band of CL halo was approximately 14 mm in width and was constant for any He+ ion dose. The width of the halo is consistent with the theoretical range of 4He ions in quartz. A quantitative response of CL intensity to He+ ion dose was obtained, leading to the application of CL halos to geodosimetrical use. The CL intensity increases exponentially in the luminescent band from the implantation surface to the inside, until it reaches a maximum at 14 mm depth, with a rapid decrease beyond this point. This result is as predicted by Bragg’s law, although we find some differences between the CL intensity and the theoretical stopping power. r 2008 Elsevier Ltd. All rights reserved. Keywords: Cathodoluminescence; Radiation-damage halo; CL halo; Geodosimeter
1. Introduction Cathodoluminescence (CL) is an optical phenomenon that is based on the generation of visible radiation through sample excitation by high-energy electrons. CL occurs as a result of radiative electronic transition, in which an electron transfers from a higher energy state to a lower one. The CL method has been used to determine the impurities of trace elements such as rare earth and transition metal elements and to detect lattice defects. Recently, CL microscopy and spectroscopy have been actively used in petrology and mineralogy (e.g., Pagel et al., 2000; Boggs and Krinsley, 2006). CL in quartz has been extensively applied to research of growth zonation (e.g., Watt et al., 1997), identification of trace elements and their distributions (e.g., Mu¨ller et al., 2002), shock effects by impact events (e.g., Boggs et al., 2001; Gucsik et al., 2003) and provenance studies (e.g., Zinkernagel, 1978; Matter and Ramseyer, 1985). Corresponding author. Tel./fax: +81 86 256 9406.
E-mail address: [email protected] (T. Okumura). 1871-1014/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2008.01.006
Radiation-damage halos in quartz are not observable with a polarized microscope. They can be detected only with a CL microscope and are called CL halos (Smith and Stenstrom, 1965; Owen, 1988; Ramseyer et al., 1988; Meunier et al., 1990; Go¨tze et al., 2001; Botis et al., 2005). Radiation-damage halos caused mainly by a-particles are recognized in mineral grains of quartz and biotite with radioactive uranium- and thorium-bearing mineral inclusions (e.g., Nasdala et al., 2001). Owen (1988) demonstrated that the CL halos in quartz grains from igneous and metamorphic rocks were created by a-radiation. Furthermore, the size of the CL halo was consistent with the predicted range of a-particles from the 238U and 232Th decay series, suggesting that CL halos can be used as a dating tool if the CL contrast can be quantified. Komuro et al. (2002) first observed CL halos in synthetic quartz irradiated experimentally by He+ ions, and revealed that the width of the halos was consistent with the theoretical range estimated by the Bragg–Kleenman rule. CCD analysis of CL halos showed a correlation between CL intensity of the halo and the He+ ion dose, suggesting the application of the CL halo in quartz to geodosimetry.
ARTICLE IN PRESS T. Okumura et al. / Quaternary Geochronology 3 (2008) 342–345
Radiation-damage halos in quartz have been investigated mainly with CL microscopes, which are used for color CL imaging. Spectral measurements and highresolution CL imaging of CL halos have not been quantitatively examined so far, however, because they are micrometers in size and because halos in natural quartz may have a complex cause due to the presence of a mixed radiation field. The purpose of this study is to quantitatively measure the intensities of CL halos due to a-particle radiation from He+ ions implanted experimentally in natural quartz, using a scanning electron microscopecathodoluminescence (SEM-CL) system. We also compare the CL halo dose-response with theoretically simulated halos to illustrate the possible applications of CL halos to radiation dosimetry and geochronology.
SE He+
CL
XX
Y Y
halo
epoxy resin
2. Samples and methods
host material
surface 1200
Y
X
1000 CL intensity (a.u.)
A single crystal of colorless optical grade quartz from Minas Gerais, Brazil, was employed for CL measurements. The sample was cut into slices (10 10 1 mm3) with the square face perpendicular to the c-axis. The slices were polished and finished with a diamond abrasive of 1 mm particle size. He+ ion implantation in the sample was performed in a 3 M-tandem ion accelerator at the Takasaki Research Center of the Japan Atomic Energy Research Institute. To enable a comparison with previous experiments on synthetic quartz (Komuro et al., 2002), the energy of He+ was set at 4 MeV. This corresponds to the energy of a-particles emitted from the decay of 238U (4.18 MeV). The homogeneous He+ ion beam was irradiated perpendicular to the surface of the slice. Implantation was made under four sets of conditions, with He+ ion doses ranging from 1.772 10 5 to 3.544 10 4 C/cm2. For the CL experiments, the sliced samples were cut vertically to the induced surfaces for cross-sectional observation of the CL halos. The cut chips were embedded together in non-luminescent resin for CL measurement under the same conditions, with their cut sections facing the surface of the resin block. After abrasive finishing of the block surface with 1 mm diamond particles, a 20 nm coat of evaporated carbon was applied to the surface to prevent the build-up of electrical charge during the CL operation. CL imaging was carried out on a SEM-CL system comprising a SEM (JEOL JSM-5410LV) with a gratingtype monochromator (Oxford Instruments MonoCL2). CL emission from the sample surface was collected using a retractable parabolic mirror with high collection efficiency and resolved by scanning grating with 1200 grooves/mm. Monochromatic light was measured by the photon counting method using a Hamamatsu R2228 photomultiplier tube (PMT). Details of the construction of the equipment and analytical system are described in Ikenaga et al. (2000). Operation conditions for SEM-CL imaging and SEM imaging were set at an acceleration voltage of 15 kV with a beam current of 1.0 nA. Images were collected from a
343
800 600 400 200 0
−5
0
5
10 15 Distance (µm)
20
25
30
Fig. 1. Secondary electron (a) and SEM-CL (b) images of natural quartz after He+ ion implantation with a dose of 3.544 10 4 C/cm2. Scale bar is 10 mm. (c) change of CL intensity with ion implantation depth from the surface to the inside, as obtained by CL line analysis.
rectangular area, of 38 27 mm in the scanning mode. CL line analysis was carried out under the same conditions as SEM imaging, by measuring 200 points across a distance of 35 mm along the line X–Y in Fig. 1b. 3. Results and discussion CL halos were observed in all samples after He+ ion implantation, whereas no radiation-damage features were detected in the SEM images (Fig. 1a) or from optical observations using a polarized microscope. Fig. 1 shows SEM and SEM-CL images of the sample irradiated by He+ ions with a dose of 3.544 10 4 C/cm2. CL halos were recognized in the SEM-CL images as a luminescent band with a width of about 14 mm parallel to the irradiated surface (Fig. 1b). This width for the CL halo is consistent with the theoretical range of a-particles resulting from disintegration of 238U in quartz, as calculated by Owen
ARTICLE IN PRESS 344
T. Okumura et al. / Quaternary Geochronology 3 (2008) 342–345
(1988), and it agrees closely with that observed previously in synthetic quartz after He+ ion implantation under the same condition using a CL microscope, LUMINOSCOPE (Komuro et al., 2002). The monochromatic CL intensity of quartz is weak and easily changed during electron beam irradiation (Ramseyer and Mullis, 1990). Komuro et al. (2002) also reported an alteration in the CL color of experimentally He+ ionimplanted quartz during observation with a CL microscope. CL line analysis in the panchromatic CL mode was carried out along the line X–Y shown on the SEM-CL image (Fig. 1b) at 200 points (spread over a distance of 35 mm) at 1 point/s. The intensity of CL emission within the halo band gradually increased from the implantation surface to a depth of 14 mm (Fig. 1c). Regardless of the He+ ion dose, this feature of CL halos was almost the same for all samples, whereas the brightness of the CL halo emission increased with He+ ion dose. An increase of He+ ion dose leads to an enhancement of the overall intensity of the CL halo area, similar to the results obtained from CL imaging. The distribution pattern of CL intensity (shown in Fig. 1c) has a similar appearance to Bragg’s curve, which relates specific energy loss to specific ionization along the track of a charged particle (see discussion below). This pattern of CL intensity is consistent with that previously obtained in a study of synthetic quartz, using a CCD camera and a CL microscope (Komuro et al., 2002). The He+ ion dose can be related to the integrated CL intensity (Fig. 2) by subtracting the measured intensity of the host material from that of the CL halo area (the shaded area in Fig. 1c). The integrated CL intensity increases with dose density, indicating that the CL intensity is a function of the population of lattice defects created by He+ ion implantation. Komuro et al. (2002) also reported that the red component of CL halos (detected by color CCD analysis) was enhanced by an increase in dose density. Here we show that the panchromatic analysis of CL halo
Fig. 2. The relationship between integrated CL intensity of the halo area (obtained by CL line analysis) and dose density.
intensity could be used to investigate the dosimetry of natural radiation and, hence, for dating using the radiation damage in quartz. As CL in quartz corresponds to lattice defects created by radiation (Botis et al., 2005), its intensity should be a function of the amount of energy deposited by radiation, which is related to stopping power. The distribution of stopping power in quartz for 4 MeV 4He ions was obtained by theoretical calculations and is shown in Fig. 3a and b, together with the distribution of measured CL intensities obtained by the present experiments. The pattern of CL distributions is similar to ‘‘Bragg’s peak’’ derived from theoretical considerations. The stopping power peaks at 12 mm from the surface and is zero at a depth of 12.7 mm, while the CL intensity peaks at depths of 13–14 mm, decreasing to zero at 16 mm depth. Such a discrepancy between theory and practice might be caused by the irradiation conditions, because the true dose depends on the position of the sample relative to the radiation source and on secondary radiation from the collision of He+ ions with the sample holder. Furthermore, the effective area
Fig. 3. The relationship between CL intensities (obtained by CL line analysis) and the stopping power for 4 MeV 4He ions (derived from theoretical calculations) for the quartz samples after He+ ion implantation at 4 MeV with dose densities of 3.544 10 4 C/cm2 (a) and 5.907 10 5 C/cm2 (b).
ARTICLE IN PRESS T. Okumura et al. / Quaternary Geochronology 3 (2008) 342–345
excited by the electron beam should be approximately 2 mm in diameter, which is several times larger than the spot size of the electron beam (o0.5 mm). Hence, measurement at steps of o0.2 mm for CL line analysis may be influenced by the capture of afterglow from a previously irradiated spot. This could result in a broadening of the peak shape and a shift in the peak position of the CL intensity distribution, relative to Bragg’s peak. The vertical scales in Fig. 3a and b have been adjusted so that the stopping power and CL intensity coincide for depths of 1–10 mm in Fig. 3a. The latter plot indicates that these two quantities are proportional to each other for stopping power in the range 350–650 MeV/mm. At the high end of this range of stopping power, the number of lattice defects created per unit of radiation energy decreases, due to the higher probability of recombination: this explains the CL intensity being lower than that for the stopping power at depths of between 10 and 12 mm in Fig. 3a. In Fig. 3b, however, the tendency is opposite: that is, the CL intensity is higher than the corresponding value of the stopping power for depths greater than 10 mm. These observations indicate that the relationships among these factors (stopping power, concentration of lattice defects, and CL intensity) are complex. Further studies, including CL spectrum and electron spin resonance (ESR) analyses, are needed to clarify these problems. 4. Conclusions Radiation-damage halos in natural quartz created by He+ ion implantation were quantitatively measured using a high-resolution CL imaging system. The band of the CL halo was about 14 mm in width, which is roughly consistent with the theoretical range. We confirmed the results of our previous study (using a CCD camera with low spatial resolution) that the width of the CL halo was constant for any He+ ion dose but that the intensity of the halo was enhanced by increasing the dose. The dose response of the integrated CL intensity of the halo was quantitatively evaluated by CL line analysis. In this way, high-resolution CL analysis of CL halos could prove useful for geodosimetry of natural radiation. A detailed study of the distribution of CL intensity with depth in the luminescent band showed that the intensity increases exponentially from the implantation surface to the interior, reaching a maximum at 14 mm depth and decreasing rapidly beyond that point. This pattern is consistent qualitatively with the curve predicted by Bragg’s law, although there are some differences between the measured CL intensities and theoretical calculations of the stopping power in quartz for 4 MeV 4He ions. Acknowledgments The authors are grateful to Kiyotaka Ninagawa, Okayama University of Science, for valuable advice in relation to SEM-CL analysis. We thank Arnold Gucsik,
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Max Plank Institute for Chemistry, for improving the manuscript. This study was supported by the InterUniversity Program for the Joint Use of the Japan Atomic Energy Research Institute (JAERI) Facilities and by the public fellowship of the Japan Nuclear Cycle Development Institute (JNC). Editorial handling by: R. Grun References Boggs Jr., S., Krinsley, D., 2006. Application of Cathodoluminescence Imaging to the Study of Sedimentary Rocks. Cambridge University Press, Cambridge, p. 165. Boggs Jr., S., Krinsley, D.H., Goles, G.G., Setedolali, A., Dypvik, H., 2001. Identification of shocked quartz by scanning cathodoluminescence imaging. Meteoritics and Planetary Science 36, 783–791. Botis, S., Nokhrin, S.M., Pan, Y., Xu, Y., Bonli, T., 2005. Natural radiation-induced damage in quartz. I. Correlations between cathodoluminescence colors and paramagnetic defects. The Canadian Mineralogist 43, 1565–1580. Go¨tze, J., Plo¨tze, M., Habermann, D., 2001. Origin, spectral characteristics and practical applications of the cathodoluminescence (CL) of quartz—a review. Mineralogy and Petrology 71, 225–250. Gucsik, A., Koeberl, C., Brandsta¨tter, F., Libowitzky, E., Reimold, W.U., 2003. Scanning electron microscopy, cathodoluminescence, and Raman spectroscopy of experimentally shock-metamorphosed quartzite. Meteoritics and Planetary Science 38, 1187–1197. Ikenaga, M., Nishido, H., Ninagawa, K., 2000. Perfomance and analytical conditions of cathodoluminescence scanning electron microscope (CLSEM). The Bulletin of Research Institute of Natural Sciences, Okayama University of Science 26, 61–75 (in Japanese with English abstract). Komuro, K., Horikawa, Y., Toyoda, S., 2002. Development of radiationdamage halos in low-quartz: cathodoluminescence measurement after He+ ion implantation. Mineralogy and Petrology 76, 261–266. Matter, A., Ramseyer, K., 1985. Cathodoluminescence microscopy as a tool for provenance studies of sandstones. In: Zuffa, G.G. (Ed.), Provenance of Arenites. D. Reidel Publishing Co., Boston, pp. 191–211. Meunier, J.D., Sellier, E., Pagel, M., 1990. Radiation-damage rims in quartz from uranium-bearing sandstones. Journal of Sedimentary Research 60, 53–58. Mu¨ller, A., Lennox, P., Trzebski, R., 2002. Cathodoluminescence and micro-structural evidence for crystallization and deformation processes of granites in the Eastern Lachlan Fold belt (SE Australia). Contributions to Mineralogy and Petrology 143, 510–524. Nasdala, L., Wenzel, M., Andrut, M., Wirth, R., Blaum, P., 2001. The nature of radiohaloes in biotite: experimental studies and modeling. American Mineralogist 86, 498–512. Owen, M.R., 1988. Radiation-damage halos in quartz. Geology 16, 529–532. Pagel, M., Barbin, V., Blanc, P., Ohnenstetter, D., 2000. Cathodoluminescence in Geosciences. Springer, Berlin, Heidelberg, New York, p. 514. Ramseyer, K., Baumann, J., Matter, A., Mullis, J., 1988. Cathodoluminescence colours of alpha-quartz. Mineralogical Magazine 52, 669–677. Ramseyer, K., Mullis, J., 1990. Factors influencing short-lived blue cathodoluminescence of alpha-quartz. American Mineralogist 75, 791–800. Smith, J.V., Stenstrom, R.C., 1965. Electron-excited luminescence as a petrologic tool. Journal of Geology 73, 627–635. Watt, G.R., Wright, P., Galloway, S., McLean, C., 1997. Cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts. Geochimica et Cosmochimica Acta 61, 4337–4348. Zinkernagel, U., 1978. Cathodoluminescence of quartz and its application to sandstone petrology. Contributions to Sedimentology 8, 1–69.