- Email: [email protected]
SIMS–AMS depth profiles for NASA Genesis samples: Preliminary measurements
Applied Surface Science 255 (2008) 1479–1481
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
SIMS–AMS depth profiles for NASA Genesis samples: Preliminary measurements C. Cetina a,*, K.S. Grabowski b, D.L. Knies b, L.T. Demoranville c a
Nova Research, Inc., Alexandria, VA 22308, United States Naval Research Laboratory, Washington, DC 20375, United States c University of Maryland, College Park, MD 20742, United States b
A R T I C L E I N F O
A B S T R A C T
Article history:
A wide variety of elements present in solar wind were collected during a 2-year space flight by the NASA Genesis Discovery mission. The high-value Genesis samples are presently analyzed by a few groups using SIMS and other techniques. For some of the more challenging measurements a combined SIMS–AMS facility may provide the advantages of accelerator mass spectrometry (AMS) to SIMS analysis, including molecular fragmentation and low-background detection. Initial results from simulating standards are presented here, demonstrating the capability of the system for this kind of surface analysis. Measurements were performed on internally produced standards consisting of Si wafers implanted with 1014 atoms/cm2 doses of Mg, Ti, Fe, Ni, Cu, and Zn. At the ion source stage, crater-edge effects were filtered by position gating, while at the spectrograph focal plane, possible interferences were rejected by coincidence position-energy detection. Thus, SIMS-like depth profiles were obtained and will be used for estimating system-specific relative sensitivity factors. ß 2008 Elsevier B.V. All rights reserved.
Available online 8 May 2008 Keywords: SIMS Accelerator mass spectrometry Trace element Genesis discovery mission Solar wind
1. Introduction To better understand the evolution of the solar system one can study the compositional differences of present-day planetary objects relative to the solar nebula as a baseline. The elemental and isotopic compositions of the sun can be inferred from solar-wind measurements. However, direct determination of present-day sun composition has been made possible only recently through solar samples returned to Earth by the NASA Genesis Discovery mission [1]. These samples consist of ultra-pure passive substrates exposed to solar wind during a 2-year flight period. Different collector materials target different elements and include Si, Ge, diamondlike carbon (DLC), epitaxial Si on sapphire (SoS), etc. [2]. Solar-wind elements, or high-energy ions streaming from the surface of the sun, have velocities in the well-understood ion implantation regime and were captured in the upper 100 nm from the collector surface, thus making implanted standards suitable for preliminary measurements. The high-value Genesis samples are presently analyzed by a few groups around the globe. Among the different techniques used for this purpose, SIMS plays a key role by having the required sensitivity to analyze many of the elements present in Genesis samples. The primary limitation of SIMS comes from molecular
* Corresponding author. Tel.: +1 202 767 4433; fax: +1 202 767 5301. E-mail address: [email protected] (C. Cetina). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.055
interferences, an aspect especially relevant for the Genesis samples where a large number of solar-wind elements are present together in the same collector material. A solution is available by providing the advantages of accelerator mass spectrometry (AMS) to SIMS analysis in a combined SIMS–AMS facility, where molecular interferences can be removed by molecular break-up. Additionally, ions accelerated at higher energies can be detected with much lower background. This type of facility has been designed at a few laboratories and has proved its utility mainly in trace-element measurements such as depth profiling analysis of electronic materials [3,4]. 2. Experimental technique The Trace Element AMS (TEAMS) facility at the Naval Research Lab (NRL) in Washington, DC has been reconfigured to incorporate a SIMS-type ion source. The details were presented elsewhere [5,6]. In summary, a modified Cameca IMS 6F SIMS provides a Cs primary beam with magnetic analysis, standard sample chamber, and secondary column with dynamic transfer optics. The accelerator facility is based on a 3-MV tandem Pelletron accelerator and supplants the mass spectrometer portion of the instrument. Additionally, the system has the unique feature of allowing for parallel mass analysis, thus improving isotope ratio precision. Selected ion beams are injected simultaneously into the accelerator through the Pretzel recombination magnet. This magnet provides mass filtering at the image plane inside the magnet by use
1480
C. Cetina et al. / Applied Surface Science 255 (2008) 1479–1481
of a mass-selecting mask. After acceleration, the ions emerge with a given charge state distribution and only one desired charge state is selected for analysis. By choosing the most probable charge state one can minimize transmission losses. All of the ions in the selected charge state are then sent to the spectrograph magnet for parallel detection. For these measurements, there were three kinds of detectors set in the focal plane of the spectrograph. The matrix beam is monitored by an ETP1420 electron multiplier. The low-concentration element’s positions and thus the mass-to-charge ratios are determined using a large-area position-sensitive microchannel plate (MCP) detector [7]. Depending on the mass region, the MCP could detect a span of up to about 18-u wide (in the actinide region) simultaneously. The MCP is used in transmission mode and is followed by a silicon-implanted energy detector. However, out of 10 cm of active MCP width, only 1.3 cm of it is followed by the energy detector. The combination of two detectors is used to remove possible mass-to-charge ambiguities. The initial measurements with the reconfigured facility proved the system’s ability to break apart injected molecules during the acceleration process and specific examples were presented previously for Pd [6] and Ag [8]. Consequently, synchronization has been implemented between primary beam location and final detection. Thus, spatial distribution and depth profile capabilities were investigated and are reported below. Further facility developments include replacing the solid-state energy detector with a gas ionization chamber equipped with thin silicon nitride window. This is expected to provide a twofold advantage: much improved energy resolution and, most importantly, the possibility to use better populated charge states. The latter was a very restrictive aspect up to now due to the solid-state detector suffering severe radiation damage, especially for the heavier elements, for which the SIMS–AMS technique is better suited. 3. Initial standard analysis The solar-wind elements present in the Genesis Si collector substrates include most of the periodic table [1]. The selection of elements suitable for analysis with the SIMS–AMS system at NRL was made based on estimated 2-year exposure fluences in the first 100 nm from the collector material surface and relative sensitivity factors (RSFs) for elements in Si. The most favorable species are P, Cl, Fe, and Ni, having concentration-to-RSF ratios of 10 8 or higher, and are to be studied directly as atomic species. At the same level are Mg, Al, Ca, and Cr, but for these cases the silicide molecules yield higher negative-ion currents. The molecules will be injected, broken into atomic components during acceleration and atomic species analyzed in the spectrograph. A third and more challenging tier include atomic F and Se at the 10 9 level. Also at this level but using the silicide molecules are Na and Mn, using the oxide
Fig. 1. Measured depth profile of 100 keV 58Ni implanted in Si with a 1014 atoms/ cm2 dose. Charge state 5+ (14.4 MeV) was analyzed.
molecules are Mn, Zn, and Ga, and using the hydride molecules are Mg, Ca, Sc, Ti, Cr, Mn, Fe, Zn, Sr, and Zr. Initial results were obtained from ion-implanted standards produced in house. The standards consist of Si wafer implanted with two-to-three ion species of 100 keV energy at 1013 to 1014 atoms/cm2 levels. The available standards relevant for solarwind studies contain Mg, Al, Ti, Cr, Fe, Ni, Cu, and Zn. Three case studies illustrating the capabilities of the facility are presented here. For all three of them the implanted dose was 1014 atoms/cm2. Measurements were done with the terminal voltage set at 2.4 MV. Of the species with a 10 8 concentration-to-RSF ratio atomic, Ni was selected as an example of a more routine measurement. A Fe– Ni–Cu implanted standard was used. 58Ni was injected, transmitted and analyzed in parallel with the 30Si matrix beam to be used for normalization. The selection mask in the injection magnet had two 1-u wide openings and the selected charge state was 5+, yielding 14.4 MeV ions. The depth profile for 58Ni is presented in Fig. 1. The irregularities in the spectrum are probably due to system instabilities not handled properly at the time of this preliminary measurement. An order of magnitude more counts are expected by using 3+ instead of 5+ when a more robust gas detector will replace the solid-state one. The case of Fe is presented as an example to illustrate the power of background reduction using filtering by coincidence between position/mass and energy data. The same Fe–Ni–Cu implanted standard was used; 56Fe was the isotope of interest; the 29Si matrix beam was analyzed simultaneously; and again charge state 5+ was selected. Fig. 2a shows the MCP position/mass spectrum. The hashed area represents the mass 56 u region and the dashed lines delimit the area backed by the energy detector. The energy spectrum is shown in Fig. 2b. It is clear that the peak at 56 u position includes not only ions in charge state 5+ (14.4 MeV) but
Fig. 2. 56Fe implanted in Si standard: (a) MCP position/mass spectrum; (b) energy spectrum in the solid-state detector; (c) position/mass vs. energy spectrum with number of events represented by the size of the box. Dashed lines in (a) show the mass region covered by the energy detector; box in (c) shows events attributed to 56Fe5+.
C. Cetina et al. / Applied Surface Science 255 (2008) 1479–1481
Fig. 3. Measured depth profile with energy cut (solid) and without (dotted) for 100 keV 56Fe implanted in Si with a 1014 atoms/cm2 dose, with charge state 5+ analyzed.
1481
The ability of the SIMS–AMS system to break apart injected molecules and analyze the resulting atomic fragments is illustrated using the example of Mg, whose silicide molecule has a higher efficiency for negative-ion production then the atomic species. An Mg–Al–Cr–Mo implanted standard was used for this case. Mass 52 u was injected, charge state 3+ (9.6 MeV) was selected, and mass 24 u was analyzed using the same technique as described for Fe. The resulting depth profile for 24Mg is shown in Fig. 4. This was only a trial measurement and a better profile is expected with the system tuned for this particular situation. Elemental compositions of solar wind are known to be fractionated relative to the photosphere, with variations depending on the three regimes of solar wind. Elemental fractionation of solar wind is most strongly influenced by the first ionization potential (FIP). Different wind regimes have different fractionation factors for elements with FIP below or above a 9-eV threshold. All of the rock-forming elements and all of the available standards (but Zn) are low-FIP elements. Of the higher-FIP elements we intend to measure F, P, Cl, and Se concentrations in Genesis samples, for which a new set of standards will be used. 4. Conclusions
Fig. 4. Measured depth profile of 100 keV 24Mg implanted in Si. Mass 52 (MgSi molecule) was injected and mass 24 (atomic Mg) in charge state 3+ (9.6 MeV) was analyzed.
The combined SIMS–AMS facility at NRL has become operational and will hopefully provide significant contributions to the SIMS field. Depth profiles were obtained for a number of elements, of which three example case studies were presented here. The Fe example proves the usefulness of energy detection in solving massto-charge ambiguities present at apparent mass 56 for this case. The example for atomic Mg injected as MgSi demonstrates unambiguously the ability of the system to analyze atomic species resulted from injected molecules fragmented during acceleration. Better results are expected after implementation of a gas detector with thin silicon nitride window, allowing use of lower charge states that are up to an order of magnitude more populated. Also, studies of post-terminal stripping will be undertaken as another way to further improve sensitivity. Acknowledgements
also in 2+ (7.2 MeV). Only the 14.4 MeV peak corresponds to 56Fe 5+ atomic beam, while the other peak originates from a molecule break-up resulting in fragments with same E/Q ratio as the iron atomic beam. The 2D mass vs. energy scatter plot in Fig. 2c is represented in boxed format, with the box size proportional with the number of events. Atomic 56Fe 5+ events are clearly identified. The low energy background under the 56Fe might be consistent with a Na 2+ fragment originating from a Na-containing molecule. Fig. 3 shows 56Fe depth profiles for two cases: including events attributed to atomic 56Fe 5+ only (solid histogram), and including all the events present at apparent mass 56 u (dotted line). For this case the energy selection provided an order of magnitude reduction in background. Further reduction is expected in the future by taking advantage of improved energy resolution offered by a gas energy detector.
The authors gratefully appreciate the technical support of Claire Kennedy and Britton Renfro. We acknowledge the financial support of NASA Sample Return Laboratory Instrument and Data Analysis Program (SRLIDAP) and ONR in acquiring the SIMS source and implementing the facility upgrade. References [1] [2] [3] [4] [5] [6] [7] [8]
D.S. Burnett, et al. Space Sci. Rev. 105 (2003) 509. A.J.G. Jurewicz, et al. Space Sci. Rev. 105 (2003) 535. J.M. Anthony, et al. Nucl. Instrum. Methods B 50 (1990) 262. S.A. Datar, et al. Nucl. Instrum. Methods B 123 (1997) 571. D.L. Knies, K.S. Grabowski, C. Cetina, Nucl. Instrum. Methods B 259 (2007) 118. K.S. Grabowski, D.L. Knies, C. Cetina, Nucl. Instrum. Methods B 259 (2007) 27. C. Cetina, et al. Nucl. Instrum. Methods A 513 (2001) 403. D.L. Knies, K.S. Grabowski, C. Cetina, Appl. Surf. Sci. 252 (2006) 7297.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
SIMS–AMS depth profiles for NASA Genesis samples: Preliminary measurements C. Cetina a,*, K.S. Grabowski b, D.L. Knies b, L.T. Demoranville c a
Nova Research, Inc., Alexandria, VA 22308, United States Naval Research Laboratory, Washington, DC 20375, United States c University of Maryland, College Park, MD 20742, United States b
A R T I C L E I N F O
A B S T R A C T
Article history:
A wide variety of elements present in solar wind were collected during a 2-year space flight by the NASA Genesis Discovery mission. The high-value Genesis samples are presently analyzed by a few groups using SIMS and other techniques. For some of the more challenging measurements a combined SIMS–AMS facility may provide the advantages of accelerator mass spectrometry (AMS) to SIMS analysis, including molecular fragmentation and low-background detection. Initial results from simulating standards are presented here, demonstrating the capability of the system for this kind of surface analysis. Measurements were performed on internally produced standards consisting of Si wafers implanted with 1014 atoms/cm2 doses of Mg, Ti, Fe, Ni, Cu, and Zn. At the ion source stage, crater-edge effects were filtered by position gating, while at the spectrograph focal plane, possible interferences were rejected by coincidence position-energy detection. Thus, SIMS-like depth profiles were obtained and will be used for estimating system-specific relative sensitivity factors. ß 2008 Elsevier B.V. All rights reserved.
Available online 8 May 2008 Keywords: SIMS Accelerator mass spectrometry Trace element Genesis discovery mission Solar wind
1. Introduction To better understand the evolution of the solar system one can study the compositional differences of present-day planetary objects relative to the solar nebula as a baseline. The elemental and isotopic compositions of the sun can be inferred from solar-wind measurements. However, direct determination of present-day sun composition has been made possible only recently through solar samples returned to Earth by the NASA Genesis Discovery mission [1]. These samples consist of ultra-pure passive substrates exposed to solar wind during a 2-year flight period. Different collector materials target different elements and include Si, Ge, diamondlike carbon (DLC), epitaxial Si on sapphire (SoS), etc. [2]. Solar-wind elements, or high-energy ions streaming from the surface of the sun, have velocities in the well-understood ion implantation regime and were captured in the upper 100 nm from the collector surface, thus making implanted standards suitable for preliminary measurements. The high-value Genesis samples are presently analyzed by a few groups around the globe. Among the different techniques used for this purpose, SIMS plays a key role by having the required sensitivity to analyze many of the elements present in Genesis samples. The primary limitation of SIMS comes from molecular
* Corresponding author. Tel.: +1 202 767 4433; fax: +1 202 767 5301. E-mail address: [email protected] (C. Cetina). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.055
interferences, an aspect especially relevant for the Genesis samples where a large number of solar-wind elements are present together in the same collector material. A solution is available by providing the advantages of accelerator mass spectrometry (AMS) to SIMS analysis in a combined SIMS–AMS facility, where molecular interferences can be removed by molecular break-up. Additionally, ions accelerated at higher energies can be detected with much lower background. This type of facility has been designed at a few laboratories and has proved its utility mainly in trace-element measurements such as depth profiling analysis of electronic materials [3,4]. 2. Experimental technique The Trace Element AMS (TEAMS) facility at the Naval Research Lab (NRL) in Washington, DC has been reconfigured to incorporate a SIMS-type ion source. The details were presented elsewhere [5,6]. In summary, a modified Cameca IMS 6F SIMS provides a Cs primary beam with magnetic analysis, standard sample chamber, and secondary column with dynamic transfer optics. The accelerator facility is based on a 3-MV tandem Pelletron accelerator and supplants the mass spectrometer portion of the instrument. Additionally, the system has the unique feature of allowing for parallel mass analysis, thus improving isotope ratio precision. Selected ion beams are injected simultaneously into the accelerator through the Pretzel recombination magnet. This magnet provides mass filtering at the image plane inside the magnet by use
1480
C. Cetina et al. / Applied Surface Science 255 (2008) 1479–1481
of a mass-selecting mask. After acceleration, the ions emerge with a given charge state distribution and only one desired charge state is selected for analysis. By choosing the most probable charge state one can minimize transmission losses. All of the ions in the selected charge state are then sent to the spectrograph magnet for parallel detection. For these measurements, there were three kinds of detectors set in the focal plane of the spectrograph. The matrix beam is monitored by an ETP1420 electron multiplier. The low-concentration element’s positions and thus the mass-to-charge ratios are determined using a large-area position-sensitive microchannel plate (MCP) detector [7]. Depending on the mass region, the MCP could detect a span of up to about 18-u wide (in the actinide region) simultaneously. The MCP is used in transmission mode and is followed by a silicon-implanted energy detector. However, out of 10 cm of active MCP width, only 1.3 cm of it is followed by the energy detector. The combination of two detectors is used to remove possible mass-to-charge ambiguities. The initial measurements with the reconfigured facility proved the system’s ability to break apart injected molecules during the acceleration process and specific examples were presented previously for Pd [6] and Ag [8]. Consequently, synchronization has been implemented between primary beam location and final detection. Thus, spatial distribution and depth profile capabilities were investigated and are reported below. Further facility developments include replacing the solid-state energy detector with a gas ionization chamber equipped with thin silicon nitride window. This is expected to provide a twofold advantage: much improved energy resolution and, most importantly, the possibility to use better populated charge states. The latter was a very restrictive aspect up to now due to the solid-state detector suffering severe radiation damage, especially for the heavier elements, for which the SIMS–AMS technique is better suited. 3. Initial standard analysis The solar-wind elements present in the Genesis Si collector substrates include most of the periodic table [1]. The selection of elements suitable for analysis with the SIMS–AMS system at NRL was made based on estimated 2-year exposure fluences in the first 100 nm from the collector material surface and relative sensitivity factors (RSFs) for elements in Si. The most favorable species are P, Cl, Fe, and Ni, having concentration-to-RSF ratios of 10 8 or higher, and are to be studied directly as atomic species. At the same level are Mg, Al, Ca, and Cr, but for these cases the silicide molecules yield higher negative-ion currents. The molecules will be injected, broken into atomic components during acceleration and atomic species analyzed in the spectrograph. A third and more challenging tier include atomic F and Se at the 10 9 level. Also at this level but using the silicide molecules are Na and Mn, using the oxide
Fig. 1. Measured depth profile of 100 keV 58Ni implanted in Si with a 1014 atoms/ cm2 dose. Charge state 5+ (14.4 MeV) was analyzed.
molecules are Mn, Zn, and Ga, and using the hydride molecules are Mg, Ca, Sc, Ti, Cr, Mn, Fe, Zn, Sr, and Zr. Initial results were obtained from ion-implanted standards produced in house. The standards consist of Si wafer implanted with two-to-three ion species of 100 keV energy at 1013 to 1014 atoms/cm2 levels. The available standards relevant for solarwind studies contain Mg, Al, Ti, Cr, Fe, Ni, Cu, and Zn. Three case studies illustrating the capabilities of the facility are presented here. For all three of them the implanted dose was 1014 atoms/cm2. Measurements were done with the terminal voltage set at 2.4 MV. Of the species with a 10 8 concentration-to-RSF ratio atomic, Ni was selected as an example of a more routine measurement. A Fe– Ni–Cu implanted standard was used. 58Ni was injected, transmitted and analyzed in parallel with the 30Si matrix beam to be used for normalization. The selection mask in the injection magnet had two 1-u wide openings and the selected charge state was 5+, yielding 14.4 MeV ions. The depth profile for 58Ni is presented in Fig. 1. The irregularities in the spectrum are probably due to system instabilities not handled properly at the time of this preliminary measurement. An order of magnitude more counts are expected by using 3+ instead of 5+ when a more robust gas detector will replace the solid-state one. The case of Fe is presented as an example to illustrate the power of background reduction using filtering by coincidence between position/mass and energy data. The same Fe–Ni–Cu implanted standard was used; 56Fe was the isotope of interest; the 29Si matrix beam was analyzed simultaneously; and again charge state 5+ was selected. Fig. 2a shows the MCP position/mass spectrum. The hashed area represents the mass 56 u region and the dashed lines delimit the area backed by the energy detector. The energy spectrum is shown in Fig. 2b. It is clear that the peak at 56 u position includes not only ions in charge state 5+ (14.4 MeV) but
Fig. 2. 56Fe implanted in Si standard: (a) MCP position/mass spectrum; (b) energy spectrum in the solid-state detector; (c) position/mass vs. energy spectrum with number of events represented by the size of the box. Dashed lines in (a) show the mass region covered by the energy detector; box in (c) shows events attributed to 56Fe5+.
C. Cetina et al. / Applied Surface Science 255 (2008) 1479–1481
Fig. 3. Measured depth profile with energy cut (solid) and without (dotted) for 100 keV 56Fe implanted in Si with a 1014 atoms/cm2 dose, with charge state 5+ analyzed.
1481
The ability of the SIMS–AMS system to break apart injected molecules and analyze the resulting atomic fragments is illustrated using the example of Mg, whose silicide molecule has a higher efficiency for negative-ion production then the atomic species. An Mg–Al–Cr–Mo implanted standard was used for this case. Mass 52 u was injected, charge state 3+ (9.6 MeV) was selected, and mass 24 u was analyzed using the same technique as described for Fe. The resulting depth profile for 24Mg is shown in Fig. 4. This was only a trial measurement and a better profile is expected with the system tuned for this particular situation. Elemental compositions of solar wind are known to be fractionated relative to the photosphere, with variations depending on the three regimes of solar wind. Elemental fractionation of solar wind is most strongly influenced by the first ionization potential (FIP). Different wind regimes have different fractionation factors for elements with FIP below or above a 9-eV threshold. All of the rock-forming elements and all of the available standards (but Zn) are low-FIP elements. Of the higher-FIP elements we intend to measure F, P, Cl, and Se concentrations in Genesis samples, for which a new set of standards will be used. 4. Conclusions
Fig. 4. Measured depth profile of 100 keV 24Mg implanted in Si. Mass 52 (MgSi molecule) was injected and mass 24 (atomic Mg) in charge state 3+ (9.6 MeV) was analyzed.
The combined SIMS–AMS facility at NRL has become operational and will hopefully provide significant contributions to the SIMS field. Depth profiles were obtained for a number of elements, of which three example case studies were presented here. The Fe example proves the usefulness of energy detection in solving massto-charge ambiguities present at apparent mass 56 for this case. The example for atomic Mg injected as MgSi demonstrates unambiguously the ability of the system to analyze atomic species resulted from injected molecules fragmented during acceleration. Better results are expected after implementation of a gas detector with thin silicon nitride window, allowing use of lower charge states that are up to an order of magnitude more populated. Also, studies of post-terminal stripping will be undertaken as another way to further improve sensitivity. Acknowledgements
also in 2+ (7.2 MeV). Only the 14.4 MeV peak corresponds to 56Fe 5+ atomic beam, while the other peak originates from a molecule break-up resulting in fragments with same E/Q ratio as the iron atomic beam. The 2D mass vs. energy scatter plot in Fig. 2c is represented in boxed format, with the box size proportional with the number of events. Atomic 56Fe 5+ events are clearly identified. The low energy background under the 56Fe might be consistent with a Na 2+ fragment originating from a Na-containing molecule. Fig. 3 shows 56Fe depth profiles for two cases: including events attributed to atomic 56Fe 5+ only (solid histogram), and including all the events present at apparent mass 56 u (dotted line). For this case the energy selection provided an order of magnitude reduction in background. Further reduction is expected in the future by taking advantage of improved energy resolution offered by a gas energy detector.
The authors gratefully appreciate the technical support of Claire Kennedy and Britton Renfro. We acknowledge the financial support of NASA Sample Return Laboratory Instrument and Data Analysis Program (SRLIDAP) and ONR in acquiring the SIMS source and implementing the facility upgrade. References [1] [2] [3] [4] [5] [6] [7] [8]
D.S. Burnett, et al. Space Sci. Rev. 105 (2003) 509. A.J.G. Jurewicz, et al. Space Sci. Rev. 105 (2003) 535. J.M. Anthony, et al. Nucl. Instrum. Methods B 50 (1990) 262. S.A. Datar, et al. Nucl. Instrum. Methods B 123 (1997) 571. D.L. Knies, K.S. Grabowski, C. Cetina, Nucl. Instrum. Methods B 259 (2007) 118. K.S. Grabowski, D.L. Knies, C. Cetina, Nucl. Instrum. Methods B 259 (2007) 27. C. Cetina, et al. Nucl. Instrum. Methods A 513 (2001) 403. D.L. Knies, K.S. Grabowski, C. Cetina, Appl. Surf. Sci. 252 (2006) 7297.