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Effect of impurities in charge-exchange carbon foil on foil thickness reduction
Vacuum 86 (2012) 899e902
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of impurities in charge-exchange carbon foil on foil thickness reduction Yasuhiro Takeda a, *, Toshiharu Kadono b a b
High Energy Accelerator Research Organization (KEK), Oho1-1, Tsukuba-shi, Ibaraki-ken 305-0801, Japan Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2010 Received in revised form 23 March 2011 Accepted 5 April 2011
Carbon thin foils are commonly used as a charge stripping material in particle accelerators. Depending on the original foil thickness, changes in thickness during beam irradiation vary: thin foils (w10 mg/cm2) thicken by build-up, medium thickness foils (w15 mg/cm2) remain unchanged, and thick foils (w20 mg/ cm2) become thinner. The thickness reduction differs even under identical manufacturing processes and conditions. The factor causing foil thinning is unknown. On the basis of the low sputtering rate of carbon, it can be said that impurities contained in the foil cause foil thinning. Carbon foils contain impurities such as water. These impurities dissociate and combine with carbon and then evaporate. Taking this into consideration, we examined the gas composition during beam irradiation, to determine which impurity causes foil thinning. As a result, we found that oxygen contained in the foil plays a role in foil thinning. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Stripper foil Carbon foil Stripping Charge-exchange Gasses
1. Introduction Self-supporting carbon foils are used as stripper foils to peel off the electrons of charged particles in order to increase the energy of accelerated particles and achieve charge conversion in an accelerator. Because of recent higher accelerating currents, the short life span of conventional stripper foils has become a serious issue, and the development of long-life foils that can withstand high currents is now under way [1,2]. However, this extension of life has led to a change in the thickness of striper foils, which was not seen in case of shorter lifetimes. Therefore, we investigated thickness changes of stripper foils in a systematic manner using the beam of the Van de Graaff accelerator of the Faculty of Science of Tokyo Institute of Technology and found out that effects vary with differences in foil thickness. As indicated in Fig. 1, when foils were radiated with a Neþ beam of 3.2 MeV, 10 mA, under vacuum of 1 104 Pa, the thickness of thin foils (10 mg/cm2) increased (build-up) while that of medium foils (w15 mg/cm2) remained unchanged (constant) and that of thick foils (20 mg/cm2) decreased (thinning) [3,4]. In the case of build-up, hydrocarbons produced by the dissociation of residual gas molecules were deposited on carbon foils. However, the cause of thinning is not yet clear. When foils are radiated with beams, heated gas is released and surface atoms sputter from foil surfaces. In the case of an ion beam
* Corresponding author. Tel.: þ81 298 864 5576. E-mail address: [email protected] (Y. Takeda). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.04.029
of 400 eV, Neþ, for example, the sputtering rate of the atoms of metals in general or that of copper in particular is 1.55 atoms/ion, whereas that of carbon atoms is very smalldas small as 0.1 atoms/ ion. Therefore, the decrease in foil thickness by sputtering is almost negligible. On the other hand, in the case of the gases released, impurities contaminated in the foil are emitted or those that have penetrated the foil reach the surface by diffusion and are then desorbed. Ion excitation caused by ion irradiation causes chemical reactions. For example, hydrogen and oxygen produce methane and carbon monoxide by reaction with carbon. The composition of the residual gas should be examined to determine the cause of thinning, which we attempted in this study. Further, we aimed to determine the source of decrease in foil thickness by studying the relations between the components of gases released from foils and such reductions. 2. Experiment Ion excitation caused by ion irradiation causes chemical reactions. For example, hydrogen and oxygen produce methane and carbon monoxide by reaction with carbon. We examined gas composition during beam irradiation to determine which impurity causes foil thinning. The experiments were conducted in the Van de Graaff laboratory at the Faculty of Science of Tokyo Institute of Technology. A beam of 3.2 MeV Neþ ions (8 mm 4 and 10 mA), accelerated by a Van de Graaff accelerator, was applied to carbon foils placed in a scattering chamber. A turbo molecular pump (TMP; 550 l/s) for vacuum
900
Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902
Foil thickness ( g/cm2)
BEAM
Q-mass
Broken
30
View
Build-up 20
Port
Constant Broken
View Port
10 Thinning
Broken
TMP
0 Integrated irradiation (mC/cm2)
Fig. 1. Different features of foil thickness by beam irradiation. Thickness of a foil of approximately 7 mg/cm2 was build-up, and the foil was damaged as a beam was irradiated. Thickness of a foil of approximately 15 mg/cm2 was constant until the foil was damaged. Thickness of a foil of approximately 30 mg/cm2 decreased, and the foil was damaged with beam irradiation. As shown here, three patterns are possible, depending on the foil thickness.
pumping, a quadrupole mass spectrometer (Q-MASS) for residual gas measurement, and a solid-state detector (SSD) for foil thickness measurement were placed in the scattering chamber, facilitating observation of changes in residual gas and in foil thickness with time. The carbon foils, five of which were placed in the scattering chamber, were affixed to stainless steel frames having 15 mm diameter holes (Fig. 2). Long-lived carbon foils, 20e30 mg/cm2 thick and formed by ion beam sputtering, were used in the experiments. The foil thickness was measured by measuring the particles dispersed by Rutherford scattering with the SSD, which was placed 22.5 from and 1.5 m downstream from the foil. The thicknesses of commercial foils were based on known standard thicknesses; foil thicknesses were determined by counting carbon particle masses scattered from the measured foils. The residual gas was measured using an ANELVA QIG-066 partial pressure vacuum gauge. The range of mass numbers measured was 1e66 amu; the residual gas was calculated on a computer on a change-with-time basis. The vacuum was maintained at 1 104 Pa at the time of measurement. 3. Results and discussion A total of ten samples were radiated in the experiments. Fig. 3 shows photographs of foils before and after irradiation. Fig. 4 shows an example of change in foil thickness in comparison with the radiation value. The vertical axis represents foil thickness (mg/ cm2) and the horizontal axis represents the integral of beam
Carbon
Movable
foils
Transfer rod
View Port Faraday cup SSD
Fig. 2. Experimental setup (vacuum chamber). By bombarding a beam on a carbon foil set at the center and using an SSD set 1.5 m downstream of the foil to measure scattering particles, foil thickness was calculated. In order to observe the composition of the gas emitted from the foil, the composition of the residual gas was measured by using Q-mass while the chamber was kept evacuated.
radiation values (mC/cm2). Two effects are apparent. First, the thickness of the foil decreased significantly just as beam irradiation commenced. Second, the change in foil thickness started slowing down when the radiation value exceeded a certain level (500 mC/ cm2). The thickness of the foil decreased to 1/4the1/5th of that before irradiation; the foil was eventually destroyed. Fig. 5 shows the time variations of major components of the residual gases. The vertical and horizontal axes represent the partial pressure (Pa) and the integral of beam radiation values (mC/
Fig. 3. Photographs of foils before and after irradiation.
Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902
901
2
Foil thickness ( g/cm )
50 40 30 20 2
Broken at 1440 mC/cm 10 0
0
500
1000
1500
2
Integrated irradiation (mC/cm ) Fig. 4. Variation foil thickness with irradiation for the initial of 26 mg/cm2. Two effects are apparent. First, the thickness of the foil decreased significantly just as beam irradiation commenced. Second, the change in foil thickness started slowing down when the radiation value exceeded a certain level (500 mC/cm2).
cm2), respectively. COþ and N2þ were assumed to be proportional to the amounts of COþ (mass number 28) and N2þ (mass number 28) released; the separation of COþ and N2þ was calculated from their count ratios. The peak in Fig. 4 corresponds to the time point at which irradiation was started. No release of carbon alone occurred. Hydrocarbon atoms and carbonic oxides were primarily released, their discharge increasing to about twice to eighteen times the background. After about 3 mC/cm2, the gas released at the time of commencement of irradiation was attenuated by vacuum pumping and shifted to an equilibrium state. Analyses of the graphs of the released gases indicate that (1) the amount of hydrocarbon gas released remains almost unchanged from commencement to completion of beam irradiation and (2) a large amount of carbonic oxides is released immediately after
a
irradiation, but this amount gradually decreases. In particular, the amount of released CO decreases when the foil thickness changes. We performed composition analysis by Rutherford backscattering (RBS) to determine the cause of foil thinning. To understand the conditions of foil elements, we analyzed them before and after irradiation with RBS by using a beam (Heþ, 2 MeV, 500 nA, and 1.5 mm 4) produced by the Van de Graaff accelerator at Kyoto University. Fig. 6(a) shows the spectrum obtained before a beam was irradiated, and Fig. 6(b) shows that obtained after irradiation. The vertical and horizontal axes represent the number of counts and the number of channels in proportion to mass numbers, respectively. Peaks of C and O were observed. As the foils were formed in a vacuum, the amount of oxygen mixed must have been small. Therefore, the conceivable reason for
b
1.E-03
Foil broken 1.E-04
Pa
1.E-04
Pa
1.E-03
STRAT
1.E-05
1.E-05 1.E-06
1.E-06
1.E-07
1.E-07 -1
0
1
2
3
4
1420
5
1440
1460
1480
2
mC/cm
2
mC/cm
1.E-03
H2 CH4
1.E-04
Pa
H2O 1.E-05
N2 CO
1.E-06
O2 CO2
1.E-07 0
200
400
600
800
mC/cm
1000
1200
1400
2
Fig. 5. Time variation of major components of residual gases. The figure shows a zoomed graph of the time point when beam bombardment of a foil was started (a) and the time point when the foil was broken (b). When the irradiation was started, a large amount of gas was emitted. When the foil broke, no significant change in the amount of gas emission was observed.
902
Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902
a
b
Fig. 6. RBS spectrum taken with 2 MeV Heþ beam. (a) shows the spectrum obtained before beam irradiation, and (b) shows that obtained after the irradiation. Peaks of C, O, Si, Cu and W were observed. The Si, Cu and W are impurities. (Si: Oil of vacuum pump, Cu: filament guide of ion beam sputtering, W: filament of ion beam sputtering). It was observed that the peaks of O were far fewer than those of any other element after irradiation.
the mixing of oxygen is that a large amount of water was used to peel off foils from the boards. The water absorbed in the foils was split into hydrogen and oxygen by beam irradiation, and then, the oxygen bonded to carbon and evaporated as carbon oxides, as described by the following chemical formula. C þ H2O / CO þ H2 Thus, the amount of CO released had a large effect on the decrease in foil thickness, since 3.2 108 Pam3/m2s of CO was released, even though the amount of vacuum pumped was excluded. A comparison of foil before and after beam irradiation reveals that the width of the carbon peak of the latter remains sharp, indicating that the foil became thinner. On the other hand, much fewer peaks of O were observed, fewer in fact than those of any other element after irradiation, indicating that O evaporated from the foil. The measured count number of oxygen decreased to about one-fourth (1920 counts before to 541 counts after irradiation); the amount of this released gas was much larger than that of other elements, which did not change. Therefore, the carbonic oxides
produced on foils appear to be a major cause of the decrease in their thicknesses. 4. Conclusions In this study, we performed a detailed analysis of the gases released from foils to clarify the close relationship between the amount of carbon oxides released and the decrease in foil thickness during their exposure to beams. Oxygen content decreased with a decrease in foil thickness. We found that the presence of oxygen is the main cause of the decrease in the thickness. To retard this reduction, it is necessary to eliminate the oxygen contained in foils. References [1] Sugai I, Takeda Y, Oyaizu M, Kawakami H, Hattori Y, Kawasaki K, et al. Nucl Instr Meth A 2002;480:191. [2] Takeda Y, Irie Y, Sugai I, Takagi A, Oyaizu M, Kawakami H, et al. Vacuum 2010; 84:1448e51. [3] Sugai I, Takeda Y, Oyaizu M, Kaweakami H, Hattori T, Kawasaki K, et al. Proc.1st Symp. Beam science and technology for an emergent network (BESTEN). Tokyo: 2001. [4] Sugai I, Oyaizu M, Takeda Y, Kawakami H, Hattori Y, Kawasaki K. Nucl Instr Meth A 2008;590:32.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of impurities in charge-exchange carbon foil on foil thickness reduction Yasuhiro Takeda a, *, Toshiharu Kadono b a b
High Energy Accelerator Research Organization (KEK), Oho1-1, Tsukuba-shi, Ibaraki-ken 305-0801, Japan Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2010 Received in revised form 23 March 2011 Accepted 5 April 2011
Carbon thin foils are commonly used as a charge stripping material in particle accelerators. Depending on the original foil thickness, changes in thickness during beam irradiation vary: thin foils (w10 mg/cm2) thicken by build-up, medium thickness foils (w15 mg/cm2) remain unchanged, and thick foils (w20 mg/ cm2) become thinner. The thickness reduction differs even under identical manufacturing processes and conditions. The factor causing foil thinning is unknown. On the basis of the low sputtering rate of carbon, it can be said that impurities contained in the foil cause foil thinning. Carbon foils contain impurities such as water. These impurities dissociate and combine with carbon and then evaporate. Taking this into consideration, we examined the gas composition during beam irradiation, to determine which impurity causes foil thinning. As a result, we found that oxygen contained in the foil plays a role in foil thinning. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Stripper foil Carbon foil Stripping Charge-exchange Gasses
1. Introduction Self-supporting carbon foils are used as stripper foils to peel off the electrons of charged particles in order to increase the energy of accelerated particles and achieve charge conversion in an accelerator. Because of recent higher accelerating currents, the short life span of conventional stripper foils has become a serious issue, and the development of long-life foils that can withstand high currents is now under way [1,2]. However, this extension of life has led to a change in the thickness of striper foils, which was not seen in case of shorter lifetimes. Therefore, we investigated thickness changes of stripper foils in a systematic manner using the beam of the Van de Graaff accelerator of the Faculty of Science of Tokyo Institute of Technology and found out that effects vary with differences in foil thickness. As indicated in Fig. 1, when foils were radiated with a Neþ beam of 3.2 MeV, 10 mA, under vacuum of 1 104 Pa, the thickness of thin foils (10 mg/cm2) increased (build-up) while that of medium foils (w15 mg/cm2) remained unchanged (constant) and that of thick foils (20 mg/cm2) decreased (thinning) [3,4]. In the case of build-up, hydrocarbons produced by the dissociation of residual gas molecules were deposited on carbon foils. However, the cause of thinning is not yet clear. When foils are radiated with beams, heated gas is released and surface atoms sputter from foil surfaces. In the case of an ion beam
* Corresponding author. Tel.: þ81 298 864 5576. E-mail address: [email protected] (Y. Takeda). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.04.029
of 400 eV, Neþ, for example, the sputtering rate of the atoms of metals in general or that of copper in particular is 1.55 atoms/ion, whereas that of carbon atoms is very smalldas small as 0.1 atoms/ ion. Therefore, the decrease in foil thickness by sputtering is almost negligible. On the other hand, in the case of the gases released, impurities contaminated in the foil are emitted or those that have penetrated the foil reach the surface by diffusion and are then desorbed. Ion excitation caused by ion irradiation causes chemical reactions. For example, hydrogen and oxygen produce methane and carbon monoxide by reaction with carbon. The composition of the residual gas should be examined to determine the cause of thinning, which we attempted in this study. Further, we aimed to determine the source of decrease in foil thickness by studying the relations between the components of gases released from foils and such reductions. 2. Experiment Ion excitation caused by ion irradiation causes chemical reactions. For example, hydrogen and oxygen produce methane and carbon monoxide by reaction with carbon. We examined gas composition during beam irradiation to determine which impurity causes foil thinning. The experiments were conducted in the Van de Graaff laboratory at the Faculty of Science of Tokyo Institute of Technology. A beam of 3.2 MeV Neþ ions (8 mm 4 and 10 mA), accelerated by a Van de Graaff accelerator, was applied to carbon foils placed in a scattering chamber. A turbo molecular pump (TMP; 550 l/s) for vacuum
900
Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902
Foil thickness ( g/cm2)
BEAM
Q-mass
Broken
30
View
Build-up 20
Port
Constant Broken
View Port
10 Thinning
Broken
TMP
0 Integrated irradiation (mC/cm2)
Fig. 1. Different features of foil thickness by beam irradiation. Thickness of a foil of approximately 7 mg/cm2 was build-up, and the foil was damaged as a beam was irradiated. Thickness of a foil of approximately 15 mg/cm2 was constant until the foil was damaged. Thickness of a foil of approximately 30 mg/cm2 decreased, and the foil was damaged with beam irradiation. As shown here, three patterns are possible, depending on the foil thickness.
pumping, a quadrupole mass spectrometer (Q-MASS) for residual gas measurement, and a solid-state detector (SSD) for foil thickness measurement were placed in the scattering chamber, facilitating observation of changes in residual gas and in foil thickness with time. The carbon foils, five of which were placed in the scattering chamber, were affixed to stainless steel frames having 15 mm diameter holes (Fig. 2). Long-lived carbon foils, 20e30 mg/cm2 thick and formed by ion beam sputtering, were used in the experiments. The foil thickness was measured by measuring the particles dispersed by Rutherford scattering with the SSD, which was placed 22.5 from and 1.5 m downstream from the foil. The thicknesses of commercial foils were based on known standard thicknesses; foil thicknesses were determined by counting carbon particle masses scattered from the measured foils. The residual gas was measured using an ANELVA QIG-066 partial pressure vacuum gauge. The range of mass numbers measured was 1e66 amu; the residual gas was calculated on a computer on a change-with-time basis. The vacuum was maintained at 1 104 Pa at the time of measurement. 3. Results and discussion A total of ten samples were radiated in the experiments. Fig. 3 shows photographs of foils before and after irradiation. Fig. 4 shows an example of change in foil thickness in comparison with the radiation value. The vertical axis represents foil thickness (mg/ cm2) and the horizontal axis represents the integral of beam
Carbon
Movable
foils
Transfer rod
View Port Faraday cup SSD
Fig. 2. Experimental setup (vacuum chamber). By bombarding a beam on a carbon foil set at the center and using an SSD set 1.5 m downstream of the foil to measure scattering particles, foil thickness was calculated. In order to observe the composition of the gas emitted from the foil, the composition of the residual gas was measured by using Q-mass while the chamber was kept evacuated.
radiation values (mC/cm2). Two effects are apparent. First, the thickness of the foil decreased significantly just as beam irradiation commenced. Second, the change in foil thickness started slowing down when the radiation value exceeded a certain level (500 mC/ cm2). The thickness of the foil decreased to 1/4the1/5th of that before irradiation; the foil was eventually destroyed. Fig. 5 shows the time variations of major components of the residual gases. The vertical and horizontal axes represent the partial pressure (Pa) and the integral of beam radiation values (mC/
Fig. 3. Photographs of foils before and after irradiation.
Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902
901
2
Foil thickness ( g/cm )
50 40 30 20 2
Broken at 1440 mC/cm 10 0
0
500
1000
1500
2
Integrated irradiation (mC/cm ) Fig. 4. Variation foil thickness with irradiation for the initial of 26 mg/cm2. Two effects are apparent. First, the thickness of the foil decreased significantly just as beam irradiation commenced. Second, the change in foil thickness started slowing down when the radiation value exceeded a certain level (500 mC/cm2).
cm2), respectively. COþ and N2þ were assumed to be proportional to the amounts of COþ (mass number 28) and N2þ (mass number 28) released; the separation of COþ and N2þ was calculated from their count ratios. The peak in Fig. 4 corresponds to the time point at which irradiation was started. No release of carbon alone occurred. Hydrocarbon atoms and carbonic oxides were primarily released, their discharge increasing to about twice to eighteen times the background. After about 3 mC/cm2, the gas released at the time of commencement of irradiation was attenuated by vacuum pumping and shifted to an equilibrium state. Analyses of the graphs of the released gases indicate that (1) the amount of hydrocarbon gas released remains almost unchanged from commencement to completion of beam irradiation and (2) a large amount of carbonic oxides is released immediately after
a
irradiation, but this amount gradually decreases. In particular, the amount of released CO decreases when the foil thickness changes. We performed composition analysis by Rutherford backscattering (RBS) to determine the cause of foil thinning. To understand the conditions of foil elements, we analyzed them before and after irradiation with RBS by using a beam (Heþ, 2 MeV, 500 nA, and 1.5 mm 4) produced by the Van de Graaff accelerator at Kyoto University. Fig. 6(a) shows the spectrum obtained before a beam was irradiated, and Fig. 6(b) shows that obtained after irradiation. The vertical and horizontal axes represent the number of counts and the number of channels in proportion to mass numbers, respectively. Peaks of C and O were observed. As the foils were formed in a vacuum, the amount of oxygen mixed must have been small. Therefore, the conceivable reason for
b
1.E-03
Foil broken 1.E-04
Pa
1.E-04
Pa
1.E-03
STRAT
1.E-05
1.E-05 1.E-06
1.E-06
1.E-07
1.E-07 -1
0
1
2
3
4
1420
5
1440
1460
1480
2
mC/cm
2
mC/cm
1.E-03
H2 CH4
1.E-04
Pa
H2O 1.E-05
N2 CO
1.E-06
O2 CO2
1.E-07 0
200
400
600
800
mC/cm
1000
1200
1400
2
Fig. 5. Time variation of major components of residual gases. The figure shows a zoomed graph of the time point when beam bombardment of a foil was started (a) and the time point when the foil was broken (b). When the irradiation was started, a large amount of gas was emitted. When the foil broke, no significant change in the amount of gas emission was observed.
902
Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902
a
b
Fig. 6. RBS spectrum taken with 2 MeV Heþ beam. (a) shows the spectrum obtained before beam irradiation, and (b) shows that obtained after the irradiation. Peaks of C, O, Si, Cu and W were observed. The Si, Cu and W are impurities. (Si: Oil of vacuum pump, Cu: filament guide of ion beam sputtering, W: filament of ion beam sputtering). It was observed that the peaks of O were far fewer than those of any other element after irradiation.
the mixing of oxygen is that a large amount of water was used to peel off foils from the boards. The water absorbed in the foils was split into hydrogen and oxygen by beam irradiation, and then, the oxygen bonded to carbon and evaporated as carbon oxides, as described by the following chemical formula. C þ H2O / CO þ H2 Thus, the amount of CO released had a large effect on the decrease in foil thickness, since 3.2 108 Pam3/m2s of CO was released, even though the amount of vacuum pumped was excluded. A comparison of foil before and after beam irradiation reveals that the width of the carbon peak of the latter remains sharp, indicating that the foil became thinner. On the other hand, much fewer peaks of O were observed, fewer in fact than those of any other element after irradiation, indicating that O evaporated from the foil. The measured count number of oxygen decreased to about one-fourth (1920 counts before to 541 counts after irradiation); the amount of this released gas was much larger than that of other elements, which did not change. Therefore, the carbonic oxides
produced on foils appear to be a major cause of the decrease in their thicknesses. 4. Conclusions In this study, we performed a detailed analysis of the gases released from foils to clarify the close relationship between the amount of carbon oxides released and the decrease in foil thickness during their exposure to beams. Oxygen content decreased with a decrease in foil thickness. We found that the presence of oxygen is the main cause of the decrease in the thickness. To retard this reduction, it is necessary to eliminate the oxygen contained in foils. References [1] Sugai I, Takeda Y, Oyaizu M, Kawakami H, Hattori Y, Kawasaki K, et al. Nucl Instr Meth A 2002;480:191. [2] Takeda Y, Irie Y, Sugai I, Takagi A, Oyaizu M, Kawakami H, et al. Vacuum 2010; 84:1448e51. [3] Sugai I, Takeda Y, Oyaizu M, Kaweakami H, Hattori T, Kawasaki K, et al. Proc.1st Symp. Beam science and technology for an emergent network (BESTEN). Tokyo: 2001. [4] Sugai I, Oyaizu M, Takeda Y, Kawakami H, Hattori Y, Kawasaki K. Nucl Instr Meth A 2008;590:32.