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Guiding and blocking of highly charged ions through a single glass capillary
Nuclear Instruments and Methods in Physics Research B 267 (2009) 2381–2384
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Guiding and blocking of highly charged ions through a single glass capillary R. Nakayama a, M. Tona a, N. Nakamura a, H. Watanabe a, N. Yoshiyasu a, C. Yamada a, A. Yamazaki a,*, S. Ohtani a, M. Sakurai b a b
CREST/JST, Institute for Laser Science, Department of Applied Physics and Chemistry, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan Department of Physics, Kobe University, Kobe 657-8501, Japan
a r t i c l e
i n f o
Article history: Received 18 July 2008 Received in revised form 8 April 2009 Available online 23 April 2009 PACS: 34.35.+a
a b s t r a c t Highly charged ions produced in an electron beam ion trap, Iqþ , q = 10–50, were transmitted through a tapered glass capillary having diameter of 50 lm at the end. We found that for a particular beam current, there exists an optimum tilting angle of the capillary in which a steady output of ions is observed, while for smaller angles, the ion counts first rise, then gradually decay on a time scale of minutes. In the case of steady transmission, the charge state distribution is found to be slightly towards the lower side. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Highly charged ion Capillary
1. Introduction Guiding of highly charged ions through capillaries (capillary arrays or a single capillary) is attracting much interest due to the possibility of placing a single highly charged ion at a desired point on the substrate, with nanometer-scale accuracy. Such an ion produces a single ‘‘nanodot,” or a hit mark with certainty [1]. When a highly charged ion interacts with a solid surface, it releases its huge potential energy on a small area of a few tens of square nanometers in a short of time 10 fs. As a result, it causes remarkable effects, such as a high yield of secondary particle emission [2–4], and formation of hillock or crater nanostructures whose morphology and sizes strongly depend on the ion charge and the materials to be exposed [5–9]. The mechanism of ion guiding inside an insulating capillary is explained tentatively as self-organized formation of patches of charged areas, which reflect succeeding ions and just allow them to pass through [10,11]. Stolterfoht et al. [12,13] were the first to recognize that insulator (polyethylene terephthalate [PET]) nano capillaries can be used to manipulate the highly charged ion beams. When the capillary axis is tilted with respect to the incident beam direction, the transmitted ions are guided through the capillary without losing their charges. Kanai et al. [14] studied the angular distribution of ions transmitted through the nano capillary array. Ikeda et al. [15] used a single glass capillary of macroscopic size for the first time.
* Corresponding author. E-mail address: [email protected] (A. Yamazaki). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.04.008
In our experiment, three features are distinctive: (1) we use a macroscopic capillary (though it tapers to an end hole of micron size). (2) The capillary is made of pyrex glass. Although these features are similar to those of the experiment of Ikeda et al. (3) We employed very highly charged ions Iqþ , q = 10–50, instead of Ne7þ [12–14], or Ar8þ [15]. Our interest was, first of all, whether the guiding is observed similarly in very highly charged ions, and if so, whether there is any difference or restriction for the very highly charged ions. 2. Experimental Highly charged ions of iodine were generated in an electron beam ion trap called the Tokyo EBIT [16,17] and transported to the observation chamber via the beam line [18]. The source material was methyl iodide ðCH3 IÞ, and the resultant ions were Iqþ with q = 10–50, mixed with the carbon ions, and residual nitrogen and oxygen. We used these ions together, i.e. no attempt was made to select a single charge-to-mass ratio. This is because the flux of the charge-selected ions is very small ð 104 ions=s mm2 Þ to sufficiently charge up the inner wall of the capillary, and therefore would not show a guiding effect. A tapered glass capillary was made by extending a glass tube IWAKI TE-32 (boro-silicate glass) having an outer diameter of 2 mm and an inner diameter of 0.8 mm while heating in a ring of Kanthal wire. By adjusting the temperature and the force of extension, it was possible to obtain a capillary of the desired length and diameter. In this experiment, we used one with a length of 30 mm
2382
R. Nakayama et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2381–2384
300
Intensity [cps]
250 200 150 100 50 0 0
50
100
150
200
Time [s] Fig. 2. Typical time dependence of the number of transmitted ions. Tilt angle was 0.25°. Fig. 1. Experimental setup 50 lmØ aperture and the glass capillary are interchangeable using Z motion. Table 1 Fitted parameters to Eqn (1).
and outlet diameter of 50 lm. The transmission experiment was performed at room temperature. Fig. 1 shows our experimental setup. The capillary was set on a z h manipulator, which enabled linear motion along the vertical axis and the rotation in the horizontal plane. The ion beam after passing through a 10 mm slit proceeded into the capillary, before which a 0:6 mmØ metal aperture was placed in close contact with the front face of the glass tube to prevent it from charging up by the ions. The area of the aperture was 0:28 mm2 . Another 50 l mØ aperture was set on the same plate, which was used to measure the ion flux. The ions after passing either the capillary or the 50 lmØ aperture were detected with a multichannel plate (MCP) having a position-sensitive detector (PSD). The MCP was placed 300 mm downstream of the outlet of the capillary. The base pressure of the vacuum chamber was 5 106 Pa. The ions were extracted from the EBIT with an acceleration voltage of 3 kV. From here, the ion current is referred to as the count per second of the ions detected after the 50 lmØ aperture, 2000 counts/s corresponding to 1 pA=mm2 , assuming an average charge state of 20þ . The angular divergence of the incident beam was about 2 mrad (at 90 intensity), and that of the beam emitting the glass capillary was estimated to be 10 mrad. The zero of the tilt angle was determined experimentally to be the angle for which the ions hit the same position on the PSD as that hit by the ions after passing the 50 l mØ aperture. 3. Results and discussion 3.1. Low ion current We observed the temporal behavior of the ion count. A typical example is shown in Fig. 2, which was obtained with a tilt angle of 0.25° and ion flux of 650 cps=50 lmØ . Similar curves were obtained for various tilt angles ð0:25—1 Þ for the same ion flux. These curves were fitted well to the following formula [13]:
t t0 JðtÞ ¼ J 1 1 exp ;
s
ð1Þ
where J1 is the saturation current, t 0 the delay time, and s the time constant. This formula has been derived by considering the ion guiding mechanism to be a two-step process. The first step is accumulation of the charge, consuming all the incident ions and the sec-
ht 0.25 0.25 0.75 1.0
J1 (cps)
t 0 (s)
s (s)
41.0 ± 0.6 220 ± 2 240 ± 2 120 ± 2
1.8 ± 1.4 0.2 ± 0.7 2.9 ± 0.8 20 ± 1
20 ± 2 12 ± 1 31 ± 2 51 ± 3
ond step is the gradual building up of a self-organized charge distribution to allow the ions to pass. As a measure of the charge necessary for stable transmission, we calculate t0 þ s, and found that this value increases rapidly with tilt angle, suggesting that more ions are needed to achieve guiding at larger tilt angles (see Table 1). This table also shows an asymmetry with respect to the tilt angle, suggesting structural asymmetry of this capillary. Moreover, it is to be noted that the transmitted ion count is 220 or 240 in the favorable cases, which is no more than the count transmitted through the 50 lmØ aperture (650 cps). Since the outlet diameter of the capillary is also 50 lm, transmission is only one-third of the geometrically possible value. The rest of the ions were consumed to compensate the discharge of the surface charge through the glass. This is in sharp contrast to the results of Ikeda et al. [15]. 3.2. High ion current Similarly, ion transmission was observed for a higher incident ion flux. The ion count was 1300—1750 cps=50 lmØ aperture, inevitably varying from run to run due to the instability of the EBIT. However, for most of the runs, the flux was nearly at the highest end of the range. It was found that at lower tilt angles, the number of transmitted ions grows at first, but soon decreases. There seemed to exist a preferable tilt angle at which a steady flow was achieved(2.25°, in this case). Transmission behavior for each tilt angle is shown in Fig. 3. This behavior, although it has not been reported, is quite understandable because for small tilt angle, i.e. for small angle of incidence with respect to the glass surface, only a small amount of charge is needed to reflect succeeding ions. Here we must be aware that for the same potential distribution, the trajectories are the same for any charge state of the ions, because we extract the ions with the same voltage, provided the incident path is the same. Such motion would further accumulate unnecessary charge on another part of the surface. Finally, the whole capillary would be positively charged, sufficient to block further ions, even if they were discharging along the surface or in the bulk of the glass
R. Nakayama et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2381–2384
2383
Fig. 3. Transmission of highly charged ions as a function of tilt angle. Tilt angles are denoted at the top-left corner of each graph. Vertical axes represent transmitted ion count, in the same scale. Horizontal axes represent time in seconds.
[10]. For a greater tilt angle, more charge is needed to reflect in the larger angle. Therefore the balance of charging and discharging is achieved at a higher current, which allows the succeeding ions to pass without touching the wall, in a self-organized manner. This effect is largely dependent on the conductivity of the glass material(temperature dependent) and also the divergence of the incident ion beam. 3.3. Charge state distribution It is interesting to compare the charge distribution before and after passing through the capillary. In order to observe this, we made use of the change of the pulse hight distribution of the MCP, with the charge state of the incident highly charged ions.
[18] It is known that, although the pulse height of the MCP signal has intrinsic statistical nature, the average or median of the pulse height distribution is almost linearly dependent on the charge state [19]. Therefore, we can recast the voltage axis of pulse-height distribution into charge state. Fig. 4 shows the difference of the net pulse-height distribution for the ions passed through the 50 lmØ aperture and through the capillary. It is evident from this figure that the pulse height is shifted towards the lower side for the capillary transmission. From a separate experiment using the same MCP, with the same voltage applied to the MCP, we found that I50þ ion produces a pulse height distribution having a peak at 2.7 V, and I10þ having 0.8 V. This may suggest the rate of charge exchange (with the glass surface) is higher for the higher charge state ions.
2384
R. Nakayama et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2381–2384
Intensity [arb. units]
(COE) Program, ‘‘Innovation in Coherent Optical Science” at the University of Electro-Communications. This work was also supported by KAKENHI 19550013. References
0
1
2
3
4
Pulse Height [V] Fig. 4. Comparison of pulse-height distributions between the ions transmitted through the 50 lmØ aperture (denoted by circles) and through the glass capillary (squares).
4. Conclusion In summary, we have observed the transmission characteristics of very highly charged ions through a glass capillary of macroscopic dimension, but having a lm-size outlet. We found a preferable angle for the steady transmission, and blocking of the ions at the smaller angles. The charge state distribution suffers from slight deterioration probably due to partial charge transfer. Acknowledgements This work was conducted partially under the auspices of the CREST programm, ‘‘Creation of Ultrafast, Ultralow Power, Superperformance Nano devices and Systems”, in the Japan Science and Technology Agency and the 21st Century Center of Excellence
[1] N. Yoshiyasu, S. Takahashi, M. Shibata, H. Shimizu, K. Nagata, N. Nakamura, M. Tona, M. Sakurai, C. Yamada, S. Ohtani, Jpn. J. Appl. Phys. 45 (2006) 995. [2] S. Takahashi, M. Tona, K. Nagata, N. Nakamura, N. Yoshiyasu, C. Yamada, S. Ohtani, M. Sakurai, Appl. Phys. Lett. 87 (2005) 063111. [3] S. Takahashi, K. Nagata, M. Tona, M. Sakurai, N. Nakamura, C. Yamada, S. Ohtani, Surf. Sci. 593 (2005) 318. [4] M. Tona, K. Nagata, S. Takahashi, N. Nakamura, N. Yoshiyasu, M. Sakurai, C. Yamada, S. Ohtani, Surf. Sci. 600 (2006) 124. [5] Y. Baba, K. Nagata, S. Takahashi, N. Nakamura, N. Yoshiyasu, M. Sakurai, C. Yamada, S. Ohtani, M. Tona, Surf. Sci. 599 (2005) 248. [6] M. Tona, H. Watanabe, S. Takahashi, N. Nakamura, N. Yoshiyasu, M. Sakurai, C. Yamada, S. Ohtani, Nucl. Instr. and Meth. B 258 (2007) 163. [7] M. Tona, H. Watanabe, S. Takahashi, N. Nakamura, N. Yoshiyasu, M. Sakurai, T. Terui, S. Mashiko, C. Yamada, S. Ohtani, Surf. Sci. 601 (2007) 723. [8] A.S. Cl-Said, R. Heller, W. Messil, R. Ritter, S. Facsko, C. Lemell, B. Solleder, I.C. Gebeshuber, G. Betz, M. Toulemonde, W. Möller, J. Burgdörfer, F. Aumayr, PRL 100 (2008) 237601. [9] M. Tona, Y. Fujita, C. Yamada, S. Ohtani, Phys. Rev. B 77 (2008) 155427. [10] K. Tökési, L. Wirtz, C. Lemell, J. Burgdörfer, Phys. Rev. A 61 (2000) 020901. [11] K. Schiessl, W. Palfinger, K. Tökési, H. Nowotony, C. Lemell, J. Burgödrfer, Phys. Rev. A 72 (2005) 062902. [12] N. Stolterfoht, J.-H Bremer, V. Hoffmann, R. Hellhammer, D. Fink, A. Petrov, B. Sulik, Phys. Rev. Lett. 88 (2002) 133201. [13] N. Stolterfoht, R. Hellhammer, J. Bundesmann, D. Fink, Y. Kanai, M. Hoshino, T. Kambara, T. Ikeda, Y. Yamazaki, Phys. Rev. A 76 (2007) 022712. [14] Y. Kanai, M. Hoshino, T. Kambara, T. Ikeda, R. Hellhammer, N. Stolterfoht, Y. Yamazaki, Nucl. Instr. and Meth. B 258 (2007) 155. [15] T. Ikeda, Y. Kanai, T.M. Kojima, Y. Iwai, T. Kambara, Y. Yamazaki, M. Hoshino, T. Nebiki, T. Narusawa, Appl. Phys. Lett. 89 (2006) 163502. [16] F.J. Currell, J. Asada, K. Ishii, A. Minoh, K. Motohashi, N. Nakamura, K. Nishizawa, S. Ohtani, K. Okazaki, M. Sakurai, H. Shiraishi, Seiji, Tsurubuchi, H. Watanabe, J. Phys. Soc. Jpn. 65 (1996) 3186. [17] H. Watanabe, J. Asada, F.J. Currell, T. Fukami, T. Hirayama, K. Motohashi, N. Nakamura, E. Nojikawa, S. Ohtani, K. Okazaki, M. Sakurai, H. Shimizu, N. Tada, S. Tsurubuchi, J. Phys. Soc. Jpn. 66 (1997) 3795. [18] H. Shimizu, F.J. Currell, S. Ohtani, E.J. Sokell, C. Yamada, T. Hirayama, M. Sakurai, Rev. Sci. Instrum. 71 (2000) 681. [19] S. Takagi, T. Iwai, Y. Kaneko, M. Kimura, N. Kobayashi, A. Matsumoto, S. Ohtani, K. Okuno, H. Tawara, S. Tsurubuchi, Nucl. Instr. and Meth. B 215 (1983) 207.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Guiding and blocking of highly charged ions through a single glass capillary R. Nakayama a, M. Tona a, N. Nakamura a, H. Watanabe a, N. Yoshiyasu a, C. Yamada a, A. Yamazaki a,*, S. Ohtani a, M. Sakurai b a b
CREST/JST, Institute for Laser Science, Department of Applied Physics and Chemistry, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan Department of Physics, Kobe University, Kobe 657-8501, Japan
a r t i c l e
i n f o
Article history: Received 18 July 2008 Received in revised form 8 April 2009 Available online 23 April 2009 PACS: 34.35.+a
a b s t r a c t Highly charged ions produced in an electron beam ion trap, Iqþ , q = 10–50, were transmitted through a tapered glass capillary having diameter of 50 lm at the end. We found that for a particular beam current, there exists an optimum tilting angle of the capillary in which a steady output of ions is observed, while for smaller angles, the ion counts first rise, then gradually decay on a time scale of minutes. In the case of steady transmission, the charge state distribution is found to be slightly towards the lower side. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Highly charged ion Capillary
1. Introduction Guiding of highly charged ions through capillaries (capillary arrays or a single capillary) is attracting much interest due to the possibility of placing a single highly charged ion at a desired point on the substrate, with nanometer-scale accuracy. Such an ion produces a single ‘‘nanodot,” or a hit mark with certainty [1]. When a highly charged ion interacts with a solid surface, it releases its huge potential energy on a small area of a few tens of square nanometers in a short of time 10 fs. As a result, it causes remarkable effects, such as a high yield of secondary particle emission [2–4], and formation of hillock or crater nanostructures whose morphology and sizes strongly depend on the ion charge and the materials to be exposed [5–9]. The mechanism of ion guiding inside an insulating capillary is explained tentatively as self-organized formation of patches of charged areas, which reflect succeeding ions and just allow them to pass through [10,11]. Stolterfoht et al. [12,13] were the first to recognize that insulator (polyethylene terephthalate [PET]) nano capillaries can be used to manipulate the highly charged ion beams. When the capillary axis is tilted with respect to the incident beam direction, the transmitted ions are guided through the capillary without losing their charges. Kanai et al. [14] studied the angular distribution of ions transmitted through the nano capillary array. Ikeda et al. [15] used a single glass capillary of macroscopic size for the first time.
* Corresponding author. E-mail address: [email protected] (A. Yamazaki). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.04.008
In our experiment, three features are distinctive: (1) we use a macroscopic capillary (though it tapers to an end hole of micron size). (2) The capillary is made of pyrex glass. Although these features are similar to those of the experiment of Ikeda et al. (3) We employed very highly charged ions Iqþ , q = 10–50, instead of Ne7þ [12–14], or Ar8þ [15]. Our interest was, first of all, whether the guiding is observed similarly in very highly charged ions, and if so, whether there is any difference or restriction for the very highly charged ions. 2. Experimental Highly charged ions of iodine were generated in an electron beam ion trap called the Tokyo EBIT [16,17] and transported to the observation chamber via the beam line [18]. The source material was methyl iodide ðCH3 IÞ, and the resultant ions were Iqþ with q = 10–50, mixed with the carbon ions, and residual nitrogen and oxygen. We used these ions together, i.e. no attempt was made to select a single charge-to-mass ratio. This is because the flux of the charge-selected ions is very small ð 104 ions=s mm2 Þ to sufficiently charge up the inner wall of the capillary, and therefore would not show a guiding effect. A tapered glass capillary was made by extending a glass tube IWAKI TE-32 (boro-silicate glass) having an outer diameter of 2 mm and an inner diameter of 0.8 mm while heating in a ring of Kanthal wire. By adjusting the temperature and the force of extension, it was possible to obtain a capillary of the desired length and diameter. In this experiment, we used one with a length of 30 mm
2382
R. Nakayama et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2381–2384
300
Intensity [cps]
250 200 150 100 50 0 0
50
100
150
200
Time [s] Fig. 2. Typical time dependence of the number of transmitted ions. Tilt angle was 0.25°. Fig. 1. Experimental setup 50 lmØ aperture and the glass capillary are interchangeable using Z motion. Table 1 Fitted parameters to Eqn (1).
and outlet diameter of 50 lm. The transmission experiment was performed at room temperature. Fig. 1 shows our experimental setup. The capillary was set on a z h manipulator, which enabled linear motion along the vertical axis and the rotation in the horizontal plane. The ion beam after passing through a 10 mm slit proceeded into the capillary, before which a 0:6 mmØ metal aperture was placed in close contact with the front face of the glass tube to prevent it from charging up by the ions. The area of the aperture was 0:28 mm2 . Another 50 l mØ aperture was set on the same plate, which was used to measure the ion flux. The ions after passing either the capillary or the 50 lmØ aperture were detected with a multichannel plate (MCP) having a position-sensitive detector (PSD). The MCP was placed 300 mm downstream of the outlet of the capillary. The base pressure of the vacuum chamber was 5 106 Pa. The ions were extracted from the EBIT with an acceleration voltage of 3 kV. From here, the ion current is referred to as the count per second of the ions detected after the 50 lmØ aperture, 2000 counts/s corresponding to 1 pA=mm2 , assuming an average charge state of 20þ . The angular divergence of the incident beam was about 2 mrad (at 90 intensity), and that of the beam emitting the glass capillary was estimated to be 10 mrad. The zero of the tilt angle was determined experimentally to be the angle for which the ions hit the same position on the PSD as that hit by the ions after passing the 50 l mØ aperture. 3. Results and discussion 3.1. Low ion current We observed the temporal behavior of the ion count. A typical example is shown in Fig. 2, which was obtained with a tilt angle of 0.25° and ion flux of 650 cps=50 lmØ . Similar curves were obtained for various tilt angles ð0:25—1 Þ for the same ion flux. These curves were fitted well to the following formula [13]:
t t0 JðtÞ ¼ J 1 1 exp ;
s
ð1Þ
where J1 is the saturation current, t 0 the delay time, and s the time constant. This formula has been derived by considering the ion guiding mechanism to be a two-step process. The first step is accumulation of the charge, consuming all the incident ions and the sec-
ht 0.25 0.25 0.75 1.0
J1 (cps)
t 0 (s)
s (s)
41.0 ± 0.6 220 ± 2 240 ± 2 120 ± 2
1.8 ± 1.4 0.2 ± 0.7 2.9 ± 0.8 20 ± 1
20 ± 2 12 ± 1 31 ± 2 51 ± 3
ond step is the gradual building up of a self-organized charge distribution to allow the ions to pass. As a measure of the charge necessary for stable transmission, we calculate t0 þ s, and found that this value increases rapidly with tilt angle, suggesting that more ions are needed to achieve guiding at larger tilt angles (see Table 1). This table also shows an asymmetry with respect to the tilt angle, suggesting structural asymmetry of this capillary. Moreover, it is to be noted that the transmitted ion count is 220 or 240 in the favorable cases, which is no more than the count transmitted through the 50 lmØ aperture (650 cps). Since the outlet diameter of the capillary is also 50 lm, transmission is only one-third of the geometrically possible value. The rest of the ions were consumed to compensate the discharge of the surface charge through the glass. This is in sharp contrast to the results of Ikeda et al. [15]. 3.2. High ion current Similarly, ion transmission was observed for a higher incident ion flux. The ion count was 1300—1750 cps=50 lmØ aperture, inevitably varying from run to run due to the instability of the EBIT. However, for most of the runs, the flux was nearly at the highest end of the range. It was found that at lower tilt angles, the number of transmitted ions grows at first, but soon decreases. There seemed to exist a preferable tilt angle at which a steady flow was achieved(2.25°, in this case). Transmission behavior for each tilt angle is shown in Fig. 3. This behavior, although it has not been reported, is quite understandable because for small tilt angle, i.e. for small angle of incidence with respect to the glass surface, only a small amount of charge is needed to reflect succeeding ions. Here we must be aware that for the same potential distribution, the trajectories are the same for any charge state of the ions, because we extract the ions with the same voltage, provided the incident path is the same. Such motion would further accumulate unnecessary charge on another part of the surface. Finally, the whole capillary would be positively charged, sufficient to block further ions, even if they were discharging along the surface or in the bulk of the glass
R. Nakayama et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2381–2384
2383
Fig. 3. Transmission of highly charged ions as a function of tilt angle. Tilt angles are denoted at the top-left corner of each graph. Vertical axes represent transmitted ion count, in the same scale. Horizontal axes represent time in seconds.
[10]. For a greater tilt angle, more charge is needed to reflect in the larger angle. Therefore the balance of charging and discharging is achieved at a higher current, which allows the succeeding ions to pass without touching the wall, in a self-organized manner. This effect is largely dependent on the conductivity of the glass material(temperature dependent) and also the divergence of the incident ion beam. 3.3. Charge state distribution It is interesting to compare the charge distribution before and after passing through the capillary. In order to observe this, we made use of the change of the pulse hight distribution of the MCP, with the charge state of the incident highly charged ions.
[18] It is known that, although the pulse height of the MCP signal has intrinsic statistical nature, the average or median of the pulse height distribution is almost linearly dependent on the charge state [19]. Therefore, we can recast the voltage axis of pulse-height distribution into charge state. Fig. 4 shows the difference of the net pulse-height distribution for the ions passed through the 50 lmØ aperture and through the capillary. It is evident from this figure that the pulse height is shifted towards the lower side for the capillary transmission. From a separate experiment using the same MCP, with the same voltage applied to the MCP, we found that I50þ ion produces a pulse height distribution having a peak at 2.7 V, and I10þ having 0.8 V. This may suggest the rate of charge exchange (with the glass surface) is higher for the higher charge state ions.
2384
R. Nakayama et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2381–2384
Intensity [arb. units]
(COE) Program, ‘‘Innovation in Coherent Optical Science” at the University of Electro-Communications. This work was also supported by KAKENHI 19550013. References
0
1
2
3
4
Pulse Height [V] Fig. 4. Comparison of pulse-height distributions between the ions transmitted through the 50 lmØ aperture (denoted by circles) and through the glass capillary (squares).
4. Conclusion In summary, we have observed the transmission characteristics of very highly charged ions through a glass capillary of macroscopic dimension, but having a lm-size outlet. We found a preferable angle for the steady transmission, and blocking of the ions at the smaller angles. The charge state distribution suffers from slight deterioration probably due to partial charge transfer. Acknowledgements This work was conducted partially under the auspices of the CREST programm, ‘‘Creation of Ultrafast, Ultralow Power, Superperformance Nano devices and Systems”, in the Japan Science and Technology Agency and the 21st Century Center of Excellence
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