- Email: [email protected]
High energy resolution PIXE analysis using focused MeV heavy ion beams
NOMB
Beam Interactions
with Materials 8 Atoms ELSEVIER
Nuclear
Instruments
and Methods
in Physics
Research
B 136138
(1998) 368-372
High energy resolution PIXE analysis using focused MeV heavy ion beams Y. Mokuno ‘**,Y. Horino ‘, T. Tad2 ‘-I, M. Terasawa b, T. Sekioka b, A. Chayahara A. Kinomura ‘, N. Tsubouchi ‘, K. Fujii a
‘,
Abstract
The possibility of chemical state microanalysis using high energy resolution PIXE was investigated using a plane crystal spectrometer installed at a heavy ion microbeam line. The spectrometer has the advantage for the analysis of an X-ray spectrum of simultaneously detecting X-rays over an energy range using position sensitive proportional counter without scanning the crystal. Though the detection efficiency is estimated to be at least four orders of magnitude lower than energy dispersive X-ray spectroscopy (EDS) using a Si(Li) detector, the resolution of the system (dE/E) is better than lo-?. because the effect of beam size on system resolution is negligible. Focused 2 MeV proton and 5 MeV Si”+ ion beams were employed for the analysis of Si KZ X-rays of Si and SiO,. In both cases, it is possible to detect chemical effects by observing relative intensities of X-ray satellite peaks. However, the use of heavy ions is considered to be more promising because the yields of satellite lines using silicon ion bombardment was much higher than that of protons. 0 1998 Elsevier Science B.V. Keworrfs:
High energy resolution
PIXE: Heavy ion microbeam
1. Introduction High energy resolution PIXE analysis is known to be a promising tool for chemical state analysis, as well as high sensitivity trace element analysis. In particular, heavy ion bombardment is considered to increase the sensitivity [1,3], because of the pro-
*Corresponding author. Tel.: +81-727-51-9534: 727-51-9631; e-mail: [email protected]. ’ On leave from “Rudjer Boskovic” Institute. 10001 Zagreb. Croatia.
fax: +81POB
1016.
0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIISO168-583X(97)00708-8
duction of intense satellite peaks as a result of multiple ionization. The intensity distribution is strongly influenced by the chemical environment of the target atoms. Therefore, it is interesting to investigate the possibility of using a heavy ion microbeam to detect chemical states of target atoms with high spatial resolution. As the energy spacing of the satellite peaks are too small, it is impossible to measure the satellite structure by energy dispersive X-ray spectroscopy (EDS) using a Si(Li) detector due to its poor energy resolution. Wave length dispersive X-ray spectroscopy (WDX) using crystal spectrometer provides high energy
Y. Mokuno et al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138
resolution. Most of WDX systems for microprobe experiments use a crystal spectrometer with a highly focusing geometry, such as the Johann and Johannson geometry to achieve a sufficient detection efficiency at a single energy setting. This type of detector has been successfully applied to a scanning electron microscope and recently applied to a proton microprobe [3] in order to increase the sensitivity for trace element analysis. However, it is necessary to use a step scanning of the crystal to detect an energy range of X-rays for the chemical state analysis which uses chemical shift or relative intensity ratio of X-ray peaks. A crystal spectrometer using a position sensitive X-ray detector has advantages over this type of spectrometer, because it allows us a simultaneous X-ray detection over an energy range without scanning the crystal. Recently, we have developed an X-ray crystal spectrometer using a position sensitive proportional counter (PSPC) and installed it on the heavy ion microbeam line at our 1.5 MV tandem accelerator [4]. In the present work, the improved energy resolution and the detection efficiency is reported, and the Si Kcc spectra of Si and SiOZ thick targets are measured using focused 2 MeV proton and 5 MeV Si3+ ion beams. These results are discussed in terms of the possibility of chemical state microanalysis by high energy resolution PIXE analysis.
2. Experimental The high energy resolution PIXE analysis was performed with a plane crystal spectrometer installed at the heavy ion microbeam line at Osaka National Research institute [5]. Details of the spectrometer have been described in our previous paper [4]. A schematic illustration of the spectrometer is shown in Fig. 1 together with the target chamber of the microbeam line. A 2 MeV proton beam and a 5 MeV Si3+ beam were focused on the target. The beam spot size was 60-l 10 pm in horizontal (H) direction and 30 pm in vertical (V) direction. The spectrometer was positioned at 105” with respect to the beam. X-rays produced in the target are dispersed by the crystal at an angle 8 (0 < 8 < 65”) into different Bragg angles 0~ (0, < Bs < 01). These X-rays are simultaneous-
369
(19981368-372 Microbeam line
4-1
e
Fig. 1. Schematic of an X-ray crystal spectrometer PSPC combined with heavy ion microbeam line.
using
a
ly detected by PSPC (Rigaku Industrial Co.) placed at an angle 28. The distance between the target and the crystal is 350 mm, which is the same as the distance between the crystal and the PSPC. The active area of the PSPC (X-ray entrance window) is 100 mm (length) x 10 mm (width) which is covered by a 9 pm PET (polyethylene terephthalate) film evaporated with 40 nm Al. The position resolution is 200 /lrn. Thick Si, SiO?, Ti and stainless steel substrates were used as targets. Si KGIXrays were analyzed by an ADP ( 101) crystal and the other X-rays were analyzed by a LiF (200) crystal.
3. Results and discussion The energy resolution and the detection efficiency of the system were investigated by measuring a peak profile of Si, Ti and Fe KU lines using a focused 2 MeV proton beam. Fig. 2 represents the Si Kcc spectra from Si. Si KU,.?, Ka3 and KQ peaks are clearly resolved in the spectrum. The energy resolution estimated from the Si Kr,,? line is 1.3 eV (FWHM). The measured resolution is summarized in Table 1 together with the calculated value. The energy resolution is limited by the position resolution of the PSPC, the effective beam size and the energy resolution of the analyzing crystal. Effective beam size is defined by the beam size and
Y Mokuno et al. I Nucl. Insrr. and Meth. in Phw. Rex B 136-138 (1998) 368-37,’
370
420 Channel
440 number
(ch)
Fig. 2. Si Ka spectrum of Si obtained by a focused 2 MeV proton beam with the size of 110 (H) x 30 (V) pm. Experimental data is fitted by nonlinear least square fitting.
the detection geometry of the sample [6], which causes the same effect on the energy resolution as the position resolution of PSPC. In the present system, the X-ray takeoff angle is 15” and it is expressed as W sin 15”, where W is the horizontal beam size. Since the beam size of the focused beam is smaller than the position resolution of the PSPC, the effective beam size is much smaller than the position resolution of the PSPC and the effect on the system resolution is negligible. Therefore, deviations of the measured resolution from calculated values are ascribed to the degradation of position resolution of the PSPC or poor energy resolution of the analyzing crystal. Detection efficiency is an important factor in microbeam experiments because of the limitation of the beam current. In our system, detection efficiency E can be expressed as follows [7]: E =
RD,&/4z.
where R is the reflectivity of the crystal, Dew is the detection efficiency of PSPC, Q is the subtended Table Energy
1
X-ray
resolution
for the Ka lines of various Crystal
elements Measured FWHM
si Kz,.z Ti Ka, Fe Ku,
solid angle of the detector which can be expressed as the product of window width of the PSPC (rad) and the resolution width of the system (rad). In the case of Ti Ka X-ray detection, the resolution width is 8.0 x 10m4rad and the subtended solid angle is 1.1 x 10m5 sr. This is three orders of magnitude lower than the Si(Li) detector which has subtended angle of lo-’ sr. In addition, detection efficiency of the PSPC is the order of 10-l. Therefore, efficiency of the system is roughly estimated to be at least four orders of magnitude lower than the Si(Li) detector. However, further improvements in detection efficiency could be expected, if the vertical focusing geometry, such as von Hamos geometry [7], is used. Table 2 shows measured detection efficiency for the Ka lines of Si and Ti expressed as Xray intensity per unit charge. To investigate the possibility of chemical state microanalysis, Si Ka X-rays from Si and SiOZ were analyzed using focused 2 MeV proton and 5 MeV Si3+ ion beams and these results were compared. Fig. 3 shows KL satellite lines in the Si Ka spectrum obtained by a focused 2 MeV proton beam. The accumulated charge was 8 uC for Si and 12 utc for SiOZ. The results are fitted by non-linear least square fitting using a Voigt function. The peak in the SiO? spectrum shifts toward the high energy side. Moreover, KaJKa4 peak intensity ratio for SiO, is smaller than that for Si. The chemical shift and the difference in peak intensity are quite similar to the Si Kcc spectra obtained by electron beam induced WDX [8]. Fig. 4 shows the Si Kcc spectra using a focused 5 MeV Si3+ ion beam. The accumulated charge was 3.3 particle uC for Si and 5 particle . uC for SiO?. KU satellite lines produced as a result of multiple ionization such as KL” lines are clearly separated. Moreover, the intensity of the satellite lines is en-
ADP LiF (2 0 0) LiF (2 0 0)
1.3 3.9 5.9
Calculated (eV)
dEIE
dEIE
7.6 x 10 + 9.5 x IO 4 1.0 x 10 3
3.2 x 10 ‘t 3.1 x Io-.j 5.3 x Io-J
311
Y. Mokuno et al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (1998) 368-372
Table 2 Measured detection efficiency for the Ka lines of Si and Ti expressed as X-ray intensity per unit charge X-ray
Yields (counts/&)
Si Kar.2 Ti Ka,
1.6 x 102 5.3 x 10’
600
hanced as a result of a level matching. The relative intensity of the satellite lines is known to reflect the chemical environment of Si atoms [93. In this case, the KL’/KL’ intensity ratio of Si02 is larger than that of Si. This can be used as indicator of chemical environment of Si. However, it is impossible to separate the X-rays produced from the target atoms from the X-rays from the incident ions. In addition, for the Si02 case, the Si X-rays are also produced form the Si-0 collisions. Therefore, the use of near symmetric collision is considered to be more effective. 350 F
I
400
450
500
550
600
Channel number (ch)
250 Fig. 4. Si KI satellite spectra cused 5 MeV Si’+ beam.
200
for Si and SiOl obtained
by a fo-
In both proton and silicon ion bombardment, it is possible to detect the chemical effect on relative intensities of Si Krl X-ray satellite. However, as the use of Si ions produce intense satellite lines than protons, it is considered to be suitable for chemical state microanalysis using a focused beam.
z!
.a, 100
>
80
4. Conclusion
60 40 20 L
Lo
*
L
I
I
I
430
440
450
460
Channel number (ch) Fig. 3. Si Ka satellite spectra for Si and SiO? obtained by a focused 2 MeV proton beam. Experimental data is fitted by nonlinear least square fitting.
It is shown that high energy resolution PIXE analysis with the crystal spectrometer can be used to observe chemical effects on Si Kcc X-ray satellite intensities. The results for Si and SiOz showed that chemical state microanalysis is possible using focused 2 MeV proton and 5 MeV Si3+ ion beams with the size of 60-I 10 (H) x 30 (V) urn. However, the use of MeV heavy ions is considered to be more promising because the intensity of satellite lines for low Z element is expected to be enhanced.
372
Y. Mokuno
et al. I Nucl. Instr. and Meth. in Phys. Res. B 136-138 (1998) 368-372
As the detection efficiency of the spectrometer is much lower than EDS setup, further improvements in detection efficiency will be necessary to reduce the focused beam size or current.
Acknowledgements
The authors would like to thank Toyo Metallizing Co., Ltd for supplying the aluminized PET film sample.
References [l]
M. Uda. 0. Benka, K. Fuwa. K. Maeda. Instr. and Meth. B 22 (1987) 5.
Y. Sasa, Nucl
[2] M. Terasawa, Int. J. PIXE 1 (1991) 251. [3] D.H. Morse, G.S. Bench, S.P.H.T. Freeman, A.E. Pontau. Nucl. Instr. and Meth. B 99 (1995) 427. (41 Y. Mokuno. Y. Horino, A. Chayahara, A. Kinomura, N. Tsubouchi. K. Fujii; M. Terasawa. T. Sekioka, T. Mitamura. Nucl. Instr. and Meth. B 130 (1997) 243. [5] Y. Horino. A. Chayahara, M. Kiuchi. K. Fujii. M. Satou. F. Fujimoto. Jpn. J. Appl. Phys. 29 (1990) 1230. [6] H. Hamanaka. M. Okane. Y. Yamamoto, Nucl. Instr. and Meth. B 45 (1990) 360. [7] C.R. Vane. E. Kallne. G. Morford, S. Raman, M.S. Smith, Nucl. Instr. and Meth. B lO/ll (1985) 190. [8] W.L. Brown, D.W. Fisher, Spectrochim. Acta 21 (1965) 1471. [9] R.L. Watson, A.K. Leeper, B.I. Sonobe. T. Chiao. F.E. Jenson, Phys. Rev. A 15 (1977) 914.
Beam Interactions
with Materials 8 Atoms ELSEVIER
Nuclear
Instruments
and Methods
in Physics
Research
B 136138
(1998) 368-372
High energy resolution PIXE analysis using focused MeV heavy ion beams Y. Mokuno ‘**,Y. Horino ‘, T. Tad2 ‘-I, M. Terasawa b, T. Sekioka b, A. Chayahara A. Kinomura ‘, N. Tsubouchi ‘, K. Fujii a
‘,
Abstract
The possibility of chemical state microanalysis using high energy resolution PIXE was investigated using a plane crystal spectrometer installed at a heavy ion microbeam line. The spectrometer has the advantage for the analysis of an X-ray spectrum of simultaneously detecting X-rays over an energy range using position sensitive proportional counter without scanning the crystal. Though the detection efficiency is estimated to be at least four orders of magnitude lower than energy dispersive X-ray spectroscopy (EDS) using a Si(Li) detector, the resolution of the system (dE/E) is better than lo-?. because the effect of beam size on system resolution is negligible. Focused 2 MeV proton and 5 MeV Si”+ ion beams were employed for the analysis of Si KZ X-rays of Si and SiO,. In both cases, it is possible to detect chemical effects by observing relative intensities of X-ray satellite peaks. However, the use of heavy ions is considered to be more promising because the yields of satellite lines using silicon ion bombardment was much higher than that of protons. 0 1998 Elsevier Science B.V. Keworrfs:
High energy resolution
PIXE: Heavy ion microbeam
1. Introduction High energy resolution PIXE analysis is known to be a promising tool for chemical state analysis, as well as high sensitivity trace element analysis. In particular, heavy ion bombardment is considered to increase the sensitivity [1,3], because of the pro-
*Corresponding author. Tel.: +81-727-51-9534: 727-51-9631; e-mail: [email protected]. ’ On leave from “Rudjer Boskovic” Institute. 10001 Zagreb. Croatia.
fax: +81POB
1016.
0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIISO168-583X(97)00708-8
duction of intense satellite peaks as a result of multiple ionization. The intensity distribution is strongly influenced by the chemical environment of the target atoms. Therefore, it is interesting to investigate the possibility of using a heavy ion microbeam to detect chemical states of target atoms with high spatial resolution. As the energy spacing of the satellite peaks are too small, it is impossible to measure the satellite structure by energy dispersive X-ray spectroscopy (EDS) using a Si(Li) detector due to its poor energy resolution. Wave length dispersive X-ray spectroscopy (WDX) using crystal spectrometer provides high energy
Y. Mokuno et al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138
resolution. Most of WDX systems for microprobe experiments use a crystal spectrometer with a highly focusing geometry, such as the Johann and Johannson geometry to achieve a sufficient detection efficiency at a single energy setting. This type of detector has been successfully applied to a scanning electron microscope and recently applied to a proton microprobe [3] in order to increase the sensitivity for trace element analysis. However, it is necessary to use a step scanning of the crystal to detect an energy range of X-rays for the chemical state analysis which uses chemical shift or relative intensity ratio of X-ray peaks. A crystal spectrometer using a position sensitive X-ray detector has advantages over this type of spectrometer, because it allows us a simultaneous X-ray detection over an energy range without scanning the crystal. Recently, we have developed an X-ray crystal spectrometer using a position sensitive proportional counter (PSPC) and installed it on the heavy ion microbeam line at our 1.5 MV tandem accelerator [4]. In the present work, the improved energy resolution and the detection efficiency is reported, and the Si Kcc spectra of Si and SiOZ thick targets are measured using focused 2 MeV proton and 5 MeV Si3+ ion beams. These results are discussed in terms of the possibility of chemical state microanalysis by high energy resolution PIXE analysis.
2. Experimental The high energy resolution PIXE analysis was performed with a plane crystal spectrometer installed at the heavy ion microbeam line at Osaka National Research institute [5]. Details of the spectrometer have been described in our previous paper [4]. A schematic illustration of the spectrometer is shown in Fig. 1 together with the target chamber of the microbeam line. A 2 MeV proton beam and a 5 MeV Si3+ beam were focused on the target. The beam spot size was 60-l 10 pm in horizontal (H) direction and 30 pm in vertical (V) direction. The spectrometer was positioned at 105” with respect to the beam. X-rays produced in the target are dispersed by the crystal at an angle 8 (0 < 8 < 65”) into different Bragg angles 0~ (0, < Bs < 01). These X-rays are simultaneous-
369
(19981368-372 Microbeam line
4-1
e
Fig. 1. Schematic of an X-ray crystal spectrometer PSPC combined with heavy ion microbeam line.
using
a
ly detected by PSPC (Rigaku Industrial Co.) placed at an angle 28. The distance between the target and the crystal is 350 mm, which is the same as the distance between the crystal and the PSPC. The active area of the PSPC (X-ray entrance window) is 100 mm (length) x 10 mm (width) which is covered by a 9 pm PET (polyethylene terephthalate) film evaporated with 40 nm Al. The position resolution is 200 /lrn. Thick Si, SiO?, Ti and stainless steel substrates were used as targets. Si KGIXrays were analyzed by an ADP ( 101) crystal and the other X-rays were analyzed by a LiF (200) crystal.
3. Results and discussion The energy resolution and the detection efficiency of the system were investigated by measuring a peak profile of Si, Ti and Fe KU lines using a focused 2 MeV proton beam. Fig. 2 represents the Si Kcc spectra from Si. Si KU,.?, Ka3 and KQ peaks are clearly resolved in the spectrum. The energy resolution estimated from the Si Kr,,? line is 1.3 eV (FWHM). The measured resolution is summarized in Table 1 together with the calculated value. The energy resolution is limited by the position resolution of the PSPC, the effective beam size and the energy resolution of the analyzing crystal. Effective beam size is defined by the beam size and
Y Mokuno et al. I Nucl. Insrr. and Meth. in Phw. Rex B 136-138 (1998) 368-37,’
370
420 Channel
440 number
(ch)
Fig. 2. Si Ka spectrum of Si obtained by a focused 2 MeV proton beam with the size of 110 (H) x 30 (V) pm. Experimental data is fitted by nonlinear least square fitting.
the detection geometry of the sample [6], which causes the same effect on the energy resolution as the position resolution of PSPC. In the present system, the X-ray takeoff angle is 15” and it is expressed as W sin 15”, where W is the horizontal beam size. Since the beam size of the focused beam is smaller than the position resolution of the PSPC, the effective beam size is much smaller than the position resolution of the PSPC and the effect on the system resolution is negligible. Therefore, deviations of the measured resolution from calculated values are ascribed to the degradation of position resolution of the PSPC or poor energy resolution of the analyzing crystal. Detection efficiency is an important factor in microbeam experiments because of the limitation of the beam current. In our system, detection efficiency E can be expressed as follows [7]: E =
RD,&/4z.
where R is the reflectivity of the crystal, Dew is the detection efficiency of PSPC, Q is the subtended Table Energy
1
X-ray
resolution
for the Ka lines of various Crystal
elements Measured FWHM
si Kz,.z Ti Ka, Fe Ku,
solid angle of the detector which can be expressed as the product of window width of the PSPC (rad) and the resolution width of the system (rad). In the case of Ti Ka X-ray detection, the resolution width is 8.0 x 10m4rad and the subtended solid angle is 1.1 x 10m5 sr. This is three orders of magnitude lower than the Si(Li) detector which has subtended angle of lo-’ sr. In addition, detection efficiency of the PSPC is the order of 10-l. Therefore, efficiency of the system is roughly estimated to be at least four orders of magnitude lower than the Si(Li) detector. However, further improvements in detection efficiency could be expected, if the vertical focusing geometry, such as von Hamos geometry [7], is used. Table 2 shows measured detection efficiency for the Ka lines of Si and Ti expressed as Xray intensity per unit charge. To investigate the possibility of chemical state microanalysis, Si Ka X-rays from Si and SiOZ were analyzed using focused 2 MeV proton and 5 MeV Si3+ ion beams and these results were compared. Fig. 3 shows KL satellite lines in the Si Ka spectrum obtained by a focused 2 MeV proton beam. The accumulated charge was 8 uC for Si and 12 utc for SiOZ. The results are fitted by non-linear least square fitting using a Voigt function. The peak in the SiO? spectrum shifts toward the high energy side. Moreover, KaJKa4 peak intensity ratio for SiO, is smaller than that for Si. The chemical shift and the difference in peak intensity are quite similar to the Si Kcc spectra obtained by electron beam induced WDX [8]. Fig. 4 shows the Si Kcc spectra using a focused 5 MeV Si3+ ion beam. The accumulated charge was 3.3 particle uC for Si and 5 particle . uC for SiO?. KU satellite lines produced as a result of multiple ionization such as KL” lines are clearly separated. Moreover, the intensity of the satellite lines is en-
ADP LiF (2 0 0) LiF (2 0 0)
1.3 3.9 5.9
Calculated (eV)
dEIE
dEIE
7.6 x 10 + 9.5 x IO 4 1.0 x 10 3
3.2 x 10 ‘t 3.1 x Io-.j 5.3 x Io-J
311
Y. Mokuno et al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (1998) 368-372
Table 2 Measured detection efficiency for the Ka lines of Si and Ti expressed as X-ray intensity per unit charge X-ray
Yields (counts/&)
Si Kar.2 Ti Ka,
1.6 x 102 5.3 x 10’
600
hanced as a result of a level matching. The relative intensity of the satellite lines is known to reflect the chemical environment of Si atoms [93. In this case, the KL’/KL’ intensity ratio of Si02 is larger than that of Si. This can be used as indicator of chemical environment of Si. However, it is impossible to separate the X-rays produced from the target atoms from the X-rays from the incident ions. In addition, for the Si02 case, the Si X-rays are also produced form the Si-0 collisions. Therefore, the use of near symmetric collision is considered to be more effective. 350 F
I
400
450
500
550
600
Channel number (ch)
250 Fig. 4. Si KI satellite spectra cused 5 MeV Si’+ beam.
200
for Si and SiOl obtained
by a fo-
In both proton and silicon ion bombardment, it is possible to detect the chemical effect on relative intensities of Si Krl X-ray satellite. However, as the use of Si ions produce intense satellite lines than protons, it is considered to be suitable for chemical state microanalysis using a focused beam.
z!
.a, 100
>
80
4. Conclusion
60 40 20 L
Lo
*
L
I
I
I
430
440
450
460
Channel number (ch) Fig. 3. Si Ka satellite spectra for Si and SiO? obtained by a focused 2 MeV proton beam. Experimental data is fitted by nonlinear least square fitting.
It is shown that high energy resolution PIXE analysis with the crystal spectrometer can be used to observe chemical effects on Si Kcc X-ray satellite intensities. The results for Si and SiOz showed that chemical state microanalysis is possible using focused 2 MeV proton and 5 MeV Si3+ ion beams with the size of 60-I 10 (H) x 30 (V) urn. However, the use of MeV heavy ions is considered to be more promising because the intensity of satellite lines for low Z element is expected to be enhanced.
372
Y. Mokuno
et al. I Nucl. Instr. and Meth. in Phys. Res. B 136-138 (1998) 368-372
As the detection efficiency of the spectrometer is much lower than EDS setup, further improvements in detection efficiency will be necessary to reduce the focused beam size or current.
Acknowledgements
The authors would like to thank Toyo Metallizing Co., Ltd for supplying the aluminized PET film sample.
References [l]
M. Uda. 0. Benka, K. Fuwa. K. Maeda. Instr. and Meth. B 22 (1987) 5.
Y. Sasa, Nucl
[2] M. Terasawa, Int. J. PIXE 1 (1991) 251. [3] D.H. Morse, G.S. Bench, S.P.H.T. Freeman, A.E. Pontau. Nucl. Instr. and Meth. B 99 (1995) 427. (41 Y. Mokuno. Y. Horino, A. Chayahara, A. Kinomura, N. Tsubouchi. K. Fujii; M. Terasawa. T. Sekioka, T. Mitamura. Nucl. Instr. and Meth. B 130 (1997) 243. [5] Y. Horino. A. Chayahara, M. Kiuchi. K. Fujii. M. Satou. F. Fujimoto. Jpn. J. Appl. Phys. 29 (1990) 1230. [6] H. Hamanaka. M. Okane. Y. Yamamoto, Nucl. Instr. and Meth. B 45 (1990) 360. [7] C.R. Vane. E. Kallne. G. Morford, S. Raman, M.S. Smith, Nucl. Instr. and Meth. B lO/ll (1985) 190. [8] W.L. Brown, D.W. Fisher, Spectrochim. Acta 21 (1965) 1471. [9] R.L. Watson, A.K. Leeper, B.I. Sonobe. T. Chiao. F.E. Jenson, Phys. Rev. A 15 (1977) 914.