- Email: [email protected]
Development of an in-air high-resolution PIXE system
Nuclear Instruments and Methods in Physics Research B 134 (1998) 418±426
Development of an in-air high-resolution PIXE system K. Maeda a
a,*
, K. Hasegawa b, H. Hamanaka b, K. Ogiwara
a
The Institute of Physical and Chemical Research (RIKEN),Wako-shi, Saitama 351-01, Japan b College of Engineering, Hosei University, Koganei-shi 184, Japan Received 28 July 1997; received in revised form 14 October 1997
Abstract A compact wavelength-dispersive crystal spectrometer system has been developed for high-resolution PIXE experiments in atmospheric air. A sample target placed in the air is irradiated with an external beam of protons. X-rays are diracted with a ¯at analyzing crystal and detected with a position-sensitive proportional counter. Nearly all of the Xray path from the target to counter is enveloped with a polyethylene bag ®lled with helium gas. The lowest X-ray energy detectable with this system is 0.9 keV and the angular resolution is 0.1° in the diracting angle. Ó 1998 Elsevier Science B.V. PACS: 07.85.-m; 33.20.Rm Keywords: In-air PIXE; High-resolution PIXE
1. Introduction The particle induced X-ray emission (PIXE) experiments in non-vacuum (external beam), especially in air of atmospheric pressure, is really suitable for biological, environmental and archaeological substances. The external beam PIXE has advantages such as (1) sample size and shape are not restricted, (2) sample handling is simple, (3) the vaporization of a volatile component is suppressed, and (4) charge buildup in insulating materials is eliminated [1].
* Corresponding author. Tel.: +81 48 4679409; fax: +81 48 4624661; e-mail: [email protected].
The external beam PIXE has been successfully used for multi-element trace analysis using energy-dispersive spectrometers (EDS) of Si(Li) or pure Ge solid state detectors. However, the highresolution PIXE experiments using wavelengthdispersive spectrometers (WDS) such as crystal spectrometers have been carried out in vacuum. Since the path of X-rays from source to detector in a WDS is, in general, much longer than that in an EDS, absorption of X-rays by the atmospheric gas in the X-ray path is much severe in WDS than in EDS. A trial of the external beam WDS±PIXE experiments was given in a short report by Isomura et al. [2]. They used a conventional single-channel step-scanning crystal spectrometer placed in vacu-
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 9 1 8 - X
K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
um and the sample placed in air. Tesauro et al. [3] developed a compact external beam WDS±PIXE system of multichannel type using a charge coupled device (CCD) as a position-sensitive X-ray detector. It was reported that the system was useful for the X-ray energies of 4±13 keV. They suggested that the X-ray absorption by air can be reduced by ¯ooding the volume from source to detector with a helium ¯ush. The lower energy limit of the system seemed to be determined by the CCD noise. Our aim is to develop an in-air WDS system which is eective to measure the PIXE spectra of low energies less than 3 keV. This is because the third period elements with K X-ray energies of 1±3 keV such as Na, Si, P and S play important roles in biological and environmental substances. For this purpose we have constructed a compact and economical WDS±PIXE system using a position-sensitive proportional counter (PSPC) as the X-ray detector. In order to detect low-energy Xrays eectively, we use a thin Mylar ®lm as the PSPC window and a helium path from near the target to the PSPC while a target sample is placed in atmospheric air. The preliminary test results were shown in our review articles concerning the position-sensitive crystal spectrometers [4,5]. In the present paper we describe the details of the construction and performance of the in-air WDS system and demonstrate its utilities in PIXE analysis. The energy resolution and sensitivity under various experimental conditions have been precisely examined. It is now possible to choose an experimental condition appropriate for the experiment of interest based on the data reported here. For the thinnest window of 1.5 lm-thick Mylar, the lower limit of X-ray detection has reached to 0.9 keV. The energy resolution of WDS is much higher than that of EDS, by one or two orders of magnitude. WDS can measure closely situated X-ray peaks, e.g. Ka lines of adjacent light elements [6] and satellite structures of Kb caused by molecular-orbital splittings [7], which cannot be separated by EDS. To make best use of the high-resolution PIXE method as a practical analytical technique, development of convenient external beam WDS systems is indispensable.
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2. Construction 2.1. Geometrical arrangement The arrangement of the PIXE system is schematically shown in Fig. 1. A multichanneltype crystal spectrometer combined with a position-sensitive X-ray detector is adopted in order to measure X-rays in a broad energy range simultaneously. A proton beam (2.5 MeV), which is collimated to a diameter of 3 mm with a cylindrical graphite collimator and taken out of a vacuum beam line of an accelerator to the air through a 7 lm-thick Al foil, bombards a sample target placed in the air. The Al-foil is strong enough for vacuum seal and irradiation of protons. We have found no damage on the Al-foil after the four year operation in which the total charge of 30 mC/mm2 was produced by 1±50 nA/mm2 proton beam. The distance between this exit Al-foil and the target surface is 15 mm. Passing through the air and Al-foil, the energy of the protons is reduced by 0.4 MeV. X-rays emitted from the target are analyzed with a ¯at single crystal and detected by a position-sensitive proportional counter (PSPC). To inhibit injection of undesirable scattered X-rays, a divergence slit of 2 mm spacing is set on the PSPC window. The analyzing crystal and PSPC are both mounted on a h±2h goniometer. A guiding frame (entrance guide Ge ) for the path of X-rays is placed between the source (proton beam spot on the tar-
Fig. 1. A compact, in-air, high-resolution PIXE system: Sd is the divergence slit; Ge and Ga , the guide frames for the X-ray path; h, the diracting angel; u, the incident angle of ions; and /, the take-o angle of X-rays.
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get) and the crystal, and another frame (acceptance guide Ga ) is placed between the crystal and PSPC. The top of the entrance guide is covered with a thin (1.5 lm-thick) Mylar window. The whole path of X-rays from the entrance guide to the PSPC window via the crystal is enveloped with a commercial polyethylene bag. In order to reduce the X-ray absorption by the atmospheric air, helium gas is introduced through a small hole made on the bag. It requires a few minutes to fully replace the air in the bag by He gas. No serious loss of He was observed during the h±2h rotation of the crystal and PSPC. The top part of the entrance guide is so small that the entrance window can be placed very close to the X-ray source, as close as 3 mm of distance. The total path length is set around 34 cm. 2.2. X-ray detector Four kinds of position-sensitive detectors, PSPC [8±10], CCD [3,11], IP (imaging plate) [12] and MCP (microchannel plate) [13,14] have been used for high-resolution PIXE experiments. Comparison of characteristics of these detectors indicates that: (1) PSPC and CCD are more useful for general PIXE study than IP and MCP; (2) CCD is appropriate for a small X-ray source because the spatial resolution of CCD (a few tens of lm) is much better than that of PSPC [4]. However, the upper limit of the counting rate of CCD is very low (<100 cps) in comparison with that of PSPC (<50 kcps), and in the in-air PIXE, the incident protons are scattered by the beam exit foil and the ambient air, resulting in the beam spot
Fig. 2. Schematic drawing of the PSPC.
spread of the order of hundreds lm. Therefore, in this system, we adopted a PSPC as the X-ray detector. The PSPC used here is a home-made, single resistance anode type shown in Fig. 2. It employs a carbon ®ber of 7 lm-diameter as the anode [15]. The resistivity of the carbon ®ber is 4 kX/cm. The anode resistance of the PSPC is about 20 kX which is suitable for position-encoding based on the charge-division method. The cathode is the window and walls of the counter tube. We have also constructed a 512 analyzer. The output pulses of the PSPC are converted into position signals by a digital divider [16] and accumulated in the memory of the analyzer. The system control and data processing are carried out with a personal computer. We prepared two sets of PSPC of the same size. Characteristics of these counters are given in Table 1. The size of the PSPC is very small in comparison to PSPCs reported in literature. The Mylar windows were coated with 50 nm-thick aluminum ®lm to perform an uniform electric ®eld in-
Table 1 Characteristics of PSPCs
Size (W ´ H ´ D, cm) Window (W ´ H, cm) Window ®lm Thickness of ®lm (lm) Sensitive width (cm) Position / channel (mm/ch) Angle (in h) / channel ( /ch) Distance from X-ray source to detector window (cm) Simultaneously detectable angular range ( )
#1 PSPC
#2 PSPC
10 3:5 4 4 0:8 Mylar 4 2.6 0.092 0.0152 34.5 4.3
10 3:5 4 4 0:8 Mylar 1.5 3.4 0.103 0.0177 33.3 5.8
K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
side the cathode. Mixed gas of Ar (90%) and CH4 (10%) is ¯owed through the PSPC under a gas pressure of 1 bar. 3. Performance 3.1. Eects of helium gas introduction Eects of introduction of helium gas into the Xray path can be seen in Fig. 3. The ®gure shows Ti K X-ray spectra of a Ti metal and Ca K X-ray spectra of soils taken from a Chinese desert (Flaming Mountains) before () and after () replacing the air in the bag by helium. As expected, the yields of Ti and Ca K lines were greatly increased (10 times for Ti K lines and 40 times for Ca K lines) when He gas was introduced. The peak to background ratios were also improved, by a factor of 2 and 10 for the Ti K and Ca K lines, respectively.
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Considering the absorption factors of X-rays by the spectrometer window (1.5 lm Mylar), PSPC window (4 or 1.5 lm Mylar), helium gas (1 bar, 33 or 34 cm) and air (1 bar, 3 mm), the lower limit of X-ray energy detectable with this system is estimated to be around 1 keV. The absorption of X-rays by the atmospheric air may be reduced by setting a vacuum spectrometer equipped with a thin X-ray entrance window close to the sample target placed in the air [2]. Our system requires no expensive vacuum system. Moreover, the analyzing crystal can easily be changed, and the arrangement of the target, crystal and PSPC is ¯exible. Flood of helium gas onto the X-ray path may be also useful to reduce the absorption eect [3]. This method has the same merits presented above. However, the ¯ood method requires a large volume of helium to reduce the absorption eectively. Helium gas required for an 8 h experiment is 50± 500 1 in our system. 3.2. PSPC The sensitivity distributions of the PSPCs were determined using a direct beam of Ti K X-rays from a Ti metal induced by 2.1 MeV protons. As shown in Fig. 4, the sensitivity in the center region of PSPC is quite uniform, i.e. the standard deviation of X-ray yield in the range of 150±350 ch is kept within 2% for #1 PSPC. The maximum deviation of the distribution of #2 PSPC in the same range is 10%, which is produced by the spatial change of the gas gain. To improve the spatial res-
Fig. 3. Ti and Ca K X-ray spectra before (s) and after (d) replacing the air in the X-ray path by He gas. The Ti K X-rays were induced by 2.7 MeV protons and analyzed with a Si (1 1 1) crystal, and the Ca K X-rays were induced by 2.1 MeV protons and analyzed with a Ge (1 1 1) crystal.
Fig. 4. Sensitivity distributions of the PSPCs.
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olution and reduce noise, we designed the analyzer to accept only pulses with the heights in the upper half range. If the gas gain at any part of the anode is either much larger or smaller than that at the middle point, the analyzed pulses will decrease: the sensitivity distribution depends on the gas gain and the pulse-height distribution. The gas gain is aected by the anode to cathode distance. The divergence slit shown in Fig. 2 helps us to ®x the distance between the window ®lm and the anode. The window ®lm of #2 PSPC is very thin and fragile. The spacing of 2 mm of the slit is not enough to keep it ¯at. This is one reason that #2 PSPC has the larger sensitivity deviation. Relationship between the PSPC position and the channel number was examined using a sliding-stage and a narrow beam of Ti K X-rays through a double slit (50 lm open width). Linear relationship with a correlation factor better than 0.99 was obtained for both PSPCs. The spatial resolution of PSPC depends on the bias voltage applied to the anode. Under the most appropriate bias voltage, the FWHM (full width at half maximum) obtained for the Ti K X-rays was 0.3 mm for both PSPCs. The PSPCs are very stable. The peak position of the X-rays had been settled within 1 channel for a ®ve hour experiment. 3.3. Energy resolution Energy resolution dE (or E/dE) of the spectrometer system depends on the angular resolution dh as dE/E dh/tan h. The angular resolution is determined by: (1) the diraction width of the analyzing crystal, (2) the spatial resolution of the PSPC; (3) the X-ray source width as viewed from the crystal, and (4) the path length of X-rays from the source to PSPC. It is known that the spatial resolution of the PSPC improves if the pressure of the counter gas is increased [17]. However, the high-pressure gas is not feasible for the present system because the PSPCs are equipped with very thin windows in order to detect the low-energy X-rays eectively. The spatial resolution of the PSPC also depends on the bias voltage applied to the anode. In Fig. 5, the FWHM for Ti K X-rays measured through the narrow slit is plotted against the bias voltage.
Fig. 5. Spatial resolution of PSPC (given by FWHM) as a function of the bias voltage.
The minimum FWHM was obtained by a bias voltage of 1100±1120 V. Fig. 6 shows the FWHM of the Ti Ka1;2 (4.5 keV) from the Ti metal and the PKa1;2 (2.0 keV) from pressed powder of BP analyzed with an InSb (1 1 1) (interplanar spacing d 0.374 nm) crystal; here the spectra were induced by the 2.1 MeV proton beam of 3 mm-diameter, and the incident angle u of protons and the take-o angle / of X-rays were set at 80 and 10 , respectively. As seen from Fig. 6, the appropriate voltage is almost the same for Ti Ka1;2 and the P Ka1;2 . This implies that it is better to adjust the pulse-heights of X-ray signals by changing the gain of linear ampli®ers but not the bias voltage. The angular resolution of the whole system depends on the take-o angle / because the X-ray source width viewed from the analyzing crystal is proportional to tan /. A relationship between
Fig. 6. FWHM of the Ti Ka1;2 (d) and P Ka1;2 (s) lines as a function of the bias voltage.
K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
the FWHM of an X-ray line and / is shown in Fig. 7(a). The data were obtained from the S Ka spectra of MoS2 . The FWHM decreases with decreasing /. The FWHM for an in®nitely narrow X-ray source is expected to be 0.10 in h by extrapolating the FWHM vs. / curve to / 0 . Since the practical limit of / is 5 ±10 , we set / 10 in most cases. The FWHM of S Ka1;2 at / 10 is 0.12 in h, corresponding to 3.3 eV. Subtracting the contributions of the natural widths (0.65 eV for Ka1 and 0.64 eV for Ka2 ) and the splitting of the Ka1;2 dou-
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blet (1.2 eV) from the observed FWHM, using the method given by Kishimoto et al. [18], the angular resolution dh for the present geometry was estimated to be 0.093 . Almost the same angular resolution of 0.10 was obtained from the FWHM of Ti Ka1;2 line measured with the InSb (1 1 1) crystal. If we use a very narrow beam, the angular resolution is expected to improve by 10±20%. The angular resolution (dh) is converted into the energy resolution (E/dE) of the whole system through the relation of dE/E dh/tan h. For example, 0.10 of dh corresponds to the energy resolution of 1000 at h 60 , 1600 at h 70 and 2100 at h 75 . We have not examined yet how the diffraction width of each analyzing crystal depends on the angle h. The FWHM of Si Ka1;2 measured with the InSb (1 1 1) crystal (h 72:3 ) was 1.8 eV, and the FWHM of P Ka1;2 measured with the Ge (1 1 1) crystal (h 70:4 ) was 2.4 eV as shown in Fig. 8(a). These values are somewhat larger (20%) than that expected from the angular resolution of 0.10 and the natural widths and the splitting of the Ka1;2 doublet. The dierences may be attributed to the changes of the diraction widths. 3.4. Sensitivity
Fig. 7. (a) FWHM and (b) X-ray yield of the S Ka1;2 line from MoS2 as functions of the X-ray take-o angle. The S Ka1;2 Xrays were induced by a 2.1 MeV proton beam of 3 mm-diameter and analyzed with the Ge (1 1 1) crystal (d 0.327 nm). The sum of the proton incident angle and the X-ray take-o angle was kept to be 90 . Theoretical yields calculated for 2.1 and 1.1 MeV protons using a computer code PIXAN [19] are given by the solid and dashed lines respectively. The observed and theoretical yields are normalized with the values at / 30 .
Until now we have measured Ka and Kb spectra of Al, Si, P and S in several compounds, La and Lb spectra of Cu, Zn and Ge of metals, and La and Lb of Mo in MoS2 . The measuring time to obtain one spectrum induced by 100 nA of 2.1 MeV protons ranged from a few minutes to one hour. Examples of the measured spectra are shown in Fig. 8. As expected, the low-energy X-rays less than 1 keV could be detected while the sample being placed in the atmospheric air. The observed intensities of representative X-ray lines are given in Table 2 together with the experimental conditions. The irradiated portion of a sample is estimated to be 1 mg from the product of the beam spot area (7 mm2 ) and the penetration depth of 2.1 MeV protons (10±20 mg/cm2 for light to medium elements). The X-ray yield increases with an increase of the take-o angle / as shown in Fig. 7(b), though the resolution becomes worse. The dependency of
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K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
Fig. 8. Examples of spectra measured with the in-air high-resolution PIXE system: (a) P Ka of BP; (b) Cu La and Lb of Cu metal; and (c) Mo La + S Ka spectra of MoS2 . The spectra were induced by a 2.1 MeV proton beam of 3 mm-diameter. The Ge (1 1 1) crystal was used for (a) and (c), and Gypsum (0 2 0) (d 0.756 nm) for (b). Q is the accumulated beam charge.
Table 2 X-ray yields induced by a 2.1 MeV proton beam of 3 mm-diameter (the distance between the center of the beam spot and the entrance window was 4±5 mm) X-ray peak
Energy (keV)
Target
Analyzing crystal
PSPC
/
X-ray yield a (counts/lC)
Cu La Zn La Ge La Al Ka Si Ka Si Ka Si Kb P Ka P Ka S Kb S Kb
0.93 1.01 1.19 1.49 1.74 1.74 1.84 2.01 2.01 2.47 2.47
Cu Zn Ge Al Si Si Si BP CaHPO4 á2H2 O Na2 SO4 Na2 SO3
Gypsum (020) Gypsum (020) Gypsum (020) ADP (101) ADP (101) InSb (111) InSb (111) Ge (111) Ge (111) NaCl (200) NaCl (200)
#2 #2 #2 #2 #1 #1 #1 #1 #1 #1 #1
25 25 25 10 20 10 10 10 10 20 20
35 140 180 800 1000 4500 150 3000 1000 250 200
a
X-ray intensity integrated over the range of twice of FWHM.
K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
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the X-ray yield on / is well estimated from the theoretical curve [19]. A narrow rectangular ion beam is preferable rather than the circular beam used here if possible. The sensitivity of the present system is enough to analyze trace elements or determine the satellite structures of major components (several %) in most cases. But the beam current must be very low for delicate sample. And, if the sensitivity is improved by one order, it will be possible to determine the satellite structures of tenth % of components. In order to improve the sensitivity of the system, we are now developing a multi-anode PSPC. Its sensitive height (56 mm) is seven times larger than the present single-anode PSPC. 4. Utilities As to element analysis, the high-resolution PIXE spectrometry using WDS is complementary to the usual PIXE method using EDS. By means of EDS±PIXE, it is dicult to measure the content of a minor element in a matrix which is rich in elements adjacent to the minor element. Fig. 9(a) shows such case, in which P is a minor component while Si is a major one [20]. The sample is a fragment of an earthenware of 2500 B . C . excavated at Fukushima prefecture, Japan. The spectrum was induced by 1.5 MeV protons and analyzed with a Si(Li) detector. The K X-ray lines of P seem to have been obscured by the strong Si K X-ray lines. On the other hand, as shown in Fig. 9(b), the Ka and Kb lines of P in the same sample were clearly detected with this WDS system using the InSb (1 1 1) crystal. The content of P in the earthenware fragment was estimated to be 0.8% by comparing the Ka yield of P in Fig. 9(b) with that of CaHPO4 á2H2 O using the program PIXAN [19]. The identi®cation of the peak located at near 190 ch in Fig. 9(a) was also somewhat ambiguous since the Ka (4.51 keV) line of Ti is very close to the La (4.47 keV) line of Ba. The WDS measurement showed that the fragment contains considerable concentration of Ti but not Ba. High-resolution PIXE provides important information on chemical bonding states. Fig. 10 shows PIXE spectra of Si Kb taken from a fused
Fig. 9. PIXE spectra of a fragment of an earthenware measured with (a) EDS and (b) WDS: (a) the target was of ground powder of the fragment and irradiated with 1.5 MeV protons in vacuum; (b) the target was as received and irradiated with 2.1 MeV protons in the atmospheric air.
Fig. 10. Si Kb spectra of SiO2 and c-Si, where Q is the accumulated beam charge.
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quartz (SiO2 ) plate and a crystalline silicon (c-Si) wafer induced by a 2.1 MeV proton beam of 3 mm-diameter and analyzed with the InSb (1 1 1) crystal. The main line Kb1 is emitted by electron transitions from molecular-orbitals composed of both Si 3p and O 2p (in the case of SiO2 ) to Si 1s vacancy. The low-energy satellite Kb0 observed in the spectrum of SiO2 is originated from the molecular-orbital composed mainly of oxygen 2s [21]. The energy dierence between the Kb0 and Kb1 , 14 eV, approximately equals the energy dierence between O 2s and O 2p. Thus the appearance of satellite Kb0 is a proof that there are silicon atoms in the target, which are chemically bound to oxygen atoms. 5. Conclusion A compact WDS system for PIXE measurements in atmospheric air has been developed. Xray spectra of low energies, as low as 0.9 keV, were measured with adequate sensitivity. Our system requires a small amount of samples (1 mg), a small target area (10 mm2 ), and a short measuring time ( 6 1 h for a spectrum). The energy resolution (1000) is enough to resolve the molecular- orbital splitting satellites which re¯ect the chemical bondings. Samples can be analyzed as received without any treatment. Using this system, in situ chemical state analysis is now possible. References [1] E.T. Williams, Nucl. Instr. and Meth. B 3 (1984) 211.
[2] T. Isomura, K. Maeda, J. Kawai, M. Takami, M. Uda, RIKEN Accel. Prog. Rep. 26 (1992) 26. [3] P. Tesauro, P.A. Mando, G. Parrini, A. Pecchioli, P. Sona, Nucl. Instr. and Meth. B 108 (1996) 197. [4] K. Maeda, H. Hamanaka, K. Hasegawa, Int. J. PIXE 6 (1996) 97. [5] K. Maeda, K. Ogiwara, H. Hamanaka, K. Hasegawa, Report of Research Center of Ion Beam Technology, Hosei Univ., Suppl. 15, 1997, p. 103. [6] H. Hamanaka, M. Ohura, Y. Yamamoto, S. Morita, K. Iwamura, K. Ishii, Nucl. Instr. and Meth. B 35 (1988) 75. [7] J. Kawai, Nucl. Instr. and Meth. B 75 (1993) 3. [8] B.L. Doyle, U. Schiebel, L.D. Ellsworth, J.R. Macdonald, Rev. Sci. Instr. 49 (1978) 760. [9] A. Hitachi, H. Kumagai, Y. Awaya, Nucl. Instr. and Meth. 195 (1982) 631. [10] H. Hamanaka, K. Hasegawa, Y. Yamamoto, Int. J. PIXE 2 (1992) 263. [11] J. Hoszowska, J.-Cl. Dousse, J. Kern, Ch. Rheme, Nucl. Instr. and Meth. A 376 (1996) 129. [12] J. Kawai, K. Maeda, Anal. Sci. 9 (1993) 179. [13] K. Maeda, Y. Sasa, M. Uda, Int. J. PIXE 2 (1992) 19. [14] F. Folkmann, H.R. Nagel, Nucl. Instr. and Meth. B 98 (1995) 589. [15] K. Hasegawa, K. Mochiki, M. Koike, Y. Satow, H. Hashizume, Y. Iitaka, Nucl. Instr. and Meth. A 252 (1986) 158. [16] M. Koike, K. Hasegawa, Nucl. Instr. and Meth. A 272 (1988) 840. [17] S. Ito, M. Tosaki, N. Maeda, N. Takahashi, R. Katano, Y. Isozumi, Nucl. Instr. and Meth. B 75 (1993) 112. [18] S. Kishimoto, Y. Isozumi, S. Ito, R. Katano, H. Takekoshi, Appl. Radiat. Isot. 40 (1989) 299. [19] E.J. Clayton, PIXAN: The Lucas Heights PIXE Analysis Computer Package, Australian Atomic Energy Commission, Menai, 1986. [20] H. Hamanaka, Y. Yamamoto, G. Ito, J. Ogura, Report of Research Center of Ion Beam Technology, Hosei Univ., no. 13, 1992, p. 33. [21] J. Kawai, E. Uda, M. Uda, Computer Aided Innovation of New Materials II, eds. M. Doyama, J. Kihara, M. Tanaka, R. Yamamoto (Elsevier, 1993) p. 229.
Development of an in-air high-resolution PIXE system K. Maeda a
a,*
, K. Hasegawa b, H. Hamanaka b, K. Ogiwara
a
The Institute of Physical and Chemical Research (RIKEN),Wako-shi, Saitama 351-01, Japan b College of Engineering, Hosei University, Koganei-shi 184, Japan Received 28 July 1997; received in revised form 14 October 1997
Abstract A compact wavelength-dispersive crystal spectrometer system has been developed for high-resolution PIXE experiments in atmospheric air. A sample target placed in the air is irradiated with an external beam of protons. X-rays are diracted with a ¯at analyzing crystal and detected with a position-sensitive proportional counter. Nearly all of the Xray path from the target to counter is enveloped with a polyethylene bag ®lled with helium gas. The lowest X-ray energy detectable with this system is 0.9 keV and the angular resolution is 0.1° in the diracting angle. Ó 1998 Elsevier Science B.V. PACS: 07.85.-m; 33.20.Rm Keywords: In-air PIXE; High-resolution PIXE
1. Introduction The particle induced X-ray emission (PIXE) experiments in non-vacuum (external beam), especially in air of atmospheric pressure, is really suitable for biological, environmental and archaeological substances. The external beam PIXE has advantages such as (1) sample size and shape are not restricted, (2) sample handling is simple, (3) the vaporization of a volatile component is suppressed, and (4) charge buildup in insulating materials is eliminated [1].
* Corresponding author. Tel.: +81 48 4679409; fax: +81 48 4624661; e-mail: [email protected].
The external beam PIXE has been successfully used for multi-element trace analysis using energy-dispersive spectrometers (EDS) of Si(Li) or pure Ge solid state detectors. However, the highresolution PIXE experiments using wavelengthdispersive spectrometers (WDS) such as crystal spectrometers have been carried out in vacuum. Since the path of X-rays from source to detector in a WDS is, in general, much longer than that in an EDS, absorption of X-rays by the atmospheric gas in the X-ray path is much severe in WDS than in EDS. A trial of the external beam WDS±PIXE experiments was given in a short report by Isomura et al. [2]. They used a conventional single-channel step-scanning crystal spectrometer placed in vacu-
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 9 1 8 - X
K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
um and the sample placed in air. Tesauro et al. [3] developed a compact external beam WDS±PIXE system of multichannel type using a charge coupled device (CCD) as a position-sensitive X-ray detector. It was reported that the system was useful for the X-ray energies of 4±13 keV. They suggested that the X-ray absorption by air can be reduced by ¯ooding the volume from source to detector with a helium ¯ush. The lower energy limit of the system seemed to be determined by the CCD noise. Our aim is to develop an in-air WDS system which is eective to measure the PIXE spectra of low energies less than 3 keV. This is because the third period elements with K X-ray energies of 1±3 keV such as Na, Si, P and S play important roles in biological and environmental substances. For this purpose we have constructed a compact and economical WDS±PIXE system using a position-sensitive proportional counter (PSPC) as the X-ray detector. In order to detect low-energy Xrays eectively, we use a thin Mylar ®lm as the PSPC window and a helium path from near the target to the PSPC while a target sample is placed in atmospheric air. The preliminary test results were shown in our review articles concerning the position-sensitive crystal spectrometers [4,5]. In the present paper we describe the details of the construction and performance of the in-air WDS system and demonstrate its utilities in PIXE analysis. The energy resolution and sensitivity under various experimental conditions have been precisely examined. It is now possible to choose an experimental condition appropriate for the experiment of interest based on the data reported here. For the thinnest window of 1.5 lm-thick Mylar, the lower limit of X-ray detection has reached to 0.9 keV. The energy resolution of WDS is much higher than that of EDS, by one or two orders of magnitude. WDS can measure closely situated X-ray peaks, e.g. Ka lines of adjacent light elements [6] and satellite structures of Kb caused by molecular-orbital splittings [7], which cannot be separated by EDS. To make best use of the high-resolution PIXE method as a practical analytical technique, development of convenient external beam WDS systems is indispensable.
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2. Construction 2.1. Geometrical arrangement The arrangement of the PIXE system is schematically shown in Fig. 1. A multichanneltype crystal spectrometer combined with a position-sensitive X-ray detector is adopted in order to measure X-rays in a broad energy range simultaneously. A proton beam (2.5 MeV), which is collimated to a diameter of 3 mm with a cylindrical graphite collimator and taken out of a vacuum beam line of an accelerator to the air through a 7 lm-thick Al foil, bombards a sample target placed in the air. The Al-foil is strong enough for vacuum seal and irradiation of protons. We have found no damage on the Al-foil after the four year operation in which the total charge of 30 mC/mm2 was produced by 1±50 nA/mm2 proton beam. The distance between this exit Al-foil and the target surface is 15 mm. Passing through the air and Al-foil, the energy of the protons is reduced by 0.4 MeV. X-rays emitted from the target are analyzed with a ¯at single crystal and detected by a position-sensitive proportional counter (PSPC). To inhibit injection of undesirable scattered X-rays, a divergence slit of 2 mm spacing is set on the PSPC window. The analyzing crystal and PSPC are both mounted on a h±2h goniometer. A guiding frame (entrance guide Ge ) for the path of X-rays is placed between the source (proton beam spot on the tar-
Fig. 1. A compact, in-air, high-resolution PIXE system: Sd is the divergence slit; Ge and Ga , the guide frames for the X-ray path; h, the diracting angel; u, the incident angle of ions; and /, the take-o angle of X-rays.
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get) and the crystal, and another frame (acceptance guide Ga ) is placed between the crystal and PSPC. The top of the entrance guide is covered with a thin (1.5 lm-thick) Mylar window. The whole path of X-rays from the entrance guide to the PSPC window via the crystal is enveloped with a commercial polyethylene bag. In order to reduce the X-ray absorption by the atmospheric air, helium gas is introduced through a small hole made on the bag. It requires a few minutes to fully replace the air in the bag by He gas. No serious loss of He was observed during the h±2h rotation of the crystal and PSPC. The top part of the entrance guide is so small that the entrance window can be placed very close to the X-ray source, as close as 3 mm of distance. The total path length is set around 34 cm. 2.2. X-ray detector Four kinds of position-sensitive detectors, PSPC [8±10], CCD [3,11], IP (imaging plate) [12] and MCP (microchannel plate) [13,14] have been used for high-resolution PIXE experiments. Comparison of characteristics of these detectors indicates that: (1) PSPC and CCD are more useful for general PIXE study than IP and MCP; (2) CCD is appropriate for a small X-ray source because the spatial resolution of CCD (a few tens of lm) is much better than that of PSPC [4]. However, the upper limit of the counting rate of CCD is very low (<100 cps) in comparison with that of PSPC (<50 kcps), and in the in-air PIXE, the incident protons are scattered by the beam exit foil and the ambient air, resulting in the beam spot
Fig. 2. Schematic drawing of the PSPC.
spread of the order of hundreds lm. Therefore, in this system, we adopted a PSPC as the X-ray detector. The PSPC used here is a home-made, single resistance anode type shown in Fig. 2. It employs a carbon ®ber of 7 lm-diameter as the anode [15]. The resistivity of the carbon ®ber is 4 kX/cm. The anode resistance of the PSPC is about 20 kX which is suitable for position-encoding based on the charge-division method. The cathode is the window and walls of the counter tube. We have also constructed a 512 analyzer. The output pulses of the PSPC are converted into position signals by a digital divider [16] and accumulated in the memory of the analyzer. The system control and data processing are carried out with a personal computer. We prepared two sets of PSPC of the same size. Characteristics of these counters are given in Table 1. The size of the PSPC is very small in comparison to PSPCs reported in literature. The Mylar windows were coated with 50 nm-thick aluminum ®lm to perform an uniform electric ®eld in-
Table 1 Characteristics of PSPCs
Size (W ´ H ´ D, cm) Window (W ´ H, cm) Window ®lm Thickness of ®lm (lm) Sensitive width (cm) Position / channel (mm/ch) Angle (in h) / channel ( /ch) Distance from X-ray source to detector window (cm) Simultaneously detectable angular range ( )
#1 PSPC
#2 PSPC
10 3:5 4 4 0:8 Mylar 4 2.6 0.092 0.0152 34.5 4.3
10 3:5 4 4 0:8 Mylar 1.5 3.4 0.103 0.0177 33.3 5.8
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side the cathode. Mixed gas of Ar (90%) and CH4 (10%) is ¯owed through the PSPC under a gas pressure of 1 bar. 3. Performance 3.1. Eects of helium gas introduction Eects of introduction of helium gas into the Xray path can be seen in Fig. 3. The ®gure shows Ti K X-ray spectra of a Ti metal and Ca K X-ray spectra of soils taken from a Chinese desert (Flaming Mountains) before () and after () replacing the air in the bag by helium. As expected, the yields of Ti and Ca K lines were greatly increased (10 times for Ti K lines and 40 times for Ca K lines) when He gas was introduced. The peak to background ratios were also improved, by a factor of 2 and 10 for the Ti K and Ca K lines, respectively.
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Considering the absorption factors of X-rays by the spectrometer window (1.5 lm Mylar), PSPC window (4 or 1.5 lm Mylar), helium gas (1 bar, 33 or 34 cm) and air (1 bar, 3 mm), the lower limit of X-ray energy detectable with this system is estimated to be around 1 keV. The absorption of X-rays by the atmospheric air may be reduced by setting a vacuum spectrometer equipped with a thin X-ray entrance window close to the sample target placed in the air [2]. Our system requires no expensive vacuum system. Moreover, the analyzing crystal can easily be changed, and the arrangement of the target, crystal and PSPC is ¯exible. Flood of helium gas onto the X-ray path may be also useful to reduce the absorption eect [3]. This method has the same merits presented above. However, the ¯ood method requires a large volume of helium to reduce the absorption eectively. Helium gas required for an 8 h experiment is 50± 500 1 in our system. 3.2. PSPC The sensitivity distributions of the PSPCs were determined using a direct beam of Ti K X-rays from a Ti metal induced by 2.1 MeV protons. As shown in Fig. 4, the sensitivity in the center region of PSPC is quite uniform, i.e. the standard deviation of X-ray yield in the range of 150±350 ch is kept within 2% for #1 PSPC. The maximum deviation of the distribution of #2 PSPC in the same range is 10%, which is produced by the spatial change of the gas gain. To improve the spatial res-
Fig. 3. Ti and Ca K X-ray spectra before (s) and after (d) replacing the air in the X-ray path by He gas. The Ti K X-rays were induced by 2.7 MeV protons and analyzed with a Si (1 1 1) crystal, and the Ca K X-rays were induced by 2.1 MeV protons and analyzed with a Ge (1 1 1) crystal.
Fig. 4. Sensitivity distributions of the PSPCs.
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olution and reduce noise, we designed the analyzer to accept only pulses with the heights in the upper half range. If the gas gain at any part of the anode is either much larger or smaller than that at the middle point, the analyzed pulses will decrease: the sensitivity distribution depends on the gas gain and the pulse-height distribution. The gas gain is aected by the anode to cathode distance. The divergence slit shown in Fig. 2 helps us to ®x the distance between the window ®lm and the anode. The window ®lm of #2 PSPC is very thin and fragile. The spacing of 2 mm of the slit is not enough to keep it ¯at. This is one reason that #2 PSPC has the larger sensitivity deviation. Relationship between the PSPC position and the channel number was examined using a sliding-stage and a narrow beam of Ti K X-rays through a double slit (50 lm open width). Linear relationship with a correlation factor better than 0.99 was obtained for both PSPCs. The spatial resolution of PSPC depends on the bias voltage applied to the anode. Under the most appropriate bias voltage, the FWHM (full width at half maximum) obtained for the Ti K X-rays was 0.3 mm for both PSPCs. The PSPCs are very stable. The peak position of the X-rays had been settled within 1 channel for a ®ve hour experiment. 3.3. Energy resolution Energy resolution dE (or E/dE) of the spectrometer system depends on the angular resolution dh as dE/E dh/tan h. The angular resolution is determined by: (1) the diraction width of the analyzing crystal, (2) the spatial resolution of the PSPC; (3) the X-ray source width as viewed from the crystal, and (4) the path length of X-rays from the source to PSPC. It is known that the spatial resolution of the PSPC improves if the pressure of the counter gas is increased [17]. However, the high-pressure gas is not feasible for the present system because the PSPCs are equipped with very thin windows in order to detect the low-energy X-rays eectively. The spatial resolution of the PSPC also depends on the bias voltage applied to the anode. In Fig. 5, the FWHM for Ti K X-rays measured through the narrow slit is plotted against the bias voltage.
Fig. 5. Spatial resolution of PSPC (given by FWHM) as a function of the bias voltage.
The minimum FWHM was obtained by a bias voltage of 1100±1120 V. Fig. 6 shows the FWHM of the Ti Ka1;2 (4.5 keV) from the Ti metal and the PKa1;2 (2.0 keV) from pressed powder of BP analyzed with an InSb (1 1 1) (interplanar spacing d 0.374 nm) crystal; here the spectra were induced by the 2.1 MeV proton beam of 3 mm-diameter, and the incident angle u of protons and the take-o angle / of X-rays were set at 80 and 10 , respectively. As seen from Fig. 6, the appropriate voltage is almost the same for Ti Ka1;2 and the P Ka1;2 . This implies that it is better to adjust the pulse-heights of X-ray signals by changing the gain of linear ampli®ers but not the bias voltage. The angular resolution of the whole system depends on the take-o angle / because the X-ray source width viewed from the analyzing crystal is proportional to tan /. A relationship between
Fig. 6. FWHM of the Ti Ka1;2 (d) and P Ka1;2 (s) lines as a function of the bias voltage.
K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
the FWHM of an X-ray line and / is shown in Fig. 7(a). The data were obtained from the S Ka spectra of MoS2 . The FWHM decreases with decreasing /. The FWHM for an in®nitely narrow X-ray source is expected to be 0.10 in h by extrapolating the FWHM vs. / curve to / 0 . Since the practical limit of / is 5 ±10 , we set / 10 in most cases. The FWHM of S Ka1;2 at / 10 is 0.12 in h, corresponding to 3.3 eV. Subtracting the contributions of the natural widths (0.65 eV for Ka1 and 0.64 eV for Ka2 ) and the splitting of the Ka1;2 dou-
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blet (1.2 eV) from the observed FWHM, using the method given by Kishimoto et al. [18], the angular resolution dh for the present geometry was estimated to be 0.093 . Almost the same angular resolution of 0.10 was obtained from the FWHM of Ti Ka1;2 line measured with the InSb (1 1 1) crystal. If we use a very narrow beam, the angular resolution is expected to improve by 10±20%. The angular resolution (dh) is converted into the energy resolution (E/dE) of the whole system through the relation of dE/E dh/tan h. For example, 0.10 of dh corresponds to the energy resolution of 1000 at h 60 , 1600 at h 70 and 2100 at h 75 . We have not examined yet how the diffraction width of each analyzing crystal depends on the angle h. The FWHM of Si Ka1;2 measured with the InSb (1 1 1) crystal (h 72:3 ) was 1.8 eV, and the FWHM of P Ka1;2 measured with the Ge (1 1 1) crystal (h 70:4 ) was 2.4 eV as shown in Fig. 8(a). These values are somewhat larger (20%) than that expected from the angular resolution of 0.10 and the natural widths and the splitting of the Ka1;2 doublet. The dierences may be attributed to the changes of the diraction widths. 3.4. Sensitivity
Fig. 7. (a) FWHM and (b) X-ray yield of the S Ka1;2 line from MoS2 as functions of the X-ray take-o angle. The S Ka1;2 Xrays were induced by a 2.1 MeV proton beam of 3 mm-diameter and analyzed with the Ge (1 1 1) crystal (d 0.327 nm). The sum of the proton incident angle and the X-ray take-o angle was kept to be 90 . Theoretical yields calculated for 2.1 and 1.1 MeV protons using a computer code PIXAN [19] are given by the solid and dashed lines respectively. The observed and theoretical yields are normalized with the values at / 30 .
Until now we have measured Ka and Kb spectra of Al, Si, P and S in several compounds, La and Lb spectra of Cu, Zn and Ge of metals, and La and Lb of Mo in MoS2 . The measuring time to obtain one spectrum induced by 100 nA of 2.1 MeV protons ranged from a few minutes to one hour. Examples of the measured spectra are shown in Fig. 8. As expected, the low-energy X-rays less than 1 keV could be detected while the sample being placed in the atmospheric air. The observed intensities of representative X-ray lines are given in Table 2 together with the experimental conditions. The irradiated portion of a sample is estimated to be 1 mg from the product of the beam spot area (7 mm2 ) and the penetration depth of 2.1 MeV protons (10±20 mg/cm2 for light to medium elements). The X-ray yield increases with an increase of the take-o angle / as shown in Fig. 7(b), though the resolution becomes worse. The dependency of
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K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
Fig. 8. Examples of spectra measured with the in-air high-resolution PIXE system: (a) P Ka of BP; (b) Cu La and Lb of Cu metal; and (c) Mo La + S Ka spectra of MoS2 . The spectra were induced by a 2.1 MeV proton beam of 3 mm-diameter. The Ge (1 1 1) crystal was used for (a) and (c), and Gypsum (0 2 0) (d 0.756 nm) for (b). Q is the accumulated beam charge.
Table 2 X-ray yields induced by a 2.1 MeV proton beam of 3 mm-diameter (the distance between the center of the beam spot and the entrance window was 4±5 mm) X-ray peak
Energy (keV)
Target
Analyzing crystal
PSPC
/
X-ray yield a (counts/lC)
Cu La Zn La Ge La Al Ka Si Ka Si Ka Si Kb P Ka P Ka S Kb S Kb
0.93 1.01 1.19 1.49 1.74 1.74 1.84 2.01 2.01 2.47 2.47
Cu Zn Ge Al Si Si Si BP CaHPO4 á2H2 O Na2 SO4 Na2 SO3
Gypsum (020) Gypsum (020) Gypsum (020) ADP (101) ADP (101) InSb (111) InSb (111) Ge (111) Ge (111) NaCl (200) NaCl (200)
#2 #2 #2 #2 #1 #1 #1 #1 #1 #1 #1
25 25 25 10 20 10 10 10 10 20 20
35 140 180 800 1000 4500 150 3000 1000 250 200
a
X-ray intensity integrated over the range of twice of FWHM.
K. Maeda et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 418±426
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the X-ray yield on / is well estimated from the theoretical curve [19]. A narrow rectangular ion beam is preferable rather than the circular beam used here if possible. The sensitivity of the present system is enough to analyze trace elements or determine the satellite structures of major components (several %) in most cases. But the beam current must be very low for delicate sample. And, if the sensitivity is improved by one order, it will be possible to determine the satellite structures of tenth % of components. In order to improve the sensitivity of the system, we are now developing a multi-anode PSPC. Its sensitive height (56 mm) is seven times larger than the present single-anode PSPC. 4. Utilities As to element analysis, the high-resolution PIXE spectrometry using WDS is complementary to the usual PIXE method using EDS. By means of EDS±PIXE, it is dicult to measure the content of a minor element in a matrix which is rich in elements adjacent to the minor element. Fig. 9(a) shows such case, in which P is a minor component while Si is a major one [20]. The sample is a fragment of an earthenware of 2500 B . C . excavated at Fukushima prefecture, Japan. The spectrum was induced by 1.5 MeV protons and analyzed with a Si(Li) detector. The K X-ray lines of P seem to have been obscured by the strong Si K X-ray lines. On the other hand, as shown in Fig. 9(b), the Ka and Kb lines of P in the same sample were clearly detected with this WDS system using the InSb (1 1 1) crystal. The content of P in the earthenware fragment was estimated to be 0.8% by comparing the Ka yield of P in Fig. 9(b) with that of CaHPO4 á2H2 O using the program PIXAN [19]. The identi®cation of the peak located at near 190 ch in Fig. 9(a) was also somewhat ambiguous since the Ka (4.51 keV) line of Ti is very close to the La (4.47 keV) line of Ba. The WDS measurement showed that the fragment contains considerable concentration of Ti but not Ba. High-resolution PIXE provides important information on chemical bonding states. Fig. 10 shows PIXE spectra of Si Kb taken from a fused
Fig. 9. PIXE spectra of a fragment of an earthenware measured with (a) EDS and (b) WDS: (a) the target was of ground powder of the fragment and irradiated with 1.5 MeV protons in vacuum; (b) the target was as received and irradiated with 2.1 MeV protons in the atmospheric air.
Fig. 10. Si Kb spectra of SiO2 and c-Si, where Q is the accumulated beam charge.
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quartz (SiO2 ) plate and a crystalline silicon (c-Si) wafer induced by a 2.1 MeV proton beam of 3 mm-diameter and analyzed with the InSb (1 1 1) crystal. The main line Kb1 is emitted by electron transitions from molecular-orbitals composed of both Si 3p and O 2p (in the case of SiO2 ) to Si 1s vacancy. The low-energy satellite Kb0 observed in the spectrum of SiO2 is originated from the molecular-orbital composed mainly of oxygen 2s [21]. The energy dierence between the Kb0 and Kb1 , 14 eV, approximately equals the energy dierence between O 2s and O 2p. Thus the appearance of satellite Kb0 is a proof that there are silicon atoms in the target, which are chemically bound to oxygen atoms. 5. Conclusion A compact WDS system for PIXE measurements in atmospheric air has been developed. Xray spectra of low energies, as low as 0.9 keV, were measured with adequate sensitivity. Our system requires a small amount of samples (1 mg), a small target area (10 mm2 ), and a short measuring time ( 6 1 h for a spectrum). The energy resolution (1000) is enough to resolve the molecular- orbital splitting satellites which re¯ect the chemical bondings. Samples can be analyzed as received without any treatment. Using this system, in situ chemical state analysis is now possible. References [1] E.T. Williams, Nucl. Instr. and Meth. B 3 (1984) 211.
[2] T. Isomura, K. Maeda, J. Kawai, M. Takami, M. Uda, RIKEN Accel. Prog. Rep. 26 (1992) 26. [3] P. Tesauro, P.A. Mando, G. Parrini, A. Pecchioli, P. Sona, Nucl. Instr. and Meth. B 108 (1996) 197. [4] K. Maeda, H. Hamanaka, K. Hasegawa, Int. J. PIXE 6 (1996) 97. [5] K. Maeda, K. Ogiwara, H. Hamanaka, K. Hasegawa, Report of Research Center of Ion Beam Technology, Hosei Univ., Suppl. 15, 1997, p. 103. [6] H. Hamanaka, M. Ohura, Y. Yamamoto, S. Morita, K. Iwamura, K. Ishii, Nucl. Instr. and Meth. B 35 (1988) 75. [7] J. Kawai, Nucl. Instr. and Meth. B 75 (1993) 3. [8] B.L. Doyle, U. Schiebel, L.D. Ellsworth, J.R. Macdonald, Rev. Sci. Instr. 49 (1978) 760. [9] A. Hitachi, H. Kumagai, Y. Awaya, Nucl. Instr. and Meth. 195 (1982) 631. [10] H. Hamanaka, K. Hasegawa, Y. Yamamoto, Int. J. PIXE 2 (1992) 263. [11] J. Hoszowska, J.-Cl. Dousse, J. Kern, Ch. Rheme, Nucl. Instr. and Meth. A 376 (1996) 129. [12] J. Kawai, K. Maeda, Anal. Sci. 9 (1993) 179. [13] K. Maeda, Y. Sasa, M. Uda, Int. J. PIXE 2 (1992) 19. [14] F. Folkmann, H.R. Nagel, Nucl. Instr. and Meth. B 98 (1995) 589. [15] K. Hasegawa, K. Mochiki, M. Koike, Y. Satow, H. Hashizume, Y. Iitaka, Nucl. Instr. and Meth. A 252 (1986) 158. [16] M. Koike, K. Hasegawa, Nucl. Instr. and Meth. A 272 (1988) 840. [17] S. Ito, M. Tosaki, N. Maeda, N. Takahashi, R. Katano, Y. Isozumi, Nucl. Instr. and Meth. B 75 (1993) 112. [18] S. Kishimoto, Y. Isozumi, S. Ito, R. Katano, H. Takekoshi, Appl. Radiat. Isot. 40 (1989) 299. [19] E.J. Clayton, PIXAN: The Lucas Heights PIXE Analysis Computer Package, Australian Atomic Energy Commission, Menai, 1986. [20] H. Hamanaka, Y. Yamamoto, G. Ito, J. Ogura, Report of Research Center of Ion Beam Technology, Hosei Univ., no. 13, 1992, p. 33. [21] J. Kawai, E. Uda, M. Uda, Computer Aided Innovation of New Materials II, eds. M. Doyama, J. Kihara, M. Tanaka, R. Yamamoto (Elsevier, 1993) p. 229.