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Realization of the simultaneous micro-PIXE analysis of heavy and light elements at a nuclear microprobe
Nuclear Instruments and Methods in Physics Research B 181 (2001) 193±198
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Realization of the simultaneous micro-PIXE analysis of heavy and light elements at a nuclear microprobe I. Uzonyi *, I. Rajta, L. Bartha, A.Z. Kiss, A. Nagy Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), P.O. Box 51, Bem t er 18/c, H-4001 Debrecen, Hungary
Abstract A new in-vacuum micro-PIXE experimental set-up has been realised at an Oxford-type scanning nuclear microprobe facility. It is based on the simultaneous use of an ultra thin windowed detector and a conventional Be-windowed one for the measurement of the characteristic X-rays of light and heavier elements in the E 0:2±6 keV and E > 4 keV energy ranges, respectively. Complete analytical characterisation of samples from carbon to uranium is possible in a single irradiation process. Performance and technical developments are described in detail. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 82.80.E; 07.85.T; 29.30.Kv Keywords: Scanning nuclear microprobe; Micro-PIXE, UTW- and Be-windowed detectors; Light and heavy elements
1. Introduction Combination of dierent ion beam analytical methods such as NRA, RBS and PIXE has been a conventional technique in microprobe-based investigations in order to realise complete characterisation of samples. The conventional procedure is to use NRA or RBS for the detection of light elements ± usually up to Na or Mg ± and PIXE for the heavier ones, mostly in a consecutive way. The results are then uni®ed into a complete concen-
*
Corresponding author. Tel.: +36-52-417-266; fax: +36-52416-181. E-mail address: [email protected] (I. Uzonyi).
tration set of data in rather sophisticated ways [1,2]. It is worth emphasising that PIXE can also be a good candidate for light element analysis due to its superior cross-sections in the respected atomic number region. Nevertheless, this possibility inherent in PIXE ± except for certain examples [3±5] ± has not been utilised suciently yet, which is chie¯y attributable to the diculties in soft X-ray detection and evaluation of low energy X-ray spectra. The major impetus to extend our micro-PIXE technique towards light elements was given by our recent studies in the ®eld of aerosol and spherule (micro meteorite) research [6,7]. Namely, in these cases, besides mineral elements, the mapping of carbon and occasionally oxygen is of vital
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 7 0 - 6
194
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
importance. Additionally, the best lateral resolution of the microprobe should be used since the size of the measurable objects are on the micrometer scale. Unfortunately, the typical beam currents (30±100 pA) which have been achieved and used at good lateral resolution (1 lm) are practically insucient for RBS or NRA measurements, but proved to be satisfactory for PIXE analysis. The aim of this paper is to present the new invacuum micro-PIXE experimental set-up developed to our Oxford-type nuclear microprobe for the simultaneous measurement of light and heavy elements. It adopts the idea of using two Si(Li) Xray detectors [4,5] and sharing tasks between them. In this arrangement, an ultra thin windowed (UTW) detector serves to characterise the matrix by measuring low energy X-ray lines (E 0:2±6 keV) while a large area Be-windowed detector is used to detect the medium or high energy X-rays (E > 4 keV) with excellent eciency. In this way elements Z > 5 can be detected simultaneously by at least one of the two detectors, reducing both irradiation damage to the sample and measurement time, compared to the consecutive analytical
techniques mentioned above. The performance and use of this new experimental set-up will be demonstrated through some examples. 2. Instrumentation and technical developments 2.1. Description of the new experimental set-up Developments have been carried out at the Debrecen scanning nuclear microprobe facility [8] produced by Oxford microbeams. A photograph of the target chamber and a schematic diagram of the new micro-PIXE experimental set-up are shown in Figs. 1 and 2, respectively. One of the basic principles of the design was to build a ¯exible system, since the microprobe is used for various purposes like grazing angle micro-RBS [9] or micro-PIGE experiments [10]. In order to facilitate an easy switch-over between dierent set-ups both the UTW- and Be-windowed Si(Li) detectors were mounted to the vacuum chamber by special mechanical interfaces (see Fig. 2) symmetrically to the incident beam at an angle of 135°. Mechanical
Fig. 1. A photograph of the instrument.
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
195
Fig. 2. Schematic view of the new experimental set-up.
slides assure optimisation of the target-detector distances and the complete retraction of the endcaps from the inner volume when necessary (during maintenance or dedicated RBS measurements). If the endcaps are fully retracted, the entrance holes to the chamber on the mechanical interfaces can be covered by small o-ring sealed lids, thus enabling vacuum-tight isolation of the Si(Li) detectors from the chamber. The arrangement for the forward-viewing zoom microscope had to be changed as compared to the original Oxford design [11] since both 135° ports were occupied by the Si(Li) detectors. In order to solve this problem a new port was ®tted to the top ¯ange of the chamber at a vertical direction of 132° to the beam. The original microscope port is still available at the 0° port of the chamber, so the rear-viewing position can be utilised for thin transparent specimens and for focussing the beam on a thin quartz, while the new top port is intended for the observation of thick samples. 2.2. Detectors, data acquisition and beam-current monitoring The UTW detector, made by Princeton gamma technology (PGT), serves to measure low energy
X-rays. It has an energy resolution of 148 eV at 5.9 keV, 3.5 mm nominal thickness, 30 mm2 active area and a light element window (0:38 lm of Polymer). The maximum solid angle is 2.5 msr. Medium or high energy X-rays are measured with a Canberra Si(Li) detector characterised with the following parameters: FWHM 190 eV; thickness 5 mm; active area 80 mm2 ; Be-window thickness 25 lm. In this case the maximum solid angle attainable is 90 msr. The pulse processing system incorporates optical reset preampli®ers, an Aptec AMP 6300X spectroscopy ampli®er for the UTW detector and an ATOMKI made NZ-870 analogue signal processor [12] for the Canberra detector respectively, as well as the Oxford microbeams data acquisition system [13] for X-ray mapping. The data acquisition system runs on a Pentium II PC under MS Windows 98. Ion beam dose measurements are carried out by using a small-sized compact beam chopper, which is described elsewhere [14]. Charge build-up in insulating samples is eliminated by operating an electron source during measurements. PIXE spectra are evaluated by a new version of the PIXEKLM program package developed by Szab o et al. [15] (ATOMKI) for standardless
196
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
PIXE analysis. The capabilities of the UTW detector can be fully utilised, using the above mentioned program package, allowing the quantitative analysis of elements down to carbon. 2.3. Detector accessories Diculties associated with the operation of UTW detectors in PIXE set-ups have been discussed in great detail in numerous publications [3,4]. Namely, protons or visible light can easily penetrate through the ultra thin window causing decreased resolution, distortion of peak shapes, loss of X-ray lines below 1 keV [4] and even damage to the detector crystal. These diculties are generally overcome by placing a magnetic de¯ection trap and an extremely thin carbon foil in front of the ultra thin window [3,4]. In order to meet the requirements of in-vacuum micro-PIXE measurements, a compact magnetic de¯ection unit was developed with a relatively large air gap size of 4 4 mm2 , a magnetic ®eld strength of 0.6 T and a length of 60 mm. The magnetic circuit is based on a NdFeB permanent magnet (Br 1:3 T, BHmax 300 kJ=m3 , 3 size 25 25 40 mm ) which is placed in a ``C''-shaped iron yoke. This unit ®ts tightly into the entrance hole of the mechanical interface and de¯ects protons up to 2 MeV energy. The application of the unusually large air gap size results a higher solid angle, thus satisfactory count rates can be achieved by the UTW detector even at low (30±100 pA) beam currents. In order to eciently protect the UTW detector from scattered light inside the vacuum chamber, a special collimator was developed. This unit perfectly ®ts into the air gap of the magnetic circuit and lets X-rays and visible light reach the UTW detector only through a tube having rectangular 4 4 mm2 cross-sectional area. It was milled from a single block of Al and it is covered with a thin layer of black paint. In this way the UTW detector can be operated even without a carbon foil for non-heavily illuminating samples. In other cases ± e.g., either a strong light source is switched on inside the chamber during sample positioning, or when the sample itself emits too much light ± further provisions are needed to preserve the operability of the detector. This dif-
®culty was overcome by the development of a three-position ®lter changer which is placed in front of the magnetic trap and is manually operated by a knob from outside the vacuum chamber. In the ®rst position no ®lter is used, allowing the X-rays to reach the detector without attenuation, while in the other two cases any ®lters can be applied. For light protection, usually a thin carbon foil is used (thickness 40±80 lg=cm2 ). Nevertheless, the thinnest available (<500 lg=cm2 ) aluminium (or other) foils can also be advantageously applied especially for the selective ®ltering of intense low energy X-ray lines (e.g. Si-Ka in geological samples). X-ray ®lters can also be placed in front of the Be-windowed detector. In most cases Al foils of 10±30 mg=cm2 thickness are used for ®ltering Xrays. 2.4. Performance of the new experimental set-up The capabilities of this new experimental set-up have been extensively tested and the results are outlined below. For demonstration purposes, the simultaneously collected spectra of an archaeological glass fragment (£ < 1 mm) are presented in Fig. 3. The reason for selecting this sample is that it contains numerous minor and trace elements from carbon to lead, and we have studied the composition of medieval glasses [16] in recent years. From the picture it can be clearly seen that the analytical range of the UTW detector is between 0.2 and 6 keV which corresponds to elements from C to Mn, whereas that of the Bewindowed large area detector is above 4 keV corresponding to elements from Ti to U. In front of the latter detector a 10 mg=cm2 Al X-ray ®lter was placed in order to eliminate the intense Si-Ka peak from the spectrum and to avoid undesirable pileup eects. It is worth noting that the energy resolution of the UTW detector (100 eV for C-Ka X-ray) is enough to resolve even the lowest Z-number elements. This example clearly demonstrates the advantage of this new experimental setup: both light and heavy elements can be measured simultaneously with uniformly high sensitivity in spite of the signi®cant dierences in their X-ray production cross-sections. Namely, the large solid
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
Fig. 3. X-ray spectra collected simultaneously from a fragment (£ 1 mm) of an archaeological cobalt-blue glass (Ep 2 MeV; Ip 100 pA; Q 200 nC; ®lters: 80 lg/g carbon (UTW) and 10 mg=cm2 aluminium (Be-windowed) foils, respectively).
angle of the Be-windowed detector compensates well for the low cross-sections of the heavier elements. In contrast, the high cross-sections for light elements make it possible to detect them with an UTW detector subtending a low solid angle. The scanning capability of this new set-up is demonstrated by the analysis of some special geological materials, namely a surface polished deep-sea sediment spherule sample and a few other magnetic spherules of impact origin. The geological aspects of these studies will be described else-
Fig. 4. O-Ka ; Fe-Ka ; Ni-Ka Fe-Kb characteristic X-ray maps for major elements of a magnetic deep-sea spherule collected by the UTW and Be-windowed detectors simultaneously (Ep 2 MeV; Ip 30 pA; Q 65 nC; UTW: no ®lter, Be-windowed: 30 g=cm2 aluminium foil). Scan size: 90 90 lm2 . Concentrations (g/g) of the elements presented are as follows: O 22%, Fe 78 % (shell); Fe 45%, Ni 45% (core).
197
where in detail. The ®rst sample is a rounded, 80 lm diameter metal ball supposedly of extraterrestrial origin (micro meteorite). In Fig. 4, elemental maps for the major constituents O, Fe and Ni are shown. The scan size was 90 90 lm2 . From the ®gure a characteristic shell structure [17] can be recognised. Namely, a perfectly round-shaped Fe± Ni core is surrounded by an FeO (wustite) mantle and only the latter one contains oxygen. The impact spherules collected at the well known Arizona crater (USA) have similar characteristics (see Fig. 5) except one major dierence that they contain carbon in minor quantities both in the core (0.17%) and shell (1.3%) of the Fe±Ni objects. It is worth emphasising that the aim of the latter measurements has been exactly the veri®cation of the presence of carbon in the spherules. These results unambiguously demonstrate the power of PIXE for light element measurement: their characteristic X-ray lines can be easily detected in the X-ray spectra and complementary RBS or NRA investigations are not necessary. By analysing some glass, biological and industrial (alloy) samples, the following typical limit of detection (LOD) values were obtained for light elements: C 500 ppm, F 600 ppm, while LOD decreases from 300 to 30 ppm in the range of Na to Ca. For N and O LOD values could not be reported due to the lack of suitable trace element standards. Nevertheless, it should be emphasized, that their Ka X-rays are seriously absorbed by the polymer window plus carbon foil system,
Fig. 5. C-Ka ; Fe-Ka ; Ni-Ka Fe-Kb X-ray maps for some characteristic elements of magnetic spherules (Q 150 nC; in another respect see Fig. 4). Scan size: 1000 1000 lm2 . Averaged concentrations (g/g) of the elements presented are as follows: C 1:3%, Fe 40%, Ni 5% (shell); C 0:17%, Fe 85%, Ni 12% (core).
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I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
therefore, their expected detection limits should be relatively high. The experimental conditions were as follows: proton energy of 2 MeV; beam current of 100 pA; total collected charge of 0:2 lC; carbon ®lter of 80 lg/g thickness. 3. Conclusion The new in-vacuum micro-PIXE experimental set-up, with the simultaneous use of UTW and Bewindowed detectors, provides an eective method for a rapid, overall characterisation of samples in a single irradiation process. Light elements such as C and O, which are of extreme importance, e.g. in geological applications, can be directly observed in the X-ray spectra even if they are in minor amounts. It can be concluded that this extended micro-PIXE technique can be a real competitor of combined RBS+NRA+conventional PIXE methods in numerous applications. Acknowledgements Support from the Hungarian Research Foundation (OTKA) under contract Nos. T 025771 and A080 is gratefully acknowledged. The authors are }r for indebted to Dr. V. Dekov and Dr. Gy. Sz oo providing us with geological samples for demonstration purposes.
References [1] T. Calligaro, J.-C. Dran, J.-P. Poirot, G. Querre, J. Salomon, J.C. Zwaan, Nucl. Instr. and Meth. B 161±163 (2000) 769. [2] Ph. Moretto, J.E. Surleve-Bazeille, D. Licu, C. Michelet, P. Stoedzel, Nucl. Instr. and Meth. B 158 (1999) 386. [3] R.G. Musket, Nucl. Instr. and Meth. B 15 (1986) 735. [4] T. Calligaro, J.D. MacArthur, J. Salomon, Nucl. Instr. and Meth. B 109±110 (1996) 125. [5] J.L. Campbell, G.K. Czamanske, L. MacDonald, W.J. Teesdale, Nucl. Instr. and Meth. B 130 (1997) 608. [6] Zs. Kertesz, I. Borbely-Kiss, I. Rajta, I. Uzonyi, A.Z. Kiss, Nucl. Instr. and Meth. B 161±163 (2000) 808. [7] I. Uzonyi, A.Z. Kiss, P. Solt, L. Dosztaly, O. Kakay Szab o, Cs.H. Detre, Nucl. Instr. and Meth. B 139 (1998) 192. [8] I. Rajta, I. Borbely-Kiss, Gy. M orik, L. Bartha, E. Koltay, A.Z. Kiss, Nucl. Instr. and Meth. B 109±110 (1996) 148. [9] A. Simon, F. Paszti, I. Uzonyi, A. Manuaba, A.Z. Kiss, Nucl. Instr. and Meth. B 136±138 (1998) 350. [10] Z. Elekes, A.Z. Kiss, Gy. Gy urky, E. Somorjai, I. Uzonyi, Nucl. Instr. and Meth. B 158 (1999) 209. [11] G.W. Grime, M. Dawson, M. Marsh, I.C. McArthur, F. Watt, Nucl. Instr. and Meth. B 54 (1991) 52. [12] T. Lakatos, Nucl. Instr. and Meth. B 62 (1991) 289. [13] G.W. Grime, M. Dawson, Nucl. Instr. and Meth. B 89 (1994) 223. [14] L. Bartha, I. Uzonyi, Nucl. Instr. and Meth. B 161±163 (2000) 339. [15] Gy. Szab o, I. Borbely-Kiss, Nucl. Instr. and Meth. B 75 (1993) 123. [16] B. Gratuze, I. Uzonyi, Z. Elekes, A.Z. Kiss, E. Mester, in: Proceedings of the 31st International Symposium on Archaeometry 1998, Budapest, Archaeolingua, in press. [17] D.E. Brownlee, B. Bates, L. Schramm, Meteoritics & Planet. Sci. 32 (1997) 157.
www.elsevier.com/locate/nimb
Realization of the simultaneous micro-PIXE analysis of heavy and light elements at a nuclear microprobe I. Uzonyi *, I. Rajta, L. Bartha, A.Z. Kiss, A. Nagy Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), P.O. Box 51, Bem t er 18/c, H-4001 Debrecen, Hungary
Abstract A new in-vacuum micro-PIXE experimental set-up has been realised at an Oxford-type scanning nuclear microprobe facility. It is based on the simultaneous use of an ultra thin windowed detector and a conventional Be-windowed one for the measurement of the characteristic X-rays of light and heavier elements in the E 0:2±6 keV and E > 4 keV energy ranges, respectively. Complete analytical characterisation of samples from carbon to uranium is possible in a single irradiation process. Performance and technical developments are described in detail. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 82.80.E; 07.85.T; 29.30.Kv Keywords: Scanning nuclear microprobe; Micro-PIXE, UTW- and Be-windowed detectors; Light and heavy elements
1. Introduction Combination of dierent ion beam analytical methods such as NRA, RBS and PIXE has been a conventional technique in microprobe-based investigations in order to realise complete characterisation of samples. The conventional procedure is to use NRA or RBS for the detection of light elements ± usually up to Na or Mg ± and PIXE for the heavier ones, mostly in a consecutive way. The results are then uni®ed into a complete concen-
*
Corresponding author. Tel.: +36-52-417-266; fax: +36-52416-181. E-mail address: [email protected] (I. Uzonyi).
tration set of data in rather sophisticated ways [1,2]. It is worth emphasising that PIXE can also be a good candidate for light element analysis due to its superior cross-sections in the respected atomic number region. Nevertheless, this possibility inherent in PIXE ± except for certain examples [3±5] ± has not been utilised suciently yet, which is chie¯y attributable to the diculties in soft X-ray detection and evaluation of low energy X-ray spectra. The major impetus to extend our micro-PIXE technique towards light elements was given by our recent studies in the ®eld of aerosol and spherule (micro meteorite) research [6,7]. Namely, in these cases, besides mineral elements, the mapping of carbon and occasionally oxygen is of vital
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 7 0 - 6
194
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
importance. Additionally, the best lateral resolution of the microprobe should be used since the size of the measurable objects are on the micrometer scale. Unfortunately, the typical beam currents (30±100 pA) which have been achieved and used at good lateral resolution (1 lm) are practically insucient for RBS or NRA measurements, but proved to be satisfactory for PIXE analysis. The aim of this paper is to present the new invacuum micro-PIXE experimental set-up developed to our Oxford-type nuclear microprobe for the simultaneous measurement of light and heavy elements. It adopts the idea of using two Si(Li) Xray detectors [4,5] and sharing tasks between them. In this arrangement, an ultra thin windowed (UTW) detector serves to characterise the matrix by measuring low energy X-ray lines (E 0:2±6 keV) while a large area Be-windowed detector is used to detect the medium or high energy X-rays (E > 4 keV) with excellent eciency. In this way elements Z > 5 can be detected simultaneously by at least one of the two detectors, reducing both irradiation damage to the sample and measurement time, compared to the consecutive analytical
techniques mentioned above. The performance and use of this new experimental set-up will be demonstrated through some examples. 2. Instrumentation and technical developments 2.1. Description of the new experimental set-up Developments have been carried out at the Debrecen scanning nuclear microprobe facility [8] produced by Oxford microbeams. A photograph of the target chamber and a schematic diagram of the new micro-PIXE experimental set-up are shown in Figs. 1 and 2, respectively. One of the basic principles of the design was to build a ¯exible system, since the microprobe is used for various purposes like grazing angle micro-RBS [9] or micro-PIGE experiments [10]. In order to facilitate an easy switch-over between dierent set-ups both the UTW- and Be-windowed Si(Li) detectors were mounted to the vacuum chamber by special mechanical interfaces (see Fig. 2) symmetrically to the incident beam at an angle of 135°. Mechanical
Fig. 1. A photograph of the instrument.
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
195
Fig. 2. Schematic view of the new experimental set-up.
slides assure optimisation of the target-detector distances and the complete retraction of the endcaps from the inner volume when necessary (during maintenance or dedicated RBS measurements). If the endcaps are fully retracted, the entrance holes to the chamber on the mechanical interfaces can be covered by small o-ring sealed lids, thus enabling vacuum-tight isolation of the Si(Li) detectors from the chamber. The arrangement for the forward-viewing zoom microscope had to be changed as compared to the original Oxford design [11] since both 135° ports were occupied by the Si(Li) detectors. In order to solve this problem a new port was ®tted to the top ¯ange of the chamber at a vertical direction of 132° to the beam. The original microscope port is still available at the 0° port of the chamber, so the rear-viewing position can be utilised for thin transparent specimens and for focussing the beam on a thin quartz, while the new top port is intended for the observation of thick samples. 2.2. Detectors, data acquisition and beam-current monitoring The UTW detector, made by Princeton gamma technology (PGT), serves to measure low energy
X-rays. It has an energy resolution of 148 eV at 5.9 keV, 3.5 mm nominal thickness, 30 mm2 active area and a light element window (0:38 lm of Polymer). The maximum solid angle is 2.5 msr. Medium or high energy X-rays are measured with a Canberra Si(Li) detector characterised with the following parameters: FWHM 190 eV; thickness 5 mm; active area 80 mm2 ; Be-window thickness 25 lm. In this case the maximum solid angle attainable is 90 msr. The pulse processing system incorporates optical reset preampli®ers, an Aptec AMP 6300X spectroscopy ampli®er for the UTW detector and an ATOMKI made NZ-870 analogue signal processor [12] for the Canberra detector respectively, as well as the Oxford microbeams data acquisition system [13] for X-ray mapping. The data acquisition system runs on a Pentium II PC under MS Windows 98. Ion beam dose measurements are carried out by using a small-sized compact beam chopper, which is described elsewhere [14]. Charge build-up in insulating samples is eliminated by operating an electron source during measurements. PIXE spectra are evaluated by a new version of the PIXEKLM program package developed by Szab o et al. [15] (ATOMKI) for standardless
196
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
PIXE analysis. The capabilities of the UTW detector can be fully utilised, using the above mentioned program package, allowing the quantitative analysis of elements down to carbon. 2.3. Detector accessories Diculties associated with the operation of UTW detectors in PIXE set-ups have been discussed in great detail in numerous publications [3,4]. Namely, protons or visible light can easily penetrate through the ultra thin window causing decreased resolution, distortion of peak shapes, loss of X-ray lines below 1 keV [4] and even damage to the detector crystal. These diculties are generally overcome by placing a magnetic de¯ection trap and an extremely thin carbon foil in front of the ultra thin window [3,4]. In order to meet the requirements of in-vacuum micro-PIXE measurements, a compact magnetic de¯ection unit was developed with a relatively large air gap size of 4 4 mm2 , a magnetic ®eld strength of 0.6 T and a length of 60 mm. The magnetic circuit is based on a NdFeB permanent magnet (Br 1:3 T, BHmax 300 kJ=m3 , 3 size 25 25 40 mm ) which is placed in a ``C''-shaped iron yoke. This unit ®ts tightly into the entrance hole of the mechanical interface and de¯ects protons up to 2 MeV energy. The application of the unusually large air gap size results a higher solid angle, thus satisfactory count rates can be achieved by the UTW detector even at low (30±100 pA) beam currents. In order to eciently protect the UTW detector from scattered light inside the vacuum chamber, a special collimator was developed. This unit perfectly ®ts into the air gap of the magnetic circuit and lets X-rays and visible light reach the UTW detector only through a tube having rectangular 4 4 mm2 cross-sectional area. It was milled from a single block of Al and it is covered with a thin layer of black paint. In this way the UTW detector can be operated even without a carbon foil for non-heavily illuminating samples. In other cases ± e.g., either a strong light source is switched on inside the chamber during sample positioning, or when the sample itself emits too much light ± further provisions are needed to preserve the operability of the detector. This dif-
®culty was overcome by the development of a three-position ®lter changer which is placed in front of the magnetic trap and is manually operated by a knob from outside the vacuum chamber. In the ®rst position no ®lter is used, allowing the X-rays to reach the detector without attenuation, while in the other two cases any ®lters can be applied. For light protection, usually a thin carbon foil is used (thickness 40±80 lg=cm2 ). Nevertheless, the thinnest available (<500 lg=cm2 ) aluminium (or other) foils can also be advantageously applied especially for the selective ®ltering of intense low energy X-ray lines (e.g. Si-Ka in geological samples). X-ray ®lters can also be placed in front of the Be-windowed detector. In most cases Al foils of 10±30 mg=cm2 thickness are used for ®ltering Xrays. 2.4. Performance of the new experimental set-up The capabilities of this new experimental set-up have been extensively tested and the results are outlined below. For demonstration purposes, the simultaneously collected spectra of an archaeological glass fragment (£ < 1 mm) are presented in Fig. 3. The reason for selecting this sample is that it contains numerous minor and trace elements from carbon to lead, and we have studied the composition of medieval glasses [16] in recent years. From the picture it can be clearly seen that the analytical range of the UTW detector is between 0.2 and 6 keV which corresponds to elements from C to Mn, whereas that of the Bewindowed large area detector is above 4 keV corresponding to elements from Ti to U. In front of the latter detector a 10 mg=cm2 Al X-ray ®lter was placed in order to eliminate the intense Si-Ka peak from the spectrum and to avoid undesirable pileup eects. It is worth noting that the energy resolution of the UTW detector (100 eV for C-Ka X-ray) is enough to resolve even the lowest Z-number elements. This example clearly demonstrates the advantage of this new experimental setup: both light and heavy elements can be measured simultaneously with uniformly high sensitivity in spite of the signi®cant dierences in their X-ray production cross-sections. Namely, the large solid
I. Uzonyi et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 193±198
Fig. 3. X-ray spectra collected simultaneously from a fragment (£ 1 mm) of an archaeological cobalt-blue glass (Ep 2 MeV; Ip 100 pA; Q 200 nC; ®lters: 80 lg/g carbon (UTW) and 10 mg=cm2 aluminium (Be-windowed) foils, respectively).
angle of the Be-windowed detector compensates well for the low cross-sections of the heavier elements. In contrast, the high cross-sections for light elements make it possible to detect them with an UTW detector subtending a low solid angle. The scanning capability of this new set-up is demonstrated by the analysis of some special geological materials, namely a surface polished deep-sea sediment spherule sample and a few other magnetic spherules of impact origin. The geological aspects of these studies will be described else-
Fig. 4. O-Ka ; Fe-Ka ; Ni-Ka Fe-Kb characteristic X-ray maps for major elements of a magnetic deep-sea spherule collected by the UTW and Be-windowed detectors simultaneously (Ep 2 MeV; Ip 30 pA; Q 65 nC; UTW: no ®lter, Be-windowed: 30 g=cm2 aluminium foil). Scan size: 90 90 lm2 . Concentrations (g/g) of the elements presented are as follows: O 22%, Fe 78 % (shell); Fe 45%, Ni 45% (core).
197
where in detail. The ®rst sample is a rounded, 80 lm diameter metal ball supposedly of extraterrestrial origin (micro meteorite). In Fig. 4, elemental maps for the major constituents O, Fe and Ni are shown. The scan size was 90 90 lm2 . From the ®gure a characteristic shell structure [17] can be recognised. Namely, a perfectly round-shaped Fe± Ni core is surrounded by an FeO (wustite) mantle and only the latter one contains oxygen. The impact spherules collected at the well known Arizona crater (USA) have similar characteristics (see Fig. 5) except one major dierence that they contain carbon in minor quantities both in the core (0.17%) and shell (1.3%) of the Fe±Ni objects. It is worth emphasising that the aim of the latter measurements has been exactly the veri®cation of the presence of carbon in the spherules. These results unambiguously demonstrate the power of PIXE for light element measurement: their characteristic X-ray lines can be easily detected in the X-ray spectra and complementary RBS or NRA investigations are not necessary. By analysing some glass, biological and industrial (alloy) samples, the following typical limit of detection (LOD) values were obtained for light elements: C 500 ppm, F 600 ppm, while LOD decreases from 300 to 30 ppm in the range of Na to Ca. For N and O LOD values could not be reported due to the lack of suitable trace element standards. Nevertheless, it should be emphasized, that their Ka X-rays are seriously absorbed by the polymer window plus carbon foil system,
Fig. 5. C-Ka ; Fe-Ka ; Ni-Ka Fe-Kb X-ray maps for some characteristic elements of magnetic spherules (Q 150 nC; in another respect see Fig. 4). Scan size: 1000 1000 lm2 . Averaged concentrations (g/g) of the elements presented are as follows: C 1:3%, Fe 40%, Ni 5% (shell); C 0:17%, Fe 85%, Ni 12% (core).
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therefore, their expected detection limits should be relatively high. The experimental conditions were as follows: proton energy of 2 MeV; beam current of 100 pA; total collected charge of 0:2 lC; carbon ®lter of 80 lg/g thickness. 3. Conclusion The new in-vacuum micro-PIXE experimental set-up, with the simultaneous use of UTW and Bewindowed detectors, provides an eective method for a rapid, overall characterisation of samples in a single irradiation process. Light elements such as C and O, which are of extreme importance, e.g. in geological applications, can be directly observed in the X-ray spectra even if they are in minor amounts. It can be concluded that this extended micro-PIXE technique can be a real competitor of combined RBS+NRA+conventional PIXE methods in numerous applications. Acknowledgements Support from the Hungarian Research Foundation (OTKA) under contract Nos. T 025771 and A080 is gratefully acknowledged. The authors are }r for indebted to Dr. V. Dekov and Dr. Gy. Sz oo providing us with geological samples for demonstration purposes.
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