- Email: [email protected]
PIXE detection limits for dental enamel from some human teeth by excitation with protons and 4He2+ ions from a 3 MeV Van der Graaff accelerator
ARTICLE IN PRESS
Vacuum 81 (2007) 1167–1170 www.elsevier.com/locate/vacuum
PIXE detection limits for dental enamel from some human teeth by excitation with protons and 4He2+ ions from a 3 MeV Van der Graaff accelerator C. Opreaa, A.P. Kobzeva, I.A. Opreaa, P.J. Szalanskib,, V. Buzgutac a
Frank Laboratory of Neutron Physics (FLNP), Joint Institute for Nuclear Research (JINR), Dubna 141980, Russian Federation b Institute of Physics, Lodz University, Lodz, Poland c Faculty of Sciences, University of Oradea, 3700 Oradea, Romania
Abstract A technique has been developed to measure elemental content in human teeth using H+ and 4He2+ ion beam analysis. Teeth of Oradea inhabitants were sampled in two stomatological clinics in Oradea in the period of 2004 and 2005 years. Tooth samples were irradiated in vacuum with 2 MeV proton and 3 MeV alpha beams from a Van der Graaff electrostatic accelerator of EG-5 experimental facility in FLNP, JINR. The particle induced X-ray emission (PIXE) analysis, apart from determination of Ca, allowed an optimised detection of Cr, Cu, Fe and Zn above the detection limits by the use of Al and Mylar filters. The detection limits for Ka X-rays using proton and alpha beams are determined and discussed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Teeth; Ion beams; H+; 4He2+; Detection limit; PIXE; RBS; Human exposure
1. Introduction It is known that a large proportion of environmental trace elements in human body are incorporated in the skeleton during life. Tooth enamel is the hardest mineralized tissue of the human body, contains neither cells nor exhibits metabolic activity. Owing to its minor organic fraction, smaller remodeling rate and strong hardness, it is considered the best bio-indicator of the cumulative human exposure [1,2]. The distribution of environmental trace elements in human teeth is derived mainly from dietary habits or environmental contamination. Zn, Sr and Pb are supplied into the human body mainly by food and water, and to a lesser extent, respiration and are enriched in tooth by substituting for Ca in hydroxyapatite [3–5]. The toxicologically interesting metals: Mn, Cu, Zn, Cd and Pb were widely studied and it is commonly known that the contamination risk is induced by these elements [6–8]. Corresponding author. Tel.: +48 42 6355636; fax: +48 42 6787087.
E-mail address: [email protected] (P.J. Szalanski). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.008
The information regarding the local environmental pollution in Oradea (Romania) and the related long-term exposure of the population are rather poor and until present reported only in papers [9,10]. This study was undertaken to estimate elemental concentrations in dental enamel by ion beam analysis (IBA) methods in an urban population, which was exposed to significant pollution levels released by local industry during the past 4 decades, compared with concentrations similarly estimated in a matched rural community without unusual exposure. Accelerator based IBA methods of particle induced X-ray emission (PIXE) and Rutherford backscattering (RBS) are ideally suited to determine over 20 different chemical elements from carbon to lead that are present in dental enamel [11–14]. Over time different approaches to the problem of obtaining low detection limits for heavy metals in different samples of interest by IBA methods have been used. In this work an approach by using H+ and 4He2+ ion beams for production of Ka radiation was employed [15]. For analytical techniques based on X-ray detection, the analysis of trace heavy metals in teeth rich in calcium [16],
ARTICLE IN PRESS 1168
C. Oprea et al. / Vacuum 81 (2007) 1167–1170
poses some difficulties due to high background and saturation effects in X-ray detectors. As selective absorbers for calcium the 50 mm Al and 25 mm metallised (Ag) Mylar filters were used [17,18]. 2. Experimental 2.1. Sample collection and preparation Teeth were collected from the patients after their teeth extraction at the clinic in the period 2004 and 2005 years in Oradea (Romania). Prior to the teeth extraction, a detailed investigation of dental history and examination were carried out for each individual. Immediately after the extraction each tooth was put into oxygenated water (to clean the organic material from its surface) into a separate polyethylene container. Then samples were stored in a deep freeze (at 20 1C). 2.2. Experimental considerations (suitability of the chosen approach) The tooth enamel samples, of only a few hundred micrograms, are ideal targets for the IBA analysis using proton and helium ion beams from a 3 MeV Van der Graaff accelerator. Typically the proton beam loses less than 40 keV in traversing 300 mg/cm2 of dental enamel. Combination of the two IBA analytical techniques (i.e. PIXE and RBS) for human tooth enamel was chosen based on the following: 1. K shell PIXE provides little depth information, has a larger cross-section for the lighter elements below calcium but poorer sensitivity than RBS for heavy elements above zirconium. 2. RBS is most useful in elemental profiling when the element being profiled is heavier than the other matrix elements and falls down badly while profiling light elements in a heavy matrix [19]. 3. Improvement of sensitivity was achieved by providing the PIXE analysis irradiating with both H+ and 4He2+ ion beams considering the elements of interest as well as the tooth matrix [20,21].
The energy resolution of the system was 260 eV FWHM at 5.89 keV MnKa line. The analyzing time was typically 1 h 30 min. The spectra were calibrated using the 241Am source. The elemental composition is deduced directly from the measured relative X-ray intensities, using known X-ray production cross-sections and correcting for projectile energy loss, X-ray attenuation in the sample and detector efficiency. The analysis procedure for human teeth was checked by the analysis of thin samples of known composition. With the optimized PIXE procedures precisions of 1–2% and an accuracy of higher than 5% are obtainable, whereas the detection limits are within 10–0.1 mg/g. The PIXE spectra were analyzed and the concentrations of trace elements were determined using the computer program ACTIV [22]. The quantitative analyses of tooth samples were obtained from the RBS measurements by simulation of the backscattered ion energy spectra using the DVBS computer program [23,24]. A total of 20 major, minor and trace elements on the dental enamel samples were determined with the two IBA methods. 3. Results and discussion The uncertainties with detection limits appear due to the interference with K X-rays of some elements always present in dental enamel, such as calcium, zinc and strontium (Fig. 1), which can be solved by calculation means [25]. Another source of errors in detection limits is due to the silicon escape peaks [26,27], that can be avoided by using a Ge(Li) detector. The background, produced mainly by the secondary emission of characteristic X-rays, secondary electrons and
2.3. Analysis The samples were irradiated in vacuum with 2 MeV proton and 3 MeV helium ion beams from the Van der Graaff accelerator of EG-5 experimental basis of FLNP in JINR. Beam diameters of around 1.5 mm and beam currents generally less than 10 mA were used. The experimental setup used was a typical PIXE/RBS arrangement, described previously [15]. The sample was placed with the surface to be irradiated at 451 with respect to beam direction and the X-rays were detected by a Si(Li) detector mounted at a right angle to the beam direction. The sample to detector gap included 10 mm of air and absorbers.
Fig. 1. Typical PIXE X-ray spectra of the external tooth enamel from the human bombarded with 2 MeV protons in the presence of a 25 mm Mylar filter (that reduces very low X-ray energy).
ARTICLE IN PRESS C. Oprea et al. / Vacuum 81 (2007) 1167–1170
bremsstrahlung gives enhancement effects to the number of pulses in certain peaks. This is the case of elements situated below the K X-ray edges of calcium [28]. pffiffiffiffiffi The detection limit is defined as S ¼ 3 N , where N is the number of pulses within an interval 71 FWHM centered on the Ka peak position, which do not belong to the peak. In this research, the detection limits were tested and improved by increasing the counting statistics using longer analyzing times or higher count rates [29]. S was found larger than 20 for most of the elements analyzed. PIXE with 2 MeV protons gives the lowest detection limits (R41) for Zo28, and PIXE with 3 MeV helium ions gives better detection limits (Ro1) for Z429 (Fig. 2). On studying Fig. 2, the effect of two different irradiations of human enamel samples on the detection limits shows the advantage of protons, in general, mainly for light elements [30]. For risk pollutants such as cadmium and lead, the results obtained in this work suggest a higher optimum projectile energy. All trace heavy metals as determined by the PIXE analysis with protons and helium ions in human dental enamel show clear rise in concentrations of 1.5–3 times for the exposed compared with the unexposed population, which reflects the quality of air/environment in two living situations. Due to slowing down of protons and absorption of Xrays in the tooth only a surface layer is analyzed by RBS, the thickness of which depends on proton energy, analyzed element and the angles between the sample surface and the sample to detector direction [31]. Concentrations of microelements in the whole enamel and in the first surface layer (50 mm depth) were compared. With exception of F, all elements show significantly higher concentrations in the first layer than in the whole enamel. The concentrations of major elements in three layers of teeth enamel decreased while F concentration increased toward the enamel–dentine junction. The concentrations of most elements were almost constant as they approached the 150 mm layer. The depth profiling analysis for some heavy metals in the surface layer of teeth support the hypothesis that they result from long-term exposure of the subject to the historical pollution during his life [10]. 2
R
1.5
1
0.5 15
20
25
30 Z
35
40
Fig. 2. Ratio R between the detection limits for the elements determined by PIXE with 2 MeV H+ and 3 MeV 4He2+ ion beams in the tooth enamel.
1169
4. Conclusions Because of its inherent characteristics, PIXE offers a great potential for trace element analysis in teeth and this was demonstrated by the selected examples through the present study. The PIXE method using H+ and 4He2+ ion impact, supplied with a standard combination of IBA techniques, allows one to determine the elemental content of tooth enamel with an accuracy higher than 5%. The detection limits obtained by using 2 MeV proton and 3 MeV helium ion beams are of the same order, with some improvements for Zo29. The specific atmospheric pollutants emitted by local industry are characterized by high retention capacities of human teeth. The results indicate that human heavy metal exposure of the investigated population is average as shown by trace heavy metal content in the analyzed teeth. Acknowledgments The authors wish thank to the EG-5 staff for technical support during the measurements. References [1] Carvalho ML, Marques JP, Marques AF, et al. X-ray Spectrom 2004;33:55. [2] Bren R, Haug C, Klar U, et al. Nucl Instrum Methods B 1999;158:270. [3] Elias M. Am J Phys Anthropol 1980;53:1. [4] Elias RW, Hirao Y, Patterson CC. Geochim Cosmochim Acta 1982;46:2561. [5] Rheingold AL, Hues S, Cohen MN. J Chem Educ 1983;60:233. [6] Grunke K, Stark H-J, Wennrich R. Fresenius J Anal Chem 1996;354:633. [7] Solis C, Oliver A, Rodrigues-Fernandez L, et al. Nucl Instrum Methods B 1996;118:359. [8] Lane DW, Peach DF. Biol Trace Elem Res 1997;60(1–2):1. [9] Oprea C, Filip S, Baluta A, et al. Environ Prog 2005;3:273. [10] Oprea C, Kobzev AP, Filip S, et al. Rev Cytol Biol Ve´g 2005;28:439. [11] Cohen DD, Clayton E, Ainsworth T. Nucl Instrum Methods 1981;188:203. [12] Rizzutto MA, Tabacniks MH, Added N, et al. Nucl Instrum Methods B 2002;190:186. [13] Andrade E, Pineda JC, Zavala EP, et al. Nucl Instrum Methods B 1998;137:908. [14] Lane DW, Duffy CA. Nucl Instrum Methods B 1996;118(1–4):392. [15] Pajek M, Kobzev AP, Sandrik R, et al. Nucl Instrum Methods B 1989;42:346. [16] Brudevold F, Soremark R. Chemistry of the mineral phase of enamel. In structural and chemical organization of teeth. New York: Academic Press; 1967. [17] Gu¨ereca G, Ruvalcaba-Sil JL. In: Proceedings of the 10th international conference on PIXE, its analysis and applications. Portorozˇ, Slovenia, June 4–8, 2004. p. 825.1 /http://pixe2004.ijs.si/S. [18] Van Grieken RE, Markovicz AJ, editors. Handbook of X-ray spectrometry. Practical spectroscopy series, vol. 29. New York: 2002. [19] Chu WK, Mayer JW, Nicolet MA. Backscattering spectrometry. New York: Academic Press; 1978. [20] Johansson SAE. Int J PIXE 1992;2:33. [21] Beck L. X-ray Spectrom 2005;34(4):393. [22] Zlokazov VB. Comput Phys Commun 1982;28:27.
ARTICLE IN PRESS 1170 [23] [24] [25] [26]
C. Oprea et al. / Vacuum 81 (2007) 1167–1170
Grime GW. Nucl Instrum Methods B 1996;109/110:170. Bohac V, Shirokov DM. Nucl Instrum Methods B 1994;84:497. Ahlberg M, Akselsson R. Int J Appl Radiat Isot 1976;27:279. Van Gysel M, Lemberge P, van Espen P. X-ray Spectrom 2003;32:139. [27] Papp T, Campbell JL. X-ray Spectrom 2001;30:77.
[28] Pallon J, Garmer M, Auzelyte V, et al. Nucl Instrum Methods B 2005;231:274. [29] Bennun L, Greaves ED, Blostein JJ. X-ray Spectrom 2002;31(4):289. [30] Orlic I, Siegele R, Hammerton K, et al. Nucl Instrum Methods B 2003;210:330. [31] Svalbe ID, et al. Nucl Instrum Methods B 1984;3:648.
Vacuum 81 (2007) 1167–1170 www.elsevier.com/locate/vacuum
PIXE detection limits for dental enamel from some human teeth by excitation with protons and 4He2+ ions from a 3 MeV Van der Graaff accelerator C. Opreaa, A.P. Kobzeva, I.A. Opreaa, P.J. Szalanskib,, V. Buzgutac a
Frank Laboratory of Neutron Physics (FLNP), Joint Institute for Nuclear Research (JINR), Dubna 141980, Russian Federation b Institute of Physics, Lodz University, Lodz, Poland c Faculty of Sciences, University of Oradea, 3700 Oradea, Romania
Abstract A technique has been developed to measure elemental content in human teeth using H+ and 4He2+ ion beam analysis. Teeth of Oradea inhabitants were sampled in two stomatological clinics in Oradea in the period of 2004 and 2005 years. Tooth samples were irradiated in vacuum with 2 MeV proton and 3 MeV alpha beams from a Van der Graaff electrostatic accelerator of EG-5 experimental facility in FLNP, JINR. The particle induced X-ray emission (PIXE) analysis, apart from determination of Ca, allowed an optimised detection of Cr, Cu, Fe and Zn above the detection limits by the use of Al and Mylar filters. The detection limits for Ka X-rays using proton and alpha beams are determined and discussed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Teeth; Ion beams; H+; 4He2+; Detection limit; PIXE; RBS; Human exposure
1. Introduction It is known that a large proportion of environmental trace elements in human body are incorporated in the skeleton during life. Tooth enamel is the hardest mineralized tissue of the human body, contains neither cells nor exhibits metabolic activity. Owing to its minor organic fraction, smaller remodeling rate and strong hardness, it is considered the best bio-indicator of the cumulative human exposure [1,2]. The distribution of environmental trace elements in human teeth is derived mainly from dietary habits or environmental contamination. Zn, Sr and Pb are supplied into the human body mainly by food and water, and to a lesser extent, respiration and are enriched in tooth by substituting for Ca in hydroxyapatite [3–5]. The toxicologically interesting metals: Mn, Cu, Zn, Cd and Pb were widely studied and it is commonly known that the contamination risk is induced by these elements [6–8]. Corresponding author. Tel.: +48 42 6355636; fax: +48 42 6787087.
E-mail address: [email protected] (P.J. Szalanski). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.008
The information regarding the local environmental pollution in Oradea (Romania) and the related long-term exposure of the population are rather poor and until present reported only in papers [9,10]. This study was undertaken to estimate elemental concentrations in dental enamel by ion beam analysis (IBA) methods in an urban population, which was exposed to significant pollution levels released by local industry during the past 4 decades, compared with concentrations similarly estimated in a matched rural community without unusual exposure. Accelerator based IBA methods of particle induced X-ray emission (PIXE) and Rutherford backscattering (RBS) are ideally suited to determine over 20 different chemical elements from carbon to lead that are present in dental enamel [11–14]. Over time different approaches to the problem of obtaining low detection limits for heavy metals in different samples of interest by IBA methods have been used. In this work an approach by using H+ and 4He2+ ion beams for production of Ka radiation was employed [15]. For analytical techniques based on X-ray detection, the analysis of trace heavy metals in teeth rich in calcium [16],
ARTICLE IN PRESS 1168
C. Oprea et al. / Vacuum 81 (2007) 1167–1170
poses some difficulties due to high background and saturation effects in X-ray detectors. As selective absorbers for calcium the 50 mm Al and 25 mm metallised (Ag) Mylar filters were used [17,18]. 2. Experimental 2.1. Sample collection and preparation Teeth were collected from the patients after their teeth extraction at the clinic in the period 2004 and 2005 years in Oradea (Romania). Prior to the teeth extraction, a detailed investigation of dental history and examination were carried out for each individual. Immediately after the extraction each tooth was put into oxygenated water (to clean the organic material from its surface) into a separate polyethylene container. Then samples were stored in a deep freeze (at 20 1C). 2.2. Experimental considerations (suitability of the chosen approach) The tooth enamel samples, of only a few hundred micrograms, are ideal targets for the IBA analysis using proton and helium ion beams from a 3 MeV Van der Graaff accelerator. Typically the proton beam loses less than 40 keV in traversing 300 mg/cm2 of dental enamel. Combination of the two IBA analytical techniques (i.e. PIXE and RBS) for human tooth enamel was chosen based on the following: 1. K shell PIXE provides little depth information, has a larger cross-section for the lighter elements below calcium but poorer sensitivity than RBS for heavy elements above zirconium. 2. RBS is most useful in elemental profiling when the element being profiled is heavier than the other matrix elements and falls down badly while profiling light elements in a heavy matrix [19]. 3. Improvement of sensitivity was achieved by providing the PIXE analysis irradiating with both H+ and 4He2+ ion beams considering the elements of interest as well as the tooth matrix [20,21].
The energy resolution of the system was 260 eV FWHM at 5.89 keV MnKa line. The analyzing time was typically 1 h 30 min. The spectra were calibrated using the 241Am source. The elemental composition is deduced directly from the measured relative X-ray intensities, using known X-ray production cross-sections and correcting for projectile energy loss, X-ray attenuation in the sample and detector efficiency. The analysis procedure for human teeth was checked by the analysis of thin samples of known composition. With the optimized PIXE procedures precisions of 1–2% and an accuracy of higher than 5% are obtainable, whereas the detection limits are within 10–0.1 mg/g. The PIXE spectra were analyzed and the concentrations of trace elements were determined using the computer program ACTIV [22]. The quantitative analyses of tooth samples were obtained from the RBS measurements by simulation of the backscattered ion energy spectra using the DVBS computer program [23,24]. A total of 20 major, minor and trace elements on the dental enamel samples were determined with the two IBA methods. 3. Results and discussion The uncertainties with detection limits appear due to the interference with K X-rays of some elements always present in dental enamel, such as calcium, zinc and strontium (Fig. 1), which can be solved by calculation means [25]. Another source of errors in detection limits is due to the silicon escape peaks [26,27], that can be avoided by using a Ge(Li) detector. The background, produced mainly by the secondary emission of characteristic X-rays, secondary electrons and
2.3. Analysis The samples were irradiated in vacuum with 2 MeV proton and 3 MeV helium ion beams from the Van der Graaff accelerator of EG-5 experimental basis of FLNP in JINR. Beam diameters of around 1.5 mm and beam currents generally less than 10 mA were used. The experimental setup used was a typical PIXE/RBS arrangement, described previously [15]. The sample was placed with the surface to be irradiated at 451 with respect to beam direction and the X-rays were detected by a Si(Li) detector mounted at a right angle to the beam direction. The sample to detector gap included 10 mm of air and absorbers.
Fig. 1. Typical PIXE X-ray spectra of the external tooth enamel from the human bombarded with 2 MeV protons in the presence of a 25 mm Mylar filter (that reduces very low X-ray energy).
ARTICLE IN PRESS C. Oprea et al. / Vacuum 81 (2007) 1167–1170
bremsstrahlung gives enhancement effects to the number of pulses in certain peaks. This is the case of elements situated below the K X-ray edges of calcium [28]. pffiffiffiffiffi The detection limit is defined as S ¼ 3 N , where N is the number of pulses within an interval 71 FWHM centered on the Ka peak position, which do not belong to the peak. In this research, the detection limits were tested and improved by increasing the counting statistics using longer analyzing times or higher count rates [29]. S was found larger than 20 for most of the elements analyzed. PIXE with 2 MeV protons gives the lowest detection limits (R41) for Zo28, and PIXE with 3 MeV helium ions gives better detection limits (Ro1) for Z429 (Fig. 2). On studying Fig. 2, the effect of two different irradiations of human enamel samples on the detection limits shows the advantage of protons, in general, mainly for light elements [30]. For risk pollutants such as cadmium and lead, the results obtained in this work suggest a higher optimum projectile energy. All trace heavy metals as determined by the PIXE analysis with protons and helium ions in human dental enamel show clear rise in concentrations of 1.5–3 times for the exposed compared with the unexposed population, which reflects the quality of air/environment in two living situations. Due to slowing down of protons and absorption of Xrays in the tooth only a surface layer is analyzed by RBS, the thickness of which depends on proton energy, analyzed element and the angles between the sample surface and the sample to detector direction [31]. Concentrations of microelements in the whole enamel and in the first surface layer (50 mm depth) were compared. With exception of F, all elements show significantly higher concentrations in the first layer than in the whole enamel. The concentrations of major elements in three layers of teeth enamel decreased while F concentration increased toward the enamel–dentine junction. The concentrations of most elements were almost constant as they approached the 150 mm layer. The depth profiling analysis for some heavy metals in the surface layer of teeth support the hypothesis that they result from long-term exposure of the subject to the historical pollution during his life [10]. 2
R
1.5
1
0.5 15
20
25
30 Z
35
40
Fig. 2. Ratio R between the detection limits for the elements determined by PIXE with 2 MeV H+ and 3 MeV 4He2+ ion beams in the tooth enamel.
1169
4. Conclusions Because of its inherent characteristics, PIXE offers a great potential for trace element analysis in teeth and this was demonstrated by the selected examples through the present study. The PIXE method using H+ and 4He2+ ion impact, supplied with a standard combination of IBA techniques, allows one to determine the elemental content of tooth enamel with an accuracy higher than 5%. The detection limits obtained by using 2 MeV proton and 3 MeV helium ion beams are of the same order, with some improvements for Zo29. The specific atmospheric pollutants emitted by local industry are characterized by high retention capacities of human teeth. The results indicate that human heavy metal exposure of the investigated population is average as shown by trace heavy metal content in the analyzed teeth. Acknowledgments The authors wish thank to the EG-5 staff for technical support during the measurements. References [1] Carvalho ML, Marques JP, Marques AF, et al. X-ray Spectrom 2004;33:55. [2] Bren R, Haug C, Klar U, et al. Nucl Instrum Methods B 1999;158:270. [3] Elias M. Am J Phys Anthropol 1980;53:1. [4] Elias RW, Hirao Y, Patterson CC. Geochim Cosmochim Acta 1982;46:2561. [5] Rheingold AL, Hues S, Cohen MN. J Chem Educ 1983;60:233. [6] Grunke K, Stark H-J, Wennrich R. Fresenius J Anal Chem 1996;354:633. [7] Solis C, Oliver A, Rodrigues-Fernandez L, et al. Nucl Instrum Methods B 1996;118:359. [8] Lane DW, Peach DF. Biol Trace Elem Res 1997;60(1–2):1. [9] Oprea C, Filip S, Baluta A, et al. Environ Prog 2005;3:273. [10] Oprea C, Kobzev AP, Filip S, et al. Rev Cytol Biol Ve´g 2005;28:439. [11] Cohen DD, Clayton E, Ainsworth T. Nucl Instrum Methods 1981;188:203. [12] Rizzutto MA, Tabacniks MH, Added N, et al. Nucl Instrum Methods B 2002;190:186. [13] Andrade E, Pineda JC, Zavala EP, et al. Nucl Instrum Methods B 1998;137:908. [14] Lane DW, Duffy CA. Nucl Instrum Methods B 1996;118(1–4):392. [15] Pajek M, Kobzev AP, Sandrik R, et al. Nucl Instrum Methods B 1989;42:346. [16] Brudevold F, Soremark R. Chemistry of the mineral phase of enamel. In structural and chemical organization of teeth. New York: Academic Press; 1967. [17] Gu¨ereca G, Ruvalcaba-Sil JL. In: Proceedings of the 10th international conference on PIXE, its analysis and applications. Portorozˇ, Slovenia, June 4–8, 2004. p. 825.1 /http://pixe2004.ijs.si/S. [18] Van Grieken RE, Markovicz AJ, editors. Handbook of X-ray spectrometry. Practical spectroscopy series, vol. 29. New York: 2002. [19] Chu WK, Mayer JW, Nicolet MA. Backscattering spectrometry. New York: Academic Press; 1978. [20] Johansson SAE. Int J PIXE 1992;2:33. [21] Beck L. X-ray Spectrom 2005;34(4):393. [22] Zlokazov VB. Comput Phys Commun 1982;28:27.
ARTICLE IN PRESS 1170 [23] [24] [25] [26]
C. Oprea et al. / Vacuum 81 (2007) 1167–1170
Grime GW. Nucl Instrum Methods B 1996;109/110:170. Bohac V, Shirokov DM. Nucl Instrum Methods B 1994;84:497. Ahlberg M, Akselsson R. Int J Appl Radiat Isot 1976;27:279. Van Gysel M, Lemberge P, van Espen P. X-ray Spectrom 2003;32:139. [27] Papp T, Campbell JL. X-ray Spectrom 2001;30:77.
[28] Pallon J, Garmer M, Auzelyte V, et al. Nucl Instrum Methods B 2005;231:274. [29] Bennun L, Greaves ED, Blostein JJ. X-ray Spectrom 2002;31(4):289. [30] Orlic I, Siegele R, Hammerton K, et al. Nucl Instrum Methods B 2003;210:330. [31] Svalbe ID, et al. Nucl Instrum Methods B 1984;3:648.