COMPARATIVE STUDY OF PIGE, PIXE AND NAA ANALYTICAL TECHNIQUES FOR THE DETERMINATION OF MINOR ELEMENTS IN STEELS ANTOANETA ENE 1, I. V. POPESCU 2, T. BÃDICÃ 3, C. BEªLIU 4 1 Department of Physics, Faculty of Sciences, Dunarea de Jos University of Galaþi, Domneasca 111, 800201 Galaþi, Romania, e-mail: aene@ugal.ro 2 Department of Physics, Faculty of Sciences, Valahia University of Târgoviºte, Romania, e-mail: ivpopes@valahia.ro 3 Horia Hulubei National Institute of Physics and Nuclear Engineering, P.O. MG-6, Mãgurele, Bucharest, Romania, e-mail: badica@ifin.nipne.ro 4 University of Bucharest, Faculty of Physics, Bucharest, Romania Received October 20, 2005 In this work the capabilities of atomic and nuclear methods PIGE (Particle- Induced Gamma-ray Emission), PIXE (Particle-Induced X-ray Emission) and NAA (Neutron Activation Analysis) used for the determination of minor and trace constituents of steels have been compared in terms of sensitivities, advantages and limitations. PIXE analyses have been carried out using a 3 MeV proton beam and PIGE determinations have been done using 3; 5.5; 6.5 and 8 MeV protons and 5 MeV deuterons. In the case of 5.5 protons and 5 MeV deuterons, in order to improve the detection limits of some minor elements in steel, neutron-gamma coincidences spectra and singular gamma spectra were measured simultaneously. The minor and trace elements identified by us in the steel samples were: S, P, Si, Al, Mn, C, Ti, Ni, Na, O by proton-pige and Co, Zn, Mn, V, Sb, Ti, As, Cr, Mo, Cu selecting only (p, n) reactions; As, Pb, Sb, P, Cr, V, Ti, Cu, Fe, Ni, O, Co, Mo, Zn, Si, S, Al, Mn, C by deuteron-pige and Mo, Cr, Ni, Pb, Fe, Co, Ti, Zn, Sb, Mn, O, S, Cu, Si, V, P, Al selecting only (d, n) reactions; Ca, Cr, Mn, Cu, Ni, Zn, W, As, Mo, Rb, Zr, Nb, Pd, In, Rh, Pb, Sn, Sb by PIXE and Mn, Al, V, As, Cu, W, Ni, Mo, Cr, Sb, Co, Na, K, Ce, La, Sm, Sc, Zr, Zn, Au, Ga, Hf, Ta, Te, Se, Ba, Rb, Yb, Tb and Ir by NAA. By applying the three methods a very good overall picture of the elemental composition of a steel sample may be obtained. Key words: particle-induced X-ray emission, particle-induced gamma-ray emission, neutron activation analysis, analysis of steels, minor elements. 1. INTRODUCTION The atomic and nuclear spectrometric techniques Particle-Induced X-ray Emission (PIXE) [1, 2], Particle-Induced Gamma-ray Emission (PIGE) [2 4] Paper presented at the National Conference of Physics, 13 17 September, 2005, Bucharest, Romania. Rom. Journ. Phys., Vol. 51, Nos. 5 6, P. 595 602, Bucharest, 2006
596 Antoaneta Ene, I. V. Popescu, T. Bãdicã, C. Beºliu 2 and Neutron Activation Analysis (NAA) [5, 6], are mature analytical techniques for trace element analysis in different complex matrices and significant advances have been made lately in already established techniques. Despite this fact, none of these techniques can be considered a panacea to tackle all sample types and/or determine all elements, but their coupling enables the obtaining of a good overall picture of a sample elemental composition. In the last years our group activities have been structured on the establishing of the methodology for multielemental analysis of iron and steel materials. In this paper a compared study of PIXE, PIGE and NAA for the analysis of steels is presented. 2. EXPERIMENTAL The steel samples and European steel standards were obtained from the Iron and Steel Works of Galaþi. Simultaneous PIXE PIGE analyses have been carried out using a 3 MeV proton beam generated with the aid of the 7 MV FN tandem accelerator of the National Institute of Physics and Nuclear Engineering (NIPNE) Bucharest. The detection system included a HPGe detector with an energy resolution of 170 ev at 5.9 kev for the X-rays and a HPGe detector with an energy resolution of 2 kev at 1.33 MeV for the γ rays. The steel target was mounted in the irradiation chamber at 45 with respect to the beam and the both detectors direction. A thin surface barrier silicon detector was also placed in the chamber, at 135 with respect to the beam direction, in order to detect the backscattered protons for spectra normalization. The X-ray, γ-ray and particle spectra were simultaneously collected and processed off-line. Separate PIGE determinations have also been done using 6.5 and 8 MeV protons. In another step there were used 5.5 MeV protons and 5 MeV deuterons both for regular and coincident PIGE measurements. Prompt γ-rays produced in the sample were measured with a HPGe detector having an active volume of about 100 cm 3, 20% detection efficiency and an energy resolution of 2 kev at 1.33 kev placed at 90 with respect to the beam. The resulted neutrons from the evaporation of the compound nucleus in (p, n) reactions or from the direct reaction mechanism in most of the (d, n) reactions, were detected with a NE213 liquid scintillation detector (Φ120 mm 100 mm 2 ) placed at 0 with respect to the beam. This scintillation detector together with the neutron-gamma pulse-shape discriminator provided the complete n-γ separation. The HPGe and the scintillation detectors have been coupled to a coincidence scheme of the type slow-fast and the resolution of the prompt coincidences curve in the gamma-ray energy range E 30 kev was 2τ = 17 ns. The energetic and timing conditionings have been set with the aid of a multiparametric acquisition program. Neutron-gamma coincidences (E γ, n, Δt) spectra, obtained with the energetic window on the neutron spectrum, and singular gamma spectra were measured simultaneously and processed off-line.
3 PIGE, PIXE and NAA analytical techniques 597 3. RESULTS AND DISSCUSION Details of a steel sample PIGE spectrum using 6.5 MeV protons and a PIXE spectrum using 3 MeV protons are presented in Fig. 1 (PIGE) and Fig. 2 (PIXE). Peaks in the PIGE spectrum are identified in Table 1 and labelled in accordance with the convention A X b (t, s) where A X is the target nuclide; b the emitted nuclear product particle in the nuclear reaction induced by protons or deuterons; t and s the level numbers in the heavy product nucleus between which the γ transition occurs. While for a given energy of the protons not all the elements of interest (especially the light ones, which influence the properties of steels, such as C, O, P, S, Al, Si etc) lead to a (p, n) reaction, for all target Fig. 1. Details of a PIGE spectrum of a steel sample using 6.5 MeV protons.
598 Antoaneta Ene, I. V. Popescu, T. Bãdicã, C. Beºliu 4 nuclides the mentioned reactions with deuterons, including the (d, n) reactions, are either exoergic or they have a low threshold. On one hand this is an advantage because of the possibility of analyzing a greater number of elements, a Fig. 2. Details of a steel PIXE spectrum using 3 MeV protons.
5 PIGE, PIXE and NAA analytical techniques 599 Table 1 Peaks in the PIGE spectrum of the steel sample presented in Fig. 1 Peak no. Identity 1 931 kev 55 Mn n (2,0) 2 955 kev 54 Cr n (6,1) 3 962 kev 63 Cu p (2,0) 4 987 kev 48 Ti γ + 49 Ti n (5,2) 5 1014 kev 27 Al p (1,0) 6 1040 kev 57 Fe γ (8,0) 7 1078 kev 63 Cu p (13,3) 8 1095 kev 58 Fe γ (1,0) 9 1099 kev 59 Co p (2,0) 10 1128 kev 214 Bi (background) 11 1155 kev 48 Ti γ (6,0) 12 1224 kev 57 Fe n (1,0) 13 1237 kev 56 Fe p (2,1) 14 1266 kev 31 P p (1,0) 15 1299 kev 64 Zn γ (8,0) 16 1315 kev 63 Cu γ (3,1) + 55 Mn n (3,0) 17 1353 kev 51 V n (4,0) 18 1377 kev 57 Fe n (2,0) 19 1408 kev 54 Fe p (1,0) + 55 Mn n (4,0) 20 1458 kev 59 Co p (5,0) 21 1499 kev 57 Fe n (14,1) 22 1552 kev 47 Ti p (1,0) 23 1588 kev 63 Cu n (16,0) 24 1600 kev 64 Zn γ (13,1) 25 1636 kev 23 Na p (2,1) 26 1727 kev 51 V γ + 52 Cr p (7,1) 27 1757 kev 57 Fe n (5,0) 28 1779 kev 28 Si p (1,0) 29 1810 kev 56 Fe p (3,1) 30 1902 kev 66 Zn p (9,1) 31 1921 kev 56 Fe γ (6,0) 32 1982 kev 18 O p (1,0) 33 2029 kev 56 Fe α + 52 Cr γ (6,1) 34 2096 kev 51 V p (5,1) 35 2113 kev 56 Fe p (4,1) 36 2149 kev 31 P p (4,1) 37 2230 kev 32 S p + 31 P γ (1,0) 38 2293 kev 56 Fe α + 52 Cr γ (9,1) 39 3088 kev 13 C p (1,0) 40 4439 kev 12 C p (1,0) greater number of γ lines being available for each element. On the other hand, the identification of the elements is more difficult in the case of deuterons because of the opening of a great number of reaction channels, especially in the singular spectrum, but the nuclear interferences due to adjacent elements in the periodic table present in the steel target are eliminated in the coincident spectrum. Peaks in the PIXE spectrum occur at energies indicative of characteristic X rays. The minor and trace elements identified by us in the iron matrix of the steel samples were: Ca, Cr, Mn, Co, Ni, Cu, Zn, W, As, Mo, Rb, Nb, Zr, Pd, In, Rh, Pb, Sn and Sb. The limitations of PIXE are the following: i) in steels, iron is the major component so that the X-rays emitted from this element will dominate the energy spectrum, as can be seen from Fig. 2a; ii) in the case of the analysis of the elements with Z 30 an interference is frequently encountered between the K α (Z+1) X-ray and the K β (Z) X-ray, which have virtually the same energy (Fig. 2a) or between the X-K lines of medium elements and X-L lines of heavy elements (Fig. 2b). Application of NAA technique in several irradiation steps at VVR-S Nuclear Reactor of NIPNE Bucharest, followed by gamma spectrometry using different source-detector distances, has led to the identification of Mn, Al, V (short irradiation), As, Cu, W, Ni, Mo, Cr, Sb, Co, Na, K, Ce, La, Sm, Sc, Zr, Zn, Au, Ga, Hf, Ta, Te, Se, Ba, Rb, Yb, Tb and Ir (long irradiation) in steels.
600 Antoaneta Ene, I. V. Popescu, T. Bãdicã, C. Beºliu 6 Peak no. E γ [kev] Radioisotope Peak no. E γ [kev] Radioisotope 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 103 134 142 145 169 192 210 220 264 299 320 329 346 364 412 479 482 559 564 603 619 630 153 Sm 187 W 59 Fe 141 Ce 177 Yb 59 Fe 192 Ir 131 Ba 75 Se 160 Tb 51 Cr 140 La 181 Hf 131 I ( 131 Te) 198 Au 187 W 181 Hf 76 As 122 Sb 122 Sb 187 W 72 Ga 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 657 686 724 740 811 834 835 847 889 1077 1099 1115 1120 1141 1172 1216 1220 1292 1332 1345 1368 76 As 187 W 95 Zr 99 Mo 58 Co ( 58 Ni) 72 Ga 54 Mn ( 54 Fe) 56 Mn 46 Sc 85 Rb 59 Fe 65 Zn 46 Sc + 182 Ta 122 Sb 60 Co 76 As 182 Ta 59 Fe 60 Co 64 Cu 24 Na Fig. 3. Detail of a NAA gamma spectrum of a deoxidized steel sample (long irradiation).
7 PIGE, PIXE and NAA analytical techniques 601 A detail of a NAA spectrum of a deoxidized steel sample, analyzed in Ref. [6], for long irradiation is presented in Fig. 3. The disadvantages of NAA are: i) there is a prolonged delay between the irradiation of the samples and the completion of the analysis; ii) some elements present in steel, such as the light ones, cannot be analysed because of the short half-life of the generated radioactive nuclei, or because the corresponding neutron-capture cross section is too small. The sensitivity was defined as the lowest concentration of the element under consideration that would give a peak under which the integrated number of counts is equal to three times the square root of the background count. The results obtained in this work for the three methods, taking into account the certified elements in steel standards or the calculated concentrations [7] by different methods, are summarized in Table 2. Table 2 Sensitivity, in ppm, for minor elements in steel in PIXE, NAA and PIGE methods Element PIXE NAA proton-pige deuteron-pige regular coincident regular coincident C 10 12 O 216 57 38 Na 7 10 Al 130 30 280 21 Si 12 35 15 P 125 188 98 S 800 890 420 Ti ND ND 375 169 202 121 V ND 12 15 16 10 Cr 110 22 53 111 51 Mn 100 15 115 18 870 400 Co ND 19 150 116 65 Ni 700 300 1802 2270 1274 Cu 80 90 330 820 630 Zn 20 8 20 10 7 As 110 1 103 134 Mo 305 1000 2345 1680 506 Sb 2 60 30 16 W NC 1 Pb 7 7 8 5 * ND not determined ** NC not calculated because of the interferences
602 Antoaneta Ene, I. V. Popescu, T. Bãdicã, C. Beºliu 8 By applying the three methods a very good overall picture of the elemental composition of a steel sample may be obtained. Our results could be used by other analysts for multielemental analysis of steel complex target. 4. CONCLUSIONS Neutron Activation Analysis (NAA) is, in general, a very sensitive method but has limited use for light elements. Also, Particle-Induced X-ray Emission (PIXE) fails in the situation where the species of interest has a low atomic number because the low K X-ray fluorescence yields are strongly attenuated by the absorption edge of higher atomic number elements present in the sample. Prompt γ-ray analysis (PIGE) offers an alternative in measuring light elements and has the advantage that γ-rays from the different light elements can be easily distinguished by their energies. The sensitivity of PIGE analysis can be improved by coincidences measurements. From this work it results that by coupling PIGE, PIXE and NAA methods, a very good overall picture of the elemental composition of a complex target such as steel may be obtained. PIXE and NAA are complements of PIGE when the determination of medium and heavy elements content with high sensitivity is necessary. REFERENCES 1. I. Popescu, T. Badica, C. Besliu, A. Olariu, A. Ene, Proc. Suppl. Balkan Phys. Letters 2 (1994) 1465. 2. Antoaneta Ene, Ph.D. Thesis, University of Bucharest, Romania, 1997. 3. T. Badica, C. Besliu, A. Ene, A. Olariu, I. Popescu, Nucl. Instr. and Meth. B111 (1996) 321. 4. Antoaneta Ene, T. Badica, A. Olariu, I. V. Popescu, C. Besliu, Nucl. Instr. and Meth. B179 (2001) 126. 5. I. Popescu, T. Badica, A. Olariu, C. Besliu, A. Ene, Al. Ivanescu, J. Radioanal. Nucl. Chem. Letters 213 (1996) 369. 6. Antoaneta Ene, C. Besliu, Agata Olariu, I. V. Popescu, T. Badica, Rom. Rep. Phys. 52 (2000) 671. 7. Antoaneta Ene, Nucl. Instr. and Meth. B222 (2004) 228.