Radiation Physics and Chemistry

Transcription

1 Radiation Physics and Chemistry 81 (2012) Contents lists available at SciVerse ScienceDirect Radiation Physics and Chemistry journal homepage: Determination of K shell fluorescence cross-section and K b /K a intensity ratios for Fe, Se, Te, FeSe, FeTe and TeSe M. Saydam a,n, C. Aksoy a, E. Cengiz a, C. Alas-alvar b, E. Tıras-oğlu a, G. Apaydın a a Department of Physics, Faculty of Science, Karadeniz Technical University, Kalkınma street, Trabzon, Turkey b Department of Physics, Faculty of Arts and Science, Giresun University, Giresun, Turkey H I G H L I G H T S c TeSe, FeSe and FeTe complexes have affected each other in terms of charge transfer. c Fe excitement and enhancement have been made by Se and Te. c Attractive interactions between electrons can help to becoming superconductivity. article info Article history: Received 10 January 2012 Accepted 12 August 2012 Available online 21 August 2012 Keywords: K-shell Superconductors Cross-sections Intensity ratios abstract The fluorescence cross-sections (s Ki ) and the intensity ratios K b /K a for pure Fe, Se, Te elements and FeSe, FeTe, TeSe complexes have been investigated. The samples were excited by 59.5 kev g-rays from 241 Am annular radioactive source and emitted X-rays. They were counted by an Ultra-LEGe detector with resolution of 150 ev at 5.9 kev. For pure elements results have been compared with the theoretical calculated values. According to our results band length and mutual interaction of atoms affected the results. We claimed that these effects would help researchers who study on superconductors, especially determining which compound can be show the superconductor properties. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction The accurate values of the X-ray fluorescence cross-sections (s Ka and s Kb ) and intensity ratios (K b /K a )areimportantastheyhave been used in atomic, molecular, radiation, materials health physics and elemental analysis, using Energy Dispersive X-ray fluorescence (EDXRF). There are many studies on the fluorescence cross-sections s Ki (i¼a,b) and the intensity ratios K b /K a for some pure elements. At 59.5 and kev, the intensity ratios K b /K a have been theoretically and experimentally determined by Cevik et al. (2007). Durak and Özdemir (2001) explained that the s Ki (i¼a,b) and yields of 14 elements in the atomic range 25rZr47 using the photo-ionization. Han et al. (2007) has measured the K X-ray parameters for some elements in the atomic range 22rZr68. Rao et al. (1986) explained that K b /K a depend on the excitation modes in 3d elements. K b /K a in atomic range 22rZr 69 at 59.5 kev have been measured by Ertuğrul et al. (2001). Ertugrul and Simsek (2002) researched K X-ray relative intensity for some high Z elements. The K b /K a intensity ratios for 3d elements change with changing crystal structure (Dagıstanlı and Mutlu, 2012). Singh (2009) investigated the electronic, n Corresponding author. Tel.: þ ; fax: þ address: m.saydam@ktu.edu.tr (M. Saydam). band, and crystal structure of iron-based superconductor. He found that Fe Fe interaction is very important for FeSe compounds. In this study, the s Ka and s Kb and K b /K a have been investigated. The measured results can show how the elements affect each other in terms of electron interactions. We claimed that the electron interactions can help to researcher in first step while determining materials which could be superconducting after the required process in terms of atomic structure. 2. Experimental procedure 2.1. Sample preparation Pure elements which commercially obtained Fe (% ), Se(% ), and Te(% ) were taken from Sigma Aldrich. TeSe, FeSe and FeTe complexes were prepared and mixed in stoichiometry (0.5; 0.5).The samples were pressed by using Perkinelmer Press to obtain samples different weight and different thickness in 13 mm Experimental method The geometry of the experimental set-up for an annular source can be seen in Fig. 1. included an Ultra-LEGe (ultra low energy X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.

2 1838 M. Saydam et al. / Radiation Physics and Chemistry 81 (2012) Sample 241 Am annular source Berilium Window Ulta- LEGe Dedector Mylar Fiber Pb annular collimator Holder Fig. 1. Geometry of the experimental setup. Table 1 Comparison of present experimental, theoretical and other experimental results for s Ka and s Kb. s Ka (b/atom) s Kb (b/atom) Sample Experimental Puri et al. (1995) Cevik et al. (2007) Theoretical Experimental Puri et al. (1995) Cevik et al. (2007) Theoretical Fe Se Te FeSe a FeSe b FeTe a FeTe c TeSe b TeSe c a For Fe. b For Se. c For Te. Table 2 Comparison of present experimental, theoretical and other experimental results for K b /K a. Theoretical values Experimental values Sample Experimental Scofield (1974a) Scofield (1974b) Manson and Kennedy (1974) Salem et al. (1974) Ertuğrul et al. (2001) Cevik et al. (2007) Hansen et al. (1970) Khan and Karimi (1980) Ertuğral et al. (2007) Fe Se Te FeSe a FeSe b FeTe a FeTe c TeSe b TeSe c a For Fe. b For Se. c For Te. Ge detector; FWHM 150 ev at 5.9 kev, active area 13 mm 2, thickness 5 mm and Be window thickness 30 mm). The out preamplifier, with a pulse pile-up rejection capability, was fed to a multi-channel analyzer interfaced with a private computer provided with suitable software for data acquisition and peak analysis. In this experimental set-up, 59.5 kev photons emitted by an annular 50 mci Am-241 radioactive sources were utilized. The experimental K shell X-ray intensity ratios K b /K a were calculated the following equation: I Kb ¼ N Kb b Ka e Kb I Ka N Ka b Kb e Ka ð1þ where N Kb and N Ka represent the net count under K b and K a peaks, b Ka and b Kb are the self-absorption correction factors of the target. e Kb and e Ka are the detector efficiencies for K b and K a X-rays. The s Ki were evacuated by using the following equation: s Ki ¼ N Ki I 0 Ge Ki b Ki t i, ði ¼ a,bþ ð2þ where N Ki and b Ki (i¼a,b) have the same meaning as Eq.(1), I 0 is the intensity of exciting radiation, G is the geometric factor, t i is the thickness of the target (g cm 2 ).

3 M. Saydam et al. / Radiation Physics and Chemistry 81 (2012) The self-absorption correction factor b Ki was calculated using the equation: b Ki ¼ 1 exp½ ðm inc =cosy 1 þm emt cosy 2 Þt i Š ðm inc =cosy 1 þm emt =cosy 2 Þt i where m inc and m emt are the total mass absorption coefficients (cm 2 /g) of incident photons and the emitted characteristic X-rays (Hubbell, 1999). The angles of incident and emitted photons X-rays with respect to the normal at the surface of samples y 1 and y 2 were equal to 451 and 901 in the present experimental set-up, respectively. 1.2x x x10 4 Kα Kβ ð3þ The term s Ki represents the K X-ray fluorescence crosssections and is given by: s Ki ¼ s KP o K f Ki ði ¼ a,bþ ð4þ where s KP is the K shell photoionization cross-section (Scofield, 1973), o K is the fluorescence yield (Krause, 1979) and f Ki is fractional X-ray emission rate f Ka taken from Broll 0 s (1986) and f Kb calculated from the equation: f Kb ¼ 1 f Ka ð5þ 3. Result and discussion The s Ki (i¼a,b) and K b /K a were investigated for pure Fe, Se, Te and their complexes FeSe, FeTe and TeSe. The results are listed in Table 1 and Table 2. According to Table 1, the values of the s Ki (i¼a,b) for pure elements are accordance with the theoretical and the other experimental values. The comparison values have been indicated in Fig. 4. Especially, Fe fluorescence cross-section values decreased in the FeSe, FeTe compounds. It means that Fe atom takes an electron from Se and Te. Since Se and Te elements have different K shell energies and these energies are much bigger than Fe, K shell energy of Fe atoms have been enhanced by Se and Te 6x10 2 7x10 2 8x10 2 9x10 2 Se Kα 4.0x x10 4 Fe Kα Se Kβ Fe Kβ Intensity ratios present Scoffield 1974 Manson 1974 Salem et al Ertugrul et al Çevik et al Hansen et al Khan 1980 Ertugral et al Fe Se Te 6.0x x x x x x x10 2 Se Kα Se Kβ Escape Peaks Te Kα 1.0x x x10 3 Te Kβ12 Fig. 2. (a) Typical K a and K b photopeak for pure Se element. (b) Typical K-X-rays spectra for FeSe complex. (c) Typical K-X-rays spectra for Te Se complex. Fig. 3. Comparison with the theoretical and the experimental values for intensity ratios. Total photoionization cross-sections values FFAST (Chantler et al ) Puri et al Theoretical value Experimentalvalue Çevik et al Fe Fig. 4. Comparison with the experimental and the theoretical values which are calculated from Eq. (4) for total photoionization cross-sections at the 59.5 kev (Puri et al., 1995, FFAST (Chantler et al., 1995, 2000), Cevik et al., (2007))). Se Te

4 1840 M. Saydam et al. / Radiation Physics and Chemistry 81 (2012) atoms, and so avalanche have become in the complexes. Se, FeSe and TeSe K-X-rays peaks have been shown in Fig. 2(a c). It is well known that s Kb and K b /K a rise with increasing atomic number. As seen in Table 2, K b to K a intensity ratios increase with increasing atomic number. Elements have different ionic size because of that, chemical substitution on Fe site and Se site have been observed different properties. Te has much bigger ionic size than Se and Fe. Bond length of Fe Se, Fe Te (Lehman et al., 2009) and Se Te are 2.39 A1, 2.55 A1 and 2.54 A1, respectively. And so, the bond length of Fe Se and Fe Te are smaller than Se Te. Therefore, binding energy of FeSe and FeTe complexes bigger than SeTe complex. It means that mutual interactions on the Fe Se and Fe Te are quite robust. The bond length of Se Te complex have been found that using the Gauss View 4.1 software and this software shapes of complexes have been shown in Fig. 5 (Frisch et al., 2003). Environmental factors, such as electronegativitiy, oxidation numbers etc., affect the compounds, complexes and alloys. Chemical effects on K and L shell production cross-sections and tranfer probabilities in Nb compounds were investigated by Cengiz et al. (2008) and s Ka cross-section is small because of the chemical effect. Charge transfer effects were explained that Zn complexes and s Ki (i¼a, b) are affected by interaction between the central atom and ligand atoms, so, chemical environmental factors influence the measurements (Aylıkcı et al., 2009). Alloying effect on the K shell fluorescence yield (o K ) in Cr x Ni 1 x and Cr x Al 1 x alloys was determined. In the Cr x Ni 1 x alloy the o K of chromium decreases with decreasing nickel concentration. On the other hand, in the Cr x Al 1 x alloy the o K of chromium increases with decrease in aluminium concentration. These differences in electronegativity, charge transition, may occur changing the binding energies. In these alloys, electrons which are in the outermost shell, interact with neighbouring ions electrostatically. This effect may change the binding energies of the outermost electrons, Auger transition probability and o K. (Büyükkasap, 1998). In the present paper, measurements have been compared with the theoretical values and the other experimental values. These values have been indicated in Figs. 3 and 4. According to the Figs. 3 and 4, our measurements are appropriate to the other values. Because of counting statistic, background determination, target thickness, absorption correction factor, and detector efficiency, evaluation of the peak area errors have observed some differences between our measurements and the other values. The errors present measurements are approximately 6% due to the counting statistics, background determination; self absorption correction and the detector efficiency QUOTE I 0 Ge determination. This error is the quadrature sum of the uncertainties in the different parameters used to evaluate the K shell fluorescence parameters, i.e. target thickness (r2%), the evaluation of the peak area (r3%), the detector efficiency I 0 Ge Ki (r3%) and the absorption correction factor (r3%).fig. 5 In this study, our measurements explain that excitement, and changing of the crystal structure of the elements arise from the mutual interaction between the electrons. Very useful results obtained from current study may be the first step before preparing materials thought to be superconducting. Acknowledgement We would like to thank Mrs. Meltem Saydam for her contributions to science as the other authors. Unfortunately, she passed away four days later when this manuscript accepted. We will always remember her. References Fig. 5. Shapes of complexes (Frisch et al., 2003) and their bond lengths are (FeSe, FeTe. and SeTe) are 2.39 A1, 2.55 A1 and 2.54 A1 respectively. Angles of FeSe and FeTe are and (Lehman et al., 2009). Aylıkcı, V., et al., Chem. Phys. 365, Broll, N., X-Ray Spectrom. 15, Büyükkasap, E., Spectrochim. Acta. B 53, Chantler, C.T., et al., X-Ray Form Factor, Attenuation and Scattering Tables (version2.0). National Institute of Standards and Technology, Gaithersburg, MD. Chantler, C.T., J. Phys. Chem. Ref. Data 24, Cengiz, E., et al., J. Radioanal. Nucl. Ch. 278 (1), C- evik, U., et al., Nucl. Instrum. Methods Phys. Res. B 262, Dagıstanlı, H., Mutlu, R.H., Radiat. Phys. Cem. 81 (7), Durak, R., Özdemir, Y., Radiat. Pyhs. Chem. 61, Ertuğral, B., et al., Radiat. Phys. Cem. 76, Ertugrul, M., et al., J. Phys. B 34, Ertugrul, M., Simsek, Ö., J. Phys. B At. Mol. Opt. Phys. 35,

5 M. Saydam et al. / Radiation Physics and Chemistry 81 (2012) Frisch, M.J., et al., Gaussian 03. Gaussian, Inc., Pittsburgh PA. Han, I., et al., Appl. Radiat. Isotop. 65, Hansen, J.S., et al., Nucl. Phys. A 142, 604. Hubbell, J.H., XCOM: Gaithersburg, MD, USA. / Khan, M.R., Karimi, M., X-Ray Spectrom. 9 (1), Krause, M.O., J. Phys. Chem. Ref. Data 8, Lehman, M.C., et al., Phys. Rev. B 81 (13), 17. Manson, S.T., Kennedy, D.J., At. Data Nucl. Data Tables 14, Puri, S., et al., At. Data Nucl.Data Tables 61, 289. Rao, V.N., et al., Physica. C 142, Salem, S.I., et al., At. Data Nucl. Data Tables 14, Scofield, H., Chem. Ref. Data 8, 307. Scofield, J.H., 1974a. Phys. Rev. A 9, Scofield, J.H, 1974b. At. Data Nucl. Data Tables 14, Singh, D.J., Physica C 469,