Investigation of K X-ray intensity ratios of some 4d transition metals depending on the temperature

Applied Radiation and Isotopes 115 (2016) 147–154

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Investigation of K X-ray intensity ratios of some 4d transition metals depending on the temperature Yüksel Özdemir, Esra Kavaz n, Nader Ahmadi, Mehmet Ertuğrul, Neslihan Ekinci Ataturk University, Faculty of Science, Department of Physics, 25240 Erzurum, Turkey

H I G H L I G H T S







The temperature effect on the Kb/Ka intensity ratios of the 4d metals was investigated. The Kβ/Kα X-ray intensity ratios of the metals vary depending on the temperature. Kα and Kβ X -ray intensity ratios of the metals generally increased with increasing temperature. FWHM, asymmetry factors and energy shifts evaluated depending on the temperature.

art ic l e i nf o

a b s t r a c t

Article history: Received 8 January 2016 Received in revised form 15 June 2016 Accepted 17 June 2016 Available online 18 June 2016

In this paper, we have studied the intensity ratios Kβ /Kα depending on the temperature for transition elements Mo, Nb, Zr and Y by 59.5 keV γ-rays from a 100 mCi 241Am radioisotope point source. The Kα and Kβ emission spectra of Mo, Nb, Zr and Y were measured by using a Si (Li) solid-state detector at temperature between 40 and 400 °C. sKα and sKβ production cross-sections, Kβ /Kα intensity ratios, asymmetry factor, energy shifts and full width half maximum (FWHM) values of the elements have been calculated. Temperature-dependent changes of the parameters are tabulated and given in the graphical forms. Based on the results obtained, Kβ /Kα X-ray intensity ratios of the elements are dependent on the temperature. It is shown that sKβ fluorescence cross sections of Mo, Nb and Zr have more increase rate than sKα fluorescence cross sections with increasing temperature. For Y, sKα and sKβ production crosssections firstly decrease, then increase. In general, Kβ /Kα X-ray intensity ratios tend to increase with increasing temperature. Some significant shifts are observed in Kα and Kβ emission spectra of Mo and Y. These results may contribute to the XRF studies of transition metals. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Kβ/Kα X-ray intensity ratio Transition element Temperature

1. Introduction Transition metals that are the key elements for the continuation of life have a great deal of responsibility in our lives. Their alloys and compounds of the d-block elements are important components of the materials the modern world depends on for its continuing technological development. Therefore valence electron structures and X-ray intensity ratios of the transition metals have attracted attention and have been investigated by many researchers (Bhuinya and Padhi, 1992; Küçükönder et al., 1993; Polasik, 1998; Mukoyama, 2000; Han and Demir, 2009). Raj et al. (2002) measured K X-ray intensity ratios of 3d transition metals from Ti to Cu and determined the structure of the valence electrons of the metals. Arndt et al. (1982) investigated n

Corresponding author. E-mail address: [email protected] (E. Kavaz).

http://dx.doi.org/10.1016/j.apradiso.2016.06.017 0969-8043/& 2016 Elsevier Ltd. All rights reserved.

change of Kβ /Kα X-ray intensity ratios of 3d elements by the form of X-ray excitation (photoionization and electron capture). Changing the physical and chemical conditions can cause shifts in the characteristic X-ray energy, satellite lines, relative X-ray intensity ratios and width of the emission lines. There are many studies which have been devoted to chemical and alloying effect on the K X-ray fluorescence parameters (Brunner et al., 1982; Chang et al., 1994; Raj et al., 2000). Mukoyama et al. (1986) investigated Mn and Cr and reported that the Kβ /Kα intensity ratio of compounds with tetrahedral symmetry is in general larger than those with octahedral symmetry. Kβ /Kα X-ray intensity ratios of Ti, V, Cr, Fe, Co (in pure metals and their disilicide compounds), V, Ni (in VxNi1  x alloys for different compositions (x; 0.00, 0.10, 0.20, 0.35, 0.50, 0.75, 1.00)) and Cr, Mn, Co (in CrSe, MnSe, MnS and CoS) were measured using 59.5 keV gamma rays by Raj et al., (1998, 1999a, 1999b). Aylıkçı et al. (2010) observed alloy effect on fluorescence parameters of Co and Zn elements. They reported that

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Fig. 1. Experimental setup.

these parameters depend on rearrange of valance electrons of the elements in which Znx Co1 − x alloys and the charge transfer process between each other. Because an important part of the transition metals have different magnetic properties, magnetic moments and magnetic interaction parameters calculations in their alloys and compounds (Bourourou et al., 2014; Pang et al., 2015; McHenry and Laughlin, 2014) are a very attractive in recent years. The K shell fluorescence parameters change when the irradiated atom is in the external magnetic field and the change with the external magnetic field of the K shell fluorescence parameters is different for types of magnetic material (Demir and Şahin, 2013). Porikli and Kurucu (2008) examined the influence of chemical state and 0.6 and 1.2 T external magnetic fields to X-ray intensity ratios of the K-series lines of compounds of nickel and cobalt on energy dispersive X-ray fluorescence analysis. The results demonstrated a clear dependence of the Kβ/Kα intensity ratios on the chemical state of the element in the sample and values of external magnetic field. In a recent paper, Kβ/Kα X-ray intensity ratios of Ni, Cr and NixCr1  x alloys without magnetic field and in 0.5 and 1 T external magnetic

field were measured following excitation by 59.5 keV γ-rays. The results of these measurements have shown that Kβ/Kα X-ray intensity ratios of Ni and Cr in NixCr1  x alloys are dependent on the external magnetic field (Perişanoğlu and Demir, 2015). Suppose that the same kind of N number of atoms found per unit volume. If the temperature is absolute zero, all of these atoms locate in the ground state. When the absolute temperature is larger than zero, some atoms distribute from the ground state to excited states of atom according to the energy levels. In thermodynamic equilibrium, the distribution of the atoms to the different energy levels is given “Boltzmann Law”. This law can be written as following equation for the number of atoms in the excited levels m and i;

Nm = Ni

⎡ E − Ei ⎤ gm exp ⎢ − m ⎥ ⎣ gi kT ⎦

(1)

where gm, gi, Em andEi , respectively, statistics weight and energy of excited energy levels m and i , k Boltzmann constant, T, temperature. As it is shown in the Eq. (1), if temperature approaches zero, the number of excited atoms approaches zero. Based on this

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Fig. 2. K X-ray spectra of Mo at different temperatures and representation of observed energy shifts.

information, it would make sense to examine the K X-ray intensity ratios depending on the temperature. Han and Demir (2010) measured Kβ /Kα X-ray intensity ratios of Fe, Ni, Ti, Co, and Cu in FexNi1  x, TixNi1  x, and CoxCu1  x alloys unannealed and thermally annealed at different temperatures following excitation by 22.69 keV X rays from a 10 mCi Cd-109 radioactive point source. The experimental data obtained after annealing treatment indicated deviations of Kβ /Kα X-ray intensity ratios for 3d transition metals in different alloy compositions from the corresponding ratios for unannealed samples. In this paper, Kβ /Kα X ray intensity ratios of pure Y, Zr, Nb and Mo elements have been measured following excitation by 59.54 keV γ -rays from a 100 mCi 241Am radioactive annular source at different temperatures as in-situ. K X-ray emission spectra of the metals have been obtained and K X-ray production cross section, asymmetry factors, energy shifts, FWHM values and Kβ /Kα X-intensity ratios have been calculated depend on the temperature.

2. Experimental procedure and calculations Pure elements which commercially obtained Y (  40 mesh, 99.999%), Zr (  325 mesh, 99.999%), Ni (  325 mesh, 99.888%), and Mo (  100 mesh, 99.955%) were taken from Alfa Aesar. The samples were pressed by using SPECAC manual hydraulic press to obtain samples different weight and different thickness in 13 mm. High purity, thin uniform samples were excited using a radioactive annular source of Am-241 of strength 100 mCi and γ-photon energy 59.54 keV. A Si(Li) detector (FWHM¼ 160 eV at 5.96 keV, active area 20 mm2 , sensitive depth 5 mm and Be window thickness 0.008 mm) with a multichannel analyser was used to detect X-rays in the measurements. The experimental setup consist of a Si (Li) detector, Am-241 radioactive annular source, ceramic and aluminium container, thermocouple and temperature controller as shown in Fig. 1. During the study, the temperature was changed by 50 °C

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the X-ray detector, were determined by collecting the Kα and Kβ X-ray spectra of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, Y, Zr, Nb and Mo with mass thickness 0.28–0.63 g/cm2 in the same geometry and calculated by using the following equation:

I0 Gε Ki =

Fig. 3. The variation of the factor I0Gɛ as a function of the mean K X-ray energy (E (Kα)) for 241Am radioactive source.

increments between 40 and 400 °C. K X-ray spectrum of the elements was measured at the given temperatures as in-situ. A typical K X-ray spectrum of Mo is shown in Fig. 2 as an example. All the X-ray spectra were carefully analysed by means of the Origin 9.0 software program using a multi-Gaussian least-square fit method in order to determine the net peak. The Kβ/Kα intensity ratios were determined from peak areas fitted to Gaussian function after applying necessary corrections to the data. For measured ratios corrections are needed because of the difference in the Kβ/ Kα self-attenuations in the sample, difference in the efficiency of the Si (Li) detector and air on the path between the sample and the Si (Li) detector window. The Kβ/Kα intensity ratio is obtained from the following equation:

IKβ NKβ βKα ϵKα = IKα NKα βKβ ϵKβ

(2)

where NKα and NKβ represent the counts under the Kα and Kβ peaks, βKα and βKβ are the self-absorption correction factors of the target for both the incident and emitted photons, and ϵKα and ϵKβ are the detector-efficiency values for the Kα and Kβ X-rays, respectively. The self-absorption correction factor β, for the incident photons and emitted K X-ray photons and is given by

βKi =

1−exp ⎡⎣ −( μ /ρ)i / cos θ1 + ( μ /ρ)e / cos θ2 ) t ⎤⎦

( ( μ/ρ) / cos θ + ( μ/ρ) / cos θ ) t i

1

e

(3)

2

2

where ( μ/ρ)i and ( μ/ρ)e are the mass attenuation coefficients (cm /g) of incident photons and emitted characteristic X-rays respectively. θ1 and θ 2 are the angles of incident photons and emitted X-rays with respect to the normal at the surface of the sample in the present setup and t is the mass thickness of the sample in g/cm2. To estimate the self-absorption correction in the sample and the absorption correction in the air path we used the mass attenuation coefficients obtained by means of a computer program named WinXCom (Gerward et al., 2001, 2004) [initially developed as XCOM (Berger and Hubbell, 1999)]. This program provides total cross-sections and attenuation coefficients of elements, compounds or mixtures as well as partial cross-sections for incoherent and coherent scattering, photoelectric absorption and pair production both in the field of nucleus and electrons at energies from 1 keV to 100 GeV. The values of the factors, I0 Gε , which contains terms related to the incident photon flux, geometrical factor and the efficiency of

NKi σ Ki β Ki ti

(3)

where NKi is the net number of counts under the corresponding photo peak, σ Ki is the σ Kα or σ Kβ fluorescence cross-section, ti is the areal mass of the sample in g/cm2 and βi is the self-absorption correction factor given by Eq.(3) (Fig. 3). The overall error in the measured σ Kα or σ Kβ fluorescence crosssection, Kβ /Kα X- ray intensity ratios is estimated to be ≤ 5%. This error is the sum of the uncertainties in different parameters used to calculate the experimental values, namely, the evaluation of peak areas (1.00–3.00%), I0 Gε product (0.5–2.00%), target mass thickness (0.50–1.00%), absorption correction factor (0.25–1.75%), the increase of the temperature ( o1%), experimental geometry (o1%), statistical error (o1%) and sample homogeneity (o 1%). The errors experimental Kβ /Kα X-ray intensity ratios are calculated between 4% and 5%.

3. Results and discussion The excitation energy can be supplied by raising the sample to a high temperature, by irradiating it with electromagnetic radiation, or by exposing it to an electrical arc or spark. The energy emitted corresponds to the energy difference between the initial and final states. If the sample is excited by radioactive sources as well as is exposed to different temperatures, how the emission spectrum of the sample is affected? Atomic emission will be severely affected by fluctuations in temperature since signal is dependent on the number of atoms in the excited state. Temperatures can have a dramatic effect on the ratio of excited and unexcited atomic particles. Emission spectra where quantitative values are being measured may have temperature sensitive. Therefore in this study, effect of the temperature changes on the K X-ray emission spectra of the some transition metals (Mo, Nb, Zr, Y) have been investigated. Kα and Kβ emission spectra of Mo, Nb, Zr and Y pure metals was determined using Si (Li) solid-state detector at between 40 and 400 °C. K X-ray production cross section, asymmetry factors, energy shifts, FWHM values and Kβ /Kα X- intensity ratios of the metals were calculated depend on the temperature and they are presented in Table 1a–d for Mo, Nb, Zr and Y respectively. Figs. 4–7a–c show these changes at graphical form. As seen in Table 1a–d, we observed temperature effect in the Kβ /Kα intensity ratios. A change at temperature leads to a variation in the metals electron structure as well as their Kβ /Kα intensity ratios. The number of excited electrons rise with increasing temperature but remains constant at a specific temperature. A 10 °C increase in temperature would increase the number of excited atoms by 4%. For Mo, while σ Kα values, firstly increase and then decrease, σ Kβ values and Kβ /Kα intensity ratios continuously increase with increasing temperature (Fig. 4a–c). Small shifts are observed Kα and Kβ emission spectra at different temperatures (Fig. 2). The energy shifts 15.767 and 17.684 eV are observed respectively for Kα and Kβ emission spectra at between 40 and 390 °C. When the results obtained for Nb element are evaluated, it is seen that the Gaussian curves of emission spectra are more prolate at high temperatures. Kβ /Kα intensity ratios and σ Kβ values of Nb show almost a linear increase with increasing temperature and tend to be stable at high temperatures (Fig. 5a and c). But σ Kα values slowly increase up to 200 °C and remain virtually constant after

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Table 1 The experimental values of sKβ and sKα (cm2/g) , IKβ/IKα, As, ΔE (eV) and FWHM (keV) for (a) Mo, (b) Nb, (c) Zr, (d) Y at different temperatures. a) Temperature °C Mo sKβa

40 90 140 190 240 290 340 390 b)

0.4188 0.4250 0.4322 0.4349 0.4354 0.4402 0.4427 0.4416

Temperature °C

Nb sKβa

40 90 140 190 240 290 340 390 c)

0.3436 0.3461 0.3516 0.3592 0.3602 0.3669 0.3660 0.3684

Temperature °C

Zr sKβa

40 90 140 190 240 290 340 390 d)

0.3172 0.3177 0.3187 0.3225 0.3265 0.3271 0.3269 0.3271

sKαa

IKβ/IKαa (As) Kα

Kβ

Kα

Kβ

Kα

Kβ

0.965 0.982 1.015 1.045 1.039 1.069 1.044 1.216

– -0.655 0.226 0.023 -0.088 0.727 9.447 15.767

– -0.187 1.524 3.728 0.436 0.976 12.028 17.684

0.2607 0.2598 0.2621 0.2608 0.2625 0.2622 0.2617 0.2607

0.2430 0.2448 0.2449 0.2519 0.2573 0.2528 0.2576 0.2561

2.2518 2.2594 2.2721 2.2752 2.2716 2.2665 2.2556 2.2559

0.1860 0.1881 0.1902 0.1912 0.1917 0.1942 0.1963 0.1958

0.957 0.972 1.017 1.033 0.938 1.010 1.012 1.014

sKαa

IKβ/IKαa

(As) Kα

Kβ

Kα

Kβ

Kα

Kβ

1.056 0.975 1.030 1.010 1.055 1.012 1.037 1.014

– 0.464 -0.176 0.880 0.288 0.400 0.608 0.432

– 0.240 0.720 0.048 0.976 0.624 1.280 1.601

0.2500 0.2500 0.2514 0.2499 0.2516 0.2475 0.2486 0.2496

0.2469 0.2465 0.2521 0.2486 0.2500 0.2497 0.2499 0.2504

0.1758 0.1758 0.1761 0.1783 0.1788 0.1813 0.1812 0.1815

0.966 0.968 0.952 1.001 1.000 1.008 1.026 1.029

sKαa

IKβ/IKαa

(As)

0.1693 0.1695 0.1698 0.1717 0.1738 0.1743 0.1740 0.1741

FWHM (keV)

ΔE (eV)

1.9548 1.9691 1.9961 2.0145 2.0145 2.0232 2.0199 2.0292

1.8729 1.8747 1.8772 1.8781 1.8787 1.8774 1.8784 1.8788

FWHM (keV)

ΔE (eV)

FWHM (keV)

ΔE (eV)

Kα

Kβ

Kα

Kβ

Kα

Kβ

1.009 0.968 1.038 0.986 0.996 1.006 0.974 1.007

0.984 0.982 0.972 1.003 0.996 0.998 0.990 0.997

– 0.575 0.054 1.465 1.465 1.561 0.150 0.438

– 0.226 2.402 1.000 0.146 1.837 4.434 2.349

0.2365 0.2394 0.2401 0.2407 0.2401 0.2418 0.2428 0.2405

0.2466 0.2475 0.2489 0.2505 0.2492 0.2501 0.2492 0.2509

Temperature °C Y sKβa

40 90 140 190 240 290 340 390 a

0.2919 0.2927 0.2924 0.2902 0.2913 0.2911 0.2958 0.2954

sKαa

1.7313 1.7311 1.7276 1.7014 1.7062 1.7069 1.7377 1.7311

IKβ/IKαa (As)

0.1686 0.1691 0.1693 0.1705 0.1707 0.1705 0.1702 0.1706

FWHM (keV)

ΔE (eV)

Kα

Kβ

Kα

Kβ

Kα

Kβ

0.974 0.996 1.009 0.998 1.004 1.007 0.991 0.991

0.997 0.981 1.015 0.996 1.001 0.999 0.973 0.996

– -5.294 -3.491 11.081 12.793 15.737 1.494 -2.538

– -4.028 -6.222 14.225 16.227 15.795 2.2542 -3.356

0.2297 0.2308 0.2349 0.2317 0.2350 0.2360 0.2305 0.2285

0.2426 0.2493 0.2427 0.2447 0.2516 0.2484 0.2482 0.2452

sKβ, sKα and IKβ/IKα subject to an estimated error of 4.5%.

200 °C. (Fig. 5b). Also Kβ /Kα intensity ratios and σ Kβ values of Zr show an increase with increasing temperature and tend to be stable at higher temperatures (Fig. 6a and c). But there is not a significant change at σ Kα values for Zr (Fig. 6b). σ Kα and σ Kβ values of Y firstly decrease with increasing temperature and are minimum at 150– 300 °C (Fig. 7b and c). Then they started to increase again with

increasing temperature. While the Kβ /Kα intensity ratios of Y increase to 300 °C, it decrease between 300 and 400 °C (Fig. 7a). Kβ X-rays originate from the transitions between 4d, 4p, 3d, 3p and 1s levels. Besides Kα X-rays consist of the transitions between 2p and 1s levels. Therefore in this study, we can expect that Kβ transitions are more affected than Kα transitions. Mo, Nb, Zr and Y elements which are 4d transition elements have the valence state

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Fig. 4. (a–c). Representation of IKβ/IKα,sKα and sKβ as a function temperature for Mo.

4d55s1, 4d45s1, 4d25s2 and 4d15s2 respectively. Since Mo, Nb and Zr have five, four and two half-filled d-orbital respectively, Kβ transitions more rose with increasing temperature. The obtained Kβ /Kα intensity ratios about these elements also confirm this review. Y have one half-filled d-orbital. An increase at temperature initially raised Kβ /Kα intensity ratios. Then, because of 4p electrons may have been more difficult to affect with increasing temperature, a decrease was observed in the Kβ /Kα intensity ratios of Y. In

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Temperature ( C ) Fig. 5. (a–c). Representation of IKβ/IKα,sKα and sKβ as a function temperature for Nb.

addition, the degree of oxidation of the samples also contributes to these changes. Some changes were observed in the asymmetry factor (As) and peak energies of K X-ray emission spectrum. The asymmetry factor is defined as the distance from the center line of the peak to the back slope divided by the distance from the center line of the peak to the front slope, with all measurements made at 10% of the

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0,329 0,328

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Temperature (οC ) Fig. 6. (a–c). Representation of IKβ/IKα,sKα and sKβ as a function temperature for Zr.

maximum peak height (Fig. 8). The As values of Mo nearly increased both of Kα and Kβ peaks. Some serious energy shifts ( ΔE (eV)) was observed at 340 and 390 °C for both of the peaks (Table 1a). For Nb, it is seen from Table 1.b that the As values tend to increase and the energy shifts in the Kβ peaks are more meaningful than the Kα peaks (Table 1b). There is not a stable increase in the As values of Zr and Y (Table 1c and d). It is also observed for Zr that the energy shifts in the Kβ peaks more evident than Kα

Fig. 7. (a–c). Representation of IKβ/IKα,sKα and sKβ as a function temperature for Y.

peaks like Nb. Significant energy shifts were determined between 190 and 340 °C for Y (Table 1d). The shifts were observed in the range 0.023–17.684, 0.048–0.880, 0.146–2.402 and 2.538– 16.227 eV for Mo, Nb, Zr and Y respectively. FWHM (full width half maximum) values of the Kα and Kβ peaks for 4d elements were also obtained (Table 1). While the

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Fig. 8. Representation and calculation of asymmetry factor.

FWHM values of Kβ peaks for Mo increase with increasing temperature, there is not a significant change in the FWHM values of Kα peaks. Similar behaviour is also valid for the FWHM values of Kα and Kβ peaks of Nb. For Zr, the FWHM values of Kα and Kβ peaks increase with increasing temperature. A meaningful relationship has not been established between temperature and FWHM values of Kα and Kβ peaks of Y.

4. Conclusions In this study, the effect of the temperature on the sKβ, sKα fluorescence cross sections and Kβ/Kα X-ray intensity ratio of pure Mo, Nb, Zr and Y were investigated by the EDXRF technique. The Kα and Kβ emission spectra of Mo, Nb, Zr and Y were measured by using solid state Si (Li) detector and evaluated with Origin 9.0 software program. It is seen that σ Kα and σ Kβ and Kβ/Kα X-ray intensity ratios of the metals vary depending on the temperature and Kβ transitions are more affected than Kα transitions. Kβ/Kα X-ray intensity ratios of almost all of the selected metals increase with the increasing temperature. Also asymmetry factor and energy shifts of the Kβ and Kα peaks for the transition metals were determined. For Mo and Y, significant energy shifts were observed at Kα and Kβ emission spectra.

Acknowledgment This work was supported by the Ataturk University Research Fund, Project no.: 2012/460. Mrs. Kavaz thank to the Scientific and Technological Research Council of Turkey (TUBITAK) for doctoral scholarships provided to her.

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