Chemical shift and intensity ratio values of dyspersium, holmium and erbium L X-ray emission lines

Radiation Physics and Chemistry 81 (2012) 113–117

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Chemical shift and intensity ratio values of dyspersium, holmium and erbium L X-ray emission lines Sevil Porikli n Erzincan University, Faculty of Arts and Sciences, Department of Physics, 24030 Erzincan, Turkey

a r t i c l e i n f o

abstract

Article history: Received 4 June 2011 Accepted 3 October 2011 Available online 13 October 2011

A systematic study of L X-ray spectrum has been made on the elements Dy, Ho and Er in the pure form elements and compounds to look into the influence of chemical effect. The vacancies were created by 59.54 keV g-rays from an 241Am radioactive source. The L X-ray emission spectra of heavy elements taken with the currently available Si(Li) detectors show four or five distinct peaks. Corrections for sample absorption and spectrometer efficiency were applied to the measured relative intensities. The results show agreement with the theoretical predictions of Scofield (Scofield, 1974a). & 2011 Elsevier Ltd. All rights reserved.

Keywords: Line position Line width Chemical shift Chemical environment changes Intensity ratio XRF

1. Introduction Chemical environment of an element affects and modifies the various characteristics of its X-ray emission spectrum. Most of the works suffer from neglecting chemical influences, and usually theoretical atomic values (Scofield, 1974a; Scofield, 1974b) are used as a reference even for quite different chemical compounds of certain elements. Theoretical calculations for solids and molecules have been done mainly to predict transition energies and line profiles, but evaluation of transition probabilities is uncommon. It is well known that the low energy X-ray spectra such as L X-ray spectra of transition metals are significantly influenced by the chemical environment. There are studies in the literature, ¨ gut ¨ which suggest that there are chemical effects on L X-rays (So˘ et al., 2002). Han et al. (2010) investigated the La, Lb and total L shell XRF cross sections for Zr, Nb, Mo, Ag, Cd, In, Sn, Sb and I using 55Fe point source at 5.96 keV. Iiahara et al. (1993) measured the L X-ray intensity ratios for some Nb and Mo compounds. When the measured Lg1/Lb1 ratios were plotted as a function of the effective number of 4d electrons, they found that the experimental data are almost on a straight line. However, it should be noted that the 4d-2p transitions are allowed dipole transition and the 4d electron is the valance shell electron, which participates directly in the X-ray emission. In this case the X-ray emission rate is proportional to the number of 4d electrons and increases with increasing effective number of 4d electrons.

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Several studies on L shell X-ray relative intensities have been reported (_Ismail and Malhi, 2000), which show fairly good agreement between the experimental results and theory. First, the X-ray production cross sections for various groups of lines are needed for quantitative estimation of the elements in various types of samples using the photon induced X-ray fluorescence technique. Second, these measurements serve to provide a check on the theoretical calculations of some of the fundamental physical parameters such as L subshell photo ionization cross sections (Shantendra et al., 1985a), fluorescence yields (Shantendra et al., 1985b), Coster–Kronig transition probabilities (Hallak, 2000) and radiative decay rates (Mann et al., 1991), the direct determination of which render (presents) many difficulties. Several attempts have been made for measuring the L X-ray fluorescence cross-sections sxi , fluorescence yield wi (i¼K, L, M, y) and intensity ratio values. L XRF cross sections and fluorescence yields are important for developing more reliable theoretical models describing the fundamental inner shell process. But these quantities are not available for all elements in all photon energies, because the L X-ray spectra is somewhat less precise and a little more complicated in nature since it originates from a shell, which has three subshells. L X-ray spectra, induced by electron impact on the elements with 73 rZr83, were measured by Goldberg with a curved crystal spectrometer, and the relative intensities of the L X-ray satellite and diagram lines were estimated by Salgueiro et al. (1974). Salgueiro et al. studied the spectra in 1965 and in 1974; these studies led to a measurement of the L1-subshell CK and fluorescence yields for some of the elements, which was quoted by Bambeynek (1985) and Krause (1979). Demir and S- ahin (2006) measured L3 subshell fluorescence yields and level widths for Gd,

S. Porikli / Radiation Physics and Chemistry 81 (2012) 113–117

Dy, Hg and Pb at 59.5 keV incident photon energy in the external magnetic field of intensities 70.75 T. Porikli (2011) conduct measurements under identical conditions using Si(Li) detector and 241Am as the source of excitation to determine the energy shifts and relative intensities of La, Lb and Lg components in La, Ce and Pr, which were in the beginning of lanthanide group. The purpose of their measurements was to characterize the dependence of the line position and line width with the chemical environment changes. XRF is an analytical method to determine the chemical composition of all kinds of materials (Berenyi et al., 1978; Rao et al., 1986; Arndt et al., 1982). The materials can be in solid, liquid, powder, filtered or other form. XRF can also sometimes be used to determine the thickness and composition of layers and coatings. Applications are very broad and they include the metal, cement, oil, polymer, plastic, food industries, along with mining, mineralogy and geology. Beside environmental analysis of water and waste materials can be included to these applications. The purpose of this paper is to study chemical effects and discuss their applications to Dy, Ho and Er in various compounds. This paper presents and discusses the measured spectra using an energy dispersive X-ray spectrometer (EDXRF). The effect of chemical state on the La, Lb, Lg and Ll X-ray energies and La/ Lb1, La/Lg and La/Ll intensity ratio values has been investigated using the 59.54 keV gamma-rays from 241Am.

2. Material and methods 2.1. Experimental arrangement The studied elements were Dy, DyBr3, DyCl3, DyF3, Dy2(C2O3)3  XH2O, Ho, Ho2(CO3)3  XH2O, Ho(NO3)3, Ho2O3, Ho2(SO4)3, Er, ErBr3, ErCl3  6H2O, Er(NO3)3  5H2O, Er2(C2O4)3  H2O and Er2(SO4)3  8H2O, Er2O3. The target samples were prepared by pressing the fine powder of compound to keep the target pure and at constant pressure. The powder was palletized to a uniform thickness of 0.10–0.20 g cm  2 range by a hydraulic press using 10 ton in  2 pressure. The diameter of the pellet was 13 mm. The experiment was repeated three times for each spectrum, using three identical samples in order to confirm the radiation decomposition effect is negligible during measurements. ¨ The experiments were performed at the Ataturk University, X-Ray Fluorescence (XRF) Laboratory. Compounds were excited by a 241Am annular source with an activity of 100 mCi. The experimental arrangement and the geometry used in the present study are shown in Fig. 1. The produced X-rays were detected by a spectroscopic amplifier (with pile up rejection) and a Si(Li) semiconductor detector with a resolution of 155 eV at 5.9 keV. The X-ray spectrum was acquired in 16384 channels of a digital spectrum analyzer DSA-1000 and the spectra were recorded for

Sample

241Am

point source

1

2

Si(Li) detector

time intervals ranging from 7200 to 40000 s in order to achieve better approximation of the X-ray peaks to a Gaussian distribution, avoid peak broadening, energy shift and non-linearity, which were the sources of statistical error. The lifetime of the analyzer was used to set the duration of each measurement and dead times throughout these measurements were always less than 1%. The integrated error including the counting statistics errors were estimated to be less than 1.0 and 1.5%. Count rates were kept enough to avoid any pile-up rejection. The counting time was adjusted all specimens to give a statistical error of o0.2 for the La and Lb peak, o0.5 for the Lg peak and o1.0 for the Ll peak. Three sets of measurements were carried out for each sample, and an average of the two measurements was found for the L X-ray intensity ratio, which is reported. The L X-ray spectra are more complex than the K X-ray spectra and they consist of many peaks, several of which strongly overlap. These complex spectra were analyzed using the least squares fitting procedure using a non-linear background subtraction. The photopeaks were assumed to consist of pure Gaussians and no tailing function. Since there is no escape peak any other undesired effects contributing to the spectrum, the mean count of twenty channels at each side of the peaks used to calculate the background and to define the net peak area. The Microcal Orgin 7.5 was used for peak resolving, background subtraction and determination of the net peak areas of L X-rays. For all the spectra, the background counting rate was approximately 2–4 counts per second. Backgrounds we used to subtract from every spectrum were the lowest counting in the spectral range. The L X-ray intensity ratios were determined from the fitted peak areas after applying necessary correction to the data. Corrections to the measured ratios mainly come from the difference in the La, Lb, Lg and Ll self attenuations in the sample, differences in the efficiency of the Si(Li) detector and air absorption on the path between the sample and the Si(Li) detector window. Our theoretically estimated efficiency was shown to be in good agreement with the measured efficiency. At the energy region of the present interest, the discrepancy between them was found to be quite small. The peaks due to La, Lb, Lg and Ll group of lines are well resolved and an example of the fitted spectra is shown in Fig. 2. Measured numbers of counts are shown as solid black circles, while the red line represents the overall fit. The background is shown as a green line. Fig. 2 shows that the observed spectrum is fitted fairly well with the fitted function, which is also indicated by the residuals being much lower than the 72s limit. The X2 value for this fitting was 0.95.

3.0x102 L

2.5x102 Count/Channel

114

L1

2.0x102

L

1.5x102 1.0x102

L2

Ll

5.0x101

L

0.0 5.5 Fig. 1. Experimental setup used for the measurement of La/Ll, La/Lb and La/Lg intensity ratios and line parameter changes.

6.0

6.5

7.0

7.5 8.0 Energy (keV)

8.5

9.0

Fig. 2. Measured Ll, La, Lb1, Lb2 and Lg emission lines of Er.

9.5

S. Porikli / Radiation Physics and Chemistry 81 (2012) 113–117

2.2. Data analysis The experimental L-shell X-ray intensity ratios I(Li)/I(La) were evaluated using the relation: ILi N T La eLa ¼ Li ILa N La T Li eLi

i ¼ l, a, b

ð1Þ

where N(Li)/N(La) represents the ratio of the counting rates under the Li and La peaks. T(Li)/T(La) is the ratio of the self absorption correction factor of the target that accounts for the absorption of the incident K X-rays and emitted L X-rays in the target material and e(La)/e(Li) is the ratio of the detector efficiency values for La and L1 X-rays. T was calculated using the relation: T¼

1exp½ððmx =cos y1 Þ þ ðmLi =cos y2 ÞÞt ððmx =cos y1 Þ þðmLi =cos y2 ÞÞt

ð2Þ

where mx and mLi are the total mass absorption coefficients taken from WinXCOM program, which is the Windows version of XCOM. XCOM is the electronic version of Berger and Hubbell’s Tables (Berger and Hubbell, 1987). The angles of incident photons and emitted characteristic X-rays with respect to the normal at the surface of the sample, y1 and y2, were equal to 451 in the present setup and t is the thickness of the target (g/cm2). The efficiency values of the detector at various energies were taken from the efficiency calibration curve of the detector, which was obtained by measuring K X-ray yields with thin samples of Zn, As, Sr, Nb, Rh, Ag, Cd and Ba using their K X-ray cross-sections (Porikli, 2011). Details of the experimental arrangement and data analysis can be found in the earlier paper by Porikli and Kurucu (2008). 3. Result and discussion It is well known that the chemical environment of an element affects and modifies the various characteristics of its X-ray emission spectrum. In general, the influence of the chemical environment results in energy shifts of the characteristic X-ray lines, formation of satellite lines and changes in the emission linewidths and relative X-ray intensities. X-ray spectroscopy can be applied to probe these phenomena efficiently, exploiting them for chemical state analysis. A systematic study of the L series lines was made on the compounds of Dy, Ho, Er and their compounds to examine the influence of chemical state on energy dispersive

115

X-ray fluorescence analysis. In Table 1 the results for their width ratios of some lanthanide compounds of L lines and their energy (chemical) shifts are given. In the following parts we discuss the results in greater details. The spectra were measured in order to examine the chemical effect (shift) on the L X-ray structure. The chemical shift was the difference between the center point of the 9/10 peak intensity of a compound and that of pure form measured before and after the measurement of the compound (DE¼Ecompound Epure). The peak position was determined at the center point of the 9/10 intensity of the emission line because the standard deviation of the peak position determined using the 9/10 intensity was less than that determined using the peak top. As can be seen in Table 1, the FWHM ratio values of La, Lb, Lg and Ll emission lines are 0.7278 and 1.2256 eV. Among all the compounds, the FWHM ratios of the Ll show large widths and Lg line is smaller than those of the other lines. The FWHM of the L lines increases monotonically from pure Dy, Ho and Er to Dy2O3, Ho2O3 and Er2O3, respectively. When the peak intensity is weak, the standard deviation is large. Therefore, instead of Lb2 and Lg2 peaks, Lb1 and Lg1 peaks were examined in the energy shift determination study. The L X-ray spectrum structures were analyzed from its relation with its chemical environment and it is observed that the line shapes of Lg1 are not symmetric. Dy, Ho and Er compounds have partially filled f subshell, e.g., electronic configuration of Dy element is [Xe] 4f106s2. Some lanthanide compounds show different shifts for normal compounds, even when they have the same oxidation state. The effect of electronegativity, the nature of the ligands and the distribution of ligands around the central emitting atoms are some of the factors, which may cause this variation. It was found that the chemical shifts of La line in DyCl3 and ErCl3  6H2O were relatively small (less than 0.1 eV with pure Dy, and Er as reference). When Lb1 peak of the Ho compounds was compared with each other, they were shifted to higher energy. The Lb1 peak shift was Ho(NO3)3 oHo2(SO4)3 oHo2(CO3)3  XH2OoHo2O3. DyCl3, Ho(NO3)3 and Er(NO3)3  5H2O element, where the Ll peak shifts are large, and show prominent asymmetry. For Lg1 lines, all oxide compounds give significant energy shifts. For all of the lanthanides measured, the chemical shifts of oxides are larger than those of carbonates and sulfates. The experimental uncertainties in the values cited in the table were determined taking into account the multiple measurements and multiple fits of each spectrum. The energy position of the

Table 1 Full width at half maximum (FWHM) ratio and chemical shift (DE) values of La, Lb1, Lg and Ll emission lines in pure dysprosium, holmium, erbium and their compounds. (FWHM)compound/(FWHM)pure

Element

Dy DyBr3 DyCl3 DyF3 Dy2(C2O3)3 XH2O Dy2O3 Ho Ho2(CO3)3 XH2O Ho(NO3)3 Ho2O3 Ho2(SO4)3 Er ErBr3 ErCl3 6H2O Er(NO3)3 5H2O Er2(C2O4)3 H2O Er2(SO4)3 8H2O Er2O3







  

DE (eV)

Ll

La

Lb1

Lc

Ll

La

Lb1

Lc

1 1.0884 0.8671 1.0673 1.0304 1.1836 1 1.0390 0.8957 1.1476 1.0489 1 0.9337 1.1745 0.9339 0.9966 1.0071 1.2256

1 0.9617 0.7989 0.9484 0.8267 0.9968 1 1.0180 0.9074 1.0484 1.0396 1 0.8773 1.1245 0.9769 1.0736 1.0551 1.1598

1 0.7468 0.7872 0.9314 0.8537 0.9634 1 0.9782 0.8731 0.9929 0.9678 1 0.9562 0.9018 0.8699 1.0219 0.9084 0.9809

1 0.7717 0.7278 0.8085 0.7512 0.8790 1 0.9238 0.8116 0.9615 0.8947 1 0.7886 0.9269 0.8741 0.9586 0.9597 0.9636

0 2.0534 3.6478 3.3827 1.9742 5.7137 0 1.1993 3.9878 4.3647 1.9372 0 2.8288 1.1891 3.9091 3.9868 2.0571 4.0690

0 0.1871 0.0838 2.2339 2.2469 2.9764 0 1.3956 0.6463 1.2488 0.6898 0 0.3054 0.0033 1.6301 2.1510 0.1779 2.4520

0 2.3765 2.8798 2.1731 4.1046 4.2853 0 0.7825 0.0856 1.0108 0.2679 0 1.3902 1.0807 0.8665 1.5716 0.3690 1.9211

0 0.2982 2.1115 0.7256 0.4801 3.1390 0 0.2264 2.4070 3.1659 1.7465 0 2.4989 0.0057 3.3673 3.8815 0.6037 3.9389

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S. Porikli / Radiation Physics and Chemistry 81 (2012) 113–117

spectral features and the half widths have a probable error of 70.05 eV. Not much work is carried out in the study of L X-ray energy shifts and intensity ratios. This is because of the complex nature of L X-ray spectrum. The intensity ratios of L X-ray emissions from several lanthanides were measured. The experimental results for the La/Lb1, La/Lg and La/Ll intensity ratios of Dy, Ho and Er for the case of pure forms and in different compounds are presented in Table 2. Within the report uncertainties, there is a good agreement between the experimental results (Scofield, 1974a) and theory ¨ zdemir, 2001; Ertu˘grul, 1996; Raghavaiah et al., (Durak and O 1987; Raghavaiah et al., 1992). The differences can be attributed

to the suggestion by Scofield (1974a) concerning the overlapping of the wavefunctions of different subshells. While comparing the measured L series intensity ratios with the theoretical values, one must note that the theoretical estimation of these ratios requires calculations involving more than one subshell of the L series and hence one has to use one subshell fluorescent yields and the Coster–Kronig transition probabilities. For Ho and Er, the differences between the experimental and theoretical values were relatively larger than Dy sample. The chemical environment has a strong effect on the transition originated in the valance band and its influence could clearly be observed in the emission spectrum structure. The intensity ratio La/Lb and La/Lg is found to show significant enhancements for the

Table 2 Experimental and theoretical values of L XRF intensity ratio values of La, Lb, Lg and Ll emission lines in pure dysprosium, holmium, erbium and their compounds. Element

Intensity ratio

Dy

DyBr3

DyCl3

DyF3



Dy2(C2O3)3 XH2O

Dy2O3

Ho



Ho2(CO3)3 XH2O

Ho(NO3)3

Ho2O3

Ho2(SO4)3

Er

ErBr3



ErCl3 6H2O



Er(NO3)3 5H2O



Er2(C2O4)3 H2O



Er2(SO4)3 8H2O

Er2O3

La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg La/Ll La/Lb La/Lg

Experimental

23.537 1.52 1.2387 0.11 7.9967 0.48 23.41 7 1.39 1.255 7 0.11 7.661 7 0.73 23.28 7 1.52 1.242 7 0.10 7.837 7 0.41 23.55 7 1.33 1.427 7 0.07 8.775 7 0.61 22.94 7 1.19 1.367 7 0.12 8.875 7 0.39 23.087 1.21 1.2307 0.10 7.874 7 0.39 23.347 1.11 1.2017 0.09 7.8237 0.71 23.21 7 1.51 1.246 7 0.12 8.141 7 0.68 23.35 7 1.66 1.269 7 0.11 8.469 7 0.83 23.507 1.39 1.347 7 0.12 8.634 7 0.59 23.46 7 1.36 1.2407 0.07 8.588 7 0.90 23.227 1.29 1.1987 0.09 7.5567 0.39 22.89 7 1.428 1.236 7 0.11 7.594 7 0.45 23.29 7 1.33 1.148 7 0.13 7.543 7 0.68 24.57 7 1.25 1.199 7 0.09 7.697 7 0.47 23.86 7 1.51 1.236 7 0.08 7.395 7 0.648 23.5317 1.43 1.211 7 0.09 7.445 7 0.65 23.18 7 1.38 1.1067 0.13 7.795 7 0.60

Theoretical (Scofield, 1974a)

¨ zdemir Durak and O (2001)

Ertu˘grul (1996)

Raghavaiah et al. (1987) Raghavaiah et al. (1992)

23.29 1.230 7.860

23.38 1.21 7.87

23.70 1.267 8.452

23.90 [31] 1.08 [32] 6.33 [32]

23.51 1.170 7.590

23.39 1.16 7.38

22.78 1.205 8.333

23.90 [31] 1.01 [32] 6.73 [32]

23.41 1.220 7.640

22.38 1.19 7.62

22.73 1.227 7.840

24.10 [31] 1.01 [32] 6.16 [32]

S. Porikli / Radiation Physics and Chemistry 81 (2012) 113–117

compound materials such as Dy2O3, Ho2O3 and Er2O3. La/Lb for Ho2O3 is observed to be higher than those for Ho and the other Ho compounds. As can be seen from the table, the La/Ll ratios of Er in ErCl3  6H2O and Ho in Ho(NO3)3 are in close agreement with the ratios of corresponding pure metals. The greatest increase of the La/Ll ratio has been observed for Er in Er(NO3)3  5H2O. No significant effect of chemical state on the La/Ll intensity ratios of Dy was observed. But the La/Lb intensity ratio for all Dy compounds shows a chemical state dependence. The intensities for the La, Lb, Lg and Ll lines were obtained for each element after applying the efficiency and self absorption corrections. The La/Lb1, La/Lg and La/Ll ratios were then computed for each element and these are shown in their final form in Table 2. The errors assigned to each case arise out of the following factors: (i) counting statistics ( o1.5%), (ii) background correction (0.5–1.0%), (iii) efficiency correction (1.0–3.5%) and (iv) self absorption correction (0.25–2.20%). The individual errors were estimated separately for each element and the compound error amounts to about 5% in the experimental values.

4. Conclusions We have presented and discussed the effect of chemical composition on the FWHM ratio values, energy shifts and La/Lb, La/Lg and La/Ll intensity ratios for some Dy, Ho and Er compounds. Some of these values have been reported for the first time. In this study, it is found that the La/Ll ratios do not depend on the chemical environment. It is also found that the relative intensity La/Lb significantly depends on the target materials. We want to show possibility of the source of intensity ratio differences between pure form and their compounds. The electronic structure of a given metal in a compound is found to be different from that of pure metal because of the presence of alien atoms. In the present measurements, only pure metals such as Dy, Ho and Er were used to neglect chemical environment effect from alien atoms. To obtain more definite conclusions on the chemical effect determination, more experimental data are clearly needed, particularly for different symmetries and for chemical compounds.

Acknowledgments This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK), under the Project no 106T045.

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