Chemical effects on the Li(i=1–3) sub-shell X-ray relative intensities for some compounds of Hg

Radiation Physics and Chemistry 80 (2011) 1166–1171

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Chemical effects on the Li(i¼1–3) sub-shell X-ray relative intensities for some compounds of Hg Anil Kumar, Sanjiv Puri n University College of Engineering, Punjabi University, Patiala 147002, Punjab, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 February 2011 Accepted 22 May 2011 Available online 16 June 2011

The intensity ratios, ILk/ILa(exp) (k¼ l, Z, b4,6, b1,2,3,15, b5,7, g1,5, g2,3,6,8, g4, b, g), have been measured for some compounds of 80Hg, namely, HgI2, Hg(C2H3O2)2 and a pure 80Hg target (liquid form) at 22.6 keV incident photon energy in order to investigate the influence of chemical effects on these ratios for a heavy transition element. The measurements have been performed using the EDXRF spectrometer involving a disk type radioactive source of Cd109 and a Peltier cooled Si-PIN X-ray detector arranged in the 901 reflection geometry. The measured intensity ratios have been compared with the theoretical ILk/ILa (Thr.) values (Kumar et al., 2010) and those calculated using the fluorescence and Coster–Kronig yields tabulated by Campbell (2003, 2009) and Krause (1979) in order to check the reliability of these available values. The measured ratios, ILb5,7/ILa (Exp.), were found to be influenced by the chemical effects. & 2011 Elsevier Ltd. All rights reserved.

Keywords: X-ray intensity ratio Chemical effects Fluorescence and Coster–Kronig yields

1. Introduction Radiative decay of the Li(i¼1–3) sub-shell vacancies results in emission of X-ray series comprising several lines in each case. Accurate data on the relative intensities of different X-ray lines are of considerable importance for investigation of atomic innershell ionization processes as well as for a variety of applications. For example, such data are required to deduce the corresponding shell and sub-shell ionization probabilities from the measured cross sections for production of X-ray lines following ion–atom collisions. For quantitative elemental analysis using X-ray emission techniques (EDXRF and PIXE), the interpretation of complex spectra comprising overlapping X-ray lines arising from a multielemental sample requires the accurate knowledge of relative intensities of different X-ray lines for all the elements. The theoretical set (Kumar et al., 2010) of intensity ratios, ILk/ ILa1(Thr.) (k¼l, Z, a2, b1, b2, 15, b3, b4, b5,7, b6, b9,10, g1,5, g6,8, g2,3, g4), for elements with 36rZ r92 at incident photon energies ranging EL1 oEinc r200 keV evaluated using the Li(i¼1–3) subshell photoionisation cross sections based on the relativistic Hartree–Fock–Slater (HFS) model (Scofield, 1973), the X-ray emission rates based on the Dirac–Fock (DF) model (Campbell and Wang, 1989), and the fluorescence and Coster–Kronig yields based on the Dirac–Hartree–Slater (DHS) model (Puri et al., 1993) have recently been reported by us. At incident photon energies

n

Corresponding author. Tel.: þ91 175 3046323; fax: þ91 175 3046333. E-mail address: [email protected] (S. Puri).

0969-806X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2011.05.011

above the K-shell ionization threshold, the contribution to the production of different L X-ray lines due to the additional Li(i¼ 1–3) sub-shell vacancies created following decay of the primary K-shell vacancies have also been included in these calculations. The characteristic X-ray intensity ratios for an element are expected to get affected by the chemical environment (coordination number, nature of the ligand attached to the central atom, etc.) and the oxidation state of the element in a given compound (Crasemann, 1985). These effects are pronounced for the X-ray lines arising from transitions involving outermost shell/sub-shell electrons. Some workers have also reported dependence of the intensity ratios on the method of characteristic X-ray generation from the target element (Kucukonder et al., 1993). Most of the reports available in literature pertained to investigations of chemical effects on the Kb/Ka intensity ratios for low and medium Z transition elements (Crasemann, 1985; Kucukonder et al., 1993; Dhal and Padhi, 1994; Kulshreshtha et al., 2005). However, similar studies for the L X-ray intensity ratios are scarce (Sawhney et al., 2000; Cengiz et al., 2010). Cengiz et al. (2010) reported the X-ray intensity ratios, ILk/ILa (k¼l, Z, b, g), for some compounds of 79Au (chloride, oxide and bromide) measured at 59.54 keV incident photon energy. They reported large spread (  40%) in the measured intensity ratios, ILk/ILa (k¼ b,g), for different compounds of gold indicating significant influence of chemical effects on these ratios. On the other hand, Sawhney et al. (2000) reported the X-ray intensity ratios, ILk/ILa (k¼l, b1, b6, b2,, b, g), for some compounds of 78Pt (chlorides with Pt in þ2 and þ4 oxidation states) and 92U (chloride and oxides with U in þ4 and þ6 oxidation states) measured at 22.6 keV incident photon

A. Kumar, S. Puri / Radiation Physics and Chemistry 80 (2011) 1166–1171

16

8 Lγ1,5 Ll

Coherent scattered Ag-Kβ

24

Incoherent scattered Ag-Kβ

Mercuric acetate target Lβ

Incoherent scattered Ag-Kα

Lα

Coherent scattered Ag-Kα

x103

Counts

energy in secondary excitation mode employing Am241 radioactive source in conjunction with the 22Ag secondary exciter as a photon source. They could not observe any significant chemical effects on measured intensity ratios. In view of the diverse observations (Sawhney et al., 2000; Cengiz et al., 2010) being reported, it was decided to measure the intensity ratios, ILk/ILa(exp) (k¼l, Z, b4,6, b1,2,3,15, b5,7, g1,5, g2,3,6,8, g4, b, g), for some compounds of 80Hg, namely, HgI2, Hg(C2H3O2)2 and a pure 80Hg (liquid form) target at 22.6 keV incident photon energy. Moreover, mercury provided a case for investigating chemical effects for a heavy transition (5d) element existing in the liquid form, whereas, its compounds in the powder form. The present measured intensity ratios have been compared with different sets of calculated values evaluated using photoionisation cross sections of Scofield (1973), two sets of the X-ray emission rates tabulated by Scofield (1974a) and Campbell and Wang (1989), and three sets of fluorescence and Coster–Kronig yields tabulated by Puri et al. (1993), Krause (1979) and Campbell (2003, 2009).

1167

Lγ2,3,4,6

Lη

0 1350

900

1800

Channel Number

2. Experimental procedure The EDXRF spectrometer used in the present measurements is shown in Fig. 1. It consisted of disk type radioisotopes of Cd109 (20 mCi) and a Peltier cooled X-ray detector arranged in 901 reflection geometry. The sealed disk source of Cd109 (4 mm  3 mm) procured from RITVERC, Russia, embedded in the lead cavity was used as a photon source. The average energy of the 22Ag K X-rays emitted from the Cd109 source was calculated to be 22.6 keV by considering weighted average of the Ag-Ka and -Kb lines in proportion to their respective intensity values (Storm and Israel, 1970). A 4 mm diameter lead collimator with aluminum lining was used with the photon source to obtain a well collimated incident photon beam. Spectroscopically pure self-supporting pressed pellets (100 dia) of HgI2 and Hg(C2H3O2)2 of thicknesses  100 mg/cm2, and a pure 80Hg (liquid form) were used as targets. The pure 80Hg target (infinite thick) was prepared by sealing liquid mercury between two layers of 10 mm thick mylar. The Hg80 in metallic form has rhombohedral crystal structure with atomic radius  1.76 A˚ and its electronic configuration is [Xe] 6 s24f145d10 (all paired

T

D

r i ∠i =∠r = 45°

Fig. 2. A typical L X-ray spectrum of 80Hg recorded at 22.6 keV incident photon energy. Six groups of L X-rays, namely, Ll, La, LZ, Lb, Lg1,5 and Lg2,3,4,6 could be distinguished.

electrons). The X-ray spectra from different targets were recorded using a Peltier cooled Si-PIN detector (AMPTEK: XR-100CR, 6 mm2  500 mm, FWHM 152 eV at 5.9 keV, Be window 0.5 mil thick) coupled to digital pulse processor (PX4, AMPTEK). An inbuilt multilayer (graded) collimator in front of the detector crystal has been provided by the manufacturer. The source-totarget and target-to-detector distances were kept as 6 and 7 cm, respectively. The digital pulse processor (PX4, AMPTEK) accepts output from the preamplifier, digitizes it, applies real-time digital processing (pulse amplification and shaping) to the signal, detects peak amplitude (digitally) and bins this value in its histogram memory thereby generating an energy spectrum, which is then viewed on the computer through a serial interface. To minimize the statistical error ( o1%) in measurements, three spectra were recorded for each target for time intervals ranging 30–35 h. A typical L X-ray spectrum taken using mercury acetate target at the 22.6 keV incident photon energy is shown in Fig. 2. It is clear from this figure that six groups of L X-rays, namely, Ll, La, LZ, Lb, Lg1,5 and Lg2,3,4,6 could be distinguished. It may be noted that the Ll, La and Lb2,5,6,7,15 groups correspond to the L3 sub-shell X-rays, i.e., those originating from filling of the L3 sub-shell vacancies; the Lg1,5,6,8 and Lb1 groups correspond to the L2 sub-shell X-rays; the Lg2,3,4, Lb3,4 and Lb9,10 groups consists of the L1 sub-shell X-rays.

S 3. Evaluation procedure 3.1. Measured relative intensities The intensity ratios, ILk/ILa(exp) (k¼l, Z, b4,6, b1,2,3,15, b5,7, g1,5, g2,3,6,8, g4, b, g) at incident photon energy of 22.6 keV, have been evaluated using the relation ILk N b eLa ¼ Lk La ILa NLa bLk eLk

Fig. 1. Experimental setup used for present measurements. S-disk type radioisotope of Cd109 embedded in Pb cavity. D-Peltier cooled Si-PIN X-ray detector. T-Target.

ð1Þ

where NLk (or NLa) is the number of counts per unit time under the Lk (or La) photo-peak, e is the detector efficiency at Lk (or La) X-ray energy and bLk (or bLa) is the self-absorption correction factor, which accounts for the absorption of incident and emitted

A. Kumar, S. Puri / Radiation Physics and Chemistry 80 (2011) 1166–1171

x103 Mercuric acetate target 20

Lβ1

16

Counts

photons in the target. It is worth noting that the measured intensity ratios (Eq. (1)) do not depend on the mass of the target and the incident photon intensity (IoG) falling on the area of the target visible to the detector. The values of bLk have been calculated as explained in our earlier paper (Puri et al., 1999) and were found to be in the range (0.0031–0.2369) for the targets under investigation. In these evaluations, the values of mass-attenuation coefficients (m) were taken from tables given by Hubbell and Seltzer (1995). It may be added that the bLk values for different targets evaluated using the mass-attenuation coefficients (m) tabulated by Chantler (1995, 2000) were found to be higher by 1.5–3% than those evaluated using the m values tabulated by Hubbell and Seltzer (1995). However, (bLa/bLk) ratios calculated using two sets of mass-attenuation coefficients (m) were found to agree with 0.5%. Further, in the present geometrical setup the source-totarget and target-to-detector solid angles are such that the large surface area of the target (100 dia) is exposed to the incident radiation and the same area is also visible to the detector, thereby minimizing any errors due to non-uniformity in the target. Each spectrum was analyzed for photo-peak areas, NLk (NLa), using software package ORIGIN 6.0 in which a non-linear least squares fitting routine based on chi-square minimization using the Marquardt’s algorithm (Bevington, 1969) has been implemented. In this code different photo-peaks can be fitted using multi-Gaussian function with the option of varying peak centroids, the FWHM as a function of energy and background under the photo-peak. The peak fitting (Puri Sanjiv et al., 1996; Chauhan et al., 2008) for different L X-ray spectra was performed in two steps. In the first step fitting was done for the Ll and Lg X-ray peaks by keeping the peak centroids fixed to their known energy values (Storm and Israel, 1970) and allowing the FWHM to vary. The values of FWHM for the Ll and the Lg1 peak (most intense peak in the Lg group) corresponding to the single (L3 M1) and (L2  N4) transitions, respectively, were obtained from spectra of different targets and fitted as function of energy. In the second step, peak fitting for different groups of the L X-rays was done by fixing, both, the peak centroids and the FWHM values interpolated from those determined in first step. Fitting of the Lb and Lg groups of X-rays for the mercury acetate target are shown in Fig. 3a and b, respectively. Different fitting parameters and associated errors are summarized in Table 1. The Lg and Lb X-ray peak shapes are well reproduced by following the above fitting procedure. The peak corresponding to very close-lying X-rays (e.g., Lg2 and Lg3; Lb2 and Lb15), originating from the same Li sub-shell, was fitted as a single Gaussian. In case of very closelying X-rays originating from different Li sub-shells (e.g., Lb4 Lb6; Lb1, Lb3 and Lb2,15; Lg2,3 and Lg6,8) the relative peak areas were found to vary significantly with the FWHM, however the total area remained the same. In such cases the combined intensities were evaluated. Similarly, the contribution of the very weak Lb9,10 X-ray group is included in Lb5,7 peak for evaluating the intensities. It may be mentioned that the observed widths of X-ray peaks are mainly due to the Gaussian response of the Si X-ray detector and has feeble dependence on the intrinsic Lorentzian width of the X-ray line. The FWHM for the intrinsic Lorentzian broadening (Campbell and Papp, 1995) of the L X-ray lines is  4–15 eV compared to the 152 eV for the Gaussian response function of the X-ray detector in the energy region of interest. In the present fitting procedure, the intrinsic width of the L X-ray lines are taken into account to a limited extent as the FWHM calibration is done using Gaussian peak shape only. The smoothly varying Lorentzian tails can effect the background estimation under the weak peaks lying in the vicinity of strong peaks, which may lead to some error in area evaluation of these components. The product, IoGe, in the photon energy range 3–17 keV was determined by measuring the K X-ray yields from self-supporting

12

8

Lβ2,3,15

4

Lβ4,6

Lβ5,7 Lβ9,10

0 880

935 Channel Number

990

x102 Lγ1

Mercuric acetate target

25

20

Counts

1168

15 Lγ2

10

Lγ3 5

Lγ5

Lγ4

0 1040

1120 1080 Channel Number

1160

Fig. 3. (a) Measured and fitted Lb X-ray components for mercury along with the residue plot. ’—measured spectra; solid/dotted curves represent the fitted peaks. (b) The fitted Lg X-ray components for mercury are shown along with the residue plot. ’—measured spectra; solid/dotted curves represent the fitted peaks.

thick pressed pellets (100 dia) of K2CO3, CaCO3, Ti, V2O5, MnS, Co, Fe, Ni, Cu, ZnSO4  7H2O, GeO2, BaBr2  2H2O, RbI, SrCO3, ZrO2, Nb2O5, MoO3 in the same geometrical setup using the relation: Io Ge ¼ NK a =ðbK a msxK a Þ

ð2Þ 2

where m is the mass thickness (g/cm ) of the target element under investigation and other symbols have same meaning as explained above. In these evaluations, the theoretical values of the K X-ray production (XRP) cross sections were taken from reference (Puri et al., 1995). The IoGe values for the present experimental setup are plotted as a function of energy in Fig. 4 along with the least-squares-fitted curve. The ratios (eLa/eLk) were determined from the fitted curve. 3.2. Calculated relative intensities The theoretical intensity ratios, ILk/ILa(Thr.), have been taken from our earlier reference (Kumar et al., 2010). The ILk/ILa(Camp.)

A. Kumar, S. Puri / Radiation Physics and Chemistry 80 (2011) 1166–1171

1169

Table 1 Summary of different fitting parameters for Hg L X-ray peaks and estimated errors in the measured intensity ratios. Chi2

 0.99–1.1 for La, Ll, LZ X-ray peaks  1.4–2.5 for Lg (Lg5, Lg1, Lg2, Lg3, Lg4) X-ray peaks  1.4–4.5 for Lb (Lb4,6, Lb1, Lb2,3,15, Lb5,7) X-ray peaks  0.995

R2 Residue always between 72s Peak widths (FWHM)  12.2–14.5 (  1.5%) channels in the energy region 8.5–14.8 keV. Error in different L X-ray photo-peak areas  1–4% Error in (IoGe) product  6% Error in (bLa/bLk) ratios o 3%

Table 2 Two sets of the L X-ray fractional emission rates (Fij) for Hg.

180 150

IoGε

120 90 60 30

3

6

15 9 12 Incident photon energy (keV)

18

Fig. 4. Plot of (IoGe) product as a function of incident photon energy (keV). (IoG) incident photon intensity falling on the area of the target visible to the detector; e detection efficiency of Si-PIN X-ray detector.

and ILk/ILa(Kr.) values have been calculated using the fluorescence (oi) and Coster–Kronig (fij) yields tabulated by Campbell (2003, 2009) and Krause (1979), respectively, by employing the formalism given in our earlier reference (Kumar et al., 2010). For calculation of both these sets of intensity ratios, the Li(i ¼1–3) sub-shell photoionisation cross sections based on the relativistic HFS model (Scofield, 1973) and the X-ray emission rates based on the DF model (Campbell and Wang, 1989) were used. The Li(i¼1–3) sub-shell photoionisation cross sections based on the relativistic Hartree–Fock–Slater (RHFS) model are available for all the elements with Z¼1–101 in the energy range 1–1500 keV (Scofield, 1973). In detailed comparisons (Saloman et al., 1988; Saloman and Hubbell, 1986), the theoretical photon absorption cross sections obtained by adding these photoionisation cross sections (Scofield, 1973) for all sub-shells/shells and small contribution of photon scattering were found to exhibit very good agreement with the measured mass attenuation cross sections at incident photon energies higher than the ionization threshold energies of different elements. It may be recalled that the incident photon energy of 22.6 keV used for present measurements is much higher than the L1 sub-shell ionization threshold energy (14.842 keV) of 80Hg. Further, it may be noted that the mass absorption coefficients (Hubbell and Seltzer, 1995) used to evaluate absorption correction factor (b) in the present work are based on the theoretical photoionisation cross sections tabulated by Scofield (1973). Two sets of the Li(i¼1–3) sub-shell X-ray emission rates are available in literature. The first set is based on the Dirac–Hartree– Slater (DHS) model (Scofield, 1974a) and the second one is based on the Dirac–Fock (DF) model (Scofield, 1974b, Campbell and Wang, 1989). In the former model, the potential is assumed to be equal for the initial and final states of the atom undergoing transitions. In the

Fractional emission rates

DF rates Scofield (1974b)

DHS rates Scofield (1974a)

% difference

F3l (L3  M1) F3a1 (L3  M5) F3a2 (L3  M4) F3b2,15 (L3  N5,4) F3b5,7 (L3  O4,5,1) F3b6 (L3  N1) F2Z (L2  M1) F2b1 (L2  M4) F1b3 (L1  M3) F1b4 (L1  M2) F1b9,10 (L1  M5,4) F2g1,5 (L2  N4,1) F1g2,3 (L1  N2,3) F2g6,8 (L2  O4,1) F1g4 (L1  O2,3) Fg Fb

0.0397 0.6972 0.0794 0.1545 0.0191 0.00987 0.0216 0.7867 0.3906 0.3431 0.0286 0.1710 0.2006 0.0203 0.0369 0.4290 1.7325

0.0401 0.7037 0.0799 0.1475 0.0166 0.00982 0.0215 0.7946 0.3798 0.3352 0.0286 0.1639 0.1898 0.0176 0.0345 0.4060 1.7124

 0.82  0.93  0.66 4.7 14.6 0.55 0.60  1.0 2.8 2.3 0.00 4.3 5.6 15.2 7.0 5.6 1.2

latter model, the potential is assumed to be different for initial and final states and hence exchange and overlap effects were included. The fractional emission rates, Fij (i¼1, 2, 3 and j¼l, a1,2, Z, b1, b2,15, b5,7, b3,4, b 6, g1,5, g2,3, g6,8, g4), representing the fraction of transitions to the ith sub-shell contributing to the jth photo-peak, evaluated using two sets of X-ray emission rates for Hg are compared in Table 2. Obviously, two sets of fractional emission rates differ by (0.5–3)% for (Li  Mn) transitions,  5% for the (Li  Nn) transitions and (7–16)% for (Li–On) transitions. Three sets of the Li(i¼1–3) sub-shell fluorescence (oi) and Coster– Kronig (fij) yields are available in literature. Puri et al. (1993) reported a comprehensive set of the oi and fij yields for all the elements with 25rZr96 based on Dirac–Hartree–Slater (DHS) model based X-ray emission rates (Scofield, 1974a) and the non-radiative transition rates (Chen et al., 1979). Krause (1979) tabulated a set of semi-empirical fitted values of oi and fij yields for all elements with 12rZr110 based on the experimental data available till 1979. Campbell (2003) provided a set of recommended values of the oi and fij yields based on experimental data available till 2003 for the elements with 62rZr96 and recently reported (Campbell, 2009) the revised set of recommended values only for the L1 sub-shell fluorescence and CK yields for all elements with 64rZr92 except for Z¼ 75 and 76. It may be noted that for 80Hg, the L1 sub-shell yields (f12, f13, o1) tabulated by Campbell (2003, 2009) and those given by Krause (1979) differ from the DHS model (Puri et al., 1993) based values by  15–45% and  30–88%, respectively (Table 3).

4. Results and discussion The present measured Li(i¼1–3) sub-shell X-ray intensity ratios, ILk/ILa(Exp.) (k¼l, Z, b4,6, b1,2,3,15, b5,7, g1,5, g2,3,6,8, g4, b, g),

1170

A. Kumar, S. Puri / Radiation Physics and Chemistry 80 (2011) 1166–1171

Table 3 Two sets of the Li(i¼ 1–3) sub-shell fluorescence and CK yields for mercury.

DHS values Puri et al. (1993) Camp. values Campbell (2003, 2009) Krause values Krause (1979)

x1

x2

x3

f12

f13

f23

0.082 0.121 0.107

0.370 0.370 0.347

0.322 0.322 0.333

0.069 0.072 0.130

0.707 0.615 0.560

0.128 0.123 0.120

Table 4 Present measured and three sets of calculated L X-ray intensity ratios at 22.6 keV incident photon energy for some compounds of mercury. The oxidation state of mercury in each compound is also mentioned. Intensity ratios IL1 ILa ILZ ILa ILb4,6 ILa ILb1,2,3,15 ILa ILb5,7 b ILa ILg1,5 ILa ILg2,3,6,8 ILa ILg4 ILa ILb ILa ILg ILa a b

Hg oxidation state¼ 0

HgI2 oxidation state¼ þ2

Hg(C2H3O2)2 oxidation state¼ þ2

ILk ILa

ILk ILa

ðThr:Þ

ILk ILa

ðCamp:Þ

ðKr:Þ

0.0480 70.0034

0.0481 7 0.0034

0.0480 7 0.0034

0.0511

0.0511

0.0511

0.0166 70.0012

0.0167 7 0.0012

0.0163 7 0.0012

0.0166

0.0172

0.0166

0.0775 70.0054

0.0761 7 0.0053

0.0777 7 0.0054

0.0553(0.0536)a

0.0778(0.0755)a

0.0697(0.0677)a

0.920 70.055

0.926 7 0.055

0.0270 70.0016

0.0295 7 0.0018

0.920 7 0.0055 0.0268 7 0.0016

0.144 70.009

0.142 7 0.009

0.141 7 0.009

0.0512 70.0041

0.05027 0.0038

0.0497 7 0.0039

0.00625 70.00050

0.006077 0.00048

0.00620 7 0.00050

1.06 70.06

1.067 0.06

1.06 7 0.06

0.193 70.012

0.188 7 0.012

0.188 7 0.012

0.848

0.897

0.868

0.0282(0.0244)a

0.0301(0.0260)a

0.0294(0.0254)a

0.131

0.136

0.132

0.0404(0.0370)a

0.0541(0.0496)a

0.00459(0.00425)

a

0.00701(0.00649)

0.0489(0.0448)a a

0.00613(0.00567)a

0.93

1.01

0.97

0.176

0.197

0.187

Values calculated using the DHS model based (Scofield 1974a) Li(i¼1–3) sub-shell X-ray emission rates. Includes the weak Lb9,10 X-rays.

for some compounds of 80Hg, namely, HgI2, Hg(C2H3O2)2 and a pure mercury (liquid form) target at 22.6 keV incident photon energy are listed in Table 4. The overall error in measured intensity ratios is calculated to be  5–8%. This error is the quadrature sum of uncertainties in different parameters used to evaluate the intensity ratios (Eq. (1)), namely, the evaluation of photo-peak areas (  1–4%), IoGe product ( 6%) and self-absorption correction factor ( o3%). The present measured intensity ratios, ILk/ILa(Exp.), for 80Hg are compared with different sets of calculated, ILk/ILa(Thr.), ILk/ ILa(Camp.) and ILk/ILa(Kr.) values in Table 4. The ILk/ILa(Exp.) (k¼l, Z, b1,2,3,15, b5,7, g1,5, g) ratios for different compounds of 80Hg are found to agree well with all these sets of calculated values. However, in case of the L1 sub-shell X-ray components (k¼ b4,6, g2,3,6, g4), the ILk/ILa(Exp.) ratios are found to be, on an average, higher than the ILk/ILa(Thr.) values by  23–40%, whereas these are found to be in general agreement with both, the ILk/ILa(Camp.) and ILk/ILa(Kr.) values. In order to check reliability of the two sets of L X-ray emission rates (Scofield 1974a, 1974b), the theoretical intensity ratios, ILk/ ILa, for the Lk X-ray groups (b5,7, g2,3,6,8 and g4) comprising of (Li  On) transitions were also calculated using the DHS model based X-ray emission rates (Scofield, 1974a) and are given in Table 4. It may be recalled that two sets of fractional X-ray emission rates differ to maximum extent (Table 2) for these X-ray groups. Obviously, the differences between these calculated and the present measured intensity ratios, ILk/ILa(k ¼ b5,7, g2,3,6,8 and g4), were found to be more as compared to the differences between present measured values and those calculated using the DF model based X-ray emission rates, thereby indicating the reliability of the DF model based emission rates as compared to the DHS model based X-ray emission rates. It is noteworthy that this observation is in accord with the earlier experimental work (Papp et al., 1993, Puri Sanjiv et al., 1996), where it was shown that in case of heavy elements the L X-ray emission rates based on

the DF model (Scofield, 1974b; Campbell and Wang, 1989) are more reliable than those based on the DHS model (Scofield, 1974a). Further, it is obvious from Table 4 that the spread in values of measured intensity ratios, ILk/ILa(Exp.)(k¼l, Z, b4,6, b1,2,3,15, g1,5, g2,3,6,8, g4, b, g), for different compounds of 80Hg is 1–3%, whereas in case of the ratio, ILb5,7/ILa(Exp.), the spread is  10%. Also, the ILb5,7/ILa(Exp.) values differ by 10% for different compounds having 80Hg in same oxidation state of (þ2). These observations indicated the influence of chemical effects on these ratios. It is noteworthy that the Lb5(L3  O4,5) and Lb7(L3  O1) X-ray lines correspond to the transitions involving outermost sub-shells of 80Hg and the corresponding electrons occupying these sub-shells are involved in chemical bond formation with the attached ligands in a given compound. Depending on the nature of the ligand and the chemical bond it formed with the metal/central atom (80Hg), the outermost shell/sub-shell electronic charge in metal/central atom may have got rearranged thereby affecting the binding energy of the corresponding shell/ sub-shell and hence transition probabilities for corresponding L X-ray components. It may be mentioned that Cengiz et al. (2010) have performed the measurements using LEGe X-ray detector, which exhibits discontinuity in the detection efficiency vs. energy curve at the K-edge energy (11.103 keV) of 32Ge. The energies of different AuLb components are in close proximity to the 32Ge K-edge energy. Therefore, measurement of the Au-(Lb/La) intensity ratio for different targets would be very difficult using LEGe detector and the reported values (Cengiz et al., 2010) may have large error. Secondly, in case of the AuBr3 target used by Cengiz et al. (2010) the Br–Ka (11.92 keV) X-ray peak overlaps with the Au-Lb peak and the Br-Kb (13.29 keV) with the Au-Lg1,5 peak and these authors did not mention any procedures/corrections applied to evaluate the L X-ray photo-peak areas in this case. It may be noted that the reported ratios, ILk/ILa(k¼ b, g) (Cengiz et al., 2010),

A. Kumar, S. Puri / Radiation Physics and Chemistry 80 (2011) 1166–1171

for AuBr3 target were shown to result in large spread. In view of these facts, the measurements reported by Cengiz et al. (2010) may be considered inconsequential. Summarizing, the present measured intensity ratios, ILb5,7/ ILa(Exp.) exhibited dependence on the oxidation state of 80Hg as well as the nature of ligand attached to it in a given compound. It may be added that in the present work, the intensity ratios were measured using thick targets prepared from compounds having 80Hg in two oxidation states (0 and þ 2) only by employing EDXRF spectrometer. For further insight into the chemical effects, similar measurements may be performed using thin/thick targets of different compounds having metallic/central atom in various oxidation states employing a high resolution spectrometer. Furthermore, the present measured intensity ratios for the L1 sub-shell X-rays (k¼ b4,6, g2,3,6, g4) exhibited agreement with both, the ILk/ILa(Camp.) and ILk/ILa(Kr.) values, whereas, these were found to be significantly higher than the ILk/ILa(Thr.) values. It may be noted that different Li(i¼1–3) sub-shell physical parameters used to deduce the theoretical intensity ratios, ILk/ ILa(Thr.), were calculated using the independent particle models, which completely ignore many-particle interactions such as electron–electron Coulomb correlations. In earlier investigations (Puri and Singh, 2006), it was found that these independent particle models significantly overestimate the L1 sub-shell CK transition probabilities. Therefore, theoretical calculations of different parameters, in particular non-radiative transition probabilities, based on existing models incorporating the many body effects are highly desirable to apprehend the observed differences between measured and theoretical values.

Acknowledgments One of the authors, S. Puri, wishes to acknowledge the financial assistance from Department of Science and Technology (DST), New Delhi in the form of a research project sanction in year 2007 for a period of three and a half years. References Bevington, P.R., 1969. Data Reduction and Error Analysis for Physical Sciences. McGraw-Hill, New York. Campbell, J.L., Wang, J.X., 1989. Interpolated Dirac–Fock values of L sub-shell X-ray emission rates including overlap and exchange effects. At. Data Nucl. Data Tables 43, 281–291. Campbell, J.L., Papp, T., 1995. Atomic level widths for X-ray spectrometry. X-ray Spectrom. 24, 307. Campbell, J.L., 2003. Fluorescence yields and Coster–Kronig probabilities for the atomic L subshells. At. Data Nucl. Data Tables 85, 291–315. Campbell, J.L., 2009. Fluorescence yields and Coster–Kronig probabilities for the atomic L subshells. Part II: The L1 sub-shell revisited. At. Data Nucl. Data Tables 95, 115–124. Cengiz, E., Tırasoglu, E., Aylıkcı, V., Apaydın, G., 2010. The investigations on K and L X-ray fluorescence parameters of gold compounds. Radiat. Phys. Chem. 79, 809–815.

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