Journal of Radioat, alytical and Nuclear Chemistry, Articles, VoL 172, No. 1 (1993) 137-143

XRF ESTIMATION OF Cd, Hf, Hg, AND Gd IN URANIUM SOLUTION

R. QADEER, J. HANIF, M. KHAN

Pakistan Institute of Nuclear Science and Technology, P ~O. Box No. 1356, Islamabad (Pakistan)

(Received September 24, 1992)

A simple X-ray fluorescence (XRF) spectrometric method for the determination of Cd, Hf, Hg, and Gd in uranium solution has been developed, where sample solutions were introduced to the XRF spectrometer directly in simple polyethylene bottles, The matrix effects have been rectified using standards of similar composition. Statistical evaluation has established it as a fast and reliable method applicable to trace analysis.

The objective of the present study was to establish a rapid, accurate and non-destructive method to determine cadmium, hafnium, mercury and gadolinium in uranium solution using X-ray fluorescence speclrometry (XRFS). In the past XRFS has been applied successfully, for the determination of the above menfoned elements in different malrices, t-14 AGRAWAL et al. 15 and SALEEM et al. 16 have determined Gd in uranium along with other elements. Wavelength dispersive X-ray fluorescence spectrometer (WDXRFS) was used in our studies for measuring the concentration of the above mentioned elements in uranium solution. The sample solution was presented directly to the spectrometer in a polyethylene bottle with wall thickness of 0.1 mm. 7,16-18 This is unlike the earlier sample presentation techniques such as pelletization, bead formation, deposition and precipitation on a substrate. The effects caused by the matrix were rectified by making the calibration curves from the simulated standard of the same composition as that of sample solution. Use of high voltage (50 kV) and current (50 mA) increased the intensities to improve the lower limit of detection.

Experimental

Chemicals and reagents. The chemicals and reagents used were cadmium nitrate (Purum, Fluka) mercury nitrate (97%, RDH), hafnium oxide (Fluka), gadolinium nitrate (99.999%, Rare Earth Product), U308 (Specpure, JMC), and nitric acid (Selectipur, Merck).

Instrumentation. A sequential WDXRF spectrometer SRS-200 from Siemens, Germany, with a Cr target X-ray tube; a Soller slit with angular divergence of 0.15 ~ Lif-100 crystal and NaI(T1) scintillation counter was used. The instrument is coupled

Elsevier Sequoia S. A., Lausanne Akadtmlal Kiadt, Budapest

R. QADEER et al.: XRF ESTIMATION OF Cd, I/f, Hg AND Gd

&

0 -~

. J : 3

,,e" T-

(J ~ III I--

9u' I I I I

10 15 20 25

. d

"" :2 3

t J

- . T . J ...J I~ c r-,

i I , . j ~ Z

--- f f . U, r, 30 35 40 45 50 55

, r

t3

:2 t3

60 65 70

20

Fig. 1. Linear XRF spectrum of Cd, I-If, Hg and Gd in uranium solution

Table 1 Optimized conditions for gross-intensity measurements

Line LBG, Peak, HBG, Counting D.L., Element

selected 20 -va lue 20-va lue 20-value time, s ~tg/ml

Cd K~ 15.00 15.33 15.70 100 8 Hg L~I 35.65 35.93 36.15 100 6 Hf Lal 45.65 45.93 46.20 100 9 Gd L~l 54.40 54.64 55.00 1000 50

with a computer PDP-11/04 through a universal interface called logic controller,

LC-200 which controls the operation and data processing.

Calibration standards. Separate standards for each of the above mentioned elements were prepared in a matrix of 1000 gg/ml of uranium and 100 pg/ml each of the remaining three elements. The concentration of the fourth element under study was

varied from 10 I.tg/ml to 100 t.tg/ml except for Gd where the range was 100 gg/ml to

1000 lxg/ml. All the dilutions were made with 3M HNO 3.

Optimization of spectral lines and their measuring conditions. Spectrum recorded with 3.0 ml solution containing 1000 gg/ml each of Cd, Hf, Hg, Gd and 10,000 pg/ml uranium is shown in Fig. 1. The lines were identified from the 20-values given in the

literature. 19 The selected analyte line for for each element has no spectral interferences

from other matrix elements and are given in Table 1. Peak analyses of these lines is performed to see the symmetric nature of the selected peak and to establish low

138

R. QADEER et al.: XRF ESTIMATION OF Cd, Hf, Hg AND Gd

-1

(A

"r

~J -r

d _J

~2o(

80

200 400 I I

4O

oc

Counting time for Gd, s

600 800 1000

Cd ~400 d oGd �9 H f ...i - H g 300

, 200

5O 100 150 2OO Counting time for Hf, Hg, Cd, s

Fig. 2. Variation of detection limit of Cd, Hf, Hg and Gd in uranium as a function of counting time

Table 2 Peak symmetry analysis of Cd Ka, Hf Lab Hg Lab and Gd L0I lines

Cd K~ Hf L~I Hg Lal Gd 1131

Peak angle (x0) 15.33 45.93 33.93 54.64 Half-width angle low (xl) 15.18 45.815 35.82 54.52 Half-width angle high (x2) 15.48 46.045 36.035 54.77 Half-width (x2-x 1) 0.30 0.23 0.215 0.25 Symmetry at half-width

(x0-x 1)/(x2-Xo) 1.00 1.00 1.04 0.93

background (LBG), peak and high background (HBG) angles for net intensity measurement. The selected values for LBG, peak and HBG are in Table 1 while the

peaks symmetric data are given in Table 2. The behavior of the detection limit (D.L.) as a function of counting time was studied and is shown in Fig. 2. The equation below is used to calculate the D.L. 16

D.L.= 3- (Rgr) ~ (I) m

where m h

R b - T -

sensitivity (c/s, %), background count rate (c/s), counting time (s).

139

R. QADEER et al.: XRF ESTIMATION OF Cd. Hf, .Hg AND Gd

The optimized counting time for analysis along with D.L. is also given in Table 1. Intensity measurement and data processing. Equal columes (3.0 ml) of standard and

sample solutions in prescribed polyethylene bottle were placed into the spectrometer. Gross intensities as number of counts were measured for each element at LBG, peak and I-IBG angles, given in Table 1, in triplicate. Net intensities were calculated from gross intensities using the following formula:

INe t = Ipeak_.+ [IHBa(20peak --20nBO)] + [ILBo(20peak - 20LBO) ] (2OPeak -- 2Orma) + (20peak -- 20LBO)

where I N e t - - net intensity, Ipeak -- peak angle intensity (gross), ILB a - lOW background angle intensity (gross), Irm a - high background angle intensity (gross), 20pe~k -- peak angle, 20LB a -- lOW background angle, 20rm a - high background angle.

Calibration curves from the simulated standard, prepared by plotting the net

intensities against the concentrations for each element, are shown in Fig. 3. The data of these curves were subjected to regression analysis. The best empirical relationship between net intensity and concentration obtained through regression are:

Icd : -- 88.45 +415.24 CCd

Inf =4464.1 + 98.286 Cnf

IHg = -1146.5 + 175.18 Cug

(3)

(4)

(5)

and IGd = 1571.1 + 26.985 CGd -- 0.0049(CGd) 2 (6)

where I - net intensity, C - concentration (Bg/ml).

Results and discussion

Equations (3) - (6) were applied to different simulated sample mixtures of various compositions to determine the concentration of Cd, Hg, Hf and Gd in the presence of uranium. The values determined matched fairly well with the known concentrations as is evident from Table 3.

140

R. QADEER et al.: XRF ESTIMATION OF Cd, I-If, Hg AND Gd

Table 3 Determined concentration of Cd, I-If, Hg and Gd in simulated mixtures containing uranium solution

Known Determined Sample Difference, No. Element concentration, Counts • 0* concentration, %

l.tg/ml ~tg/ml

S-1

S-2

S-3

S-4

Cd 100 41 690 • 204 100.40 - 0.4 Hf 100 11 190 • 106 99.00 + 1.0 Hg 100 16 608 • 129 101.40 - 1.4 Gd 500 13 883 • 118 502.00 - 0.4 U 10 000 N.D.

Cd 80 32 890 • 181 79.50 + 0.6 Hf 40 5 360• 73 39.60 + 1.0 Hg 60 9 300-+ 96 59.65 + 0.6 Gd 400 11 546 • 108 398.50 + 0.4 U 1 000 N.D.

Cd 40 16 240 • 127 39.25 + 1.9 Hf 60 7 560• 87 62.00 -3.3 Hg 30 4 180• 65 30.40 - 1.3 Gd 200 6 850• 83 203.00 - 1.5 U 1 000 - N.D. -

Cd 10 4 001• 63 9.85 + 1.5 Hf 20 3 500• 59 20.70 -3.5 Hg 20 2540• 50 21.00 -5.0 Gd 100 4 330• 66 104.00 - 4.0 U 500 N.D. -

*~ - standard deviation. N.D. - Not determined.

It is observed in Fig. 1 that the background is more prominent on lower 20 values.

General ly , the radiation responsible for the background in the spectrum is due to

inelastic col l is ions o f the pr imary X-ray photons with the e lements in the samples, their

container and the envi ronment . 7 M o I ~ , Cu K~, Zn K~, Ni K~, Cu K~,Ni K~, Fe KI~,

Cr K~, and Cr K~ lines in the spect rum are from the sample chamber and sample holder

of stainless steel. Se lec ted peaks o f Cd, Hf, Hg and Gd are symmetr ic , as is seen in

Table 2. The cal ibrat ion curves in Fig. 3 are l inear for Cd, Hf, Hg and slightly curved

for Gd.

The precis ion o f the me thod is calculated in terms o f relat ive standard deviat ion

(%RSD). Ten measurements were m a d e under opt imized condit ions g iven in Table 1

for each e l emen t o f different concentra t ion in a matrix of 1000 gg /ml o f uranium and

100 g g / m l each o f the remain ing three elements . The varation o f % R S D o f Cd, Hf, Hg

141

R. QADEER et al.: XRF ESTIMATION OF Cd, Hf, Hg AND Gd

Gd concentration,tJg/mr .̂~ 200. tOO 600 800 1000

z ~ [ i I I / j 4 s

o.C J .0o �9 Hf . / ' / .d xi

�9 ~ IO >, 79

o I I o 0 20 z,o 6o 8o 10o

Hf, Hg, Cd concentration, pg/ml

Fig. 3. Calibration curves of Cd, Hf, Hg and Gd in uranium solution

Gd, concentration, p g/m{

o~. 200 ZOO 600 8OO T000 . ~ . u I I I I

~ \ ,~ Z5

1,5

05

I l I I 0q 20 40 60 80 100

I-if, Hg, Cd concentration, IJg/m[

Fig. 4. Variation of percentage RSD with concentration of Cd, Hf, Hg and Gd in uranium solution

and Gd with concentration is given in Fig. 4. It is seen that percentage RSD is concentration dependent and is better at higher concentration, which reflects precision

of the technique.

Conclusion

Measurement of Cd, Hf, Hg and Gd in uranium solution can be carried out precisely and accurately using an X-ray fluorescence spectrometer. The method developed does not require chemical separation or any special sample preperation technique. Solutions in simple polyethylene bottle can be used.

:r

142

R. QADEER et al.: XRF ESTIMATION OF Cd, Hf, Hg AND Gd

The authors wish to thank P.A.E.C. authorities for their cooperation and encouragement during the course of this work. Thanks are also due to Mr. GUL for typing the manuscript.

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