Q t WSRC-MS-96-0569

Development of Total Uranium Analytical Method by L X-Ray Fluorescence

by R. A. Dewberry Westinghouse Savannah Rier Company Savannah Riier Site Aiken, South Carolina 29808

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WSRC-MS-96-0569

DEVELOPMENT OF TOTAL U,RANIUM ANALYTICAL METHOD BY 1 X-RAY FLUORESCENCE

I R . A. Dewberry Savannah River Technology Center Westinghouse Savannah River Company Aiken , SC 29801

ABSTRACT

Thi s paper describes devel perform t o t a l uranium anal added direct ly t o t h e samp U samples i n t h e the advantages o f normally be found zn an ext determined by counting L x- Cd-109 added d i rec t ly to source from t h e detector is included i n the analy

INTRODUCTION

Thi s paper de method of anal range of 1 g/ described use fluoresce t h e fluorescence d ce. The technique of x-ray fluorescence ly t i ca l instruments ex is t which provide anayl ime, inc luding uraniuml. These instruments u measure each element by fluorescence .of the K x-ray d River S i t e we have dev oped and demonstrated an on-l uses a co-57 source t o prescence o f Pu and seve ins t ruments it has bee source and then t o rem detector is not exposed t o it.

The use of Cd-109 as an excitation source for t h e U L x-rays i advantageous. Its decay p r f ive x-ray and 7-ray t r a i n Table 1. absorption edge fo r uraniu ionize the L electrons and not energetic enough t o f l and we can expect t o obtair; active L fluorescence spectra which a r e very clean of anything'but t h e uranium transit ions.

t i n g of an L x-ray fluorescence content i n the

not require an external x-ray

nd generally a re designed t o let. A t t h e Savannah type instrument which

-rays t o measure t o t a l uranium i n the ch case for these e t o t h e exc so tha t the

The Cd-109 t r a ar 22 kev are jus t la

L x-rays of any of the Z > 92 elements,

c

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I n the technique we describe here, the Cd-109 spiked activity is introduced directly into t h e l i q u i d sample, and the sample and excitation source are counted simultaneously. large enough that we are able t o count. the fluoresced L x-rays without shielding t h e detector from-the Cd-109 spiked activity. t h i s technique over others sensitive i n t h i s range are simplicity, speed, and that t h i s technique empirically subtracts interferences. If t h e Pu content is low, no chemical preparation of the sample is necessary. No interaction w i t h the counting instrument is necessary after the analysis is initiated.

Note we require t h a t the Pu content of ti7 t o U and produces large quantities o f t h e photons we wish t o count i n t h i s fluoresc Since a l l Pu isotope decay w i t h half-lives much shorter than eith or U-238, atom contents of PU up t o ~ 1 0 0 t h r s u l t i n significa interference i n t h i s analysi t h i s interference is that the L x-rays from Pu decay are passive transitions and do not contribute t o the active fluorescence spectrum. Th the blank sample spectrum and ca active spectrum.

I n t h e event that the Pu content of t h e introduced i n the analysis. Rather t h e i n t h e measuremen t o dominate t h e results. That is the s u

the active s p e e ld a n e t small a ls that t h e sample must be

The cross section for ionization of t h e U L x-rays is .

The advantages of

ample is very low. L x-rays, which are the same

Pu a-decays

o large, no bias is

the passive bac

nty i n t h e data

PRINCIPAL OF MEASUREMENT - The mode of fluorescence used i n t h i s techniq depicted i n Figure 1. tates of Ag-10 emitting K x-rays w i t h energies o f 21.990-, y-ray of energy 88.034-keV. The two lowest are particularly suited i n energy t o fluore n cross section for t h i s fluorescence can estimated from available d be 43 cmz/g. Thus a 1 g/l U solution w i t h a 1 cm absorption 1 absorb these K x-rays a t t h e rate

Cd-109 decays w i t h a 463-day half-li .603-keV and a

U L x-rays.

I = Io exp [(-1)(43)(0.001)] (1)

where IO is t h e i n i t i a l intensity of Cd-109 K x-rays. The f for these L x-ray holes is approximately 0.5 41, so $he rat L x-rays from a 1pCi sp ike of Cd-109 is

0.51(1 - 0.958)(3.7 x lo4) = 790 <U L x-rays)/sec. (2)

Thus t h e ra t io of U x-rays t o transmitted Ag x-rays is i n the range of 0.02, and we Can count t h e fluoresced L x-rays i n the presence of the excitation source without s h i e l d i n g the detector from the source. relative intensit ies of t h e U L x-rays obtdined are shown i n Tqble 2.

The energies and

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Since t h e technique is based on the sample absorption of 22xkeV x-rays, it is obvious that the fluoresced L x-rays of approximately 17 keV x-rays w i l l themselves be strongly self-absorbed by the sample. method t o measure the sample self-absorption and t o correct for it. describe our sample self-absorption correction below based on the approach used i n reference 5.

We require a We

EXPERIMENT AND RESULTS

To demonstrate the fluorescence technique we made five U solutions i n 2M HN03 w i t h U contents of 1-, 4-, 6-, 8-, and 15-g/l. Then 5ml aliquots of each were pipetted in to p las t ic v i a l s o f 25 m i capacity. items w i t h a 4-cm circular base and a wall thickness of 2 mm.

exposure of t h e l iqu i#d sample t o the detector active area and t o minimize absorption of photons by the vial .

The v ia l s are screw They are

t o sit on an up-looking photon detector provide maximum surface

Each of the samples was placed on the up-looking surface of a Si(Li) low energy photon detector w i t h an active surface area of 80 mm2 and an operating

calibration. keV was taken for each sa Canberra Series 90 Multic

Each sample was then s p i counted i n - t h e same conf blank sample was a l so s p sample is shown i n Figure 3. 3.4% was obtained. The previously s tor normalized t o 1000 seconds and subtract spectrum. For each o f t h and 17.2-keV L x-ray peak 22.2-keV K x-ray peak was'obtained. tabulated as xs i n Table 3.

The data of xs are shown p t ted i n Figure 4 i n the 1 the curve is significantly concave downward-due t o t h

It is clear that sample abs causes a negative bias i n the detected fluorescence r

1000 V. The detector had a 2 u b l e t of K x-rays a t 5.9 keV an

A 300 sec passive x-

Note i n the f i g u

The r a t io between these two quantit ies is

- o f the uranium L x-rays. -

increases.

To correct fo r t h e sample self-abs ption effect, we constr shown i n Figure 2(b). T h i s v ia l h a 2 gram sample of nat by epoxy t o the under surface of the screw cap; The uraniu suspended 1 cm above the surface of the sample, and t h e ac t spectrum was again collected for 1000 sec for each sample a w i t h t h e f o i t i n p l is U L x-ray t o Ag K x-ray ra t io is tabulated i n '

, I 4

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I n the conf igurat ion o f Figure 2(b), Ag K x-rays escape the sample and fluoresce the U L x-rays i n the U f o i l i n the same mechanism t h a t occurs i n the sample. reach the detector, and i n t h i s way the act ive spectrum i s a sum o f sample fluorescence and f o i l fluorescence. Since sample x-rays and f o i l L x-rays both su f fe r from sample absorption, t h i s technique o f analysis provides a very good method o f empir ica l ly subtracting the e f fec t o f sample self-absorption.

Using the method o f referen 5 for photon transmissio transmission correct ion fac s and corrected detect io as shown i n the fol lowing. We determin the sample se a(i) f o r each sample from

These fo i l - f luoresced L x-rays must then traverse the sample t o

(3) x 0

where xo i s the x t value obtained f o r the blank.

For example, since x t f o r sample 1 i s 0. 0.01397, then

, and xs i s 0.01246, and xo i s

= 0.7974. 0.02360 - 0.01246’ 0.01397

a(i) =

We then determine a correct ion factor CF(i) f o r each sample by

(4) -In a (i) 1 -a (i)

CF(i) =

So the correct ion fac to r f o r sample 1 i s 1.1 . The correct fluorescent y i e l d f o r each sample i s

CC(i) = CF(i) x xs(i), (51

where CC(i ) i s the corrected count (L t o K ra t io) o f sample i , the calculated correct ion factor. reference 5 pages 1-1 through 1-12.

The resu l t i ng correcte These data show t h a t t orrect ion has aightened the rescence curve. The di f ference between the two curve demonstrates the ma ude o f the se l f - absorption e f fec t , which i n the case f the 15 g / l sample factor o f two correction.

Equations (3) through (5) a r

unt data are p n Figure 4 i n the upper curve.

greater than a

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CONCLUSION

T h i s experiment has demonstrated the u t i l i t y of the i x-ray fluorescence analysis of uranium u s i n g an internal excitation source The samples i n the range of 1 g/ l t o 15 g / l can be dete counting time o f less than one hour. section for,uranium u s i n g 22.1-keV x-rays as the excitation source i (43 cm*/g) that the detector can be exposed t o the excitation source causing an unacceptable dead time.

T h i s technique requires no chemical treatment of t h e sample when it less than -0.lg Pu/ l . ins t rument is ini t ia t ion of each count. analyzer of the instrument can be set up tomatically store the t h e appropriate memory segment, acquire t ive spectra, perform subtraction, and provide the 5, x t , and xo for eac run, w i t h one task command. can also be set up t o calculate and the precision obtained for each measurement.

The advantages that t h i s technique of U dete dependent techniques and over external excit techniques are elaborated below.

i k e i n the sample. ned w i t h an overall orption cross The fluorescence

The only operator interaction w i t h t h e analys With preparation t h e multic

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nation provide over matrix fluorescence

The system is simple t o operate, as no excitati source instrument exis complicate operation. Initiation of each _spect collection w i t h a s i n task command can be the only operator interaction required. automatically collects the spectrum, s u b t c t s background, and displays the peak ra t io data and errors needed for sub quent calculations using equations (3) through (5)

The technique empirically subtracts interferences from U self-absorption and any other matrix absorption effect. Larg interferences from too much Pu i n t h e sample are signalled by large impreci alues xs and x t . For samples w i t h large U L x-ray contribution y, the values xs and x t will be large and very near each other i n v the uncertainty i n these values w i l l become larger an the difference between them, and t h e self-absorption factor a(i) cal ated i n (3) can go negative from random variation.

The system

Using t h e ra t io of U fluorescent yield t o transmitted Ag x-rays automatically normalizes the data t o remove deadtime corrections and decay of excitation source s t r e n g t h . Thus exact knowledge o e sp iked activ not required. The Cd-109 source has a long (463 day) h everal years of free excitation.

l i f e , t h u s ass

The data from t h i s experiment have demonstrated t h e u t i l i t y over the l i m i t e d range of 1 g/l t o 15 g/ l .

Table 3

Fluorescent yield data and co range 1 g/l to 15 g/l . The u corrected data are tabulated as xt. Corrected and uncorrected yields are explained in

cted yield results fo rrected data are tabu CC.

quid U samples in the d as XS, and the

The sample plus U foil data are tabulated The correction factors are listed in the urth column as CF.

U XS xt 0 (no units1 (no units1

0 ----- 0.01397 1.000 0.01397

0.992 3.970 6.000 7.937 15.00

0.01246 0.03976 0.05600 0.07300 0.11187

0.02360 0.04730 0.06319 0.07627 0.11400

1.117 1.340 1.369 1.896 2.219

, 0.01392 0.05327 0.07664 0.1384 0 - 24826

L

,

. Determination

""Cd

\ 463d O9Ag + 22.1 63' keV X-Ray

X-Rays 1 09 Ag u- U + LX-Rays

FIGURE 1. Schematic R8preS8ntatiOn of U L X-Ray Fluorescence by"39Cd Decay

' ,. * --

0 .

r 3

Detector X-PHP(

. . . . . . . . _ _ . ... - - .

. .

Sample Vial U +- loSCd Spike

Sampie Vial U + '%d Spike

Dead Time = 3.4% IUI = a gtr ' Cd

I

I - 0) C . 8

a . 2 ~ -- 3 0 0

-. - 4.1K logCd -

1 32.5

4 4 ~ -1 1

2.8 17.7 E (keV)

- : , FIGURE 3. U L X-Ray Spectrum Obtained using the Counting Configuration of Figure 2(a) with an 8 g/l U Solution and a 2pCi lWCd Spike in the Sample as Described in the Text

. Y

r: 8

I

3 . L . .

1

F