LARGE FIELD X-RAY ABSORPTION MICRO-ANALYSIS NEAR AN ABSORPTION EDGE BY IMAGE PROCESSING

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LARGE FIELD X-RAY ABSORPTION

MICRO-ANALYSIS NEAR AN ABSORPTION EDGE BY IMAGE PROCESSING

E. Bigler, F. Polack

To cite this version:

E. Bigler, F. Polack. LARGE FIELD X-RAY ABSORPTION MICRO-ANALYSIS NEAR AN AB- SORPTION EDGE BY IMAGE PROCESSING. Journal de Physique Colloques, 1984, 45 (C2), pp.C2- 83-C2-86. �10.1051/jphyscol:1984220�. �jpa-00223876�

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LARGE FIELD X-RAY ABSORPTION MICRO-ANALYSIS NEAR AN ABSORPTION EDGE BY IMAGE PROCESSING

E. Bigler and F. Polackr

I n s t i t u t d1@tique, B.P. 43, 91406 Orsay Cedex, France

*L.u.R.E., Universite' Paris-Sud, B a t . 209 C, 91405 Grsay Cedex, France Resum& - La microradiographie de contact en rayonnement X mono- chromatique accordable(synchrotron) permet de f a i r e de 1 'analyse

chimique. On obtient par absorption s e l e c t i v e au voisinage d'un seuil des cartes quantitatives de r e p a r t i t i o n des elements s u r des champs de 1 'ordre de 3 cm2.

Nous exposons l e s p o s s i b i l i t e s actuelles e t l e s developpements futurs de l a methode : champ e t resolution s p a t i a l e , s e n s i b i l i t e e t facteurs c o r r e c t i f s .

Abstract

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Chemical microanalysis i s performed by contact micro- radio raphy with a tunable synchrotron X-ray beam. Large f i e l d ( 3 cm

!

) quantitativechartsof the elements a r e obtained by s e l e c t i v e qbsorption near an edge. We expose here present perfor- mances and future developments of the method : f i e l d and s p a t i a l resolution! s e n s i t i v i t y and corrective factors.

Microanalytical study of objects with complex s t r u c t u r e s requires both a large f i e l d and a high s p a t i a l resolution. In t h i s paper, we examine how these two opposite q u a l i t i e s can be obtained by X-ray d i f f e r e n t i a l absorption near an edge and contact imaging.

I

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PRINCIPLES

1 -1.

~hnmlcal-analrsls-~.)!~X1r~r~abs_1!retI1!!-near-an-eE!~e-i1~2~

The transmittance of a sample i s measured with a monochromatic X-ray illumina- tion. The beam i s successively tuned on each side of an absorption edge of a given element. The concentration C of the analyzed element i s given by :

where ( u / p ) i s the mass absorption edge-jump of the analyzed element.

m i s the mass per unit area of the whole sample. N1 and N2 a r e the number 'of transmitted photons on each side of the absorption edge. The r e s u l t can a l s o be expressed as a mass per unit area

1.2. Experjm$ntal-s_etIup

The X-ray source i s the synchrotron beam delivered by,the storage ring D.C.I. i n Orsay. A tunable monochromatic beam (1 - 3 A ) i s delivered by a channel -cut monochromator (Si : 220). Two contact microradiographs

of the sample (one of each side of an absorption edge) a r e recorded on a high resolution photographic plate. When the concentration of the analyzed element i s greater than ¹10 %, a simple visual comparison of the two images shows the l o c a l i s a t i o n zones of the element (31.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984220

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C2-84 JOURNAL DE PHYSIQUE

The f i e l d covered i s l i m i t e d o n l y by the s e c t i o n o f t h e synchrotron beam, i . e 3 cm2.

II - IMAGES PROCESSING : EXAMPLE OF RESULTS 2.1. Microdeqsi~omgter_gng1ys_is_

An area o f i n t e r e s t i s v i s u a l l y s e l e c t e d on enlarged p r i n t s . The o r i g i n a l p l a t e s a r e scanned w i t h a microdensitometer. The a n a l y t i c a l s i g n a l ,given by Eq. (1) ,is obtained by comparison o f o p t i c a l d e n s i t i e s o f t h e two contact images.

A c a l i b r a t i o n scale,recorded on each plate,gives t h e s e n s i t o m e t r i c data. The curve p l o t t i n g o p t i c a l d e n s i t i e s vs number o f r e c e i v e d photons i s l i n e a r f o r o p t i c a l d e n s i t i e s < 1.5. A simple a n a l y t i c a l model o f t h e response curve a l l o w s a p l a t e c a l i b r a t i o n up t o d e n s i t i e s o f 2.5. I t must be p o i n t e d o u t t h a t we o n l y need a r e l a t i v e c a l i b r a t i o n , which i s much more simple

.

For example, i n e q u a l i - t i e s o f exposure between homologous images can be c o r r e c t e d a p o s t e r i o r i .

A c a r e f u l r e g i s t r a t i o n o f images p a i r s i s necessary t o ensure t h a t t h e number o f photons N1 and N2 i n Eq.1 correspond e x a c t l y t o t h e same p i x e l o f t h e sample.

This problem i s solved d u r i n g t h e r e c o r d i n g o f the second image by ddi'splqying t h e image o f l o c a l differences

.

The b e s t r e g i s t r a t i o n i s obtained when d i f f e r e n t i a t i o n a r t e f a c t s disappear near t h e sharp l i n e s o f t h e image.

2.2. Corn~ute~-~roce??jng

The mass per u n i t area o f t h e analyzed element i s computed f o r each p o i n t o f the image (Eq. ( 2 ) ) . The r e s u l t i s a numeric a r r a y which i s displayed as an image i n grey l e v e l s . F i g u r e 1. shows 3 a n a l y t i c a l c h a r t s o f Mn, Fe and N i i n a 30 um-thick p o l y m e t a l l i c nodule section. White areas correspond t o h i g h e r concentrations o f the analyzed element. Large f i e l d c o r r e l a t i o n s between t h e concentrations zones o f t h e elements appear c l e a r l y on these p i c t u r e s . I t would be r a t h e r d i f f i c u l t t o o b t a i n such images w i t h a microprobe : a l a r g e f i e l d method i s h i g h l y i n t e r e s t i n g t o study o b j e c t s w i t h complex s t r u c t u r e s l i k e p o l y m e t a l l i c nodules.

2.3.

quantltatjye-output-~-~i_s_~u_s_s_~o~

The sample shown on F i g u r e 1. has a l s o been analyzed w i t h a S.E.M.

The c o n c e n t r a t i o n o f Mn, Fe and N i have been measured i n t h e i r zone o f predomi- nance. The sample i s known t o have an average d e n s i t y o f 4 : comparison can be made i n c o n c e n t r a t i o n terms.

Mn Fe N i

X-ray a b - s o r ~ t i o n 28 % 10 % 3 %

SEM-fl uorescence 35 % 16 % 4 %

The systematic under e v a l u a t i o n o f our r e s u l t s can be explained by d i f f e r e n t f a c t o r s :

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the c o n t r a s t between t h e two images i s reduced.

x the channel-cut monochromator tuned on wavelength a l s o d e l i v e r s t h e harmonics 112, A/3

...

These h i g h energy components o f t h e beam are n o t a f f e c t e d by t h e

presence o f the a b s o r p t i o n edge. Thus, another a d d i t i v e s i g n a l i s recorded on each image. This phenomenon y i e l d s another r e d u c t i o n o f t h e d i f f e r e n c e s between t h e two images, m o s t l y w i t h t h i c k samples. L e t us f i n a l l y mention t h a t the Dresence o f monocrystals i n t h e sample can b r i n g topographic e f f e c t s i n the c o n t a c t images. T h i s has never been observed w i t h the nodule samples

(Fig. 1.) which a r e amorphous.

I11

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PRESENT PREFORMANCES AND IMPROVEMENTS 3.1.

Zpcitjal-resolution

High r e s o l u t i o n p l a t e s y i e l d 1 vm r e s o l u t i o n , b u t the f i n a l p i x e l s i z e i s determined by t h e s m a l l e s t a v a i l a b l e microdensitometer s l i t , i .e 5 x 5 pm2.

I t must be emphasized t h a t t h e photographic p l a t e remains one o f t h e o n l y X-ray images d e t e c t o r w i t h good s p a t i a l r e s o l u t i o n and q u a n t i t a t i v e output.

With the new photoelectron s o f t X-ray microscope I41, we hope t o g e t a -1 urn r e s o l u t i o n and q u a n t i t a t i v e s images.

3.2.

%nsjti"ty

S e n s i t i v i t y i s mainly determined by t h e number o f avai 1 a b l e photons d u r i n g an absorption measurement. For example, i n t h e case o f I r o n a n a l y s i s near t h e K edge i n a 30 um t h i c k sample (average d e n s i t y : 4), t h e minimum d e t e c t a b l e c o n c e n t r a t i o n i n a 20 x 20 pm2 c e l l i s C m i n = 3 x 10- w i t h a 4 min synchrotron exposure and an i d e a l d e t e c t o r . Experimentally , t h e minimum c o n c e n t r a t i o n i s found t o be = 1 % i n t h e same c o n d i t i o n s , w i t h a photographic d e t e c t o r . To improve s e n s i t i v i t y , we are developping a non s a t u r a t i n g s c i n t i l l a t i o n image d e t e c t o r . Our aim i s t o g e t 10 vm r e s o l u t i o n w i t h counting o u t p u t . CONCLUSION

New synchrotron sources such as ondulators w i l l d e l i v e r 10 to100 times more photons than the f i r s t synchrotron sources. These very i n t e n s e beams w i l l o f f e r t h e p o s s i b i - l i t y o f X-ray imaging microanalysis w i t h very s h o r t exposures. One must chose between two complementary t o o l s : X-ray absorption, and fluorescence. According t o J . KIRZ C51, X-ray absorption

i s

b e t t e r i n t h e s o f t X-ray r e q i o n w h i l e fluorescence i s more s e n s i t i v e i n t h e 1

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^{3 A} ^region.

X-ray absorption w i t h c o n t a c t imaging o f f e r s h i g h q u a l i t y images ; on t h e o t h e r hand fluorescence a n a l y s i s can be more s e n s i t i v e 161, b u t image r e c o n s t r u c t i o n needs a scanning microscope. A t present, t h e r e i s no a v a i l a b l e microscope o f t h i s k i n d w i t h a few vm r e s o l u t i o n ( a p a r t from a pionneer work o f HOROWITZ and HOWELL {73), A l l t h e p r o j e c t s o f scanning X-ray microscopes i n c l u d e an X-ray focussing o p t i c s , and t h e r e f o r e w i l l be l i m i t e d t o t h e s o f t X-ray r e g i o n . I n t h e hard X-ray domain, c o n t a c t imaging remains t h e s i m p l e s t method.

We have shown t h a t q u a n t i t a t i v e a n a l y t i c a l images can be obtained w i t h a photogrtphic d e t e c t i o n . We t h i n k t h a t t h e next years w i l l b r i n g i m p o r t a n t improvements o f X-ray images d e t e c t o r s and t h e r e f o r e X-ray a b s o r p t i o n m i c r o a n a l y s i s w i l l be more e f f i c i e n t . REFERENCES

{l } ENGSTROM A., Acta Radiol. Suppl

.

s(19.46)

{2} COSSLETT V.E. etNIXON W.C., X ray microscopy Cambridge U n i v e r s i t y Press, Cambridge England (1960).

{ 3 } POLACK F. e t a l . Appl. Phys. L e t t . 31 (1977) 785. -

( 4 ) POLACK F. This conference

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C2-86 JOURNAL DE PHYSIQUE

(5) KIRZ J. ,Ann. N.Y. Acad. Sci. =,(1980) 273.

16) KNOCHEL 4. e t al., Nucl. r n s t r . Methods,

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(1983) 659.

(7) HOROWrTZ P. and HOWELL J.A., Science 178, (1972) 608-611.