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HAL Id: jpa-00223977

https://hal.archives-ouvertes.fr/jpa-00223977

Submitted on 1 Jan 1984

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X-RAY FLUORESCENCE ANALYSIS WITH MONOCHROMATIC X-RAYS

M. Kotera, D. Wittry

To cite this version:

M. Kotera, D. Wittry. X-RAY FLUORESCENCE ANALYSIS WITH MONOCHROMATIC X-RAYS.

Journal de Physique Colloques, 1984, 45 (C2), pp.C2-281-C2-284. �10.1051/jphyscol:1984263�. �jpa- 00223977�

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JOURNAL D E PHYSIQUE

Colloque C2, suppl6ment au n02, Tome 45, f6vrier 1984 page C2-281

X-RAY FLUORESCENCE A N A L Y S I S WITH MONOCHROMATIC X-RAYS

M. Kotera and D.B. Wittry

Departments of MateriaZs Science and EZectricaZ Engineering, University of Southern California, Los Angeles, CA 90089-0241, U.S.A.

Rdsumd - On calcule l'intensitd du rayonnement de fluorescence de cibles dldmentaires Gpaisses et d'dldments pr6sents en traces dans des cibles minces. Le calcul est effectud pour tmois rayonnements primaires excita- teurs.

Abstract - The intensities of fluorescence x-rays from thick elemental targets and from trace elements in three thin targets are calculated for three monochromatic primary x-rays.

X-ray fluorescence analysis has many advantages compared to electron probe micro- analysis. For example, the efficiency of excitation of characteristic x-rays by photons is greater than the efficiency for excitation by electrons for energies less than 20 keV. Also, charging of insulating specimens is not a critical problem, and x-rays produce less damage in radiation-sensitive materials. Furthermore in x-ray fluorescence analysis the background is mainly due to scattered primary x-rays so that a significant improvement in signal to background ratio can be obtained by excitation with monochromatic x-rays. Finally, the use of monochromatic x-rays greatly simplifies the calculations for quantitation. A variety of methods have been used to obtain monochromatic x-rays including secondary fluorescorsl, electron bombardment of thin targets using the transmitted X-rays2,3 and crystal diffract or^.^

In this paper we give the results of calculations of the efficiency of fluorescence excitation by monochromatic radiation.

Basically fluorescence x-ray intensity is expressed by eq. (1).

r-l Xi

If = I;W~X~ .-[l-exp{-(~~+~~)pt}l -p Xi + Xf

where 0i is the angle of primary x-ray incidence, 0f is the take-off angle of the fluorescence x-ray, wi is the fluorescence yield, r is the absor~tion jump ratio, Xf is the fraction of radiation emitted in a given line of a series, p is the density and t is the thickness of the specimen, ( u / P ) ~ is the mass absorption coefficient of the specimen for primary x-ray, and ( ~ / p ) ~ is the mass absor~tion coefficient of the specimen for fluorescence x-ray. This equation takes account of absorption of the primary x-ray in the specimeb, excitation of the fluorescence x-ray of an element, and absorption of the fluorescence x-ray emitted at a certain take-off angle from the surface.

Figure 1 shows the excitation efficiency If/Io and Oi = Of for thick elemental tar- gets. The right set and the left set of the three solid lines show the cases for L line and K line excitation, by MoK,, C r G , and MoL, primary x-rays. Points in the figure show calculated results and solid lines are drawn through the points. Because there is a fluctuation in Xf, the points scatter around the lines. The broken line shows a theoretical maximum efficiency, that is, when wave lengths of a primary x-ray and a fluorescence x-ray are just on the shorter and the longer side of an absorption edge of the element. In this case the fluorescence x-ray is excited most efficiently and undergoes less absorption in the specimen. Each solid line

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984263

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C2-282 JOURNAL DE PHYSIQUE

approaches the broken line at the threshold atomic number. If Z decreases from that of the threshold case, the wave length of the primary x-ray is smaller than the wave length at the absorption edge and the solid line drops below the broken line.

For L lines, because of the presence of LI, L11 and LIII edges, solid lines at high Z show jumps.

PURE ELEMENT

1 . E 0 t I

.

I I I 4

ATOMIC NUMBER

Fig. 1 - X-ray fluorescence excitation efficiency for elemental targets with MO%, CrK,, and MoL, primary x-rays.

If the specimen is a compound, eq. (2) is applied with the following form:

If = CA(u/pIA cosec Bi wi

.

Xf ' - r-l I,

-

- ~l-exp{-(~~+x~)~til Xi + Xf

where,

C is the weight fraction of a tracer element, ( u / ~ ) ~ A is the mass absorption coefficient of the element for primary x-rays. When the concentration of a trace element is negligible, xi and xf are calculated only for the matrix. Setting C A = 1 for simplicity, the relative fluorescence x-ray intensity is obtained. If ( u / P ) ~ cosec 81 > ~ i . + ~Xf.matrix, the relative intensity may be larger than 1. ~ ~ ~ i ~ Figure 2 shows an example of the relative intensity of trace elements in a Si specimen 250 pm thick and Bi = Bf = 9 0 ' . The primary x-rays are MoK,, CrK,, and MoL, lines. There is a jump for each of the and L, lines due to the absorption edge of Si.

GaAs is also an important semiconductor and impurities in this material at low concentrations are of interest. Because both Ga and As are heavier than Si, the jump in the relative Intensity appears at higher Z and there are two jumps corres- ponding to the K edges of Ga and As as shown in Fig. 3.

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- - - .

0 20 40 60 80 100

ATOMIC NUMBER

Fig. 2 - The relative intensities of fluorescence x-rays for tracer elements in Si matrix. The same primary x-rays are used as in Fig. 1.

Fig. 3 - The relative intensities of fluorescence x-rays for tracer elements in GaAs matrix. The same primary x-rays are used as in Fig. 1

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C2-284 JOURNAL DE PHYSIQUE

I n t h e same manner a s f o r F i g . 2 and F i g . 3 , c a l c u l a t i o n s were made f o r a 30 um t h i c k specimen of A l b i t e , NaAlSi308. I t i s w e l l known t h a t Na i o n s move e a s i l y i n s u c h specimens under e l e c t r o n beam i r r a d i a t i o n , b u t i n x-ray f l u o r e s c e n c e e x c i t a t i o n t h i s k i n d of damage s h o u l d b e n e g l i g i b l e and h i g h a c c u r a c y i n a n a l y s i s of t r a c e e l e m e n t s s h o u l d b e p o s s i b l e . I n A l b i t e , t h e weight p e r c e n t of S i i s 32% and a s l i g h t jump i s s e e n i n F i g . 4 a t t h e same Z a s i t was found i n F i g . 2. While t h e r e a r e o t h e r e l e m e n t s i n A l b i t e , t h e i r a t o m i c numbers a r e low o r t h e w e i g h t p e r c e n t i s low s o t h a t jumps due t o t h e i r a b s o r p t i o n e d g e s a r e n o t a p p a r e n t .

ATOMIC NUMBER

F i g . 4 - The r e l a t i v e i n t e n s i t i e s of f l u o r e s c e n c e x-rays f o r t r a c e r e l e m e n t s i n A l b i t e m a t r i x . The same primary x-rays a r e used a s i n F i g . 1.

I n summary, i t c a n b e s a i d t h a t i t i s p o s s i b l e t o c o v e r a l l e l e m e n t s i n t h e p e r i o d i c t a b l e above Z = l l by u s i n g MoK,, C r Q , and MoL, r a d i a t i o n w i t h a maximum d i f f e r e n c e i n s e n s i t i v i t y by a f a c t o r o f 2.10-100. By u s i n g t h e c a l c u l a t i o n s g i v e n h e r e i t would b e p o s s i b l e t o q u a n t i t a t e t h e amount o f a t r a c e element i n S i , GaAs, and A l b i t e . However i t is a l s o n e c e s s a r y t o t a k e i n t o a c c o u n t t h e i n f l u e n c e of s e c o n d a r y f l u o r e s c e n c e caused by r a d i a t i o n produced i n t h e m a t r i x and t o v e r i f y t h e p r e s e n t c a l c u l a t i o n s w i t h e x p e r i m e n t a l r e s u l t s .

R e f e r e n c e s

(1) WOIDSETH, R., X-ray Energy S p e c t r o m e t r y , Kevex Corp. F o s t e r C i t y , CA (1973)257.

(2) ZULLIGER, H.R. and STEWART, W.D., American L a b o r a t o r y , A p r i l (1977).

(3) RIESSEN, A. VAN and TERRY, K.W., JEOL News, 20E 3 (1982) 19

(4) DESPUJOLS, J . , ROULET, H . and SENEMAUD, G . , X-ray O p t i c s and X-ray Micro- a n a l y s i s , P a t t e e , H. H . , C o s s l e t t , V . E., Engstrum, A., Eds., Academic P r e s s , NY (1963) 445.

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