HIGH RESOLUTION EXAFS-XANES SPECTROMETER

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HIGH RESOLUTION EXAFS-XANES SPECTROMETER

A. Balerna, R. Buschert, M. Giardina, D. Inzaghi, S. Mobilio

To cite this version:

A. Balerna, R. Buschert, M. Giardina, D. Inzaghi, S. Mobilio. HIGH RESOLUTION EXAFS- XANES SPECTROMETER. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-117-C8-120.

�10.1051/jphyscol:1986821�. �jpa-00226121�

Colloque C8, suppl6ment au n o 12, Tome 47, dbcembre 1986

HIGH RESOLUTION EXAFS-XANES SPECTROMETER

A. BALERNA, R. BUSCHERT*, M.D. GIARDINA*", D. INZAGHI** and S. MOBILIO

INFN, Laboratori Nazionali di Frascati, C.P. 1 3 , I-00044 Frascati, Italy

* ~urner Laboratory, Goshen College, U. S. A.

* * Physics Division, JRC Ispra, I-21020 Ispra (Varese), Italy

Introduction

A double crystal (n,+n asymmetric) spectrometer with high resolution (0.1-0.2 ev) for analysing near edge XANES and EXAFS spectra is described. The spectra obtained on this instrument using a rotating anode source compare favorably with those obtained at a synchrotron.

Discussion and results

This spectrometer is a double crystal in the (n,+n) dispersive mode using two asymmetric reflections of Silicon (111).The standard way to obtain high energy resolution is with double crystals in the (n,+n) dispersive mode. The primary controlling factor in energy resolution is the diffraction line widths (Darwin) of the exit beam from the first crystal and the incident acceptance width of the second crystal.This can be understood by reference to the Dumond diagrams shown in Fig.1, 2. The wave length (and hence energy) resolution is given by the size of the overlap or window region a-b-c-d- shown in the figure for a particular setting of the two crystals. Rotating one of the crystals is equivalent to sliding its curve (band or window) horizontally and in this way the transmitted band moves up and down the (energy) scale. It is obvious that to obtain better energy resolution the a-b-c-d- window must be reduced. This can be accomplished by using higher order reflections to obtain a narrower line width but at the cost of some intensity. For this reason it was decided to use asymmetric reflections of (111) silicon as shown in Fig. 3.

In this arrangement the narrow (Darwin width) asymmetric exit beam of the first crystal feeds into the narrow incident asymmetric reflection of the second crystal. This reduces the overlap region a-b-c-d- as shown in Fig.2. In addition the wider acceptance angle of the first crystal means that more energy is channelled into the system (input energy to window Fig. 2).

This has the advantage of theoretically increasing both the Intensity and resolution over symmetric reflections by a factor of 5 to 10 times, depending on the degree of asymmetry. A measured increase in intensity of 7 times was obtained.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986821

JOURNAL DE PHYSIQUE

FIG.1

first-crystal exit

-angular-width first crystal

e

schematic

0

(b) asymmetric

snowing only central part of the Dumond diagram

FIG.2

The mechanical arrangement of the spectrometer involves elastic torsion bearings with torsion couplings. To provide 0 , 2 3, ^{3 9} , 4 9 motions for the crystals, sample and counters with only one stepping motor.

The automatization of our system and the data acquisition is obtained by a computer.

A

acceptance width

acceptance width I st crystal

--- -

9

I I

I ^I I

I

1

. .

0 1102E 05 K-edge of G e in amorphous

~bsorotton coenioisnt nmes thlc*nsss w pholon-energy 0.1 7726.05

FXGG 03SB3E 01 AMP K 3 DK 2 5-1 2 8JG

iLIL t

0 OOOOE W 0600OE 01

0 WWE 00 Fourtsr franotormatlon of the EXAFS of Ge # n amornhous slate

PHOTON ENERGY E(ev) X Ray abSOrDtlon K-edges of Fe oxides

i

-.?75OE-01 0 17071.05

FIG.4

0 ~ 7 3 9 ~ - 0 5 u ~ 4g(absowtaon e coenlclenf time th~ckners v r photon BnergYl

The s p e c t r o m e t e r f u n c t i o n s r e a s o n a b l y w e l l i n t h e l a b o r a t o r y with a

r o t a t i n g anode s o u r c e . However, even with t h e seven f o l d i n c r e a s e i n

i n t e n s i t y due t o t h e asymmetric c r y s t a l arrangement, t h e counting

times a r e long. I t t a k e s 2 1 / 2 days f o r an EXAFS spectrum and one day

f o r an XANES spectrum. A synchrotron s o u r c e would g i v e f a s t e r and

b e t t e r d a t a . This would r e q u i r e t h e a d d i t i o n o f counter weights f o r

o p e r a t i o n of t h e s p e c t r o m e t e r i n t h e v e r t i c a l p l a n e a t t h e

synchrotron.

JOURNAL DE PHYSIQUE

The q u e s t i o n o f t h e a c t u a l s p e c t r o m e t e r r e s o l u t i o n compared t o t h e c a l c u l a t e d i s d i f f i c u l t t o answer. Two methods, one i n v o l v i n g a Mossbauer s o u r c e and t h e o t h e r i n v o l v i n g d o u b l e c r y s t a l r e f l e c t i o n a r e under c o n s i d e r a t i o n t o a c t u a l l y measure t h e s p e c t r o m e t e r r e s o l u t i o n e x p e r i m e n t a l l y .

References

1. R.W. James, The O p t i c a l P r i n c i p l e s o f t h e D i f f r a c t i o n o f X-Rcys, London, G. B e l l and Sons Ltd ( 1 9 6 2 )

2. J . W . M . Dumond : Phys. Rev. 52 ( 1 9 3 7 ) - 872

3 . J . A . Bearden and J . S . Thomsen : J . Appl. C r y s t . 4 , 130-138 ( 1 9 7 1 )

4 . S. K i k u t a and K . Kohra, J . o f t h e Phys. Soc. J a p . Vol. 29.