AN X-RAY MONOCHROMATOR SUITABLE FOR STUDYING BIOLOGICAL SYSTEMS

HAL Id: jpa-00226214

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Submitted on 1 Jan 1986

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AN X-RAY MONOCHROMATOR SUITABLE FOR STUDYING BIOLOGICAL SYSTEMS

R. Pettifer, C. Hermes

To cite this version:

R. Pettifer, C. Hermes. AN X-RAY MONOCHROMATOR SUITABLE FOR STUDYING BIOLOGICAL SYSTEMS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-127-C8-133.

�10.1051/jphyscol:1986823�. �jpa-00226214�

AN X-RAY MONOCHROMATOR SUITABLE FOR STUDYING BIOLOGICAL SYSTEMS

R.F. PETTIFER" a n d ^C. HERMES

EMBL, c/o DESY, Notkestrasse 85, 0-2000 Hamburg 52, F.R.G.

"and Department of Physics, University of Warwick, GB-Coventry, CV4 7 A L , Great-Britain

Abstract

An X-ray monochromator which is now i n r o u t i n e u s e a t t h e El-1BL O u t s t a t i o n i n Hamburg i s d e s c r i b e d . The d e v i c e u s e s two s e p a r a t e (220) s i l i c o n c r y s t a l s , t h e f i r s t of which i s t h e r m a l l y i s o l a t e d from t h e body of t h e m o n o c h r o n a t o r . The r e l a t i v e a n g u l a r s e t t i n g of t h e two c r y s t a l s i s a d j u s t e d by a s m a l l p i e z o e l e c t r i c s t a c k mounted on a s t i f f b e r y l l i u m - c o p p e r s p r i n g . Rapid s e t t l i n g (120 msecs) r n i n i m i s e s t h e t i m e l o s t f o r t h e monochromator t o s t a b i l i s e a n d h e n c e X-ray beam damage on t h e specimen. The r e l a t i v e a n g u l a r s e t t i n g o f t h e two c r y s t a l s i s c o n t r o l l e d v i a s i g n a l s from a p h o t o - e m i s s i o n d e v i c e a n d t h e f i r s t i o n i s a t i o n chamber. These s i g n a l s a r e combined i n a s e c o n d o r d e r c o n t r o l s y s t e m .

By d r i v i n g t h e monochromator p i e z o e l e c t r i c w i t h a p e r i o d i c ramping s i g n a l , t h e p e r f o r m a n c e a n d a l i g n m e n t of t h e d e v i c e c a n b e checked.

The e f f e c t s o f a t e m p e r a t u r e d i f f e r e n c e between t h e c r y s t a l s c a u s e d by beam h e a t i n g i s a l s o d i s c u s s e d .

I t h a s become a p p a r e n t t o us o v e r t h e l a s t few y e a r s t h a t s p e c t r a l p u r i t y from a n X-ray monochromator i s of paramount i m p o r t a n c e t o a c h i e v e h i g h q u a l i t y X-ray a b s o r p t i o n s p e c t r a . T h i s i s p a r t i c u l a r l y t r u e when s t u d y i n g d i l u t e s y s t e m s s u c h a s b i o l o g i c a l m o l e c u l e s . The EPlBL EXAFS s t a t i o n i n I-IASYLAB [11 was o r i g i n a l l y e q u i p p e d w i t h a c h a n n e l c u t monochromator-mirror system. The p o s t d i f f r a c t i o n m i r r o r s e r v e d , n o t o n l y t o f o c u s t h e monochromatic beam, b u t a l s o t o remove unwanted t h i r d harmonic c o n t a m i n a t i o n from t h e S i ( l l 1 ) c h a n n e l c u t monochromator. T h i s c o m b i n a t i o n , however, p r o v e d t o be i n a d e q u a t e . R a d i a t i o n e m a n a t i n g from Bragg p l a n e s n o t p a r a l l e l t o t h e c r y s t a l s u r f a c e i s e x c i t e d by t h e h a r d X-ray f l u x p r o d u c e d by DORIS 11, p a r t i c u l a r l y i n p a r a s i t i c mode. T h i s c a u s e s a s m a l l amount of c o n t a m i n a t i n g r a d i a t i o n t o be r e f l e c t e d a b o v e t h e c u t - o f f w a v e l e n g t h of t h e m i r r o r . Although n o t p r o d u c i n g s e r i o u s d i s t o r t i o n , t h i s unwanted c o n t a m i n a t i o n was l i a b l e t o f l u c t u a t e owing t o t h e e x t r e m e l y n a r r o w v e r t i c a l f a n o f r a d i a t i o n p r o d u c e d by t h e s t o r a g e r i n g a t h i g h e n e r g i e s . T h i s a d d e d t o t h e n o i s e a n d was p a r t i c u l a r l y t r o u b l e s o m e when u s i n g t h e e q u i p m e n t i n t h e f l u o r e s c e n c e mode.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986823

C8-128 JOURNAL DE PHYSIQUE

DORIS I1 c a n o p e r a t e a t e n e r g i e s a s h i g h as 5.3 GeV and w i t h l i f e t i m e s a s s h o r t a s one hour. I n a d d i t i o n , o u r b i o l o g i c a l s p e c i m e n s f r e q u e n t l y o n l y s u r v i v e e x p o s u r e f o r one hour a n d a r e d i f f i c u l t a n d c o s t l y t o p r o d u c e . I t i s t h u s of i m p o r t a n c e t o d e s i g n .a monochromator which i n c r e a s e s t h e y i e l d of u s e f u l s p e c t r a and c a n c o p e w i t h t h e h i g h power d e n s i t i e s a n d s h o r t l i f e t i m e of t h e s t o r a g e r i n g . Or7ing t o t h e s h a r e d n a t u r e of o u r b e a m l i n e , t h e monochromator h a s t o o p e r a t e u n d e r h i g h vacuum c o n d i t i o n s a n d t o be rugged and t r o u b l e f r e e .

Our monochromator u s e s t h e w a v e l e n g t h dependence of t h e r e f r a c t i v e i n d e x t o remove harmonic c o n t a m i n a t i o n a s o r i g i n a l l y s h o i n i n r e f e r e n c e [21. T h i s b a s i c i d e a h a s been implemented i n v a r i o u s d e s i g n s t y p i f i e d by a weak l i n k s i l i c o n s p r i n g s y s t e m [ 3 1 , 141 o r a s e p a r a t e f a c e monochromator 151, 161, 171. Our d e s i g n combines some of t h e a t t r a c t i v e f e a t u r e s of a m o n o l i t h , e.g. freedom from microphony e f f e c t s , r a p i d s e t t l i n g and r u g g e d n e s s , w i t h t h e c a p a b i l i t y of harmonic and s i d e beam s u p r e s s i o n . I n t h i s p a p e r we p r e s e n t o u r d e s i g n , which h a s been i n t r o u b l e - f r e e u s e f o r one y e a r , t o g e t h e r w i t h o u r s o l u t i o n t o p r o b l e m s s u c h a s beam h e a t i n g . T h e s e p r o b l e m s w i l l become of i n c r e a s i n g i m p o r t a n c e w i t h t h e n e x t g e n e r a t i o n o f s y n c h r o t r o n s o u r c e s .

F i g . 1 shows a d r a w i n g of t h e monochromator whose c e n t r a l f e a t u r e i s t h e s t i f f b e r y l l i u m - c o p p e r s p r i n g ( a ) which i s f l e x e d by a p i e z o - e l e c t r i c s t a c k ( p ) t o f i n e l y a d j u s t . t h e a n g l e s between t h e s e p a r a t e c r y s t a l s . The s l o t which r e t a i n s t h e p i e z o s t a c k i s m a n u f a c t u r e d s l i g h t l y s m a l l e r t h a n t h e s t a c k which i s mounted u n d e r permanent c o m p r e s s i o n . The r a n g e o f a n g l e s s p a n s approx. 50 a r c s e c . C o u r s e a n g u l a r a d j u s t m e n t of t h e f i r s t s i l i c o n c r y s t a l i s a c h i e v e d w i t h s t e p p e r m o t o r s b l l a n d M2. These m o t o r s r o t a t e f i n e t h r e a d s c r e w s a n d a d j u s t t h e h e i g h t o f b a l l b e a r i n g s which form t h e f e e t of a k i n e m a t i c mount f o r t h e f i r s t c r y s t a l s u p p o r t i n g t r a y . The f i r s t c r y s t a l t r a y i s t h e r m a l l y i s o l a t e d . f r o m t h e body of t h e d e v i c e e x c e p t t h r o u g h c o n t a c t v i a t h e k i n e m a t i c mount. M 1 and ~2 a r e u s e d d u r i n g i n i t i a l a l i g n m e n t o n l y a n d a d j u s t t h e Eragg a n g l e a n d h o r i z o n t a l t i l t a n g l e t o g u i d e t h e d i f f r a c t e d b e a n c e n t r a l l y t h r o u g h t h e b e a m l i n e .

S i ( 2 2 0 ) c r y s t a l s a r e c u r r e n t l y i n u s e a l t h o u g h a n y c u t may be used.

The c r y s t a l s a r e m a n u f a c t u r e d w i t h s t r a i n r e l i e f g r o o v e s a n d mounted u s i n g l o n g b e r y l l i u m - c o p p e r s t r i p s w i t h 30 pm mylar s h e e t s between t h e c r y s t a l s a n d r e t a i n i n g s p r i n g s . The f i r s t c r y s t a l r e s t s o n a n Aluminium t r a y (shown i n n o r e d e t a i l i n v i e w s B and C of F i g . 1) a n d t h e s e c o n d c r y s t a l i s a t t a c h e d i n a s i m i l a r manner t o t h e body of t h e monochromator. The t r a y i s e q u i p p e d w i t h e l e c t r i c a l h e a t i n g f o i l s a n d embedded r e s i s t a n c e t h e r m o m e t e r s . Both a r e u s e d t o m a i n t a i n t h e f i r s t c r y s t a l a t a c o n s t a n t t e m p e r a t u r e of 1 4 2 ^OC. T h i s i s t h e l o w e s t t e m p e r a t u r e a t which we c a n c o n t r o l t h e e f f e c t s of beam h e a t i n g .

D e s p i t e t h e t h e r m a l i s o l a t i o n of t h e f i r s t c r y s t a l , t h e main body of t h e d e v i c e g e t s h o t a n d t h i s i s l i m i t e d by a c o p p e r h e a t c o n d u c t i o n b r a i d ( i n F i g . 1 ) . The e n t i r e s y s t e n i s mounted o n a r o t a t i o n t a b l e w i t h a b r a c k e t a t t a c h e d t o t h e s e c o n d c r y s t a l s i d e of t h e m6nochromator body. The s h a f t of t h e b r a c k e t i s s u p p o r t e d by a c o l l a r which r e t a i n s vacuum g r e a s e t o damp o u t microphony t r a n s m i t t e d t h r o u g h t h e s u p p o r t s from t h e f l o o r .

( a ) , t i l t motor ( ~ 1 2 1 , c o a r s e € ) - o f f s e t a b j u s t ~ e n t 1 , p i e z o e l e c t r i c t r a n s d u c e r ( p ) , and h e a t - s i n k b r a i d (h). An expanded view of t h e c r y s t a l s a n d t r a y s u p p o r t i s shown i n ( B ) , and t h e u n d e r s i d e of t h e t r a y i s i l l u s t r a t e d i n view (C), which a l s o shows t h e a t t a c h e d h e a t i n g f o i l s t o m a i n t a i n a c o n s t a n t t e m p e r a t u r e of t h e f i r s t c r y s t a l and t h e k i n e m a t i c mounting p o i n t s f o r t h e t r a y . (Dl i s a c r o s s - s e c t i o n v i e w o f t h e d e v i c e .

F i g . 2 g i v e s a s c h e m a t i c d e s i g n o f t h e c o n t r o l s y s t e m . S i g n a l s a r e d e r i v e d f r o n a pkiotoemission d e v i c e ( s c r e e n e d A l - f o i l ) s i t u a t e d i n t h e w h i t e beam b e h i n d t h e f i r s t beam d e f i n i n g s l i t s , anci t h e f i r s t i o n i s a t i o n chamber. Compensation i s t h u s made f o r t h e p r i n c i p a l t i m e v a r i a b l e , namely t h e d e c a y i n g , ( a n d f l u c t u a t i n g ) X-ray i n t e n s i t y from t h e s t o r a g e r i n g . The s i g n a l s a r e f e d v i a c u r r e n t t o v o l t a g e b u f f e r a n p l i f i e r s ( R e i t h l e y 4 2 7 ) t o a c t i v e low p a s s B e s s e l f i l t e r s ( l i n e a r p h a s e r e s p o n s e , a p p r o x . 10 I!z b a n d w i d t h ) . The l a t t e r a r e n e c e s s a r y t o l i m i t e l e c t r i c a l i n t e r f e r e n c e . The s i g n a l s a r e combined i n a d i v i d e r t o y i e l d a n o u t p u t a t (A), which i s n o r m a l i s e d t o 1 V. T h i s i s c o a p a r e d w i t h a r e f e r e n c e v o l t a g e , c u r r e n t l y a p o t e n t i o m e t e r ( B ) , t o p r o v i d e t h e b a s i c d i f f e r e n c e c o n t r o l s i g n a l . A l o o p g a i n s t a g e i s n e c e s s a r y t o compensate f o r t h e v a r i a t i o n o f r o c k i n g c u r v e s l o p e w i t h w a v e l e n g t h . F o l l o w i n g t h e l o o p g a i n i s a s t a n d a r d s e c o n d o r d e r c o n t r o l s y s t e m which f e e d s s i g n a l s back v i a t h e N.T. a m p l i f i e r t o t h e p i e z o - e l e c t r i c f i n e t u n i n g s t a c k .

The i n i t i a l a l i g n m e n t p r o c e d u r e c o n s i s t s of o p e n i n g a l l s l i t s of t h e s y s t e m a p a r t from t h a t which c o n t r o l t h e v e r t i c a l beam i n c i d e n t o n t o t h e monochromator. i l l is a d j u s t e d u n t i l a r e f l e c t i o n i s o b s e r v e d from

C8-130 JOURNAL DE PHYSIQUE

t h e monochromator w i t h t h e C.T. a n p l i f i e r s e t a t mid-range of 400 V.

C l e a r l y it i s n o t p o s s i b l e t o a l i g n t h e o p t i c s w h i l s t t h e c o n t r o l s y s t e m f u n c t i o n s . IIowever, d r i f t c a n be accornnodated by f e e d i n g a ramping s i g n a l t o t h e f1.T. a m p l i f i e r which sweeps t h e monochromator t h r o u g h i t s r o c k i n g c u r v e . T h i s p u l s a t i n g s i g n a l may t h e n be u s e d f o r p o s t - d i f f r a c t i o n s l i t a n d d e t e c t o r a l i g n m e n t . A t y p l c a l r o c k i n g c u r v e o b s e r v e d i n t h e i n c i d e n t beam d e t e c t o r s i s shown i n F i g . 3 a , w h i l s t t h e r o c k i n g c u r v e o b s e r v e d i n t h e t r a n s m i t t e d beam d e t e c t o r a f t e r 2 0 A l - f o i l s of 50

mi

C l e a r l y , t h e d o t t e d l i n e i s p r i n c i p a l l y t h e s e c o n d o r d e r r o c k i n g c u r v e of t h e n o n o c h r o n a t o r . To w i t h i n e x p e r i m e n t a l e r r o r t h e w i d t h of t h e s e c u r v e s c o r r e s p o n d , t o c h o s e c a l c u l a t e d (

+

X RAY BEAM

XTAL

1 4s7

^pi-q

F i g . 2 S c h e m a t i c diagram of t h e c o n t r o l s y s t e m , w i t h p h o t o e n i s s i o n d e t e c t o r (P. E. , f i r s t ionchamber

( I o I C ) , c u r r e n t a m p l i f i e r s (K) and low p a s s C e s s e l f i l t e r s (PILT)

.

F i g . 3 Rocking c u r v e s of S i ( 2 2 0 ) rhonochronator measured w i t h f i r s t ( s o l i d l i n e ) a n d s e c o n d ( b r o k e n l i n e ) i o n c h a m b e r s r e s p e c t i v e l y . Puncianental X-ray e n e r g y 9 G G 1 eV.

a ) no a b s o r b e r

L ) 20 A l - f o i l s of 50 pn t h i c k n e s s a s a b s o r b e r .

Fundarlental X-ray e n e r g y 9661 e V . a ) no a b s o r b e r

b ) 20 A l - f o i l s o f 5 0 pm t h i c k n e s s a s a b s o r b e r .

I t was f o u n d t h a t p a i r s of r o c k i n g c u r v e s s u c h a s t h o s e of F i g . 3b a r e v e r y u s e f u l d i a g n o s t i c a i d s . U i t h m i s a l i g n e d s l i t s i t i s p o s s i b l e t o o b s e r v e p s e u d o - r o c k i n g c u r v e s w i t h t h e harmonic d i s p l a c e d from t h e peak of t h e f u n d a m e n t a l r o c k i n g c u r v e . Using measurements s u c h a s t h o s e of F i g . 3 a a n d F i g . 3b, we c a n form t h e r a t i o of t h e t r a n s m i t t e d t o i n c i d e n t i n t e n s i t y shown i n F i g . 4a and 4b. Even i n t h e c a s e of no a b s o r b e r f o i l s between t h e d e t e c t o r s , t h e h a r m o n i c s p r o d u c e a s i g n i f i c a n t n o n - l i n e a r i t y of r e s p o n s e . I t i s a l s o c l e a r t h a t a 508 d e t u n e of t h e i n c i d e n t i n t e n s i t y p r o d u c e s e s s e n t i a l l y a h a r m o n i c f r e e X-ray bean. The n o n - l i n e a r i t y i s f u r t h e r shown by f o r m i n g t h e a b s o r p t i o n t h i c k n e s s p r o d u c t f t f o r a s e r i e s of A l - f o i l s i n F i g . 5.

owing t o t h e f a c t t h a t t h e two c r y s t a l s a r e a t d i f f e r e n t t e m p e r a t u r e s , we h a v e a s l i g h t l y d i s p e r s i v e s y s t e m , i . e . t h e d - s p a c i c g s of t h e c r y s t a l s d i f f e r s l i g h t l y . T h i s c a u s e s a s i g n i f i c a n t v e r t i c a l s h i f t of t h e d i f f r a c t e d beam a t t h e f i r s t i o n i s a t i o n d e t e c t o r . Over t h e 20 m d i s t a n c e from t h e monochromator t o t h e f i r s t d e t e c t o r , t h i s amounts t o a d i s p l a c e m e n t of 2 mn, a s t h e f i r s t c r y s t a l r e a c h e s i t s e q u i l i b r i u m t e m p e r a t u r e . Hence, i t i s v i t a l t o c o n t r o l t h e t e m p e r a t u r e of t h e f i r s t c r y s t a l .

The e l e c t r o n i c feed-back s y s t e m i s a d j u s t e d by v a r y i n g t h e p o t e n t i a l a t X i n F i g . 2 u n t i l a 1 V o u t p u t i s o b s e r v e d a t ( A ) f o r t h e r o c k i n g c u r v e peak. The r e f e r e n c e s i g n a l (B) t h e n g i v e s t h e f r a c t i o n a l t u n i n g d i r e c t l y i n v o l t s . The i n t e g r a l , p r o p o r t i o n a l and d e r i v a t i v e s i g n a l s a r e f e d t o t h e summing a m p l i f i e r and a d j u s t e d t o g i v e minimum r e s p o n s e t i m e t o g e t h e r w i t h minimum d r i f t w i t h o u t o s c i l l a t i o n , A s e t t l i n g t i m e of 1 2 0 msecs c a n be o b t a i n e d when t h e monochromator i s r o t a t e d . The microphony r e c o r d e d , c o r r e s p o n d s t o a 0.05 a r c s e c mismatch between c r y s t a l s , due t o 50 I-Iz v i b r a t i o n s . w i t h t h i s d e v i c e t h e r e was a l s o a h u n d r e d f o l d r e d u c t i o n of background r a d i a t i o n compared w i t h t h e c h a n n e l c u t c r y s t a l , a s m e a s s u r e d w i t h o u r c a l i b r a t o r [ 8 1 . F u r t h e r t h e r e i s a n i n c r e a s e i n y i e l d of u s e f u l s p e c t r a a n d a c o n s i d e r a b l e improvement i n s i g n a l t o n o i s e r a t i o .

JOURNAL DE PHYSIQUE

HER ff FOILS

F i g . 5 A b s o r p t i o n t h i c k n e s s p r o d u c t p t f o r a s e r i e s o f 50 t h i c k A l - f o i l s . The s t r a i g h t l i n e c o r r e s p o n d s

<:

open r e c t a n g l e s r e f e r t o no d e t u n e , i . e . t h e p e a k s of F i g . 4 . Fundamental X-ray e n e r g y 9661 eV.

The monochromator h a s b e e n i n t r o u b l e f r e e o p e r a t i o n f o r one y e a r and h a s y i e l d e d h i g h q u a l i t y s p e c t r a , f r e e from s p u r i o u s r e f l e c t i o n s . On t h e o t h e r hand, improvements c a n s t i l l be made. I n p a r t i c u l a r , t h e way of mounting t h e c r y s t a l s s t i l l o n l y a l l o w s o n e h a l f of t h e h o r i z o n t a l f a n o f r a d i a t i o n t o be used. T h i s i s a problem of m e c h a n i c a l s t r a i n r a t h e r t h a n a h e a t i n g e f f e c t . F u r t h e r , m a t c h i n g t h e t e m p e r a t u r e s o f b o t h c r y s t a l s c o u l d e l i m i n a t e t h e beam d e f l e c t i o n s o b s e r v e d . F i n a l l y , we p l a n t o compensate f o r t h e w a v e l e n g t h dependence o f t h e i o n i s a t i o n chamber c u r r e n t by c o r r e c t i n g t h e r e f e r e n c e v o l t a g e i n F i g . 2 v i a computer c o n t r o l .

We w i s h t o t h a n k E. Robrahn a n d R. K l a e r i n g f o r e l e c t r o n i c a n d m e c h a n i c a l a s s i s t a n c e , and M.J.A. Smith f o r d i s c u s s i o n s c o n c e r n i n g c o n t r o l c i r c u i t s . One of u s (RFP) t h a n k s t h e U n i v e r s i t y of Warwick f o r l e a v e of a b s e n c e .

[21 BOMSE, U., IlATERLIR, G. a n d SCEIROEDER, IT., J. Appl.

c r y s t .

e

[31 IIPART, El. a n d RODRIGUES, A.R.D., J. Appl. C r y s t . (1978) 248.

[ 4 1 IIATERLIK, G. a n d I<OSTROUM, V.O., Rev. S c i . I n s t r .

a

(1980) DG.

[51 GOLO\7CHEEJKOI I . A .

,

S c i . I n s t r . a ( 4 ) (1981) 509.

[GI GOULOPJI J. LEI'IOETBIEBI El. CORTESI R. RETOUPJJARDI A.

a n d RAOUX, D., PJucl. I n s t r . I l e t h . 2M (1983) 625.

171 GREAVES, G.N., DIAICUII, G.P., QUIMHI P.D., HART, !I. and SIDDOEJS, D.P., Nucl. I n s t r . Meth. (1983) 335.

[81 P E T T I F E R , R.F. a n d I-ICRI,ES, C., J. Appl. C r y s t . JJi (1985) 404.