0 45.5 58 67.8 Distance from the source (m) ⊗ 27-m Undulator Double-crystal monochromator Ultrahigh resolution monochromator 4-jaw slit Avalanche photo diodes 1 & 2 Coincidence circuit X-ray Intensity Interferometry using ∆ ∆ E = 120 μ μ eV Monochromatized Beam Intensity interferometry developed by Hanbury- B r o w n a n d T w i s s [ 1 ] i s a p o w e r f u l m e t h o d t o investigate the statistical properties of light (higher- order coherence). In particular, when it is applied to chaotic light, the spatial and temporal coherence (first-order coherence) can be determined with very fa st t im e re so lu ti on , le ss t ha n ns . Th es e ad va nt ag es h a v e p r o m o t e d i n t e r e s t i n g a p p l i c a t i o n s i n v a r i o u s f i e l d s i n c l u d i n g a s t r o n o m y , q u a n t u m o p t i c s , l a s e r p h y s i c s , a n d n u c l e a r p h y s i c s . N o w a d a y s , application of the method to the X-ray region is of g r e a t i m p o r t a n c e b o t h f o r d i a g n o s i n g m o d e r n synchrotron light sources and for utilizing coherent X-rays. We report on our recent development of X- r a y i n t e n s i t y i n t e r f e r o m e t r y a n d i t s a p p l i c a t i o n t o characterizing the spatial coherence of synchrotron radiation [2]. We briefly present the principle. If one took a beam image insta ntane ously for chaot ic light , one would observe a number of bright and dark spots in a s p e c k l e p a t t e r n . S u c h i n t e n s i t y d i s t r i b u t i o n results from the interference of light, and the spatial c o h e r e n c e l e n g t h c a n b e d e t e r m i n e d s i m p l y b y m e a s u r i n g t h e s p o t p r o f i l e s . B e c a u s e t h e a c t u a l profile varies quite rapidly (with a time scale of the t e m p o r a l c o h e r e n c e t i m e ) , t h e c o i n c i d e n c e t e c h n i q u e i s u s e f u l f o r f a s t d e t e c t i o n . H e r e t h e interference is simply observed as an enhancement o f t h e c o i n c i d e n c e r a t e . W h e n t h e t e c h n i q u e i s a p p l i e d t o p u l s e d l i g h t s u c h a s s y n c h r o t r o n r a d i a t i o n , o n e c a n g r e a t l y i m p r o v e t h e d e t e c t i n g t i m e r e s o l u t i o n a s s h o r t a s t h e i n c o m i n g p u l s e width, which is 10 to 100 ps in our case [3]. N e v e r t h e l e s s , t h e e x t e n s i o n o f i n t e n s i t y i n t e r f e r o m e t r y t o t h e X - r a y r e g i o n [ 4 ] h a s b e e n di ff ic ul t. Th e pr im ar y re as on is th at th e te mp or al coh ere nce tim e, whi ch is inv ers ely pro por tio nal to the energy bandwidth ∆ E , is much shorter than the incident pulse width. This has obstructed the clear de te ct io n of th e en ha nc em en t of th e co in ci de nc e. Fi g. 1. Sc he ma ti c vi ew of th e ex pe ri me nt al se tu p. Un du la to r ra di at io n was pre-monochromatized with the Si 111 double-crystal monochromator. The ultrahigh resolution monochromator was installed in the experimental hutch 1. The coincidence signals between outputs of two detectors were counted with changing aperture of the 4-jaw slit. 91 Makina Yabashi SPring-8 / JASRI E-mail: yabashi @ spring8.or.jp In addition, the brightness of the X-ray source was too low to obtain a good signal to noise ratio within a reasonable measurement time. R e c e n t l y , w e a c h i e v e d a s i g n i f i c a n t i m p r o v e m e n t o f e n e r g y r e s o l u t i o n b y d e v e l o p i n g a n u l t r a h i g h r e s o l u t i o n m o n o c h r o m a t o r : a t 1 4 . 4 1 k e V , 1 2 0 - μ e V b a n d w i d t h w a s r e a l i z e d u s i n g f o u r - b o u n c e d a s y m m e t r i c r e f l e c t i o n s [ 5 ] . T h e t e m p o r a l coherence time is expected to reach ~ 40 % o f t h e i n c i d e n t p u l s e w i d t h . W i t h t h e m o n o c h r o m a t o r t h e e x p e r i m e n t w a s pe rf or me d at be am li ne BL 19 LX U fo r th e 27 - m undulator [6], which is the brightest X-ray source. The setup is schematically shown in Fig. 1 . The coincidence counts between two d e t e c t o r s w e r e m e a s u r e d a s a f u n c t i o n o f v e r t i c a l s l i t w i d t h , a s p l o t t e d i n F i g . 2 . T h e s p a t i a l c o h e r e n c e l e n g t h w a s c o n s e q u e n t l y determined to be 72.6 μ m. A vertical source size of 12.8 μ m was obtained from the value w i t h v a n C i t t e r t - Z e r n i k e ’ s t h e o r e m . T h e s o u r c e s i z e a l m o s t a g r e e d w i t h t h a t i n d e p e n d e n t l y m e a s u r e d b y t h e a c c e l e r a t o r g r o u p . F u r t h e r m o r e , w e c h a r a c t e r i z e d t h e c o h e r e n c e d e g r a d a t i o n i n t h e t r a n s m i t t e d b e a m t h r o u g h a f i l t e r a n d i n t h e d i f f r a c t e d beam with diamond crystals [2]. T o s u m m a r i z e , t h e c o m b i n a t i o n o f t h e narrowest bandwidth monochromator with the b r i g h t e s t X - r a y s o u r c e p r o v e d t h a t X - r a y intensity interferometry can be applied to the ta sk of de te rm in in g X- ra y sp at ia l co he re nc e p r o p e r t i e s . T h i s o p e n s u p n e w a n d b r o a d op po rt un it ie s to ch ar ac te ri ze be am qu al it ie s n o t o n l y o f t h e t h i r d - g e n e r a t i o n s y n c h r o t r o n sources but of the next generation ones. Normalized coincidence Vertical slit width ( μ μ m ) 1.3 1.2 1.1 1.0 0 100 200 300 400 500 Fig. 2. Normalized coincidence as a function of the vertical slit width. The line shows a fit based on a Gaussian source profile. References [ 1 ] R . H a n b u r y - B r o w n a n d R . Q . T w i s s , N a t u r e (London) 177 (1956) 27. [ 2 ] M . Y a b a s h i , K . T a m a s a k u a n d T . I s h i k a w a , Phys. Rev. Lett. 87 (2001) 140801. [3] E. Ikonen, Phys. Rev. Lett. 68 (1992) 2759. [ 4 ] Y . K u n i m u n e e t a l . , J . S y n c h r o t r o n R a d . 4 (1997) 199; E. Gluskin et al. , ibid. 6 (1999) 1065. [ 5 ] M . Y a b a s h i , K . T a m a s a k u , S . K i k u t a a n d T . Ishikawa, Rev. Sci. Instrum. 72 (2001) 4080. [6] H. Kitamura et al. , Nucl. Instrum. Meth. Phys. Res. Sect. A 467-468 (2001) 110; M. Yabashi et al , ibid. 467-468 (2001) 678; T. Hara et al. , Rev. Sci. Instrum. 73 (2002) 1125. 92
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