N u c l e a r r e s o n a n t s c a t t e r i n g ( N R S ) o f synchrotron radiation [1] provides several methods f o r i n v e s t i g a t i n g m a t e r i a l s . O n o n e h a n d , t i m e differential measurements allow access to nearly all of the hyperfine and materials science information o f c o n v e n t i o n a l M ö s s b a u e r e x p e r i m e n t s , w i t h a d d e d s t r u c t u r a l i n f o r m a t i o n p o s s i b l e d u e t o t h e u s e o f a s c a t t e r i n g g e o m e t r y a n d t h e e x t r e m e l y brilliant X-ray beam. On the other, nuclear inelastic scattering (NIS) uses the narrow nuclear resonance as a probe of the nuclear motions within a material, allowing access to phonon spectra on meV energy scales. Most NRS experiments provide information specifically about the excited resonant nuclei ( i.e . , hy pe rf in e fi el ds at th e re so na nt nu cl eu s, ph on on s t h a t i n c l u d e t h e r e s o n a n t n u c l e u s , s t r u c t u r a l a n d mo ti on al co rr el at io ns am on g th e re so na nt nu cl ei ). T h i s s p e c i f i c i t y a l l o w s o n e t o o b t a i n v e r y p r e c i s e i n f o r m a t i o n . I t a l s o p r o v i d e s c l e a r i m p e t u s f o r e x t e n d i n g t h e n u m b e r o f a v a i l a b l e t r a n s i t i o n s beyond the few that are now commonly used. NRS experiments with new resonant transitions a r e l a r g e l y l i m i t e d b y i n s t r u m e n t a t i o n . W h i l e Nuclear Resonant Scattering from 161 Dy at 25.65 keV str ong er syn chr otr on sou rce s alw ays pre sen t new o p p o r t u n i t i e s , i n f a c t p r e s e n t s o u r c e s a r e s t r o n g e n o u g h ( a t l e a s t f o r l o w t o m i d - r a n g e X - r a y energies) to provide rather high flux in the resonant b a n d w i d t h o f m a n y t r a n s i t i o n s . U s i n g t h a t f l u x , w i t h o u t l a r g e l o s s e s e i t h e r i n o p t i c s o r d e t e c t i o n , h o w e v e r , i s a s i g n i f i c a n t c h a l l e n g e . H e r e w e des cri be a new set up, com mis sio ned at BL3 5XU , for nuclear scattering with the 25.65 keV resonance of 161 Dy. In principle, this resonance is convenient fo r sy nc hr ot ro n ba se d st ud ie s, ha vi ng a re la ti ve ly long (42 ns ) lifetime and a large cross-section (low internal conversion). In addition, the difficulties in obtaining narrow lines in conventional (radioactive s o u r c e ) M ö s s b a u e r m e a s u r e m e n t s m a k e t h e s y n c h r o t r o n b a s e d w o r k e s p e c i a l l y a p p e a l i n g . H o w e v e r , p r a c t i c a l l y , s i g n i f i c a n t i n s t r u m e n t a t i o n work is required to make experiments feasible. We have developed a new monochromator and a new detector optimized for this resonance [2]. Our monochromator uses a thin coupling crystal p l a c e d i n s i d e o f a h i g h o r d e r c h a n n e l c u t c r y s t a l o p e r a t i n g n e a r b a c k s c a t t e r i n g ( s e e F i g . 1 ) . B y u s i n g t h e c o u p l i n g c r y s t a l b o t h i n r e f l e c t i o n a n d t r a n s m i s s i o n , t h i s d e s i g n a l l o w s o n e t o g e t e x t r e m e l y c l o s e t o b a c k s c a t t e r i n g ( B r a g g a n g l e s near 90 degrees) without the very big crystals that wou ld be nee ded for pre vio us des ign s [3] . Thu s, Fig. 1. Schematic of the setup for nuclear resonant scattering with 161 Dy summarizing properties of important components. High Resolution Monochromator Nuclear Inelastic Scattering Element Selective Phonon Density of States ∆ E = 0.5 meV 1 - 2 × 10 8 photons/s Nuclear Forward Scattering Hyperfine Structure, Probe of Local Fields Sample Backscattering channel cut crystal Coupling Crystal Out Extremely Compact Design for Near Exact Backscattering APD Detector: 10 mm × 10 mm × 200 μ m Good Signal to Noise Ratio High Efficiency, Large Area 16 Channel Array Device Used at Grazing Incidence ~ 180 ps Time Resolution ( FWHM ) 17% Efficiency at 26 keV 2 3 In 1 96 o n e c a n t a k e f u l l a d v a n t a g e o f t h e l a r g e a n g u l a r a c c e p t a n c e s a v a i l a b l e n e a r b a c k s c a t t e r i n g , w h i l e r e t a i n i n g a c o m p a c t , i n - l i n e , g e o m e t r y . I n particular, working at 25.65 keV (using the (6 2 0) a n d ( 1 8 1 2 6 ) r e f l e c t i o n s – B r a g g a n g l e s o f 1 6 . 3 and 87.4 degrees, respectively) we have obtained a 0.52 meV bandwidth ( ∆ E/E = 2 × 10 -8 ) and a peak throughput of about 2 × 10 8 photons/s, both in good a g r e e m e n t w i t h t h e o r y ( a d d i t i o n a l d i s c u s s i o n c a n be found in [4]). The monochromator was used to me as ur e nu cl ea r in el as ti c sc at te ri ng fr om se ve ra l s a m p l e s , a n d r e s u l t s f r o m a D y B 2 C 2 s a m p l e (natural abundance, 19% 161 Dy ) are shown in Fig. 2 . I n p a r t i c u l a r , t h e r e l a t i v e l y s i m p l e p h o n o n d e n s i t y o f s t a t e s m a k e s i t e a s y t o i d e n t i f y t h e v a r i o u s m u l t i - p h o n o n c o n t r i b u t i o n s a p p e a r i n g a t high temperature, while the inset shows the derived density of states, with some softening evidence at room temperature. In general, this monochromator design should be easy to use in the 20 - 30 keV range and work o n a m o n o c h r o m a t o r f o r t h e S n r e s o n a n c e a t 2 4 ke V is in pr og re ss [5 ]. At lo we r en er gi es , ab so rp ti on F i g . 2 . N u c l e a r i n e l a s t i c s c a t t e r i n g fr om Dy B 2 C 2 at lo w te mp er at ur e an d r o o m t e m p e r a t u r e . T h e s o l i d l i n e s i n t h e f i g u r e a r e c a l c u l a t i o n s b a s e d o n t h e d e r i v e d p a r t i a l d e n s i t y o f s t a t e s D O S ( s h o w n i n t h e i n s e t – 2 2 . 5 m e V full scale). See text for discussion. in the silicon coupling crystal becomes a problem, but this might be avoided by using a less absorbing material ( e.g. diamond or beryllium). Extension to h i g h e r e n e r g y i n s i l i c o n i s a l s o p o s s i b l e , b u t , t h e b a c k s c a t t e r i n g r e f l e c t i v i t y o f t h e s i l i c o n f a l l s o f f q u i c k l y , s o o t h e r m a t e r i a l s ( e . g . s a p p h i r e ) w i t h a higher Debye temperature might be advantageous, if a suitable quality crystals can be found. At exact backscattering, this design becomes reminiscent of an X-ra y Fabr y-Pe rot inte rfer omet er, sugg esti ng a w a y o f c o n t r o l l i n g t h e c o u p l i n g i n t o / o u t o f s u c h a n i n t e r f e r o m e t e r t h a t i s i n d e p e n d e n t o f t h e backscattering mirrors. Development of a proper detector for nuclear forward scattering ( NFS ) from 161 Dy is challenging. This is because one needs both high efficiency (as the resonance is narrow and count rates are small) a n d e x t r e m e l y g o o d t i m e r e s o l u t i o n ( a s t h e hyperfine splitting of the 161 Dy resonance can lead t o b e a t f r e q u e n c i e s o f ~ 1 0 G H z ) . T h e s e t w o c o n d i t i o n s are usually mutually exclusive in silicon avalanche pho tod iod es ( APD s ) sin ce mak ing the m thi cke r to improve the efficiency degrades the time resolution: -40 -30 -20 -10 0 10 20 30 40 299K 18 K 18 K Single Phonon Creation Two Phonon Creation Single Phonon Creation Two Phonon Creation Creation and Annihilation Single Phonon Annihilation Two Phonon Annihilation 299 K Energy Transfer ( meV ) 97 F i g . 3 . N u c l e a r f o r w a r d s c a t t e r i n g f r o m a ( n o n - e n r i c h e d ) 16 1 D y f o i l a t l o w t e m p e r a t u r e . T h e a g r e e m e n t between the fit (solid line) and the data serve to highlight the good performance of the detector. Note both log and linear plots are shown and that the axis labels are for the log plot. References [ 1 ] E . G e r d a u a n d H . d e W a a r d , H y p e r f i n e I n t e r a c t i o n s 123-125 (2000) and references therein. [2] A.Q.R. Baron, Y. Tanaka, D. Ishikawa, D. Miwa, and T. Ishikawa, to be published. [3] T. Ishikawa et al. , Rev. Sci. Instrum. 63 (1992) 10 15 ; M. Ya ba sh i an d T. Is hi ka wa , SP ri ng -8 An nu al Report 1999 (2000) 151. [4] A.Q.R. Baron, Y. Tanaka, D. Ishikawa, D. Miwa, M. Yabashi and T. Ishikawa, J. Synchrotron Rad. 8 (2001) 1127. [5] D. Miwa, D. Ishikawa, A. Baron et al. , work in progress. th e rat io of th e act iv e thi ck ne ss to th e tim e res ol ut io n for an APDs is appr oxim atel y cons tant , and equa l to the saturation drift velocity of the electrons in the APD ( ~ 100 μ m / ns for silicon). However, this trade- o f f m a y b e c i r c u m v e n t e d u s i n g a n a r r a y o f t h i n devices at grazing incidence, allowing a long path l e n g t h t h r o u g h t h e s i l i c o n , w i t h o u t m a k i n g t h e electron transit time spread longer. An array of 16 e l e m e n t s ( e a c h 1 × 2 . 5 m m 2 o n a 1 . 1 m m p i t c h ) allowed us to achieve 180 ps resolution with about a 0.5 mm path length in the silicon, corresponding to ~ 17% efficiency at 25.65 keV. While the 180 ps r e s o l u t i o n i s n o t q u i t e s u f f i c i e n t t o r e s o l v e t h e fastest beats from 161 Dy it is sufficient so that there a r e n o i s o l a t e d l i n e s i n t h e r e s p o n s e – b e a t s w i l l appear from all excited levels. The time response m e a s u r e d f r o m a D y f o i l a t l o w t e m p e r a t u r e i s s h o w n i n F i g . 3 , t h e e x c e l l e n t t i m e r e s o l u t i o n i s clearly evident, and the general quality of the data is confirmed by the good agreement with theory. A l f r e d Q . R . B a r o n a , Y o s h i k a z u T a n a k a b a n d Tetsuya Ishikawa a,b (a) SPring-8 / JASRI (b) SPring-8 / RIKEN E-mail: baron@ spring8.or.jp 1 10 100 1000 10000 Time After Excitation ( ns ) Log Scale (0 - 60 ns ) Linear Scale (5 - 35 ns ) Intensity (counts/channel) 0 10 20 30 40 50 60 98
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