Fig. 1. Experimental setup. For the hyperfine-interaction studies of nuclear r e s o n a n t s c a t t e r i n g , s y n c h r o t r o n r a d i a t i o n h a s a number of distinct advantages when compared with a conventional radioactive source. In addition, the u s e o f s y n c h r o t r o n r a d i a t i o n a l l o w s a d i r e c t measurement of the spectrum of phonon energy in s o l i d s o r t h e d i f f u s i v e m o t i o n o f a t o m s i n l i q u i d s . A l t h o u g h t h e e n e r g y t u n a b i l i t y o f s y n c h r o t r o n r a d i a t i o n a l l o w s p e r f o r m i n g n u c l e a r r e s o n a n c e e x c i t a t i o n e x p e r i m e n t s o f v a r i o u s n u c l i d e s , t h e resonance excitation with synchrotron radiation has been confined only for a few isotopes. Therefore, t h e i n c r e a s e o f t h e n u c l i d e s t h a t c a n b e e x c i t e d wi th sy nc hr ot ro n ra di at io n is ex tr em el y im po rt an t. A m o n g t h e s e n u c l i d e s , p o t a s s i u m i s o n e o f t h e most important elements in material and biological s c i e n c e s , a n d a M ö s s b a u e r r e s o n a n c e h a s b e e n observed for the potassium isotope 40 K [1,2] . Due t o t h e l a c k o f a n y r a d i o a c t i v e p a r e n t n u c l i d e p o p u l a t e d f o r t h e 2 9 . 8 3 k e V f i r s t e x c i t e d s t a t e o f 40 K, it wa s im po ss ib le to ob se rv e th e Mö s sb au er ef fe ct by im pl em en ti ng or di na ry ra di oa ct iv e so ur ce s. NUCLEAR RESONANT SCATTERING OF SYNCHROTRON RADIATION BY 40 K Si mo no ch ro ma to r Io ni za ti on c ha mb er X-ray Slit Al pl at e KC L s am pl e AP D de te ct or F o r t h i s r e a s o n , i n b e a m m e t h o d s f o r n u c l e a r r e a c t i o n s { ( d , p ) a n d ( n , γ ) r e a c t i o n s } o n t h e pred omin ant pota ssiu m isot ope 39 K were adop ted t o o b s e r v e t h e M ö s s b a u e r e f f e c t , h o w e v e r , resulting in serious damage of the sample, which is frequently used as a source. This brought the 40 K nuclide to be one of the most important nuclides for n u c l e a r r e s o n a n c e e x c i t a t i o n e x p e r i m e n t s w i t h synchrotron radiation. We have now measured the nuclear resonant scattering of synchrotron radiation using 40 K for the first time [3] . The experiments were performed at the JAERI beamline BL11XU . The storage ring was operated in a 11 6- bu nc h mo de , gi vi ng a bu nc h di st an ce of 41.3 ns. The measured sample was KCl powder, whos e isot ope rati o of 40 K was 4.03 %. A doub le crystal Si(3 3 3) monochromator produced incident X - r a y r a d i a t i o n w i t h a b a n d w i d t h o f ~ 2 e V a t a n energy of ~ 29.83 keV. An Al plate of 2 mm thick wa s pl ac ed in fr on t of th e sl it s to re du ce th e fi rs t harmonic radiation (9.94 keV ) passing through the m o n o c h r o m a t o r . A S i - a v a l a n c h e p h o t o d i o d e d e t e c t o r Fig. 2. Energy spectrum of delayed emission from 40 K in KCl. Solid circles r e p r e s e n t t h e d e l a y e d c o u n t s n o r m a l i z e d b y t h e c o u n t s o f t h e p r o m p t scattering. The solid line is a Gaussian fit to the normalized delayed counts. Normalized Intensity (10 -5 / 30s) Energy ( keV ) sp ec tr um , no qu an tu m be at wa s ob se rv ed , wh ic h c o i n c i d e d w i t h t h e c u b i c - s y m m e t r i c a l e l e c t r o n i c s t a t e s o f K + i o n s i n K C l c r y s t a l . T h e r e f o r e , t h e o b s e r v e d s p e c t r u m c a n b e f i t t e d w i t h a s i m p l e exponential time distribution. From this spectrum, the half-life of the first excited state was found as 4.13 0.12 ns, b e i n g c o n s i s t e n t w i t h t h e v a l u e o f 4.24 0.09 ns previously reported [5] . In conclusion, we were capable of observing the nuclear resonant excitation of the first excited state of 40 K and have measured the time evolution of the decay. Our observation of the nuclear resonance e x c i t a t i o n o f 4 0 K i n d i c a t e s t h e p o s s i b i l i t i e s i n e l e c t r o n i c a n d v i b r a t i o n a l s t u d i e s o n p o t a s s i u m . T h i s p r o m i s e s a b r i g h t f u t u r e i n t h e s t u d y o f p o t a s s i u m in both material and life sciences. w i t h a d i a m e t e r o f 3 m m w a s u s e d t o d e t e c t t h e scattering from the KCl sample [4] . Figure 1 shows t h e e x p e r i m e n t a l s e t u p . T o s e a r c h t h e n u c l e a r r e s o n a n c e e x c i t a t i o n o f 4 0 K , w e p e r f o r m e d a n e n e r g y s c a n o f t h e i n c i d e n t X - r a y r a d i a t i o n b y c h a n g i n g t h e B r a g g a n g l e o f t h e S i ( 3 3 3 ) mon och rom ato r. The mea sur ed ene rgy spe ctr um is s ho wn i n Fi g. 2 . T he n uc le ar r es on an ce s ca tt er in g by 40 K is cle arl y obs erv ed. In thi s mea sur eme nt, the absolute energy of the first excited state of 40 K was obtained as 29.834 0.011 keV, which was in good agreement with the previous reported value of 2 9 . 8 2 9 9 0 . 0 0 0 6 k e V [ 5 ] . T h e m e a s u r e d t i m e spectrum of nuclear resonant scattering from 40 K in t h e K C l s a m p l e i s s h o w n i n F i g . 3 . I n t h e t i m e 29.82 29.83 29.84 29.85 2.0 1.5 1.0 0.5 0.0 F i g . 3 . T i m e d i s t r i b u t i o n o f t h e 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 o f s y n c h r o t r o n r a d i a t i o n b y 4 0 K i n K C l . T h e m e a s u r e d s p e c t r u m (solid circles) is fitted by an exponential function, and the half-life of the first excited state of 40 K was evaluated to be 4.13 0.12 ns . Makoto Seto Kyoto University E-mail: seto @ rri.kyoto-u.ac.jp 23 22 21 20 19 18 17 Time ( ns ) References [1] S.L. Ruby and R.E. Holland, Phys. Rev. Lett. 14 (1965) 591. [2] D.W. Hafemeister and E. Brooks Shera, Phys. Rev. Lett. 14 (1965) 593. [3] M. Seto , S. Kita o, Y. Koba yash i, R. Haru ki, T. M i t s u i , Y . Y o d a , X . W . Z h a n g a n d Y u . M a e d a , Phys. Rev. Lett. 84 (2000) 566. [4] S. Kishimoto, Nucl. Instrum. Meth. A 309 (1991) 603. [5] P. M. Endt, Nucl. Phys. A 521 (1990) 1. 10 100 Counts / 240 min
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