- Email: [email protected]
Systematics of fission product X-ray intensities
Volume24B,
number
7
PHYSICS
LETTERS
r e a c t i o n . The t e n s o r p o l a r i z a t i o n of t h e p o s i t i v e d e u t e r o n b e a m had b e e n m e a s u r e d e a r l i e r a n d f o u n d to b e P33 = - 0 . 9 3 5 ± 0.029. In the f i r s t m e a s u r e m e n t t h e r e w a s no m a g n e tic f i e l d in t h e p o t a s s i u m o v e n . The o b s e r v e d r a t i o of c o u n t i n g r a t e s in t h e two c o u n t e r s , n o r m a l i z e d to the r a t i o o b s e r v e d f o r u n p o l a r i z e d i o n s w a s f o u n d to c o r r e s p o n d to P 3 3 = - 0 . 3 5 + 0.06. T h i s v a l u e a g r e e s w e l l w i t h t h e a m o u n t of d e p o l a r i z a t i o n o b s e r v e d at W i s c o n s i n [3]. In the n e x t m e a s u r e m e n t a m a g n e t i c f i e l d of 435 g a u s s w a s a p p l i e d o v e r the exchange region, and a t e n s o r p o l a r i z a t i o n of P 3 3 = - 0 . 8 7 ~ 0.08 w a s d e t e r m i n e d . T h i s v a l u e i s e q u a l to t h e v a l u e f o u n d f o r the p o s i t i v e b e a m w i t h i n the s t a t i s t i c a l u n c e r t a i n t y . In o r d e r to i n v e s t i g a t e f u r t h e r the d e p o l a r i z a t i o n p r o c e s s , m e a s u r e m e n t s w e r e t a k e n w i t h d i f f e r e n t v a l u e s of the m a g n e t i c f i e l d . The r e s u l t s a r e s h o w n in fig. 1. A l s o s h o w n i s the r e s u l t of a n e a r l i e r m e a s u r e m e n t w i t h a t h i n c a r b o n foil. D u r i n g the e x p e r i m e n t t h e o v e n h a d a t e m p e r a t u r e of a b o u t 180°C. T h e b e a m i n t e n s i t y at the t a r g e t w a s a f e w t i m e s 109 d e u t e r i u m i o n s p e r s e c o n d , w h i c h i s a p p r o x i m a t e l y the s a m e we f o u n d e a r l i e r f o r t h e c h a r g e e x c h a n g e in a c a r b o n f o i l . T h e p o l a r i z e d n e g a t i v e b e a m c o u l d b e f o c u s e d on a b e a m s p o t of a b o u t 3 m m d i a m e t e r at a d i s t a n c e 3 m f r o m the p o t a s s i u m o v e n .
SYSTEMATICS
OF
FISSION
3April1967
In c o n c l u s i o n we h a v e f o u n d t h a t u s i n g p o t a s sium vapour c o m b i n e d with a s t r o n g m a g n e t i c field as a charge-exchanger for a polarized beam l e a d s to l i t t l e o r no d e p o l a r i z a t i o n of the b e a m . F u r t h e r , a n e g a t i v e b e a m p r o d u c e d by a g a s e x c h a n g e r is of b e t t e r q u a l i t y t h a n a b e a m p r o d u c e d by a f o i l e x c h a n g e r , and w o u l d b e e x p e c t e d to h a v e a h i g h e r t r a n s m i s s i o n through the a c c e l e r a t o r . We w o u l d like to t h a n k M r . H. K n o t h f o r the h e l p in the d e s i g n of t h e p o t a s s i u m o v e n .
References 1. W.Griiebler, H. Haeberli and P.Schwandt, Phys. Rev. L e t t e r s 12 {1964) 595. 2. B . L . Donnally and R. Becker, Bull.Am. Phys. Soc. 12 (1967) 29. 3. W. Haeberli, W. Griiebler, P. Extermann and P. Schwandt, P h y s . R e v . L e t t e r s 15 (1965) 267. 4. H.G. Clausnitzer, Symp. on Recent p r o g r e s s in nuc l e a r physics with tandems, Heidelberg, 1966. 5. W. Grtiebler, V. K~nig and P. M a r m i e r , Phys. L e t t e r s 24B (1967) 280. 6. W. Haeberli, P r o c . 2nd Intern. Symp. on Polarization phenomena of nucleons, K a r l s r u h e , eds. P . H u b e r and H. Schopper (Birkh~iuser Verlag, 1965) p. 64.
PRODUCT
X-RAY
INTENSITIES
*
W. J O H N , R. MASSEY ** a n d B. G. SAUNDERS
Lawrence Radiation Laboratory, University of California, Livermore, California Received
6 March
1967
The K X-rays of f i s s i o n products f r o m t h e r m a l neutron capture in 235U have been studied with a h i g h - r e s o lution, bent crystal s p e c t r o m e t e r . Systematic variations of the X - r a y intensities with atomic number are found. R e c e n t e x p e r i m e n t s on n u c l e a r f i s s i o n h a v e d e m o n s t r a t e d t h a t t h e d e t a i l e d s t u d y of t h e 7 r a y s , X r a y s , and c o n v e r s i o n e l e c t r o n s can p r o v i d e new i n f o r m a t i o n on the f i s s i o n p r o c e s s a n d on t h e n u c l e a r s t r u c t u r e of t h e n e u t r o n - r i c h f i s s i o n p r o d u c t s [1-4]. In the p r e s e n t w o r k , h i g h - r e s o l u t i o n * Work p e r f o r m e d under the auspices of the U.S. Atomic Energy Commission. ** Lt. C m d r . , U.S. Navy. 336
m e a s u r e m e n t s of the i n t e n s i t i e s of t h e K X r a y s e m i t t e d by f i s s i o n p r o d u c t s r e v e a l c e r t a i n s y s t e m a t i c s of t h e i n t e n s i t i e s . A 2-meter, bent-crystal spectrometer viewed a U-A1 a l l o y f o i l c o n t a i n i n g 0.4 g c m -2 235U in t h e t h r o u g h tube of t h e L i v e r m o r e p o o l - t y p e r e a c t o r . A f t e r c o l l i m a t i o n , the f i s s i o n p r o d u c t X r a y s w e r e B r a g g - r e f l e c t e d f r o m the (310) p l a n e s of q u a r t z . The f o c u s s e d l i n e s w e r e s c a n n e d by a m o v a b l e s l i t f o l l o w e d by a t h i n - w i n d o w G e ( L i )
Volume 24/3, n u m b e r 7
PHYSICS
d e t e c t o r *. L i n e w i d t h s w e r e 20 s e c of a r c , c o r r e s p o n d i n g to a b o u t 30 e V i n t h i s e n e r g y r a n g e . O w i n g to a b s o r p t i o n , t h e m e a s u r e m e n t s w e r e r e s t r i c t e d to e n e r g i e s g r e a t e r t h a n 25 k e V a n d h e n c e to f i s s i o n p r o d u c t s on t h e h e a v y m a s s p e a k . F r o m the f a c t t h a t t h e l i n e w i d t h s of t h e f i s s i o n p r o d u c t X r a y s w e r e f o u n d to b e n o r m a l , it f o l l o w s t h a t the o b s e r v e d X r a y s w e r e e m i t t e d a t times greater than 1 ps since this is the estimated s t o p p i n g t i m e of t h e f r a g m e n t s in t h e foil. T h e full-velocity Doppler spread would have resulted in a l i n e w i d t h a b o u t 100 t i m e s t h e o b s e r v e d width. T h e l o w e r l i m i t s e t h e r e on t h e t i m e of e m i s s i o n i s c o n s i s t e n t w i t h t h e o r i g i n of t h e X r a y s b e i n g the internal conversion process [1-4]. I d e n t i f i c a t i o n of t h e X r a y s w a s b a s e d p r i m a - r i l y on t h e e n e r g i e s , w h i c h c o u l d b e d e t e r m i n e d to w i t h i n ~ 2 eV. T h e X - r a y e n e r g i e s a g r e e c l o s e l y w i t h t h o s e o b t a i n e d u n d e r o r d i n a r y c o n d i t i o n s [5], w h i c h i s to b e e x p e c t e d if t h e e l e c t r o n v a c a n c y i s c r e a t e d a f t e r t h e f r a g m e n t c o m e s to r e s t a n d i s n e u t r a l i z e d . It w a s i n c i d e n t a l l y p o s s i b l e to m a k e n e w p r e c i s i o n m e a s u r e m e n t s of t h e K X r a y e n e r g i e s of Xe a n d P m . F o r a l l e l e m e n t s e x c e p t Sb a n d E u , two of the KOtl, Kc~2 o r K(t I i n t e n s i t i e s were measured for comparison with the known int e n s i t y r a t i o s to d e t e c t a n y a c c i d e n t a l o v e r l a p of lines. Even at the present high resolution, overlap of X - r a y c o m p o n e n t s f r o m d i f f e r e n t e l e m e n t s occurs in some cases. Overlap with nuclear T rays was rarely encountered. A prior measurem e n t of t h e l o w - e n e r g y f i s s i o n s p e c t r u m w i t h t h e b e n t c r y s t a l , m a d e by u s i n g e m u l s i o n d e t e c t o r s , s h o w e d o n l y s o m e 20 d e t e c t a b l e y r a y s in t h e p r e s e n t e n e r g y r a n g e [6]. B e c a u s e the f i s s i o n f o i l r e m a i n e d in t h e n e u t r o n f l u x , b e t a d e c a y of the p r i m a r y f i s s i o n p r o d u c t s b u i l t up a d d i t i o n a l c o n t r i b u t o r s to t h e t o t a l X-ray intensity. The fission foil was irradiated c o n t i n u o u s l y e x c e p t f o r w e e k e n d s f o r 30 w e e k s . In a d d i t i o n , t h e i n t e n s i t y r u n s w e r e t a k e n n e a r t h e e n d of t h e w e e k s o t h a t m o s t of t h e f i s s i o n product activities would be near equilibrium. C o u n t s w e r e t a k e n w i t h t h e s p e c t r o m e t e r s l i t on t h e B r a g g p e a k of a l i n e a n d t h e n off t h e l i n e to evaluate background. Net counting rates were corrected for absorption, quartz crystal reflectivity, and Ge detector efficiency. The results a r e p l o t t e d in fig. l a f o r odd Z a n d in fig. l b f o r even Z. F o r c o m p a r i s o n to t h e d a t a , t h e c u m u l a t i v e y i e l d s of f i s s i o n p r o d u c t s w e r e c a l c u l a t e d f r o m * This Ge d e t e c t o r was s e n s i t i v e down to 10 keV. In the p r e s e n t application the resolution of the Ge d e t e c t o r is used only to improve the s i g n a l - t o - n o i s e ration, the energy being d e t e r m i n e d by the B r a g g angle.
LETTERS
3 A p r i l 1967 bl J,/
io l
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I I I I (a) ODD Z
I
\
I
I
I
I
I
I
(b) EVEN Z
\ I I I I I
52 56 60 55 59 65 Te Xe Be Ce Nd Sm I Cs La Pr PmEu ATOMIC NUMBER Fig. 1. Relative intensities of the K X rays emitted by fission products. Statistical e r r o r s a r e shown on the m e a s u r e d points where they a r e l a r g e r than the points. Calculated points a r e connected by the line. F o r the e v e n - Z case in (b). the normalization of tile calculated points is one-half that for the o d d - Z case in (a). 51 Sb
t h e i n d e p e n d e n t f i s s i o n y i e l d s I7] a n d h a l f - l i v e s . W e c a l c u l a t e d t h e t o t a l f i s s i o n y i e l d of t h e i s o t o p e s of a p a r t i c u l a r e l e m e n t ; no a c c o u n t w a s t a k e n of t h e p r o b a b i l i t y of i n t e r n a l c o n v e r s i o n in each isotope. After a small correction for fluore s c e n t y i e l d , the c a l c u l a t e d i n t e n s i t i e s a r e p l o t t e d a n d c o n n e c t e d b y l i n e s in the f i g u r e s . F o r the o d d - Z c a s e i n fig. l a t h e c a l c u l a t i o n w a s n o r m a l i z e d to the L a p o i n t . F o r t h e e v e n - Z c a s e in fig. lb the same normalization was further multiplied by 0.5 to f i t t h e m e a s u r e m e n t s . T h e f a c t o r 0.5 thus arises from an empirical odd-even effect, t h e r e b e i n g no o d d - e v e n v a r i a t i o n in t h e c a l c u l a t e d f i s s i o n p r o d u c t y i e l d s . T h e a g r e e m e n t of t h e c a l c u l a t i o n w i t h m o s t of the m e a s u r e d p o i n t s is unexpectedly good, indicating that variations in the i n t e r n a l c o n v e r s i o n p r a b a b i l i t y in i n d i v i d u a l i s o t o p e s t e n d to a v e r a g e out. T h e o d d - e v e n e f f e c t found here confirms earlier qualitative results f r o m b e n t c r y s t a l s p e c t r o m e t e r s m a d e by u s i n g e m u l s i o n d e t e c t o r s [6, 8]. A l i k e l y e x p l a n a t i o n f o r t h e o d d - e v e n e f f e c t i s t h a t the l e v e l d e n s i t y in o d d - Z n u c l e i i s g r e a t e r t h a n in e v e n - Z n u c l e i , p a r t i c u l a r l y j u s t a b o v e the K e l e c t r o n b i n d i n g e n e r g y w h e r e t h e p r o b a b i l i t y of i n t e r n a l c o n v e r s i o n is highest. F r o m t h e c a l c u l a t e d f i s s i o n y i e l d s we e s t i m a t e t h a t f o r o u r e x p e r i m e n t , the f r a c t i o n of the t o t a l yield which is pre-beta-decay decreases from 0.66 a t Sb to 0.01 a t Eu. S i n c e t h e o v e r a l l a c c u r a c y of o u r m e a s u r e m e n t s i s a b o u t 5%, we c a n detect the contribution from pre-beta-decay X r a y s f o r o v e r h a l f of t h e e l e m e n t s s t u d i e d . T h i s implies that the observed systematics, i.e. the o d d - e v e n e f f e c t , s h o u l d b e a p p l i c a b l e a t l e a s t in 337
Volume 24B. n u m b e r 7
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LETTERS
p a r t to t h e p r e - b e t a - d e c a y i n t e n s i t i e s . A c u r r e n t e x p e r i m e n t i n B e r k e l e y [9] on t h e p r e - b e t a - d e c a y X r a y s f r o m 252Cf s h o u l d h e l p to c l a r i f y t h i s p o i n t . It i s i n t e r e s t i n g t h a t t h e o d d - e v e n e f f e c t was not observed for the pre-beta-decay X rays o n t h e l i g h t m a s s p e a k f r o m f i s s i o n of 235U [10]. L e a v i n g t h e s u r p r i s i n g o v e r a l l r e g u l a r i t y of t h e i n t e n s i t i e s , we now c o n s i d e r t h e e x c e p t i o n s . At t h e low Z e n d , t h e Sb, T e a n d Xe p o i n t s f a l l considerably below the calculated line. This poss i b l y r e s u l t s f r o m p r o x i m i t y to t h e m a g i c p r o t o n n u m b e r 50. T h e a b r u p t n e s s of t h e d r o p a t Xe i s a striking unexplained feature, as well as the fact t h a t t h e I p o i n t i s n o t low. T h e P r p o i n t i s s i g n i f i c a n t l y a b o v e t h e l i n e . A l a r g e f r a c t i o n of t h e Pr X ray intensity comes from 143pr which has a h i g h l y c o n v e r t e d , 57 k e V t r a n s i t i o n . We n o t e a l s o t h a t t h e P r i s o t o p e s b o r d e r a r e g i o n of n u clear deformation [11,3]. Thus, the present results give the systematics of t h e Z d e p e n d e n c e of t h e p r o b a b i l i t y of i n t e r n a l c o n v e r s i o n f o r t h e b r o a d s a m p l e of n u c l e i s p a n n e d by the heavy fission products.
LIFETIME OF BY PROXIMITY
3 A p r i l 1967
We w o u l d l i k e to t h a n k R. J e w e l l f o r c a r r y i n g out computer calculations.
RefCrCnCCS 1. R . L . W a t s o n , H . R . B o w m a n , S . G . T h o m p s o n and J. O. R a s m u s s e n , Phys. Rev. L e t t e r s 14 (1965) 183. 2, L . E . G l e n d e n i n a n d J . R . U n i k , Phys. Rev. 140 (1965) B1301. 3. S.S. Kapoor, H.R. Bowman and S. G. Thompson, Phys. Rev. 140 (1965) B1310. 4. R . A . A t n e o s e n , J . D . T h o m a s , W . M . G i b s o n and M.L. P e r l m a n . P h y s . R e v . 148 (1966) 1206. 5. J , A , B e a r d e n , X - r a y wavelengths, NYO-10586, U.S. Atomic Energy Commission, Oak Ridge, T e n n e s s e e (1964). 6. W. John and B . G . S a u n d e r s , unpublished. 7. L . E . W e a v e r , P . O . S t r o m a n d P . A . K i l l e e n , U.S. Naval Radiologieal Defense L a b o r a t o r y Report USNRDL-TR-633 (1963), unpublished. 8. J . E . C a n t y , C , D . C o r y e l l , L . L e i f e r and N . C . R a s mussen, B u l l . A m . Phys.Soc. 10 (1965) 481. 9. H. Bowman and R.Watson, private communication, 10. B. W. Wehring, Ph. D. T h e s i s , University of Illinois (1966). 11. E . M a r s h a l e k , L.W. P e r s o n and R , K . S h e l i n e , Rev. Mod. Phys. 35 (1963) 108.
THE 3.46 MeV SCATTERING
STATE OF 41Sc MEASUREMENTS
L. L A S S E N a n d W. Z A H R A D N I K H. Physikalisches Institut der Universit~it Heidelberg and B. Z E I T N I T Z H. Institut f l i t Experi~nentalphysik der Universiti~t Hamburg Received 23 F e b r u a r y 1967
Neutron-proton s c a t t e r i n g following the sequential decay 40Ca(d,n)41Sc*(p)40Ca has been m e a s u r e d to d e t e r m i n e the lifetime of the i n t e r m e d i a t e state at 3.46 MeV excitation energy in 41Se. A value of (4+21.5) × 10 -21 s has been obtained f r o m the evaluation of the n - p r e s c a t t e r i n g intensity.
In a s e q u e n t i a l d e c a y of t h e t y p e A ( x , y ) B * f o l l o w e d by t h e d e c a y of t h e u n s t a b l e n u c l e u s B* z + C, p a r t i c l e s z s h o w a d i s c r e t e e n e r g y s p e c t r u m w h i c h c o r r e s p o n d s to t h e e x c i t e d s t a t e s of B. In t h i s c a s e t h e e m i s s i o n of z i s d e l a y e d b y t h e l i f e t i m e T of t h e e x c i t e d s t a t e B* w i t h r e s p e c t to t h e p r i m a r y e m i t t e d n u c l e o n y. If ~ i s s u f f i c i e n t l y s h o r t (~ 1 0 - 2 0 s ) a n d p a r t i c l e s y a n d z a r e e m i t t e d n e a r l y p a r a l l e l a n d w i t h v e l o c i t i e s v z > Vy a y - z 338
rescattering process can take place in the neighb o u r h o o d of t h e n u c l e u s C c a l l e d " p r o x i m i t y s c a t t e r i n g i n s e q u e n t i a l d e c a y " [1]. P r o x i m i t y s c a t t e r i n g a l l o w s u s to m e a s u r e s h o r t n u c l e a r l i f e t i m e s d i r e c t l y . U s i n g t h i s m e t h o d L a n g e t al. [2, 3] d e t e r m i n e d t h e m e a n l i f e t i m e of t h e ( u n r e s o l v e d ) l e v e l g r o u p a t 3.51 r e s p . 3.56 M e V i n 13N by n - p rescattering measurements from the reaction casc a d e 12C(d, n ) 1 3 N * ( p ) 1 2 C .
number
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r e a c t i o n . The t e n s o r p o l a r i z a t i o n of t h e p o s i t i v e d e u t e r o n b e a m had b e e n m e a s u r e d e a r l i e r a n d f o u n d to b e P33 = - 0 . 9 3 5 ± 0.029. In the f i r s t m e a s u r e m e n t t h e r e w a s no m a g n e tic f i e l d in t h e p o t a s s i u m o v e n . The o b s e r v e d r a t i o of c o u n t i n g r a t e s in t h e two c o u n t e r s , n o r m a l i z e d to the r a t i o o b s e r v e d f o r u n p o l a r i z e d i o n s w a s f o u n d to c o r r e s p o n d to P 3 3 = - 0 . 3 5 + 0.06. T h i s v a l u e a g r e e s w e l l w i t h t h e a m o u n t of d e p o l a r i z a t i o n o b s e r v e d at W i s c o n s i n [3]. In the n e x t m e a s u r e m e n t a m a g n e t i c f i e l d of 435 g a u s s w a s a p p l i e d o v e r the exchange region, and a t e n s o r p o l a r i z a t i o n of P 3 3 = - 0 . 8 7 ~ 0.08 w a s d e t e r m i n e d . T h i s v a l u e i s e q u a l to t h e v a l u e f o u n d f o r the p o s i t i v e b e a m w i t h i n the s t a t i s t i c a l u n c e r t a i n t y . In o r d e r to i n v e s t i g a t e f u r t h e r the d e p o l a r i z a t i o n p r o c e s s , m e a s u r e m e n t s w e r e t a k e n w i t h d i f f e r e n t v a l u e s of the m a g n e t i c f i e l d . The r e s u l t s a r e s h o w n in fig. 1. A l s o s h o w n i s the r e s u l t of a n e a r l i e r m e a s u r e m e n t w i t h a t h i n c a r b o n foil. D u r i n g the e x p e r i m e n t t h e o v e n h a d a t e m p e r a t u r e of a b o u t 180°C. T h e b e a m i n t e n s i t y at the t a r g e t w a s a f e w t i m e s 109 d e u t e r i u m i o n s p e r s e c o n d , w h i c h i s a p p r o x i m a t e l y the s a m e we f o u n d e a r l i e r f o r t h e c h a r g e e x c h a n g e in a c a r b o n f o i l . T h e p o l a r i z e d n e g a t i v e b e a m c o u l d b e f o c u s e d on a b e a m s p o t of a b o u t 3 m m d i a m e t e r at a d i s t a n c e 3 m f r o m the p o t a s s i u m o v e n .
SYSTEMATICS
OF
FISSION
3April1967
In c o n c l u s i o n we h a v e f o u n d t h a t u s i n g p o t a s sium vapour c o m b i n e d with a s t r o n g m a g n e t i c field as a charge-exchanger for a polarized beam l e a d s to l i t t l e o r no d e p o l a r i z a t i o n of the b e a m . F u r t h e r , a n e g a t i v e b e a m p r o d u c e d by a g a s e x c h a n g e r is of b e t t e r q u a l i t y t h a n a b e a m p r o d u c e d by a f o i l e x c h a n g e r , and w o u l d b e e x p e c t e d to h a v e a h i g h e r t r a n s m i s s i o n through the a c c e l e r a t o r . We w o u l d like to t h a n k M r . H. K n o t h f o r the h e l p in the d e s i g n of t h e p o t a s s i u m o v e n .
References 1. W.Griiebler, H. Haeberli and P.Schwandt, Phys. Rev. L e t t e r s 12 {1964) 595. 2. B . L . Donnally and R. Becker, Bull.Am. Phys. Soc. 12 (1967) 29. 3. W. Haeberli, W. Griiebler, P. Extermann and P. Schwandt, P h y s . R e v . L e t t e r s 15 (1965) 267. 4. H.G. Clausnitzer, Symp. on Recent p r o g r e s s in nuc l e a r physics with tandems, Heidelberg, 1966. 5. W. Grtiebler, V. K~nig and P. M a r m i e r , Phys. L e t t e r s 24B (1967) 280. 6. W. Haeberli, P r o c . 2nd Intern. Symp. on Polarization phenomena of nucleons, K a r l s r u h e , eds. P . H u b e r and H. Schopper (Birkh~iuser Verlag, 1965) p. 64.
PRODUCT
X-RAY
INTENSITIES
*
W. J O H N , R. MASSEY ** a n d B. G. SAUNDERS
Lawrence Radiation Laboratory, University of California, Livermore, California Received
6 March
1967
The K X-rays of f i s s i o n products f r o m t h e r m a l neutron capture in 235U have been studied with a h i g h - r e s o lution, bent crystal s p e c t r o m e t e r . Systematic variations of the X - r a y intensities with atomic number are found. R e c e n t e x p e r i m e n t s on n u c l e a r f i s s i o n h a v e d e m o n s t r a t e d t h a t t h e d e t a i l e d s t u d y of t h e 7 r a y s , X r a y s , and c o n v e r s i o n e l e c t r o n s can p r o v i d e new i n f o r m a t i o n on the f i s s i o n p r o c e s s a n d on t h e n u c l e a r s t r u c t u r e of t h e n e u t r o n - r i c h f i s s i o n p r o d u c t s [1-4]. In the p r e s e n t w o r k , h i g h - r e s o l u t i o n * Work p e r f o r m e d under the auspices of the U.S. Atomic Energy Commission. ** Lt. C m d r . , U.S. Navy. 336
m e a s u r e m e n t s of the i n t e n s i t i e s of t h e K X r a y s e m i t t e d by f i s s i o n p r o d u c t s r e v e a l c e r t a i n s y s t e m a t i c s of t h e i n t e n s i t i e s . A 2-meter, bent-crystal spectrometer viewed a U-A1 a l l o y f o i l c o n t a i n i n g 0.4 g c m -2 235U in t h e t h r o u g h tube of t h e L i v e r m o r e p o o l - t y p e r e a c t o r . A f t e r c o l l i m a t i o n , the f i s s i o n p r o d u c t X r a y s w e r e B r a g g - r e f l e c t e d f r o m the (310) p l a n e s of q u a r t z . The f o c u s s e d l i n e s w e r e s c a n n e d by a m o v a b l e s l i t f o l l o w e d by a t h i n - w i n d o w G e ( L i )
Volume 24/3, n u m b e r 7
PHYSICS
d e t e c t o r *. L i n e w i d t h s w e r e 20 s e c of a r c , c o r r e s p o n d i n g to a b o u t 30 e V i n t h i s e n e r g y r a n g e . O w i n g to a b s o r p t i o n , t h e m e a s u r e m e n t s w e r e r e s t r i c t e d to e n e r g i e s g r e a t e r t h a n 25 k e V a n d h e n c e to f i s s i o n p r o d u c t s on t h e h e a v y m a s s p e a k . F r o m the f a c t t h a t t h e l i n e w i d t h s of t h e f i s s i o n p r o d u c t X r a y s w e r e f o u n d to b e n o r m a l , it f o l l o w s t h a t the o b s e r v e d X r a y s w e r e e m i t t e d a t times greater than 1 ps since this is the estimated s t o p p i n g t i m e of t h e f r a g m e n t s in t h e foil. T h e full-velocity Doppler spread would have resulted in a l i n e w i d t h a b o u t 100 t i m e s t h e o b s e r v e d width. T h e l o w e r l i m i t s e t h e r e on t h e t i m e of e m i s s i o n i s c o n s i s t e n t w i t h t h e o r i g i n of t h e X r a y s b e i n g the internal conversion process [1-4]. I d e n t i f i c a t i o n of t h e X r a y s w a s b a s e d p r i m a - r i l y on t h e e n e r g i e s , w h i c h c o u l d b e d e t e r m i n e d to w i t h i n ~ 2 eV. T h e X - r a y e n e r g i e s a g r e e c l o s e l y w i t h t h o s e o b t a i n e d u n d e r o r d i n a r y c o n d i t i o n s [5], w h i c h i s to b e e x p e c t e d if t h e e l e c t r o n v a c a n c y i s c r e a t e d a f t e r t h e f r a g m e n t c o m e s to r e s t a n d i s n e u t r a l i z e d . It w a s i n c i d e n t a l l y p o s s i b l e to m a k e n e w p r e c i s i o n m e a s u r e m e n t s of t h e K X r a y e n e r g i e s of Xe a n d P m . F o r a l l e l e m e n t s e x c e p t Sb a n d E u , two of the KOtl, Kc~2 o r K(t I i n t e n s i t i e s were measured for comparison with the known int e n s i t y r a t i o s to d e t e c t a n y a c c i d e n t a l o v e r l a p of lines. Even at the present high resolution, overlap of X - r a y c o m p o n e n t s f r o m d i f f e r e n t e l e m e n t s occurs in some cases. Overlap with nuclear T rays was rarely encountered. A prior measurem e n t of t h e l o w - e n e r g y f i s s i o n s p e c t r u m w i t h t h e b e n t c r y s t a l , m a d e by u s i n g e m u l s i o n d e t e c t o r s , s h o w e d o n l y s o m e 20 d e t e c t a b l e y r a y s in t h e p r e s e n t e n e r g y r a n g e [6]. B e c a u s e the f i s s i o n f o i l r e m a i n e d in t h e n e u t r o n f l u x , b e t a d e c a y of the p r i m a r y f i s s i o n p r o d u c t s b u i l t up a d d i t i o n a l c o n t r i b u t o r s to t h e t o t a l X-ray intensity. The fission foil was irradiated c o n t i n u o u s l y e x c e p t f o r w e e k e n d s f o r 30 w e e k s . In a d d i t i o n , t h e i n t e n s i t y r u n s w e r e t a k e n n e a r t h e e n d of t h e w e e k s o t h a t m o s t of t h e f i s s i o n product activities would be near equilibrium. C o u n t s w e r e t a k e n w i t h t h e s p e c t r o m e t e r s l i t on t h e B r a g g p e a k of a l i n e a n d t h e n off t h e l i n e to evaluate background. Net counting rates were corrected for absorption, quartz crystal reflectivity, and Ge detector efficiency. The results a r e p l o t t e d in fig. l a f o r odd Z a n d in fig. l b f o r even Z. F o r c o m p a r i s o n to t h e d a t a , t h e c u m u l a t i v e y i e l d s of f i s s i o n p r o d u c t s w e r e c a l c u l a t e d f r o m * This Ge d e t e c t o r was s e n s i t i v e down to 10 keV. In the p r e s e n t application the resolution of the Ge d e t e c t o r is used only to improve the s i g n a l - t o - n o i s e ration, the energy being d e t e r m i n e d by the B r a g g angle.
LETTERS
3 A p r i l 1967 bl J,/
io l
-
I I I I (a) ODD Z
I
\
I
I
I
I
I
I
(b) EVEN Z
\ I I I I I
52 56 60 55 59 65 Te Xe Be Ce Nd Sm I Cs La Pr PmEu ATOMIC NUMBER Fig. 1. Relative intensities of the K X rays emitted by fission products. Statistical e r r o r s a r e shown on the m e a s u r e d points where they a r e l a r g e r than the points. Calculated points a r e connected by the line. F o r the e v e n - Z case in (b). the normalization of tile calculated points is one-half that for the o d d - Z case in (a). 51 Sb
t h e i n d e p e n d e n t f i s s i o n y i e l d s I7] a n d h a l f - l i v e s . W e c a l c u l a t e d t h e t o t a l f i s s i o n y i e l d of t h e i s o t o p e s of a p a r t i c u l a r e l e m e n t ; no a c c o u n t w a s t a k e n of t h e p r o b a b i l i t y of i n t e r n a l c o n v e r s i o n in each isotope. After a small correction for fluore s c e n t y i e l d , the c a l c u l a t e d i n t e n s i t i e s a r e p l o t t e d a n d c o n n e c t e d b y l i n e s in the f i g u r e s . F o r the o d d - Z c a s e i n fig. l a t h e c a l c u l a t i o n w a s n o r m a l i z e d to the L a p o i n t . F o r t h e e v e n - Z c a s e in fig. lb the same normalization was further multiplied by 0.5 to f i t t h e m e a s u r e m e n t s . T h e f a c t o r 0.5 thus arises from an empirical odd-even effect, t h e r e b e i n g no o d d - e v e n v a r i a t i o n in t h e c a l c u l a t e d f i s s i o n p r o d u c t y i e l d s . T h e a g r e e m e n t of t h e c a l c u l a t i o n w i t h m o s t of the m e a s u r e d p o i n t s is unexpectedly good, indicating that variations in the i n t e r n a l c o n v e r s i o n p r a b a b i l i t y in i n d i v i d u a l i s o t o p e s t e n d to a v e r a g e out. T h e o d d - e v e n e f f e c t found here confirms earlier qualitative results f r o m b e n t c r y s t a l s p e c t r o m e t e r s m a d e by u s i n g e m u l s i o n d e t e c t o r s [6, 8]. A l i k e l y e x p l a n a t i o n f o r t h e o d d - e v e n e f f e c t i s t h a t the l e v e l d e n s i t y in o d d - Z n u c l e i i s g r e a t e r t h a n in e v e n - Z n u c l e i , p a r t i c u l a r l y j u s t a b o v e the K e l e c t r o n b i n d i n g e n e r g y w h e r e t h e p r o b a b i l i t y of i n t e r n a l c o n v e r s i o n is highest. F r o m t h e c a l c u l a t e d f i s s i o n y i e l d s we e s t i m a t e t h a t f o r o u r e x p e r i m e n t , the f r a c t i o n of the t o t a l yield which is pre-beta-decay decreases from 0.66 a t Sb to 0.01 a t Eu. S i n c e t h e o v e r a l l a c c u r a c y of o u r m e a s u r e m e n t s i s a b o u t 5%, we c a n detect the contribution from pre-beta-decay X r a y s f o r o v e r h a l f of t h e e l e m e n t s s t u d i e d . T h i s implies that the observed systematics, i.e. the o d d - e v e n e f f e c t , s h o u l d b e a p p l i c a b l e a t l e a s t in 337
Volume 24B. n u m b e r 7
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LETTERS
p a r t to t h e p r e - b e t a - d e c a y i n t e n s i t i e s . A c u r r e n t e x p e r i m e n t i n B e r k e l e y [9] on t h e p r e - b e t a - d e c a y X r a y s f r o m 252Cf s h o u l d h e l p to c l a r i f y t h i s p o i n t . It i s i n t e r e s t i n g t h a t t h e o d d - e v e n e f f e c t was not observed for the pre-beta-decay X rays o n t h e l i g h t m a s s p e a k f r o m f i s s i o n of 235U [10]. L e a v i n g t h e s u r p r i s i n g o v e r a l l r e g u l a r i t y of t h e i n t e n s i t i e s , we now c o n s i d e r t h e e x c e p t i o n s . At t h e low Z e n d , t h e Sb, T e a n d Xe p o i n t s f a l l considerably below the calculated line. This poss i b l y r e s u l t s f r o m p r o x i m i t y to t h e m a g i c p r o t o n n u m b e r 50. T h e a b r u p t n e s s of t h e d r o p a t Xe i s a striking unexplained feature, as well as the fact t h a t t h e I p o i n t i s n o t low. T h e P r p o i n t i s s i g n i f i c a n t l y a b o v e t h e l i n e . A l a r g e f r a c t i o n of t h e Pr X ray intensity comes from 143pr which has a h i g h l y c o n v e r t e d , 57 k e V t r a n s i t i o n . We n o t e a l s o t h a t t h e P r i s o t o p e s b o r d e r a r e g i o n of n u clear deformation [11,3]. Thus, the present results give the systematics of t h e Z d e p e n d e n c e of t h e p r o b a b i l i t y of i n t e r n a l c o n v e r s i o n f o r t h e b r o a d s a m p l e of n u c l e i s p a n n e d by the heavy fission products.
LIFETIME OF BY PROXIMITY
3 A p r i l 1967
We w o u l d l i k e to t h a n k R. J e w e l l f o r c a r r y i n g out computer calculations.
RefCrCnCCS 1. R . L . W a t s o n , H . R . B o w m a n , S . G . T h o m p s o n and J. O. R a s m u s s e n , Phys. Rev. L e t t e r s 14 (1965) 183. 2, L . E . G l e n d e n i n a n d J . R . U n i k , Phys. Rev. 140 (1965) B1301. 3. S.S. Kapoor, H.R. Bowman and S. G. Thompson, Phys. Rev. 140 (1965) B1310. 4. R . A . A t n e o s e n , J . D . T h o m a s , W . M . G i b s o n and M.L. P e r l m a n . P h y s . R e v . 148 (1966) 1206. 5. J , A , B e a r d e n , X - r a y wavelengths, NYO-10586, U.S. Atomic Energy Commission, Oak Ridge, T e n n e s s e e (1964). 6. W. John and B . G . S a u n d e r s , unpublished. 7. L . E . W e a v e r , P . O . S t r o m a n d P . A . K i l l e e n , U.S. Naval Radiologieal Defense L a b o r a t o r y Report USNRDL-TR-633 (1963), unpublished. 8. J . E . C a n t y , C , D . C o r y e l l , L . L e i f e r and N . C . R a s mussen, B u l l . A m . Phys.Soc. 10 (1965) 481. 9. H. Bowman and R.Watson, private communication, 10. B. W. Wehring, Ph. D. T h e s i s , University of Illinois (1966). 11. E . M a r s h a l e k , L.W. P e r s o n and R , K . S h e l i n e , Rev. Mod. Phys. 35 (1963) 108.
THE 3.46 MeV SCATTERING
STATE OF 41Sc MEASUREMENTS
L. L A S S E N a n d W. Z A H R A D N I K H. Physikalisches Institut der Universit~it Heidelberg and B. Z E I T N I T Z H. Institut f l i t Experi~nentalphysik der Universiti~t Hamburg Received 23 F e b r u a r y 1967
Neutron-proton s c a t t e r i n g following the sequential decay 40Ca(d,n)41Sc*(p)40Ca has been m e a s u r e d to d e t e r m i n e the lifetime of the i n t e r m e d i a t e state at 3.46 MeV excitation energy in 41Se. A value of (4+21.5) × 10 -21 s has been obtained f r o m the evaluation of the n - p r e s c a t t e r i n g intensity.
In a s e q u e n t i a l d e c a y of t h e t y p e A ( x , y ) B * f o l l o w e d by t h e d e c a y of t h e u n s t a b l e n u c l e u s B* z + C, p a r t i c l e s z s h o w a d i s c r e t e e n e r g y s p e c t r u m w h i c h c o r r e s p o n d s to t h e e x c i t e d s t a t e s of B. In t h i s c a s e t h e e m i s s i o n of z i s d e l a y e d b y t h e l i f e t i m e T of t h e e x c i t e d s t a t e B* w i t h r e s p e c t to t h e p r i m a r y e m i t t e d n u c l e o n y. If ~ i s s u f f i c i e n t l y s h o r t (~ 1 0 - 2 0 s ) a n d p a r t i c l e s y a n d z a r e e m i t t e d n e a r l y p a r a l l e l a n d w i t h v e l o c i t i e s v z > Vy a y - z 338
rescattering process can take place in the neighb o u r h o o d of t h e n u c l e u s C c a l l e d " p r o x i m i t y s c a t t e r i n g i n s e q u e n t i a l d e c a y " [1]. P r o x i m i t y s c a t t e r i n g a l l o w s u s to m e a s u r e s h o r t n u c l e a r l i f e t i m e s d i r e c t l y . U s i n g t h i s m e t h o d L a n g e t al. [2, 3] d e t e r m i n e d t h e m e a n l i f e t i m e of t h e ( u n r e s o l v e d ) l e v e l g r o u p a t 3.51 r e s p . 3.56 M e V i n 13N by n - p rescattering measurements from the reaction casc a d e 12C(d, n ) 1 3 N * ( p ) 1 2 C .