INVESTIGATION OF CHANGES IN HYPERFINE INTERACTION AND DEBYE-WALLER-FACTOR BY α-RADIATION SELF DAMAGE IN 241Am METAL

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INVESTIGATION OF CHANGES IN HYPERFINE INTERACTION AND DEBYE-WALLER-FACTOR BY

α-RADIATION SELF DAMAGE IN 241Am METAL

L. Asch, W. Potzel, G. Kalvius, J .C. Spirlet, W. Müller

To cite this version:

L. Asch, W. Potzel, G. Kalvius, J .C. Spirlet, W. Müller. INVESTIGATION OF CHANGES IN HYPERFINE INTERACTION AND DEBYE-WALLER-FACTOR BY α-RADIATION SELF DAMAGE IN 241Am METAL. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-544-C2-546.

�10.1051/jphyscol:19792188�. �jpa-00218565�

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JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n o 3, Tome 40, mars 1979, page

C2-544

INVESTIGATION OF CHANGES IN HYPERFINE INTERACTION AND DEBYE-WALLER-FACTOR

BY a-RADIATION SELF DAMAGE IN 2 4 ' ~ m METAL*

L. Asch, W. ~ o t z e l ' , G.K. ~ a l v i u s + . J.C. S p i r l e t

++

and W. ~ ~ l l e r + +

Laboratoire de Chimie Nuc Zdaire, F 67037 STRASBOURG, France

' ~ h y s i k Department, Technische U n i v e r s i t t i t Miinchen, D8046 GARCHING, Germany

++

I n s t i t u t Europden des Transuraniens, D7500 KARLSRUHE, Germany

Rdsum6.- Les e f f e t s d e l ' a u t o - i r r a d i a t i o n a d e 2 ' 1 ~ m m d t a l o n t e t 6 d t u d i d s p a r s p e c t r o s c o p i e Miiss- b a u e r d ' d m i s s i o n . Nous avons maintenu l e s d c h a n t i l l o n s B 4 , 2 K p e n d a n t 230 h d ' a f f i l d e e t r e l e v d d e s s p e c t r e s Miissbauer t o u t e s l e s 10 2 20 h e u r e s . I1 n ' y a aucune v a r i a t i o n du f a c t e u r f , p a r con-

t r e l a l a r g e u r de r a i e augmente p r o g r e s s i v e m e n t p o u r a t t e i n d r e s a v a l e u r B s a t u r a t i o n a p r a s 180 h . Un b r e f r e c u i t B t e m p e r a t u r e o r d i n a i r e r b g 6 n S r e l a l a r g e u r i n i t i a l e . L ' a u g m e n t a t i o n d e l a r g e u r de r a i e r e f l a t e un changement de l ' i n t e r a c t i o n q u a d r u p o l a i r e l i 6 B l a c r d a t i o n de d l f a u t s . La v a l e u r c o n s t a n t e du f a c t e u r f i n d i q u e que l a s t r u c t u r e c r i s t a l l i n e n ' e s t p a s r a d i c a l e m e n t m o d i f i l e . A b s t r a c t . - The Miissbauer e m i s s i o n s p e c t r u m o f 241Am m e t a l was i n v e s t i g a t e d f o r i n f l u e n c e s o f r a d i a - t i o n s e l f damage. Samples were k e p t c o n t i n u o u s l y f o r 230 h a t 4 . 2 K and s p e c t r a w e r e t a k e n e a c h 10 t o 20 h . No change i n f - f a c t o r was o b s e r v e d , w h i l e t h e l i n e w i d t h i n c r e a s e d m o n o t o n i c a l l y and r e a - ched s a t u r a t i o n a f t e r ^'l.180 h . The o r i g i n a l w i d t h c o u l d b e r e p r o d u c e d by a b r i e f warming t o room

t e m p e r a t u r e . The i n c r e a s e i n w i d t h r e f l e c t s t h e c h a n g e i n q u a d r u p o l e i n t e r a c t i o n due t o t h e c r e a - t i o n of l a t t i c e d e f e c t s . The c o n s t a n c y o f t h e f - f a c t o r i n d i c a t e s t h a t t h e b a s i c c r y s t a l l i n e s t r u c - t u r e i s r e t a i n e d .

The commonly used s o u r c e f o r Mzssbauer measurements w i t h 2 3 7 ~ p i s 241Am which d e c a y s i n t o t h e 60 keV e x c i t e d s t a t e ( T l I 2 = 6 3 n s ) by t h e e m i s s i o n of a 5.5 MeV a - p a r t i c l e . I t i s a s t o n i s h i n g , t h a t t h e ob- s e r v a t i o n o f a c l e a r G s s b a u e r s i g n a l i s p o s s i b l e some t e n nanoseconds a f t e r s u c h a v i o l e n t a c t a s a n a-decay which i m p a r t s a b o u t 100 keV r e c o i l e n e r g y t o the. n e p t u n i u m n u c l e u s . A f t e r e f f e c t s o f t h e p r e c e e - d i n g a-decay a r e known t o e x i s t . B e s t e s t a b l i s h e d

i s t h e o b s e r v a t i o n o f m u l t i p l e e m i s s i o n l i n e s i n s o u r c e s l i k e Am02 / l / . I n t h e p r e s e n t e x p e r i m e n t we look f c r c h a n g e s i n t i m e of t h e r e s o n a n c e l i n e o f Am m e t a l s o u r c e s k e p t a t l i q u i d h e l i u m t e m p e r a t u r e f o r p r o i o n g e d t i m e s . Such c h a n g e s c o u l d b e c a u s e d by r a d i a t i o n s e l f damage which i s known t o p l a y a r o l e i n measurements o f t h e h e a t c a p a c i t y / 2 / and t h e e l e c t r i c a l r e s i s t i v i t y 131. Measurements were p e r - formed u s i n g a 100 mg/cm2 a b s o r b e r o f NpOz a t 7 7 K.

A f a s t p u l s e d e t e c t i o n s y s t e m / 4 / a l l o w e d t h e con- p i l a t i o n o f p r e c i s e s p e c t r a i n s h o r t t i m e . Data were l e a s t s q u a r e s f i t t e d t o a s i n g l e L o r e n t z i a n m o d i f i e d by a d i s p e r s i o n a m p l i t u d e o f

5

= -0.034 /5/. The r e c o i l f r e e f r a c t i o n i s p r o p o r t i o n a l t o t h e

mined by a p o i n t by p o i n t summation o v e r t h e whole v e l o c i t y r a n g e a f t e r t h e b a s e l i n e had b e e n f i x e d i n t h e l e a s t s q u a r e s p r o c e d u r e . Two d i f f e r e n t s o u r c e s o f " + ' ~ m m e t a l were s t u d i e d . They were b o t h produ- c e d by vacuum e v a p o r a t i o n of Am m e t a l o n t o a Ta d i s c . They d i f f e r i n t h e t o t a l amount o f Am m e t a l (15.5 mg and 8 9 mg) and t h e i r r a d i o a c t i v i t y ( 5 3 mCi and 306 mCi). T h e i r mass ( a c t i v i t y ) p e r u n i t a r e a , however, i s r o u g h l y t h e same (- 100 mg/cm2).

F i g u r e l d i s p l a y s f o r t h e 53 mCi s o u r c e t h e l i n e - w i d t h , peak h e i g h t and t o t a l a r e a a s a f u n c t i o n o f t i m e . The a r e a remained c o n s t a n t , b u t t h e l i n e w i d t h showed a monotonic i n c r e a s e r e a c h i n g s a t u r a t i o n a f t e r

-

180 h . A f t e r 10 d a y s a t 4.2 K t h e sample was b r i e f l y warmed t o room t e m p e r a t u r e , t h e n recoo-

l e d . W i t h i n t h e l i m i t s o f e r r o r t h e i n i t i a l v a l u e s o f w i d t h , h e i g h t and a r e a were r e p r o d u c e d . S i m i l a r r e s u l t s were o b t a i n e d f o r t h e 306 mCi s a m p l e . I n a n o t h e r , y e t u n p u b l i s h e d , measurement we found t h a t t h e r e s o n a n t l i n e s h a p e f o r Am a t 4.2 K c a n b e f i t t e d t o a n u n r e s o l v e d , p u r e l y a x i a l q u a d r u p o l e i n t e r a c t i o n o f e Z q q

=

⁶^mm/s

=

³⁰⁰^MHz.Such a n in- t e r a c t i o n i s e x p e c t e d from t h e d o u b l e hcp s t r u c t u r e t o t a l a r e a u n d e r t h e r e s o n a n c e c u r v e which was d e t e r - o f b u l k Am m e t a l 1 6 1 . O t h e r s o u r c e s o f l i n e b r o a -

d e n i n g , l i k e a d i s t r i b u t i o n o f isomer s h i f t s , w i l l

*

Work s u p p o r t e d i n p a r t by t h e B u n d e s m i n i s t e r i u m a l s o b e p r e s e n t b u t t h e i r s i z e i s d i f f i c u l t t o e s - fGr Forschung und T e c h n o l o g i e , F e d e r a l R e p u b l i c

of Germany. t i m a t e . The o b s e r v e d change i n l i n e w i d t h i s i n t e r -

p r e t e d a s a g r a d u a l i n c r e a s e o f q u a d r u p o l e i n t e r -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792188

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a c t i o n c o n n e c t e d w i t h t h e r a d i a t i o n damage o f t h e s i m i l a r i t y o f t h e i r b e h a v i o u r i s u n d e r s t a n d a b l e . l a t t i c e .

0 0' X.,

-

E-

^0-^-U^-Height

0 - ^Q- U- ,a- -1

A r e a

Time [hours]

F i g . 1 : Change o f s p e c t r a l p a r a m e t e r s w i t h t i m e f o r t h e 53 mCi sample o f *"~m m e t a l k e p t a t 4 . 2 K.

The c r o s s h a t c h e d d a t a were t a k e n a t t h e end o f t h e r u n a f t e r a warm-up t o 300 K.

Both, t h e a - p a r t i c l e s o f 5.5 MeV and t h e Np r e c o i l n u c l e i w i l l p r i m a r i l y c r e a t e v a c a n c i e s and i n t e r - s t i t i a l ~ i n t h e i r p a s s a g e t h r o u g h t h e l a t t i c e 171.

I f l o c a t e d i n t h e v i c i n i t y o f t h e e m i t t i n g Am atom, t h e s e l a t t i c e d e f e c t s w i l l g i v e r i s e t o a n a d d i - t i o n a l e l e c t r i c f i e l d g r a d i e n t 181. The p r i m a r y r e - c o i l s w i l l t r a v e l o n l y - 8 nm and s t o p i n a t i m e s h o r t compared t o t h e h a l f l i f e o f t h e 60 keV e x c i - t e d s t a t e . They a r e s u r r o u n d e d m a i n l y by a c l o u d o f v a c a n c i e s w h i l e t h e i n t e r s t i t i a l s have moved f u r t h e r away by c o r r e l a t e d c o l l i s i o n p r o c e s s e s . The damages r e m a i n i n g i n t h e v i c i n i t y o f t h e e m i t t i n g atom it- s e l f c o u l d a t b e s t c o n t r i b u t e t o t h e i n i t i a l l i n e - w i d t h b u t s h o u l d n o t show a b u i l d - u p i n t i m e . T h i s i s d i f f e r e n t w i t h t h e g e n e r a l r a d i a t i o n damage i n - f l i c t e d on t h e Am l a t t i c e by b o t h t h e n u c l e a r re- c o i l s and t h e a - p a r t i c l e s . The r e c o i l s c r e a t e damage i n h i g h c o n c e n t r a t i o n w h i l e t h e a - p a r t i c l e s produce v a c a n c i e s and i n t e r s t i t i a l s o v e r a much l a r - g e r volume. These damages w i l l b e f r o z e n i n a t he- l i u m t e m p e r a t u r e and c a n n o t e a s i l y a n n i h i l a t e to- g e t h e r . T h e i r c o n c e n t r a t i o n w i l l r i s e and s o o n t h e whole Am l a t t i c e i s h e a v i l y a f f l i c t e d . F i n a l l y t h e v a c a n c i e s and i n t e r s t i t i a l s w i l l come c l o s e enough t o g e t h e r t o a n n i h i l a t e e v e n a t low t e m p e r a t u r e s and a s t a t i o n a r y c o n c e n t r a t i o n of d e f e c t s w i l l r e s u l t s . The t h i c k n e s s o f b o t h Am s a m p l e s i s l a r g e compa- r e d t o t h e mean s t o p p i n g d i s t a n c e o f e i t h e r t h e n u c l e a r r e c o i l s o r t h e a - p a r t i c l e s ( - 8

urn).

Thus t h e number o f damaged s i t e s produced p e r u n i t volume w i l l b e t h e same f o r b o t h samples and t h e

T h i s view o f t h e o r i g i n o f t h e t i m e d e p e n d e n t l i n e b r o a d e n i n g i s f u r t h e r s u p p o r t e d by measurements o f

t r a n s p o r t p r o p e r t i e s / 2 , 3 / . I n p a r t i c u l a r , a n ex- p o n e n t i a l growth o f r e s i s t i v i t y w i t h a time cons- t a n t o f 5 0 h was found. Warming t o room t e m p e r a t u r e n e a r l y b u t n o t e x a c t l y r e p r o d u c e d t h e o r i g i n a l r e - s i s t i v i t y . The s m a l l amount o f remanent r a d i a t i o n damage i s a t t h e l i m i t of s e n s i t i v i t y o f a ~ 6 s s b a u - e r measurement.

The c o n s t a n c y o f t h e f - f a c t o r w i t h time i n d i c a t e s on a l o c a l p i c t u r e t h a t no d i f f u s i o n a n d s u b s e q u e n t t r a p p i n g a t t h e Np s i t e o c c u r f o r t h e i n t e r s t i t i a l s w i t h i n t h e l i f e t i m e o f t h e 60 keV s t a t e . I t i s known t h a t i m p u r i t y - i n t e r s t i t i a l complexes have low v i b r a t i o n a l f r e q u e n c y modes c a u s i n g a r e d u c t i o n i n f - f a c t o r 191. One e x p e c t s l i t t l e m i g r a t i o n a t 4 . 2 K . F u r t h e r m o r e , t h e e l e c t r o n i c s t r u c t u r e s o f neptunium and americium a r e n o t v e r y d i f f e r e n t and t h e f o r m e r w i l l n o t b e a n e f f i c i e n t t r a p . I n terms of non- l o c a l p r o p e r t i e s one may e x p e c t t h a t t h e c r y s t a l s t r u c t u r e w i l l s e v e r e l y be d e s t r o y e d u n d e r t h e i n - f l u e n c e o f s e l f - i r r a d i a t i o n . Our r e s u l t makes a n

" a n o r p h i z a t i o n " o f Am m e t a l r a t h e r u n l i k e l y , s i n c e n o t i c e a b l e d i f f e r e n c e s i n t h e f - f a c t o r u s u a l l y e x i s t between c r y s t a l l i n e and amorphous p h a s e s / 1 0 / . The m a t e r i a l may w e l l become m i c r o c r y s t a l l i n e u n d e r t h e i n f l u e n c e o f s e l f - i r r a d i a t i o n b u t r e t a i n s i t s b a s i c c r y s t a l l i n e s t r u c t u r e .

We t h a n k 0 . Beck o f t h e I n s t i t u t f u r Radiochemie d e r T e c h n i s c h e n ~ n i v e r s i t g t ~ i i n c h e n f o r making a v a i l a - b l e t h e NpOn a b s o r b e r , U . Gonser and F . J . L i t t e r s t f o r h e l p f u l d i s c u s s i o n s .

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