With the help of the isomer shifts measured on the same sources an estimate of the radial difference (A r2) was obtained which is rather independent of assumptions

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5 7F e I s o m e r S h i f t C a l i b r a t i o n E x p e r i m e n t H. Daniel, F. J. H a r t m a n n , a n d B. Pitesa Physics Department, Technical University of Munich, Munich

Z. Naturforsch. 40 a, 5 3 9 - 5 4 0 (1985);

received March 30, 1985

The 4 s electron contact density was measured on 57Fe atoms embedded in Au and in graphite. With the help of the isomer shifts measured on the same sources an estimate of the radial difference (A r2) was obtained which is rather independent of assumptions: — (A r2) < 9 x 10"3 fm2 at 80 per cent confidence.

T h e c a l i b r a t i o n of t h e 5 7F e i s o m e r s h i f t is certain- ly the most i m p o r t a n t task of M ö ß b a u e r spectro- metry. D e s p i t e t h e m a n y a t t e m p t s t h e s i t u a t i o n is not at all satisfactory. T h e r e p o r t e d r a d i a l c h a n g e s {Ar2) vary b e t w e e n - 3 8 . 0 x 1 0 '3 f m2 a n d - 8 . 2 x 1 0- 3 f m2; f o r reviews cf. [ l ] - [ 3 ] . R e c e n t l y a large value with small e r r o r [(— 33 ± 3) x 10~3 f m2] has been publised [4], It w a s t h e goal of t h e p r e s e n t work to o b t a i n an u p p e r limit of \{Ar2)\ with as little v a g u e a s s u m p t i o n s as possible. T h e m e t h o d consisted in m e a s u r i n g t h e conversion r a t i o a4 s/ a3 s

b e t w e e n 4 s a n d 3 s electrons on 5 7F e in two d i f f e r - ent e n v i r o n m e n t s , e v a l u a t i n g t h e two spectra in t h e s a m e way a n d use t h e 4 s electron contact density as a lower limit of t h e i n d u c e d total electron c o n t a c t density c h a n g e b e c a u s e t h e core " a m p l i f i c a t i o n f a c t o r " [1] is certainly larger t h a n unity b u t very uncertain [2].

5 7C o was i m p l a n t e d w i t h t h e G a r c h i n g m a s s s e p a r a t o r [5] into e i t h e r A u (two sources) or C - g r a p h i t e (one source). T h e conversion electron spectra f r o m 5 7F e w e r e t a k e n w i t h a rt y T 3 / 2 p- ray s p e c t r o m e t e r [6] set to 0.5 p e r cent m o m e n t u m resolution ( f w h m ) . Least s q u a r e s fits i n c l u d i n g as a block the right half of t h e L, conversion line, t h e o t h e r L lines, M , , M2,3, a n d N , w e r e p e r f o r m e d . O t h e r p r o c e d u r e s yielded s o m e w h a t d i f f e r e n t single values but almost the s a m e d i f f e r e n c e between A u and g r a p h i t e . T h e a b s o l u t e v a l u e of t h e 4 s contact

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density was o b t a i n e d f r o m the e x p e r i m e n t a l r a t i o 24 s/ a3 s and the calculated 3s v a l u e ; t h e error of t h e calculation is negligible c o m p a r e d to t h a t of t h e e x p e r i m e n t . M ö ß b a u e r spectra of t h e s p e c t r o m - eter sources were t a k e n a f t e r t h e conversion elec- tron runs. F i g u r e s 1 and 2 show M ö ß b a u e r a n d con- version electron spectra of o n e of the sources of 5 7Fe in Au. T h e results a r e s u m m a r i z e d in T a b l e 1, S d e n o t i n g the i s o m e r shift as o b t a i n e d in o u r ex- p e r i m e n t .

S [mm/sl

Fig. 1. Mößbauer spectrum of one of the sources 57Fe in Au.

Bp [G cm]

Fig. 2. Conversion electron spectrum of the source of Figure 1. One channel corresponds to 0.17 G cm.

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540 Nc

Table 1. Experimental deviations.

data; stated errors are standard

Source *4s/»3s S (mm)

57Fe in Au 0.061 ±0.005 0.65

57Fe in graphite 0.17 ± 0 . 1 0 0.25

T h e final result, -(Ar2) < 9 x 1(T3 f m2 at 80 p e r cent c o n f i d e n c e , strongly favors t h e smaller ( A r2) values r e p o r t e d [ 1 . 3 ] and disagrees with t h e large ones, in p a r t i c u l a r with the most recent precise

[1] R. Ingalls, F. van der Wonde, and G. A. Sawatzky, in:

Mößbauer Isomer Shifts (eds.: G. K. Shenoy and F. E.

Wagner), North Holland. Amsterdam 1978, p. 361 ff.

[2] F. Pleiter and H. de Waard. ibid [1], p. 251 ff.

[3] H. Daniel, Atom. Energy Rev. 172,287 (1979).

[4] A. Meykens, R. Coussement, J. Ladriere, M. Cogneau, M. Boge, P. Auric, R. Bouchez, A. Benabed, and J.

Godard. Phys. Rev. B 21, 3816 (1980).

value [4], T h e a u t h o r s of [4], h o w e v e r , d i d not calculate and h e n c e d i d not t a k e into account t h e e x c h a n g e and o v e r l a p corrections necessary in t h e i r case of electron c a p t u r e decay. T h e s e corrections were c a l c u l a t e d [7] for t h e n e i g h b o r i n g Z decay of C u to a m o u n t to a factor of 1.28; they m a y very well r e d u c e t h e final v a l u e of — ( A r2) f r o m [4] by an a m o u n t of t h e o r d e r of 30 p e r cent.

It is a p l e a s u r e to t h a n k E. Hechtl f o r t h e i m p l a n - tations, N . S c h u h e s and F. W a g n e r for t h e i r h e l p in t a k i n g t h e M ö ß b a u e r spectra, F. W a g n e r or eluci- d a t i n g discussions, and t h e D e u t s c h e F o r s c h u n g s - g e m e i n s c h a f t f o r f i n a n c i a l s u p p o r t .

[5] E. Hechtl and J. Kemmer, Rad. Eff. 44,191 (1979).

[6] H. Daniel, P. Jahn, M. Kuntze, and B. Martin, Nucl.

Instrum. Meth. 82,29 (1970).

[7] W. Gnettner, V. Dornow, and H. Daniel, Hyperf. Int.

12,45 (1982).

N a c h d r u c k — a u c h a u s z u g s w e i s e — n u r mit s c h r i f t l i c h e r G e n e h m i g u n g d e s V e r l a g e s g e s t a t t e t V e r a n t w o r t l i c h f ü r d e n I n h a l t : A. KLEMM

S a t z u n d D r u c k : K o n r a d T r i l t s c h . W ü r z b u r g