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A NEW ATTEMPT TO OBSERVE THE RESONANCE IN Ag109

W. Wildner, U. Gonser

To cite this version:

W. Wildner, U. Gonser. A NEW ATTEMPT TO OBSERVE THE RESONANCE IN Ag109. Journal

de Physique Colloques, 1979, 40 (C2), pp.C2-47-C2-48. �10.1051/jphyscol:1979216�. �jpa-00218534�

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JOURNAL DE PHYSIQUE

Collogue C2, supplement au n° 3, Tome 40, mars 1979, page C2-47

A NEW ATTEMPT TO OBSERVE THE RESONANCE IN A

g 1 0 9

W. Wildner and U. Gonser

Fackbereieh Angewandte Phyaik, Universitdt des Saavlandes, 6600 Saavbrileken, Germany

Résumé.- Le rayonnement gamma de 88 keV d'Ag émis par un monocristal d'argent dopé par du Cd

109

a é t é mesuré en fonction de l a température. La diminution de l ' i n t e n s i t é d'émission entre 78 K e t 4.2 K é t a i t plus grande q u ' e n t r e l a température ambiante e t 78 K. Ce comportement s'explique par l a v a r i a t i o n de l ' a u t o - a b s o r p t i o n résonante (effet Mossbauer) avec l a température.

Abstract.- The counting rate of the 88 keV Ag1 0 9 my-rays emitted from a Cd109-doped silver single crystal was measured as a function of temperature. The decrease in emission intensity between 78 K and 4.2 K was greater than the decrease between room temperature and 78 K. This behaviour is explai- ned in terms of the temperature dependence of the resonant self absorption (Mossbauer effect).

The long-lived nuclear excited states with their sharp resonance lines have intrigued scientists since the discovery of the Mossbauer effect. The first excited states of the two silver isotopes Ag and A g1 0 9 have extremely long life times, of the or- der of minutes. The mean life time of the 88 keV ex- cited state of A g1 0 9 is 57.7 s corresponding to a natural line width of T = 1.1 x 10~17eV and a theo- retical energy resolution of T/Ey = 1.3 x 10~22. This is nearly 108 times smaller than the value for Fe5 7: T/Ey - 3 x 10~1 3. Of course, common techniques of Doppler modulation are completely inadequate for such a case. In the past, two attempts on the silver iso- topes have been published : on A g1 0 7 in 1964 /l/ and on A g1 0 9 in 19 77 111. A technique was used where source and absorber were brought together and after removing the source the induced activity was measu- red. However, the number of counts was rather small and the results were inconclusive.

The essential feature of our experiment is that source and absorber form one stationary unit consisting of a silver single crystal. This crystal contains the absorber A g1 0 9 with a natural abundance of 48.6 % and the parent isotope for the source, Cd , which has a mean life time of 470 days and populates the 88 keV excited state of A g1 0 9 which decays with a conversion coefficient of 12.5. The Mossbauer effect was searched for in the temperature dependence of the self-absorption of the 88 keV y- rays. The emitted y-rays experience electronic and resonant nuclear absorption. The latter has the tem- perature dependence of the Mossbauer fraction f (f- factor). Taking the Debye temperature of Ag as 0 = 226 K, the f-factor has been evaluated as shown in figure 1.

Fig. 1 : Temperature dependence of the f-factor of A g1 0 9 in silver.

For our experiment this figure represents the theo- retical limit on the amount by which the y-emission can be reduced by resonant absorption. From 78 K to 4.2 K the f-factor increases from 8 x i0~3 to 5.5 x 10- 2. If the geometrical conditions are kept the same the expected increase in resonant absorp- tion at the lower temperature would be reflected in a decrease in the counting rate. In our first ap- proach the parent radio isotopes were produced in the Ag single crystal by the nuclear reactions Ag1 0 7>1 0 9(p,n) C d1 0 7'1 0 9. The irradiation was carried out in the Karlsruhe cyclotron. Because both Cd109

and C d1 0 7 were produced it seemed possible to obtain resonances simultaneously in both A g1 0 9 and in Ag1 0 7, the latter having also a long-lived excited state with similar nuclear parameters. However, the cyclo-

tron irradiation causes strong background radioacti- vity even after the short-lived isotopes have de- cayed and it was difficult to distinguish the 88keV

(Ag1 0 9) and 93.5 keV (Ag107) y-radiation from the

background radiation. In addition, there exists the problem of radiation damage caused by the nuclear

5

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979216

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C2-48

JOURNAL DE PHYSIQUE

r e a c t i o n . Even w i t h subsequent a n n e a l i n g treatment e q u a l t o t h e observed e f f e c t , which means t h a t t h e r e one cannot exclude t h e p o s s i b i l i t y t h a t some l a t t i c e

damage remains around l a t t i c e s i t e s where t h e nu- c l e a r r e a c t i o n s took p l a c e . Therefore we used t h e a l t e r n a t i v e of t h e r m a l l y d i f f u s i n g the cd109 i n t o t h e Ag s i n g l e c r y s t a l . The d i f f u s i o n parameters ( t e m p e r a t u r e and time) were chosen i n such a way t h a t t h e mean p e n e t r a t i o n d e p t h was 125 ym. The emission spectrum of such an

A ~ : c ~ ' "

source o b t a i - ned w i t h a hyperpure Ge d e t e c t o r i s shown i n f i g u r e 2. The 88 keV l i n e s t a n d s o u t c l e a r l y w i t h t h i s Ge d e t e c t o r which has a r e s o l u t i o n of 500 eV.

i s v e r y l i t t l e i f any change i n t h e n u c l e a r absor- p t i o n between room temperature and 78 K , a s i s t o be expected from f i g u r e 1 . But t h e a d d i t i o n a l e l e c t r o - n i c e f f e c t f o r t h e d e c r e a s e from 78 K t o 4.2 K i s c a l c u l a t e d t o be only 0.03 %, which i s much l e s s t h a n t h e observed e f f e c t . T h e r e f o r e , most of t h e ob- served e f f e c t between 78 K and 4.2 K must be due t o t h e expected i n c r e a s e i n n u c l e a r a b s o r p t i o n .

86 88 90

energy

I

keVI

Fig. 2 : Emission spectrum of a C d ' 0 9 - ~ g source as measured by a Ge d e t e c t o r . The source was produ- ced by thermal d i f f u s i o n .

I n t h e experiments t h e counting r a t e was mea- sured a t t h r e e temperatures : 4.2 K , 78 K and room temperature. Each run l a s t e d n e a r l y a day, and t h e c o u n t s (about 1.7 x 107)were accumulated f o r 80000 s (22h 13' 20") s t a r t i n g a t t h e same time e a c h day.

Temperature changes up o r down were made between runs. The r e s u l t s a r e p l o t t e d i n f i g u r e 3. The s l o - pes of t h e curves i n d i c a t e t h e decay of cd'09. The lower counting r a t e s a t the lower temperatures can be due p a r t l y t o i n c r e a s e d n u c l e a r resonance r e s u l - t i n g from t h e expected i n c r e a s e i n t h e f - f a c t o r , and p a r t l y t o i n c r e a s e d e l e c t r o n i c a b s o r p t i o n r e s u l t i n g from thermal c o n t r a c t i o n o f , t h e - s i l v e r l a t t i c e . For t h e d e c r e a s e from room temperature t o 78 K, t h e cal- c u l a t e d e l e c t r o n i c e f f e c t i s 0.14 %. This i s about

time [dl

F i

.

3 : Counting r a t e of t h e 88 keV y - r a d i a t i o n of Cdqo9-Ag source a t room temperature (O), a t 78 K

(a)

and a t 4.2 K ( A ) .

We b e l i e v e t h a t t h i s e s t a b l i s h e s t h e MEssbauer e f - f e c t i n t h i s i s o t o p e . F i n a l l y , t h e e f f e c t i v e l i n e width may be e s t i m a t e d . By t a k i n g i n t o account t h e mean e f f e c t i v e resonance absorber t h i c k n e s s i n our experiment an h y p o t h e t i c a l l i n e i n t e n s i t y ( d i p ) of 2.8 % i s c a l c u l a t e d on t h e b a s i s of a n a t u r a l l i n e width. A c t u a l l y an e f f e c t of only 0.1 % was observed.

I f we assume f o r t h i s resonance a L o r e n t z i a n l i n e shape where t h e product of l i n e width and depth i s conserved we o b t a i n an e f f e c t i v e width

reff

of a- bout 30 times t h e n a t u r a l l i n e w i d t h , o r

reff/Ey

=

4 x 1

o - ~ '.

F u r t h e r experiments a r e i n p r o g r e s s , i n p a r t i c u l a r t o d e t e c t t h e i n f l u e n c e of t h e g r a v i - t a t i o n a l f i e l d on t h e photons.

Acknowledgement.- We a r e t h a n k f u l t o P r o f . W. Klose and D r . W. Schweikert f o r making t h e Karlsruhe cy- c l o t r o n a v a i l a b l e f o r t h e i r r a d i a t i o n of t h e Ag sin- g l e c r y s t a l s . We a r e g r a t e f u l t o P r o f . R.S. P r e s t o n f o r h e l p f u l d i s c u s s i o n s .

References

/ I / B i z i n a , G.E., Beda, A.G., Burgov, N.A., Davydov, A.V., Sov. Phys. JETP

2

(1964) 973.

/ 2 / Alpatov, V.G., Beda, A.G., B i z i n a , G.E., Davydw, A.V., Korotokov, Y.M., Proc. I n t . Conf. on Moss- bauer S p e c t r . Bucharest, Romania (1977) 43.

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