HAL Id: jpa-00218534
https://hal.archives-ouvertes.fr/jpa-00218534
Submitted on 1 Jan 1979
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
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�
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 9W. 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
109a 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
C2-48
JOURNAL DE PHYSIQUEr 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
keVIFig. 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 rreff/Ey
=
4 x 1o - ~ '.
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.