PRECISION DETERMINATION OF THE ISOMER SHIFT OF ZN-67 IN ZNO

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PRECISION DETERMINATION OF THE ISOMER SHIFT OF ZN-67 IN ZNO

T. Katila, K. Riski, J. Ylä-Jääski

To cite this version:

T. Katila, K. Riski, J. Ylä-Jääski. PRECISION DETERMINATION OF THE ISOMER SHIFT OF ZN-67 IN ZNO. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-121-C1-122.

�10.1051/jphyscol:1980125�. �jpa-00219693�

JOURNAL DE PHYSIQUE Colloque C1, supplkment au no 1, Tome 41, janvier 1980, page C1-121

PRECISION DFTERMINATION OF

ME

T. Katila, K.J. Riski and J. Yla-Jagski

Department o f Technical Physics, Helsinki U n i v e r s i t y o f Technology, SF-02150 Espoo 15, Finland.

Mdssbauer e x p e r i m e n t s a r e s u i t a b l e f o r a c - c u r a t e d e t e r m i n a t i o n o f s h i f t s o f t h e gam- ma-ray e n e r g y . I n s u c h e x p e r i m e n t s , t h e i s o m e r s h i f t o f t h e r e s o n a n c e l i n e h a s t o b e known p r e c i s e l y . I s o m e r s h i f t i s u s u a l l y a c h a r a c t e r i s t i c p r o p e r t y o f t h e c h e m i c a l c o m p o s i t i o n , b u t it i s i n f l u e n c e d by v a r i - a b l e s s u c h a s t e m p e r a t u r e , p r e s s u r e e t c . I s o t o p i c e n r i c h m e n t / I / o r i m p u r i t i e s a r e a l s o known t o c h a n g e t h e i s o m e r s h i f t .

The 93.3 keV Mdssbauer t r a n s i t i o n o f 6 7 ~ n i s e s p e c i a l l y s u i t a b l e f o r a c c u r a t e d e t e r - m i n a t i o n o f e n e r g y s h i f t s , s i n c e t h e n a t u - r a l l i n e w i d t h i s o n l y 0.32 pm/s. The r e s o - n a n c e i s somewhat t e d i o u s t o o b s e r v e due t o t h e h i g h gamma-ray e n e r g y and s m a l l r e s o - n a n c e e f f e c t e v e n a t low t e m p e r a t u r e s . Most 6 7 ~ n Mdssbauer measurements h a v e been p e r f o r m e d w i t h a 6 7 ~ a : Z n 0 s o u r c e and w i t h a ZnO a b s o r b e r . U s i n g s i n g l e c r y s t a l s , n a t - u r a l l i n e w i d t h h a s b e e n o b s e r v e d / 2 / . The l i n e w i d t h i n powders i s much b r o a d e r . No a c c u r a t e d e t e r m i n a t i o n of t h e i s o m e r s h i f t i s g i v e n i n t h e l i t e r a t u r e . We h a v e p e r - formed p r e c i s i o n measurements o f t h e i s o m e r s h i f t ( I S ) o f ZnO. The I S d e p e n d s on t h e sample and c h a n g e s d u r i n g h e a t t r e a t m e n t . EXPERIMENTS

The c e n t r a l p a r t o f t h e measurement s y s t e m i s shown i n F i g . 1 . The s o u r c e and t h e ab-

DRIVE SOURCE ABSORBER He GAS AT REDUCED PRESSLRE

---

/--

F i g . 1. The c e n t r a l p a r t s o f t h e measure- ment s y s t e m .

The a b s o r b e r s w e r e e i t h e r n a t u r a l o r en- r i c h e d p o w d e r s , o r s i n g l e w y s t a l s . Some s a m p l e s w e r e g r o u n d m e c h a z i c a l l y . Samples w e r e a n n e a l e d a t v a r i o u s T e m p e r a t u r e s be- tween 700 ^{O C} and 1 2 3 0 O C . The p a r t i c l e s i z e s w e r e d e t e r m i n e d by e l e c t r o n m i c r o s c o - py. The a v e r a g e s i z e w a s a b o u t 1 um f o r e n r i c h e d r ~ o w d e r s arid l e s s t h a n 1 um f o r n a t u r a l powders. The s i n g l e c r y s t a l s w e r e grown froin v a p o u r p h a s e , t h e y w e r e t r a n s - p a r e n t and s y m m e t r i c p r i s m s of h i g h p u r i t y . The t h i c k n s s s of t h e n o n - e n r i c h e d a b s o r b e r s w a s a b o u t 2 g ZnO / c n 2 and t h a t o f e n r i c h - ed a b s o r b e r s 0 . 4

-

.

s o r b e r a r e l o c a t e d i n a b r a s s chamber s u s -

The D o p p l e r v e l o c i t y was g e n e r a t e d w i t h a pended by a s p r i n g t o r e d u c e h i g h f r e q u e n -

p i e z o e l e c t r i c PZT-4 t u b e . A s i n u s o i d a l c y v i b r a t i o n s . The chamber i s s u r r o u n d e d by

v o l t a g e a t 330 Hz was a p p l i e d b e t w e e n t h e 4 ~ e g a s a t 4 . 2 K.

i n n e r and o u t e r s u r f a c e s o f t h e t u b e . The The s o u r c e s were made by i r r a d i a t i n g ZnO v e l o c i t y c a l i b r a t i o n was made w i t h t h e a i d s i n g l e c r y s t a l s w i t h 1 0 MeV d e u t e r o n s . A o f t h e known q u a d r u p o l e s p l i t t i n g o f ZnO.

t y p i c a l d o s e was 30 pAh. A f t e r i r r a d i a t i o n The gamma-rays w e r e d e t e c t e d w i t h a NaI- t h e c r y s t a l s were a n n e a l e d a t 700 ^{O C} f o r s c i n t i l l a t o r and t h e p u l s e s w e r e c o l l e c t e d

1 2 h i n oxygen a t m o s p h e r e . i n MCS-mode.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980125

c1-122 JOURNAL Dl3 PHYSIQUE

RESULTS AND DISCUSSION

The centre lines of the Mbssbauer-spectra of two different ZnO absorbers are shown in Fig. 2. The isomer shift of the lower spectrum is clearly negative. For all ab- sorbers investigated, the isomer shifts were either negative or close to zero. En addition to the IS-values extracted from the computer fits, the shifts were also measured by operating the drive in constant velocity mode. The velocities +v and 0 were utilized, the results were in agreement with those obtained from the computer fits.

+A -a2 o 0.2 OL

VELOCITY (pmlsl

Fig. 2. Mbssbauer spectra of 6 7 ~ n 0 absor- bers at 4.2 K. The source is Ga-67 in a ZnO single crystal. The velocity sweep is si- nusoidal. Only the centre line is shown in the figure. a) A single crystal absorber, b) An absorber made of enriched powder and sintered for two hours at 1200 C.

Results for the isomer shift and for the linewidth are presented in Table 1. Sam- ples 1-4 were isotopically enriched pow- ders. Samples 5-9 were non-enriched, 5-6 were powders, 7 was made of single crys- tals, 8-9 were ground from these crystals until the particle size was below 10 um.

Results from X-ray measurements indicate that the c/a ratio of the hexagonal unit cell decreases and the volurhe of the cell increases during sintering. Grinding changes the colour to yellow and broadens the X-ray lines. Annealing restores white colour and sharpens the X-ray lines.

0.52(3)

-

0.54(3) 900 (air) 0.54(2) 1200 ( ~ r - 0 2 ) 0.57(5) grind.

0.49(6)

-

0.69(71 1000 (air) 0.31 ( 2 )

-

0.29(10) grind.

0.40(81 700 (0,)

L

T a b l e 1.Results from Mdssbauer measurements

of different ZnO absorbers.

Since 6 7 ~ n is a natural candidate for high precision Mdssbauer measurements, it is im- portant to note that at least for ZnO the isomer shift is not constant. Although the variation of the isomer shift may be unusu- ally high since ZnO is nonstoichiometric, similar effects are to be expected for oth- er Zn compounds and for other MCjssbauer iso'topes as well. The nonstoichiometry changes lattice parameters and hence the isomer shift and the electric field gradi- ent. We have observed the largest isomer shifts for sintered samples, and variation of lattice parameters due to sintering.

A report has been published on the search of the change of the second order Doppler shift due to isotopic enrichment /3/. The expected shift is only about one half of the shifts of our enriched samples (1-4) of Table 1 ./I/ Our results show that this type of experiment should be made with enriched single crystals as an absorber.

Part of the isomer shift variation, espe- cially in crushed samples, iS caused by large deformations of the lattice. Such deformations have been observed in ZnS.

ACKNOWLEDGEMENTS

We thank the Academy of Finland for finan- cial support and Dr. M. Hirvonen and Mr. P.

Helistb for help during the experiments.

One of us (KR) wishes to thank

Dr. W.

REFERENCES

3 1 Lipkin, H.J., Ann. Phys. %(I963128

2 ) Potzel,

W.,

G.M., J. Physique =(1976)C6-691 3) deWaard, H. and Perlow, G.J.,

Phys. Rev. Letters 2(1970)566