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Temperature effects on positron annihilation in metallic single crystals
Volume 38A, number 5
PHYSICS LETTERS
TEMPERATURE
EFFECTS
IN METALLIC
ON P O S I T R O N SINGLE
28 February 1972
ANNIHILATION
CRYSTALS
J. L. CAMPBELL, C.'W. SCHULTE and I. K. MACKENZIE
Department of Physics, University of Guelph, Guelph, Ontario. Canada Received 11 January 1972 A Ge(Li) spectrometer was used to measure the temperature-dependence of the intensity of gamma rays resulting from positron annihilation in single crystals of Pb, Sn, Ge and Cu. In contrast to recent angular distribution results, no significant temperature effects were observed.
V a r i o u s w o r k e r s have r e p o r t e d e x p e r i m e n t a l studies of the effect of t e m p e r a t u r e c h a n g e s on the a n g u l a r d i s t r i b u t i o n of the two g a m m a r a y s r e s u l t i n g f r o m the a n n i h i l a t i o n of p o s i t r o n s i n m e t a l l i c c r y s t a l s . Dekhtyar and Mikhalenkov[1] f i r s t r e p o r t e d the o b s e r v a t i o n of a 30% d e c r e a s e in the a r e a u n d e r the 2y a n g u l a r d i s t r i b u t i o n c u r v e when a b i s m u t h single c r y s t a l was cooled f r o m 300°K to 90°K. F a r a c i et al.[2, 3] then studied single c r y s t a l s and p o l y c r y s t a l s of v a r ious m e t a l s including A1, Zn, Pb and Bi, e m p l o y ing t e m p e r a t u r e s of 300°K and 77OK. They o b s e r ved l a r g e d e c r e a s e s (8 - 20%) in the a r e a u n d e r the a n g u l a r d i s t r i b u t i o n c u r v e (0 - 15mr) when single c r y s t a l s w e r e cooled, the r e d u c t i o n obeying a Z 3/2 law. No effect was o b s e r v e d for p o l y c r y s t a l s . F a r a c i et a l . s u g g e s t e d that the o b s e r ved effect is due to c h a n n e l i n g of p o s i t r o n s b e tween c r y s t a l p l a n e s r e s u l t i n g in enhanced a n n i h i l a t i o n in flight; the effect would be washed out at r o o m t e m p e r a t u r e by l a t t i c e v i b r a t i o n s and would i n c r e a s e with d e c r e a s i n g t e m p e r a t u r e as was o b s e r v e d [ 2 ] in the c a s e of Bi. Dekhtyar [4] has invoked a d i f f r a c t i o n a l r e f l e c tion of p o s i t r o n s f r o m l a t t i c e p l a n e s which would r e s u l t in a p r e d i c t a b l e degree of escape f r o m a single c r y s t a l . New a n g u l a r d i s t r i b u t i o n m e a s u r e m e n t s by Dekhtyar et al.[5, 8] support this hypot h e s i s . However l i f e t i m e m e a s u r e m e n t s by F a r aci et al.[7] give r e s u l t s in d i s a g r e e m e n t with the hypothesis. F i n a l l y , Berko et al.[8, 9] have twice p e r f o r m e d a n g u l a r d i s t r i b u t i o n m e a s u r e m e n t s with single c r y s t a l s and have been unable to o b s e r v e any significant t e m p e r a t u r e effects. In view of this p u z z l i n g situation, we have studied the t e m p e r a t u r e - d e p e n d e n c e of the i n t e n sity of the a n n i h i l a t i o n g a m m a ray peak c e n t r e d
at 511 keV, o b s e r v e d with a Ge(Li) s p e c t r o m e t e r . This method has the advantage that if enhanced a n n i h i l a t i o n i n flight, whether due to c h a n n e l i n g or some other p r o c e s s , does indeed occur, an i n c r e a s e in h i g h - e n e r g y g a m m a ray i n t e n s i t y would be o b s e r v e d , r e l a t e d to the d e c r e a s e in the the 511 keV peak. Such o b s e r v a t i o n of high e n ergy events is e x t r e m e l y difficult with the a n g u l a r c o r r e l a t i o n technique. Single c r y s t a l s and p o l y c r y s t a l s of Pb, Sn, Cu and Ge were studied. Samples were mounted on the cold f i n g e r of a c o m m e r c i a l c r y o s t a t which had f o r m e r l y housed a Ge(Li) detector. A v e r y thin 68Ge s o u r c e deposited on 0.8 rail Kapton foil was sandwiched between two c r y s t a l s . 68Ge was chosen since its high p o s i t r o n energy (1.9 MeV m a x i m u m ) would m i n i m i s e s u r f a c e effects, but n e v e r t h e l e s s each c r y s t a l was lapped and etched before mounting and the c r y o s t a t was i m m e d i a t e l y evacuated to p r e v e n t oxidation. Sample t e m p e r a t u r e was m o n i t o r e d by a t h e r m o couple. G a m m a r a d i a t i o n was detected in a coaxial Ge(Li) s p e c t r o m e t e r whose energy r e s o l u t i o n was 1.50 keV (fwhm) for the 514 keV g a m m a ray of 85Sr. The a n n i h i l a t i o n line c e n t r e d at 511 keV was r e c o r d e d i n a 4096-channel pulseheight a n a l y s e r , i n c o r p o r a t i n g a digital s t a b i l i s e r which held both the c e n t r o i d s of the 511 keV line and the 122 keV line of 57Co fixed to within a few eV. The 1078 keV g a m m a ray e m i t t e d in the decay of 68Ge to 68Zn was r e c o r d e d s i m u l t a n e o u s l y as a m o n i t o r of conditions that might a l t e r with t e m p e r a t u r e , such as s o u r c e motion due to c o n t r a c tion of the cold finger. In a t~pical 2 - h o u r run, about 1.5 × 104 and 1.5 x 10 ~' counts were a c c u m u l a t e d in the 1078and 511 keV peaks r e s p e c t i v e l y . As shown in 377
Volume 38A, number 5
PHYSICS LETTERS
Table 1 Percentage increases in 511 keV intensity (A) and in intensity ratio of 511 and 1078 keV gamma rays (R) when temperature is lowered from 300°K to 77°K. (Experimental e r r o r is one standard deviation). Single crystals A Lead Tin Copper Germanium
R
0.'7 ~ 0 . i 1.2±1.2 3.0±0.1 2.3~ 1.2 2.2 ± 0.1 1.3±1.4 1.2 ± 0.1 1.3±0.8
Polycrystals A
R
1.8~0.1 0.8±1 1.6±0.1 1.6±1.2 1.3 ~0.1 0.8±1.4
t a bl e 1, f o r e v e r y s a m p l e l o w e r i n g the t e m p e r a t u r e r e s u l t e d in slightly i n c r e a s e d i n t e n s it y of the annihilation line, an o b s e r v a t i o n opposite to that of o t h e r w o r k e r s [1-3]. H o w e v e r the 1078 keV i n t e n s i t y a l s o i n c r e a s e d in a ll but one c a s e by a m o u n t s c o m p a r a b l e to the a s s o c i a t e d s t a t i s t i c a l e r r o r . We a l s o give in table 1 the i n c r e a s e in the r a t i o n (R) of t h e s e i n t e n s i t i e s , which i s e f f e c t i v e ly the i n c r e a s e in annihilation, c o r r e c t e d f o r p o s sible g e o m e t r y e f f e c t s . T h e r e is c l e a r l y no e v i dence fo r any c h a n g e s i n R within the e r r o r l i m i t of about 1% s t a n d a r d deviation. We a r e t h e r e f o r e led to a g r e e with B e r k o [9] that any t e m p e r a t u r e - i n d u c e d c h a n g e s in the i n t e nsit y of annihilation r a d i a t i o n f r o m m e t a l l i c s i n g l e c r y s t a l s a r e s m a l l , c e r t a i n l y at m o s t an o r d e r of magnitude l e s s than t h o s e r e p o r t e d by F a r a c i et al. We note that the l a t t e r a u t h o r s u s e d a l o w - e n e r g y p o s i t r o n s o u r c e (22Na), r e n d e r i n g crystal surface effects more probable; further, s m a l l v a r i a t i o n s of s o u r c e p o s i t i o n due to cooling could have s e r i o u s c o n s e q u e n c e s in the a n g u l a r d i s t r i b u t i o n e x p e r i m e n t s while the method u s e d
378
28 February 1972
h e r e enabled us to m o n i t o r and c o r r e c t for such variations. Of c o u r s e our r e s u l t does not r u l e out a c h a n neling effect f o r p o s i t r o n s ; it simply i n d i c a t e s that l o w e r i n g of the t e m p e r a t u r e does not r e s u l t in the o c c u r r e n c e of any p r o c e s s which s u b s e quently m a n i f e s t s i t s e l f by en h an cem en t of an n i hilation in flight. H o w e v e r if channeling i s to be c i t e d as such a p r o c e s s , then o b s e r v a t i o n of the effect must depend v e r y s t r o n g l y on the e x p e r i m e n t a l g e o m e t r y used. This w o r k was su p p o r t ed by the National R e s e a r c h Council of Canada. We thank Mr. A. R. Singh f o r his e x t r e m e l y p ai n st ak i n g p r e p a r a t i o n of the m e t a l s a m p l e s u s e d and Dr. R. B a r c l a y f o r h i s ad v i ce.
References [1] I.Ya. Dekhtyar and V. S. Mikhalenkov, SOv.Phys. Doklady 6 (1961) 31. [2] G. Faraei, I.F. Quercia, M. Spadoni and E. Turrisi, Nuovo Cimente 60]3 (1969) 228. [3] G. Faraei, G. Foti, I.F.Quereta and E.Turrisi, Phys. Rev. B2 (1970) 2335. [4] I. Ya~Dekhtyar, Phys. Lett. 30A (1969) 462. [5] I. Ya.Dekhtyar, V. S. Mikhalenkov, Phys. Star. Sol. (b) 47 (1971) Kl17. [6] I. Ya.Dekhtyar and V. S. Mikhalenkov, Phys. Stat. Sol. (b) 47 (1971) K121. [7] G. Faraci, G. Foti and E. Turrisi, Phys. Lett. 31A (1970) 427. [8] F.I. Davis and S. Herko, Bull. Am. Phys. Soe., 2,8 (1963) 74. [9] S. Berko, Proe. Second Intern. Conf. on Positron annihilation eds. B. G. Hogg and A. T. Stewart (Queen's University, Kingston, Canada, 1971), p.175.
PHYSICS LETTERS
TEMPERATURE
EFFECTS
IN METALLIC
ON P O S I T R O N SINGLE
28 February 1972
ANNIHILATION
CRYSTALS
J. L. CAMPBELL, C.'W. SCHULTE and I. K. MACKENZIE
Department of Physics, University of Guelph, Guelph, Ontario. Canada Received 11 January 1972 A Ge(Li) spectrometer was used to measure the temperature-dependence of the intensity of gamma rays resulting from positron annihilation in single crystals of Pb, Sn, Ge and Cu. In contrast to recent angular distribution results, no significant temperature effects were observed.
V a r i o u s w o r k e r s have r e p o r t e d e x p e r i m e n t a l studies of the effect of t e m p e r a t u r e c h a n g e s on the a n g u l a r d i s t r i b u t i o n of the two g a m m a r a y s r e s u l t i n g f r o m the a n n i h i l a t i o n of p o s i t r o n s i n m e t a l l i c c r y s t a l s . Dekhtyar and Mikhalenkov[1] f i r s t r e p o r t e d the o b s e r v a t i o n of a 30% d e c r e a s e in the a r e a u n d e r the 2y a n g u l a r d i s t r i b u t i o n c u r v e when a b i s m u t h single c r y s t a l was cooled f r o m 300°K to 90°K. F a r a c i et al.[2, 3] then studied single c r y s t a l s and p o l y c r y s t a l s of v a r ious m e t a l s including A1, Zn, Pb and Bi, e m p l o y ing t e m p e r a t u r e s of 300°K and 77OK. They o b s e r ved l a r g e d e c r e a s e s (8 - 20%) in the a r e a u n d e r the a n g u l a r d i s t r i b u t i o n c u r v e (0 - 15mr) when single c r y s t a l s w e r e cooled, the r e d u c t i o n obeying a Z 3/2 law. No effect was o b s e r v e d for p o l y c r y s t a l s . F a r a c i et a l . s u g g e s t e d that the o b s e r ved effect is due to c h a n n e l i n g of p o s i t r o n s b e tween c r y s t a l p l a n e s r e s u l t i n g in enhanced a n n i h i l a t i o n in flight; the effect would be washed out at r o o m t e m p e r a t u r e by l a t t i c e v i b r a t i o n s and would i n c r e a s e with d e c r e a s i n g t e m p e r a t u r e as was o b s e r v e d [ 2 ] in the c a s e of Bi. Dekhtyar [4] has invoked a d i f f r a c t i o n a l r e f l e c tion of p o s i t r o n s f r o m l a t t i c e p l a n e s which would r e s u l t in a p r e d i c t a b l e degree of escape f r o m a single c r y s t a l . New a n g u l a r d i s t r i b u t i o n m e a s u r e m e n t s by Dekhtyar et al.[5, 8] support this hypot h e s i s . However l i f e t i m e m e a s u r e m e n t s by F a r aci et al.[7] give r e s u l t s in d i s a g r e e m e n t with the hypothesis. F i n a l l y , Berko et al.[8, 9] have twice p e r f o r m e d a n g u l a r d i s t r i b u t i o n m e a s u r e m e n t s with single c r y s t a l s and have been unable to o b s e r v e any significant t e m p e r a t u r e effects. In view of this p u z z l i n g situation, we have studied the t e m p e r a t u r e - d e p e n d e n c e of the i n t e n sity of the a n n i h i l a t i o n g a m m a ray peak c e n t r e d
at 511 keV, o b s e r v e d with a Ge(Li) s p e c t r o m e t e r . This method has the advantage that if enhanced a n n i h i l a t i o n i n flight, whether due to c h a n n e l i n g or some other p r o c e s s , does indeed occur, an i n c r e a s e in h i g h - e n e r g y g a m m a ray i n t e n s i t y would be o b s e r v e d , r e l a t e d to the d e c r e a s e in the the 511 keV peak. Such o b s e r v a t i o n of high e n ergy events is e x t r e m e l y difficult with the a n g u l a r c o r r e l a t i o n technique. Single c r y s t a l s and p o l y c r y s t a l s of Pb, Sn, Cu and Ge were studied. Samples were mounted on the cold f i n g e r of a c o m m e r c i a l c r y o s t a t which had f o r m e r l y housed a Ge(Li) detector. A v e r y thin 68Ge s o u r c e deposited on 0.8 rail Kapton foil was sandwiched between two c r y s t a l s . 68Ge was chosen since its high p o s i t r o n energy (1.9 MeV m a x i m u m ) would m i n i m i s e s u r f a c e effects, but n e v e r t h e l e s s each c r y s t a l was lapped and etched before mounting and the c r y o s t a t was i m m e d i a t e l y evacuated to p r e v e n t oxidation. Sample t e m p e r a t u r e was m o n i t o r e d by a t h e r m o couple. G a m m a r a d i a t i o n was detected in a coaxial Ge(Li) s p e c t r o m e t e r whose energy r e s o l u t i o n was 1.50 keV (fwhm) for the 514 keV g a m m a ray of 85Sr. The a n n i h i l a t i o n line c e n t r e d at 511 keV was r e c o r d e d i n a 4096-channel pulseheight a n a l y s e r , i n c o r p o r a t i n g a digital s t a b i l i s e r which held both the c e n t r o i d s of the 511 keV line and the 122 keV line of 57Co fixed to within a few eV. The 1078 keV g a m m a ray e m i t t e d in the decay of 68Ge to 68Zn was r e c o r d e d s i m u l t a n e o u s l y as a m o n i t o r of conditions that might a l t e r with t e m p e r a t u r e , such as s o u r c e motion due to c o n t r a c tion of the cold finger. In a t~pical 2 - h o u r run, about 1.5 × 104 and 1.5 x 10 ~' counts were a c c u m u l a t e d in the 1078and 511 keV peaks r e s p e c t i v e l y . As shown in 377
Volume 38A, number 5
PHYSICS LETTERS
Table 1 Percentage increases in 511 keV intensity (A) and in intensity ratio of 511 and 1078 keV gamma rays (R) when temperature is lowered from 300°K to 77°K. (Experimental e r r o r is one standard deviation). Single crystals A Lead Tin Copper Germanium
R
0.'7 ~ 0 . i 1.2±1.2 3.0±0.1 2.3~ 1.2 2.2 ± 0.1 1.3±1.4 1.2 ± 0.1 1.3±0.8
Polycrystals A
R
1.8~0.1 0.8±1 1.6±0.1 1.6±1.2 1.3 ~0.1 0.8±1.4
t a bl e 1, f o r e v e r y s a m p l e l o w e r i n g the t e m p e r a t u r e r e s u l t e d in slightly i n c r e a s e d i n t e n s it y of the annihilation line, an o b s e r v a t i o n opposite to that of o t h e r w o r k e r s [1-3]. H o w e v e r the 1078 keV i n t e n s i t y a l s o i n c r e a s e d in a ll but one c a s e by a m o u n t s c o m p a r a b l e to the a s s o c i a t e d s t a t i s t i c a l e r r o r . We a l s o give in table 1 the i n c r e a s e in the r a t i o n (R) of t h e s e i n t e n s i t i e s , which i s e f f e c t i v e ly the i n c r e a s e in annihilation, c o r r e c t e d f o r p o s sible g e o m e t r y e f f e c t s . T h e r e is c l e a r l y no e v i dence fo r any c h a n g e s i n R within the e r r o r l i m i t of about 1% s t a n d a r d deviation. We a r e t h e r e f o r e led to a g r e e with B e r k o [9] that any t e m p e r a t u r e - i n d u c e d c h a n g e s in the i n t e nsit y of annihilation r a d i a t i o n f r o m m e t a l l i c s i n g l e c r y s t a l s a r e s m a l l , c e r t a i n l y at m o s t an o r d e r of magnitude l e s s than t h o s e r e p o r t e d by F a r a c i et al. We note that the l a t t e r a u t h o r s u s e d a l o w - e n e r g y p o s i t r o n s o u r c e (22Na), r e n d e r i n g crystal surface effects more probable; further, s m a l l v a r i a t i o n s of s o u r c e p o s i t i o n due to cooling could have s e r i o u s c o n s e q u e n c e s in the a n g u l a r d i s t r i b u t i o n e x p e r i m e n t s while the method u s e d
378
28 February 1972
h e r e enabled us to m o n i t o r and c o r r e c t for such variations. Of c o u r s e our r e s u l t does not r u l e out a c h a n neling effect f o r p o s i t r o n s ; it simply i n d i c a t e s that l o w e r i n g of the t e m p e r a t u r e does not r e s u l t in the o c c u r r e n c e of any p r o c e s s which s u b s e quently m a n i f e s t s i t s e l f by en h an cem en t of an n i hilation in flight. H o w e v e r if channeling i s to be c i t e d as such a p r o c e s s , then o b s e r v a t i o n of the effect must depend v e r y s t r o n g l y on the e x p e r i m e n t a l g e o m e t r y used. This w o r k was su p p o r t ed by the National R e s e a r c h Council of Canada. We thank Mr. A. R. Singh f o r his e x t r e m e l y p ai n st ak i n g p r e p a r a t i o n of the m e t a l s a m p l e s u s e d and Dr. R. B a r c l a y f o r h i s ad v i ce.
References [1] I.Ya. Dekhtyar and V. S. Mikhalenkov, SOv.Phys. Doklady 6 (1961) 31. [2] G. Faraei, I.F. Quercia, M. Spadoni and E. Turrisi, Nuovo Cimente 60]3 (1969) 228. [3] G. Faraei, G. Foti, I.F.Quereta and E.Turrisi, Phys. Rev. B2 (1970) 2335. [4] I. Ya~Dekhtyar, Phys. Lett. 30A (1969) 462. [5] I. Ya.Dekhtyar, V. S. Mikhalenkov, Phys. Star. Sol. (b) 47 (1971) Kl17. [6] I. Ya.Dekhtyar and V. S. Mikhalenkov, Phys. Stat. Sol. (b) 47 (1971) K121. [7] G. Faraci, G. Foti and E. Turrisi, Phys. Lett. 31A (1970) 427. [8] F.I. Davis and S. Herko, Bull. Am. Phys. Soe., 2,8 (1963) 74. [9] S. Berko, Proe. Second Intern. Conf. on Positron annihilation eds. B. G. Hogg and A. T. Stewart (Queen's University, Kingston, Canada, 1971), p.175.