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Experimental evidence for the nuclear structure effect in internal conversion of N- and O-shell electrons
Volume 29B, n u m b e r 4
PHYSICS
EXPERIMENTAL IN INTERNAL
LETTERS
12 May 1969
EVIDENCE FOR THE NUCLEAR STRUCTURE EFFECT CONVERSION OF N- AND O-SHELL ELECTRONS
O. D R A G O U N and Z. P L A J N E R Nuclear R e s e a r c h Institute, !~e~ near Prague, Czechoslovakia
and B. M A R T I N a n d A. v. K A P - H E R R M a x - P l a n c k - I n s t i t u t f l i t K e r n p h y s i k , Heidelberg, Germany Received 10 April 1969 I n t e r n a l c o n v e r s i o n line intensities were m e a s u r e d for the hindered 58 keV E1 transition in 180Hf. Anomalies were o b s e r v e d for the N - s u b s h e l l and O-total conversion coefficients and were explained with a nuclear c u r r e n t p a r a m e t e r ?7 = - 6.5 • 0.7 derived f r o m the L- and M-subshell ratios.
The internal conversion process is known to b e u s u a l l y i n d e p e n d e n t of n u c l e a r s t r u c t u r e . F o r hindered transitions, however, the internal conversion coefficients (ICC) can deviate substantially f r o m t h e t h e o r e t i c a l v a l u e s c a l c u l a t e d w i t h o u t terms involving nuclear wave functions. For a n u m b e r of t r a n s i t i o n s , t h e e f f e c t w a s o b s e r v e d f o r t h e c o n v e r s i o n e l e c t r o n s e m i t t e d f r o m t h e K, L a n d M a t o m i c s h e l l s [e.g.1]. T h e a i m of t h i s letter is to demonstrate that the nuclear struct u r e c a n i n f l u e n c e t h e ICC of t h e N a n d O a t o m i c s h e l l s too. T h e 58 k e V E1 t r a n s i t i o n ( 8 - ~ 8 +) in 180Hf w i t h a h i n d r a n c e f a c t o r 1016 w a s c h o s e n f o r t h e investigation, as the experimental L-subshell ICC r a t i o s [2,3] a r e a b o u t t w i c e a s l a r g e a s t h e t h e o r e t i c a l p r e d i c t i o n s . T e n s o u r c e s of 180Hf were prepared by neutron irradiation (flux 9 × × 1013 c m - 2 s - l ) of 95% e n r i c h e d 179Hf e v a p o r a t e d in v a c u o o n t o a t h i n c a r b o n f o i l . T h e conversion electron spectrum was measured with a m a g n e t i c i o n - f r e e s p e c t r o m e t e r [4] ( s i x r u n s f o r t h e M, N a n d O g r o u p s a n d t w o f o r t h e L l i n e s ) . T h e e x p e r i m e n t a l ICC r a t i o s g i v e n in table 1 were determined with the improved method of t a b u l a t e d s t a n d a r d l i n e s [5]. T h e ICC a i n c l u d i n g t h e n u c l e a r s t r u c t u r e e f f e c t c a n b e e x p r e s s e d in t h e f o r m [1]: a = a (0) ( l + a l T ? + a 2 ~ 2 + a 3 ~ + a 4 ~ a 5
~2) ,
(1)
w h e r e a (0) i s t h e n u c l e a r s t r u c t u r e i n d e p e n d e n t ICC a n d a i a r e t h e c o r r e c t i o n c o e f f i c i e n t s c a l c u l a t e d f r o m t h e e l e c t r o n w a v e f u n c t i o n s only. T h e 77 a n d ~ a r e t h e n u c l e a r c u r r e n t a n d n u c l e a r charge parameters, respectively, which depend o n t h e n u c l e a r w a v e f u n c t i o n s . T h e a(0) a n d a i
I
o -5 -lo -15
I
l
I
-~- - 6 . 5 ± 0 . 7 ~
L2/L3
=5 0 - ' 2 2 0 ~ I
~ I
l
I
LllL 3 I
I
~" 0
® E -5 -lO ~ ~15 P_
~1= _ 6 . 6 ± 0 . 7 ~ ~=3 0 ± 2 5 0 ~ l
I
M2]M3 MI/M3
~ I
|
l
O ~ -5 2 -10 -15 -20
T1--9.0±2.0~ ~=50-'720~
N21N3
NI/N3
I
I
-2000
-1000
I
I
I
0 1000 2O00 nuclear charge parameter
Fig. 1. Determination of the nuclear s t r u c t u r e ~ a r a m e t e r s ?)and ~ for the 58 keV E1 t r a n s i t i o n in 180Hf from the e x p e r i m e n t a l ICC ratios. v a l u e s f o r t h e L, M a n d N s u b s h e l l s w e r e o b t a i n e d w i t h a c o m p u t e r p r o g r a m [6] u s i n g t h e n o n - r e l a tivistic self-consistent field atomic model. The a p p r o x i m a t e O - s h e l l ICC w e r e c a l c u l a t e d a c c o r d i n g to r e f . [7]. T h e c o m p a r i s o n of e x p e r i m e n t a l a n d t h e o r e t i c a l ICC r a t i o s p e r f o r m e d in t a b l e 2 s h o w s t h a t t h e a n o m a l i e s o b s e r v e d f o r t h e L, M and N shells are equal within the experimental 221
Volume 29B. number 4
PHYSICS
Table 1 Experimental ICC ratios for the 58 keV E1 transition in 180ttf. Atomic shell
I/II
Subshell ratio I/III
II/III
L M N
3.9 • 0.3 3.8 • 0.4 3.7 4= 0.5
4.4 • 0.4 4.6 i 0.4 6.6 ± 1.7
1.13 ~ 0.10 1.22 ~ 0.14 1.8 ~ 0.5
Table 2 Comparison of experimental and theoretical ICC ratios calculated without nuclear s t r u c t u r e effect for the 58 keV E1 transition in 180Hf. Atomic shell L M N
(Xi/Xk)exp/(Xi/Xk)te°r I/II 1.72 =~0.13 1.71 ~ 0.20 1.55=L 0.23
I/III 2.40 ± 0.21 2.56 ± 0.23 3.5 *_ 1.0
II/III 1.40 , 0.11 1.50 ± 0.17 2.3 ± 0.6
errors. Corresponding nuclear structure parame t e r s a r e a l s o c o n s i s t e n t ( s e e fig. 1). T h e y a r e in a g r e e m e n t w i t h t h e v a l u e s d e t e r m i n e d in t h i s w o r k f r o m t h e e x p e r i m e n t a l L - s u b s h e l l r a t i o s of r e f . 2 (7/ = - 7 . 8 ~ 1,4, ~ = - 2 5 0 ~ 510) a n d r e f . 3 (7 = - 6 . 4 + 0 . 5 , ~ = 1 7 0 ± 170). T h e e x p e c t e d v a l u e ~ = 0 [1] a g r e e s w i t h all e x p e r i m e n t a l d a t a . In o r d e r t o i n v e s t i g a t e t h e e x i s t e n c e of t h e nuclear structure effect for the O-shell electrons, the O/N intensity ratios were measured for the 58 k e V E1 a n d 93 k e V p u r e E2 t r a n s i t i o n s in 180Hf. T h e s e c o n d o n e w a s u s e d to t e s t t h e n e c e s s a r y s c r e e n i n g c o r r e c t i o n [7] f o r t h e O s h e l l ICC. T h e o b s e r v e d v a l u e ( O / N ) 9 3 = 0.14 ± 0.01 a g r e e s w i t h t h e t h e o r e t i c a l p r e d i c t i o n
222
LETTERS
12 May 1969
0.13. C o n s i d e r i n g t h e n u c l e a r s t r u c t u r e e f f e c t f o r t h e N - I C C o n l y 0 / = - 6 . 5 + 0.7, ~ = 0 ) , w e g e t ( O / N ) 5 8 = 0.07 in d i s a g r e e m e n t w i t h t h e e x p e r i m e n t a l r e s u l t 0.19 + 0.02. T h e d i s c r e p a n c y c a n be r e m o v e d a s s u m i n g t h a t t h e e f f e c t i s a l s o present for the O electrons. As the correction p a r a m e t e r s ai of eq. (1) f o r t h e L , M a n d N e l e c trons are roughly equal, the N-shell values were a p p l i e d to c a l c u l a t e t h e O - I C C . T h e v a l u e ( O / N ) 5 8 = 0.14 t h u s d e r i v e d a g r e e s r e a s o n a b l y w e l l w i t h t h e m e a s u r e d r a t i o 0.19 ~: 0.02. T h e c o n t r i b u t i o n of t h e P - s h e l l e l e c t r o n s to t h e o b s e r v e d O / N r a t i o w a s e s t i m a t e d [7] to b e a b o u t 10% w h i c h c a n n o t c h a n g e t h e p r e v i o u s c o n c l u s i o n . If t h e e f f e c t w o u l d b e o b s e r v e d a l s o f o r t h e v a l e n c e s h e l l e l e c t r o n s ( P e l e c t r o n s in Hf), o n e h a s an i n t e r e s t i n g c a s e w h e r e t h e e l e c t r o n s a r e i n f l u e n c e d by t h e c h e m i c a l s t a t e of t h e a t o m a s w e l l a s b y t h e s t r u c t u r e of t h e n u c l e u s . T h a n k s a r e due t o P r o f . W. G e n t n e r f o r h i s support.
References 1. H.C. Pauli and K. Alder, Z. Physik 202 (1967) 255. 2. G. Scharff-Goldhaber and M. M e K e o ~ , Phys. Rev. 158 {1967) 1105. 3. V. S. Gvozdev, V. N. Grigoryev and Y. V. Sergeenkov. Soviet J. Nucl. Phys. 8 (1968) 3. 4. H.Daniel, P. Jahn, M. Kuntze and G. Spannagel, Nuel I n s t r . and Method. 35 (1965) 171. 5. O. Dragoun, C1. Ribordy and O. Huber, Nucl. Phys. A124 (1969) 337. 6. H.C. Pauli, The Niels Bohr Institute Report {Copenhagen, 1968). 7. O,Dragoun et al., Phys. L e t t e r s 28B (1968) 251.
PHYSICS
EXPERIMENTAL IN INTERNAL
LETTERS
12 May 1969
EVIDENCE FOR THE NUCLEAR STRUCTURE EFFECT CONVERSION OF N- AND O-SHELL ELECTRONS
O. D R A G O U N and Z. P L A J N E R Nuclear R e s e a r c h Institute, !~e~ near Prague, Czechoslovakia
and B. M A R T I N a n d A. v. K A P - H E R R M a x - P l a n c k - I n s t i t u t f l i t K e r n p h y s i k , Heidelberg, Germany Received 10 April 1969 I n t e r n a l c o n v e r s i o n line intensities were m e a s u r e d for the hindered 58 keV E1 transition in 180Hf. Anomalies were o b s e r v e d for the N - s u b s h e l l and O-total conversion coefficients and were explained with a nuclear c u r r e n t p a r a m e t e r ?7 = - 6.5 • 0.7 derived f r o m the L- and M-subshell ratios.
The internal conversion process is known to b e u s u a l l y i n d e p e n d e n t of n u c l e a r s t r u c t u r e . F o r hindered transitions, however, the internal conversion coefficients (ICC) can deviate substantially f r o m t h e t h e o r e t i c a l v a l u e s c a l c u l a t e d w i t h o u t terms involving nuclear wave functions. For a n u m b e r of t r a n s i t i o n s , t h e e f f e c t w a s o b s e r v e d f o r t h e c o n v e r s i o n e l e c t r o n s e m i t t e d f r o m t h e K, L a n d M a t o m i c s h e l l s [e.g.1]. T h e a i m of t h i s letter is to demonstrate that the nuclear struct u r e c a n i n f l u e n c e t h e ICC of t h e N a n d O a t o m i c s h e l l s too. T h e 58 k e V E1 t r a n s i t i o n ( 8 - ~ 8 +) in 180Hf w i t h a h i n d r a n c e f a c t o r 1016 w a s c h o s e n f o r t h e investigation, as the experimental L-subshell ICC r a t i o s [2,3] a r e a b o u t t w i c e a s l a r g e a s t h e t h e o r e t i c a l p r e d i c t i o n s . T e n s o u r c e s of 180Hf were prepared by neutron irradiation (flux 9 × × 1013 c m - 2 s - l ) of 95% e n r i c h e d 179Hf e v a p o r a t e d in v a c u o o n t o a t h i n c a r b o n f o i l . T h e conversion electron spectrum was measured with a m a g n e t i c i o n - f r e e s p e c t r o m e t e r [4] ( s i x r u n s f o r t h e M, N a n d O g r o u p s a n d t w o f o r t h e L l i n e s ) . T h e e x p e r i m e n t a l ICC r a t i o s g i v e n in table 1 were determined with the improved method of t a b u l a t e d s t a n d a r d l i n e s [5]. T h e ICC a i n c l u d i n g t h e n u c l e a r s t r u c t u r e e f f e c t c a n b e e x p r e s s e d in t h e f o r m [1]: a = a (0) ( l + a l T ? + a 2 ~ 2 + a 3 ~ + a 4 ~ a 5
~2) ,
(1)
w h e r e a (0) i s t h e n u c l e a r s t r u c t u r e i n d e p e n d e n t ICC a n d a i a r e t h e c o r r e c t i o n c o e f f i c i e n t s c a l c u l a t e d f r o m t h e e l e c t r o n w a v e f u n c t i o n s only. T h e 77 a n d ~ a r e t h e n u c l e a r c u r r e n t a n d n u c l e a r charge parameters, respectively, which depend o n t h e n u c l e a r w a v e f u n c t i o n s . T h e a(0) a n d a i
I
o -5 -lo -15
I
l
I
-~- - 6 . 5 ± 0 . 7 ~
L2/L3
=5 0 - ' 2 2 0 ~ I
~ I
l
I
LllL 3 I
I
~" 0
® E -5 -lO ~ ~15 P_
~1= _ 6 . 6 ± 0 . 7 ~ ~=3 0 ± 2 5 0 ~ l
I
M2]M3 MI/M3
~ I
|
l
O ~ -5 2 -10 -15 -20
T1--9.0±2.0~ ~=50-'720~
N21N3
NI/N3
I
I
-2000
-1000
I
I
I
0 1000 2O00 nuclear charge parameter
Fig. 1. Determination of the nuclear s t r u c t u r e ~ a r a m e t e r s ?)and ~ for the 58 keV E1 t r a n s i t i o n in 180Hf from the e x p e r i m e n t a l ICC ratios. v a l u e s f o r t h e L, M a n d N s u b s h e l l s w e r e o b t a i n e d w i t h a c o m p u t e r p r o g r a m [6] u s i n g t h e n o n - r e l a tivistic self-consistent field atomic model. The a p p r o x i m a t e O - s h e l l ICC w e r e c a l c u l a t e d a c c o r d i n g to r e f . [7]. T h e c o m p a r i s o n of e x p e r i m e n t a l a n d t h e o r e t i c a l ICC r a t i o s p e r f o r m e d in t a b l e 2 s h o w s t h a t t h e a n o m a l i e s o b s e r v e d f o r t h e L, M and N shells are equal within the experimental 221
Volume 29B. number 4
PHYSICS
Table 1 Experimental ICC ratios for the 58 keV E1 transition in 180ttf. Atomic shell
I/II
Subshell ratio I/III
II/III
L M N
3.9 • 0.3 3.8 • 0.4 3.7 4= 0.5
4.4 • 0.4 4.6 i 0.4 6.6 ± 1.7
1.13 ~ 0.10 1.22 ~ 0.14 1.8 ~ 0.5
Table 2 Comparison of experimental and theoretical ICC ratios calculated without nuclear s t r u c t u r e effect for the 58 keV E1 transition in 180Hf. Atomic shell L M N
(Xi/Xk)exp/(Xi/Xk)te°r I/II 1.72 =~0.13 1.71 ~ 0.20 1.55=L 0.23
I/III 2.40 ± 0.21 2.56 ± 0.23 3.5 *_ 1.0
II/III 1.40 , 0.11 1.50 ± 0.17 2.3 ± 0.6
errors. Corresponding nuclear structure parame t e r s a r e a l s o c o n s i s t e n t ( s e e fig. 1). T h e y a r e in a g r e e m e n t w i t h t h e v a l u e s d e t e r m i n e d in t h i s w o r k f r o m t h e e x p e r i m e n t a l L - s u b s h e l l r a t i o s of r e f . 2 (7/ = - 7 . 8 ~ 1,4, ~ = - 2 5 0 ~ 510) a n d r e f . 3 (7 = - 6 . 4 + 0 . 5 , ~ = 1 7 0 ± 170). T h e e x p e c t e d v a l u e ~ = 0 [1] a g r e e s w i t h all e x p e r i m e n t a l d a t a . In o r d e r t o i n v e s t i g a t e t h e e x i s t e n c e of t h e nuclear structure effect for the O-shell electrons, the O/N intensity ratios were measured for the 58 k e V E1 a n d 93 k e V p u r e E2 t r a n s i t i o n s in 180Hf. T h e s e c o n d o n e w a s u s e d to t e s t t h e n e c e s s a r y s c r e e n i n g c o r r e c t i o n [7] f o r t h e O s h e l l ICC. T h e o b s e r v e d v a l u e ( O / N ) 9 3 = 0.14 ± 0.01 a g r e e s w i t h t h e t h e o r e t i c a l p r e d i c t i o n
222
LETTERS
12 May 1969
0.13. C o n s i d e r i n g t h e n u c l e a r s t r u c t u r e e f f e c t f o r t h e N - I C C o n l y 0 / = - 6 . 5 + 0.7, ~ = 0 ) , w e g e t ( O / N ) 5 8 = 0.07 in d i s a g r e e m e n t w i t h t h e e x p e r i m e n t a l r e s u l t 0.19 + 0.02. T h e d i s c r e p a n c y c a n be r e m o v e d a s s u m i n g t h a t t h e e f f e c t i s a l s o present for the O electrons. As the correction p a r a m e t e r s ai of eq. (1) f o r t h e L , M a n d N e l e c trons are roughly equal, the N-shell values were a p p l i e d to c a l c u l a t e t h e O - I C C . T h e v a l u e ( O / N ) 5 8 = 0.14 t h u s d e r i v e d a g r e e s r e a s o n a b l y w e l l w i t h t h e m e a s u r e d r a t i o 0.19 ~: 0.02. T h e c o n t r i b u t i o n of t h e P - s h e l l e l e c t r o n s to t h e o b s e r v e d O / N r a t i o w a s e s t i m a t e d [7] to b e a b o u t 10% w h i c h c a n n o t c h a n g e t h e p r e v i o u s c o n c l u s i o n . If t h e e f f e c t w o u l d b e o b s e r v e d a l s o f o r t h e v a l e n c e s h e l l e l e c t r o n s ( P e l e c t r o n s in Hf), o n e h a s an i n t e r e s t i n g c a s e w h e r e t h e e l e c t r o n s a r e i n f l u e n c e d by t h e c h e m i c a l s t a t e of t h e a t o m a s w e l l a s b y t h e s t r u c t u r e of t h e n u c l e u s . T h a n k s a r e due t o P r o f . W. G e n t n e r f o r h i s support.
References 1. H.C. Pauli and K. Alder, Z. Physik 202 (1967) 255. 2. G. Scharff-Goldhaber and M. M e K e o ~ , Phys. Rev. 158 {1967) 1105. 3. V. S. Gvozdev, V. N. Grigoryev and Y. V. Sergeenkov. Soviet J. Nucl. Phys. 8 (1968) 3. 4. H.Daniel, P. Jahn, M. Kuntze and G. Spannagel, Nuel I n s t r . and Method. 35 (1965) 171. 5. O. Dragoun, C1. Ribordy and O. Huber, Nucl. Phys. A124 (1969) 337. 6. H.C. Pauli, The Niels Bohr Institute Report {Copenhagen, 1968). 7. O,Dragoun et al., Phys. L e t t e r s 28B (1968) 251.