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Characteristic X-ray production from metals by 90 keV Ar+ ion bombardment
Volume26A. number 12
PHYSICS LETTERS
6May1968
CHARACTERISTIC X-RAY PRODUCTION FROM B Y 90 k e V A r + I O N B O M B A R D M E N T
METALS
F . W . SARIS, W . F . V A N DER WEG and J. KISTEMAKER FOM
-
I n s t i t u u t v o o r A t o o m - en M o l e c u u l f y s i c a , A m s t e r d a m ,
The Netherlands
Received 5 April 1968
Soft X-rays are observed during bombardment of metal surfaces with 90 keY Ar + ions. The wavelength involved is correlated to the inelastic energy loss during the ion-atom collision.
B e s i d e s e l e c t r o n s and photons, heavy c h a r g e d p a r t i c l e s can a l s o be employed for X - r a y p r o d u c tion [1]. To c r e a t e an i n n e r - s h e l l excitation, the i n e l a s t i c energy l o s s d u r i n g the c o l l i s i o n of the p r o j e c t i l e and the t a r g e t p a r t i c l e has to be m o r e than the energy of the i n n e r - s h e l l l e v e l . Snoek et al. [2] showed that the i n e l a s t i c e n e r g y l o s s d u r ing a c o l l i s i o n of a 90 keV Ar + ion with a Cu a t o m can be as high as 1500 eV. So t h e s e Ar + i o n s might be able to c r e a t e a v a c a n c y i n the Cu L shell which is about 930 eV below the g r o u n d level. The s u b s e q u e n t d e - e x c i t a t i o n may go v i a an A u g e r - t y p e t r a n s i t i o n which gives r i s e to s e c o n d a r y e l e c t r o n s , or via a r a d i a t i v e t r a n s i t i o n i.e. e m i s s i o n of a photon of 13.3 A (in the c a s e of Cu L). Using photographic detection we s u c c e e d e d in p r o v i n g the e m i s s i o n of t h e s e soft X - r a y s . In a pinhole c a m e r a an 8 ~ thick A1 foil and a Schumann f i l m w e r e mounted. An 8 ~ thick A1 foil i s t r a n s p a r e n t below abouto25 A except at the A1 K a b s o r p t i o n edge n e a r 8 A [4]. We b o m b a r d e d a nurr,b e r of m e t a l s with 90 keV Ar + ions with a b e a m i n t e n s i t y of 400 ~ A / c m 2 for one hour. T h e v a c u u m was 10 -6 T o r r . During the ion b o m b a r d ment many s e c o n d a r y p a r t i c l e s a r e r e l e a s e d f r o m the t a r g e t . T h e r e will be: r e f l e c t e d Ar a t o m s and ions of about 5 keV; s e c o n d a r y e l e c t r o n s of 0 until ~ 1000 eV; and s p u t t e r e d t a r g e t a t o m s of a few eV. The p e n e t r a t i o n depth of Ar p a r t i c l e s of 5 keV in A1 i s l e s s than 100/~ [7]. Only fast e l e c t r o n s can give l u m i n e s c e n c e of the A1 foil in the s p e c t r a l r e g i o n involved. T h e r e fore a m a g n e t i c field of 600 g a u s s i s a p p l i e d to deflect t h e s e p a r t i c l e s f r o m the foil. With an Ar ion b e a m of 40 # A and a s p u t t e r i n g r a t i o of 5 (for i n s t a n c e Cu) about 10~15~target p a r t i c l e s / s a r e sputtered. Through the a p e r t u r e of the c a m e r a only 1012 p a r t i c l e s / s will r e a c h the foil. 592
In a copper s u r f a c e the d e n s i t y of Cu a t o m s is 1.5 x 1015 a t o m s / c m 2. T h u s a f t e r 1.5 x 103 s e c o n d s one a t o m i c l a y e r of s p u t t e r e d Cu a t o m s i s f o r m e d on the foil. T h e Cu, Zn and A1 t a r g e t s gave an i m a g e on the f i l m , while Au, Ag and Be did not. Nor did b o m b a r d i n g Cu with 40 keV A r + ions give any d a r k e n i n g of the film. I n fig. 1 a r e v i e w i s given of the K, L, M and N s e r i e s of the e l e m e n t s i n volved, and the t r a n s p a r e n c y of the A1 foil i s a l s o shown. The soft X - r a y s f r o m A1 and Zn t a r g e t s may come either f r o m excitation of the A1 K shell (8 ~1 and Zn L s h e l l (13 ~ ) or f r o m the O K shell (23 A) due to oxidation of t h e s e m e t a l s u r f a c e s . Using a 16 ~ thick A1 foil one e l i m i n a t e s wavelengths longer than about 15 ~ . In doing this we did not o b s e r v e a d a r k spot on the f i l m a f t e r b o m b a r d i n g the A1 t a r g e t with 90 keV Ar + ions, but the Zn t a r g e t s t i l l gave an image.
i
100 ~/J
,
4E~ 13AI 29Cu )0Zn ~?Ag 79Au
L
L
It
80 18At
L ,o
xm
- [x]" ~
Fig. 1. A review of the K, L, M and N series of the used elements as a function of wavelength (~) and e n e r g i e (E). No details of subshells are given. The opacity of the A1 foil is shown by shading.
Volume26A, number 12
PHYSICS LETTERS
The above mentioned experimental results j u s t i f y t h e following c o n c l u s i o n s : a) 90 keV h e a v y i o n s can p r o d u c e X - r a y s c o m i n g f r o m i n n e r s h e l l s of the b o m b a r d e d a t o m s . A c t u a l l y t h i s i s the c a s e f o r the O K s h e l l and Cu and Zn L s h e l l s ; b) N e i t h e r 40 keV A r + on Cu n o r 90 keY A r + on A1, Au and Ag give r i s e to enough i n e l a s t i c e n e r g y t r a n s f e r to c r e a t e e x c i t a t i o n of t h e Cu L, A1 K, Au L, Ag L and A r K s h e l l s r e s p e c t i v e l y . T h i s i s in a c c o r d a n c e with m e a s u r e m e n t s of Shock et al. [2] and Dahl [3]; c) One can expect an emission s p e c t r u m f r o m ion b o m b a r d m e n t Cu s u r f a c e s going f r o m the v i s i b l e s p e c t r a l r e g i o n [5] down into the soft X - r a y r e g i o n ; d) Still m o r e
X-RAY
SMALL
6May1968
s t r u c t u r e i s to be e x p e c t e d in t h e s e c o n d a r y e l e c t r o n s p e c t r u m than Snoek et a l . m e a s u r e d [6]. 1. J. MI Khan, D.L. Potter, R.D. Worley and H. P. Smith Jr., Phys. Rev. 163 (1967) 81. 2. C. Shock, W.F. Van der Weg, R.Geballe and P.K. Rol, Physica 35 (1967) 1. 3. P.Dahl and J.Magyar, Phys. Rev. 140 (1965) A 1420. 4. J . A . R . Samsom (John Wiley and Sons, Inc., New York, 1967). 5. J. Kistemaker and C. Shock, Le Bombardment ionique, (C.N.R.S., Paris, 1961) p. 52. 6. C.Snoek, R.Geballe, W.F.Van der Weg, P.K.Rol and D.J.Bierman, Physica 31 (1965) 1553. 7. J.A,.Davies, F.Brown and M.McCargo, Can. J. Phys. 41 (1963) 829.
ANGLE SCATTERING MEASUREMENT OF CLUSTERING IN CuNi
A. KIDRON Department of Physics, Technion, Israel Institute of Technology, Haifa, Israel Received 5 April 1968
Small angle X-ray diffuse scattering of an almost equiatomic CuNi alloy was measured using CuKot radiation. The scattering shows a diffuse ring due to clustered zones. Fourier analysis of the intensity yields the probability of like neighbours as a function of the distance from a given atom.
Magnetic s u s c e p t i b i l i t y m e a s u r e m e n t s m a d e by Ryan et al. [1] on CuNi showed the e x i s t e n c e of s u p e r p a r a m a g n e t i s m in t h e s e s o l u t i o n s . T h i s was a t t r i b u t e d to the s e g r e g a t i o n of Ni a t o m s , and it was a l s o shown that d i f f e r e n t m e a s u r e m e n t s m a d e by o t h e r w o r k e r s a l s o fit b e t t e r to t h i s p i c t u r e than to any o t h e r model. T h e m o s t d i r e c t m e a s u r e m e n t of s e g r e g a t i o n in a l l o y s i s by the s m a l l a n g l e diffuse s c a t t e r i n g of X : r a y s [2]. Unfortunately the i n t e n s i t y h e r e i s p r o p o r t i o n a l to t h e s q u a r e of the difference in the a t o m i c s c a t t e r i n g f a c t o r s of the two kinds of a t o m s . T h i s f a c t m a k e s it p r a c t i c a l l y i m p o s s i b l e to u s e a l l o y s w h e r e the a t o m s a r e c o n s e c u t i v e in the p e r i o d i c t a b l e a s in t h e c a s e of CuNi. In o r d e r to o v e r c o m e t h i s difficulty we u s e d C u K ~ radiation. This radiation gives a dispersion corr e c t f o n to the s c a t t e r i n g of the Ni a t o m s , giving a d i f f e r e n c e of about 2e.u. between the s c a t t e r i n g f a c t o r s of Cu and Ni and thus making the i n t e n s i t y 4 t i m e s h i g h e r and s o l e s s difficult to o b s e r v e . The a l l o y was p r e p a r e d f r o m high p u r i t y Cu and
Ni by m e l t i n g t h e m both in a high t e m p e r a t u r e f u r n a c e u n d e r He (10%H) a t m o s p h e r e . I t was then h o m o g e n i z e d f o r 5 d a y s at 1000°C. S u b s e quent c h e m i c a l a n a l y s i s showed it to have 50.8 a t o~ Cu and f r e e of c o n t a m i n a t i o n . P o w d e r s p e c i m e n s give at s m a l l a n g l e s [3] m u l t i p l e B r a g g s c a t t e r i n g which shadows the s i g n i f i c a n t s c a t t e r i n g . To o v e r c o m e t h i s , we g r e w a s i n g l e c r y s t a l of the s p e c i m e n by c o l d r o l l i n g i t and then heating i t a t 900°C f o r a few h o u r s in an evacuated quartz capsule. T h e e x p e r i m e n t a l s e t - u p included a P i c k e r d i f f r a c t o m e t e r with a L i F m o n o c h r o m a t o r put between the s p e c i m e n and the d e t e c t o r [4] and a l s o with a Ni f i l t e r between the tube and the s p e c i m e n . T h e f i l t e r and the m o n o c h r o m a t o r s u p p r e s s e d a l m o s t a l l the unwanted f l u o r e s c e n c e f r o m the s p e c i m e n . T h e -~k component of the r a d i a t i o n and the f l u o r e s c e n c e that was left was s u p p r e s s e d by a p u l s e height a n a l y z e r coming after a proportional counter. The measurement w a s m a d e by t r a n s m i s s i o n of the X - r a y s through
593
PHYSICS LETTERS
6May1968
CHARACTERISTIC X-RAY PRODUCTION FROM B Y 90 k e V A r + I O N B O M B A R D M E N T
METALS
F . W . SARIS, W . F . V A N DER WEG and J. KISTEMAKER FOM
-
I n s t i t u u t v o o r A t o o m - en M o l e c u u l f y s i c a , A m s t e r d a m ,
The Netherlands
Received 5 April 1968
Soft X-rays are observed during bombardment of metal surfaces with 90 keY Ar + ions. The wavelength involved is correlated to the inelastic energy loss during the ion-atom collision.
B e s i d e s e l e c t r o n s and photons, heavy c h a r g e d p a r t i c l e s can a l s o be employed for X - r a y p r o d u c tion [1]. To c r e a t e an i n n e r - s h e l l excitation, the i n e l a s t i c energy l o s s d u r i n g the c o l l i s i o n of the p r o j e c t i l e and the t a r g e t p a r t i c l e has to be m o r e than the energy of the i n n e r - s h e l l l e v e l . Snoek et al. [2] showed that the i n e l a s t i c e n e r g y l o s s d u r ing a c o l l i s i o n of a 90 keV Ar + ion with a Cu a t o m can be as high as 1500 eV. So t h e s e Ar + i o n s might be able to c r e a t e a v a c a n c y i n the Cu L shell which is about 930 eV below the g r o u n d level. The s u b s e q u e n t d e - e x c i t a t i o n may go v i a an A u g e r - t y p e t r a n s i t i o n which gives r i s e to s e c o n d a r y e l e c t r o n s , or via a r a d i a t i v e t r a n s i t i o n i.e. e m i s s i o n of a photon of 13.3 A (in the c a s e of Cu L). Using photographic detection we s u c c e e d e d in p r o v i n g the e m i s s i o n of t h e s e soft X - r a y s . In a pinhole c a m e r a an 8 ~ thick A1 foil and a Schumann f i l m w e r e mounted. An 8 ~ thick A1 foil i s t r a n s p a r e n t below abouto25 A except at the A1 K a b s o r p t i o n edge n e a r 8 A [4]. We b o m b a r d e d a nurr,b e r of m e t a l s with 90 keV Ar + ions with a b e a m i n t e n s i t y of 400 ~ A / c m 2 for one hour. T h e v a c u u m was 10 -6 T o r r . During the ion b o m b a r d ment many s e c o n d a r y p a r t i c l e s a r e r e l e a s e d f r o m the t a r g e t . T h e r e will be: r e f l e c t e d Ar a t o m s and ions of about 5 keV; s e c o n d a r y e l e c t r o n s of 0 until ~ 1000 eV; and s p u t t e r e d t a r g e t a t o m s of a few eV. The p e n e t r a t i o n depth of Ar p a r t i c l e s of 5 keV in A1 i s l e s s than 100/~ [7]. Only fast e l e c t r o n s can give l u m i n e s c e n c e of the A1 foil in the s p e c t r a l r e g i o n involved. T h e r e fore a m a g n e t i c field of 600 g a u s s i s a p p l i e d to deflect t h e s e p a r t i c l e s f r o m the foil. With an Ar ion b e a m of 40 # A and a s p u t t e r i n g r a t i o of 5 (for i n s t a n c e Cu) about 10~15~target p a r t i c l e s / s a r e sputtered. Through the a p e r t u r e of the c a m e r a only 1012 p a r t i c l e s / s will r e a c h the foil. 592
In a copper s u r f a c e the d e n s i t y of Cu a t o m s is 1.5 x 1015 a t o m s / c m 2. T h u s a f t e r 1.5 x 103 s e c o n d s one a t o m i c l a y e r of s p u t t e r e d Cu a t o m s i s f o r m e d on the foil. T h e Cu, Zn and A1 t a r g e t s gave an i m a g e on the f i l m , while Au, Ag and Be did not. Nor did b o m b a r d i n g Cu with 40 keV A r + ions give any d a r k e n i n g of the film. I n fig. 1 a r e v i e w i s given of the K, L, M and N s e r i e s of the e l e m e n t s i n volved, and the t r a n s p a r e n c y of the A1 foil i s a l s o shown. The soft X - r a y s f r o m A1 and Zn t a r g e t s may come either f r o m excitation of the A1 K shell (8 ~1 and Zn L s h e l l (13 ~ ) or f r o m the O K shell (23 A) due to oxidation of t h e s e m e t a l s u r f a c e s . Using a 16 ~ thick A1 foil one e l i m i n a t e s wavelengths longer than about 15 ~ . In doing this we did not o b s e r v e a d a r k spot on the f i l m a f t e r b o m b a r d i n g the A1 t a r g e t with 90 keV Ar + ions, but the Zn t a r g e t s t i l l gave an image.
i
100 ~/J
,
4E~ 13AI 29Cu )0Zn ~?Ag 79Au
L
L
It
80 18At
L ,o
xm
- [x]" ~
Fig. 1. A review of the K, L, M and N series of the used elements as a function of wavelength (~) and e n e r g i e (E). No details of subshells are given. The opacity of the A1 foil is shown by shading.
Volume26A, number 12
PHYSICS LETTERS
The above mentioned experimental results j u s t i f y t h e following c o n c l u s i o n s : a) 90 keV h e a v y i o n s can p r o d u c e X - r a y s c o m i n g f r o m i n n e r s h e l l s of the b o m b a r d e d a t o m s . A c t u a l l y t h i s i s the c a s e f o r the O K s h e l l and Cu and Zn L s h e l l s ; b) N e i t h e r 40 keV A r + on Cu n o r 90 keY A r + on A1, Au and Ag give r i s e to enough i n e l a s t i c e n e r g y t r a n s f e r to c r e a t e e x c i t a t i o n of t h e Cu L, A1 K, Au L, Ag L and A r K s h e l l s r e s p e c t i v e l y . T h i s i s in a c c o r d a n c e with m e a s u r e m e n t s of Shock et al. [2] and Dahl [3]; c) One can expect an emission s p e c t r u m f r o m ion b o m b a r d m e n t Cu s u r f a c e s going f r o m the v i s i b l e s p e c t r a l r e g i o n [5] down into the soft X - r a y r e g i o n ; d) Still m o r e
X-RAY
SMALL
6May1968
s t r u c t u r e i s to be e x p e c t e d in t h e s e c o n d a r y e l e c t r o n s p e c t r u m than Snoek et a l . m e a s u r e d [6]. 1. J. MI Khan, D.L. Potter, R.D. Worley and H. P. Smith Jr., Phys. Rev. 163 (1967) 81. 2. C. Shock, W.F. Van der Weg, R.Geballe and P.K. Rol, Physica 35 (1967) 1. 3. P.Dahl and J.Magyar, Phys. Rev. 140 (1965) A 1420. 4. J . A . R . Samsom (John Wiley and Sons, Inc., New York, 1967). 5. J. Kistemaker and C. Shock, Le Bombardment ionique, (C.N.R.S., Paris, 1961) p. 52. 6. C.Snoek, R.Geballe, W.F.Van der Weg, P.K.Rol and D.J.Bierman, Physica 31 (1965) 1553. 7. J.A,.Davies, F.Brown and M.McCargo, Can. J. Phys. 41 (1963) 829.
ANGLE SCATTERING MEASUREMENT OF CLUSTERING IN CuNi
A. KIDRON Department of Physics, Technion, Israel Institute of Technology, Haifa, Israel Received 5 April 1968
Small angle X-ray diffuse scattering of an almost equiatomic CuNi alloy was measured using CuKot radiation. The scattering shows a diffuse ring due to clustered zones. Fourier analysis of the intensity yields the probability of like neighbours as a function of the distance from a given atom.
Magnetic s u s c e p t i b i l i t y m e a s u r e m e n t s m a d e by Ryan et al. [1] on CuNi showed the e x i s t e n c e of s u p e r p a r a m a g n e t i s m in t h e s e s o l u t i o n s . T h i s was a t t r i b u t e d to the s e g r e g a t i o n of Ni a t o m s , and it was a l s o shown that d i f f e r e n t m e a s u r e m e n t s m a d e by o t h e r w o r k e r s a l s o fit b e t t e r to t h i s p i c t u r e than to any o t h e r model. T h e m o s t d i r e c t m e a s u r e m e n t of s e g r e g a t i o n in a l l o y s i s by the s m a l l a n g l e diffuse s c a t t e r i n g of X : r a y s [2]. Unfortunately the i n t e n s i t y h e r e i s p r o p o r t i o n a l to t h e s q u a r e of the difference in the a t o m i c s c a t t e r i n g f a c t o r s of the two kinds of a t o m s . T h i s f a c t m a k e s it p r a c t i c a l l y i m p o s s i b l e to u s e a l l o y s w h e r e the a t o m s a r e c o n s e c u t i v e in the p e r i o d i c t a b l e a s in t h e c a s e of CuNi. In o r d e r to o v e r c o m e t h i s difficulty we u s e d C u K ~ radiation. This radiation gives a dispersion corr e c t f o n to the s c a t t e r i n g of the Ni a t o m s , giving a d i f f e r e n c e of about 2e.u. between the s c a t t e r i n g f a c t o r s of Cu and Ni and thus making the i n t e n s i t y 4 t i m e s h i g h e r and s o l e s s difficult to o b s e r v e . The a l l o y was p r e p a r e d f r o m high p u r i t y Cu and
Ni by m e l t i n g t h e m both in a high t e m p e r a t u r e f u r n a c e u n d e r He (10%H) a t m o s p h e r e . I t was then h o m o g e n i z e d f o r 5 d a y s at 1000°C. S u b s e quent c h e m i c a l a n a l y s i s showed it to have 50.8 a t o~ Cu and f r e e of c o n t a m i n a t i o n . P o w d e r s p e c i m e n s give at s m a l l a n g l e s [3] m u l t i p l e B r a g g s c a t t e r i n g which shadows the s i g n i f i c a n t s c a t t e r i n g . To o v e r c o m e t h i s , we g r e w a s i n g l e c r y s t a l of the s p e c i m e n by c o l d r o l l i n g i t and then heating i t a t 900°C f o r a few h o u r s in an evacuated quartz capsule. T h e e x p e r i m e n t a l s e t - u p included a P i c k e r d i f f r a c t o m e t e r with a L i F m o n o c h r o m a t o r put between the s p e c i m e n and the d e t e c t o r [4] and a l s o with a Ni f i l t e r between the tube and the s p e c i m e n . T h e f i l t e r and the m o n o c h r o m a t o r s u p p r e s s e d a l m o s t a l l the unwanted f l u o r e s c e n c e f r o m the s p e c i m e n . T h e -~k component of the r a d i a t i o n and the f l u o r e s c e n c e that was left was s u p p r e s s e d by a p u l s e height a n a l y z e r coming after a proportional counter. The measurement w a s m a d e by t r a n s m i s s i o n of the X - r a y s through
593