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Nonequilibrium charge states of 60 MeV I-ions channeled in Au single crystals
Vol me 33A, number 5
PHYSICS L E T T E R S
NONEQUILIBRIUM
16 November 1970
CHARGE STATES OF 60 MeV IN Au SINGLE CRYSTALS*
I-IONS
CHANNELED
H. O. LUTZ **, S. DATZ, C.D. MOAK and T. S. NOGGLE Oak Ridge National Laboratory, Oak Ridge, Tennesse 37830, USA Received 29 September 1970
Iodine ions (60 MeV) penetrating Au crystals in (100) planar channeling directions require ca 1200 to reach equilibrium, implying a reduction of the charge changing cross sections. Differences in most probable charge of up to 3.6 units in 20 are observed.
The ionic charge of an e n e r g e t i c p a r t i c l e t r a v e r s i n g a solid or gaseous t a r g e t r e f l e c t s the b a l a n c e of the effective c r o s s s e c t i o n s for c a p t u r e and loss of e l e c t r o n s . Condensed t a r g e t s yield higher e q u i l i b r i u m charge s t a t e s ; this has b e e n a t t r i b u t e d to an i n c r e a s e d p r o b a b i l i t y for e l e c t r o n l o s s [1] or a d e c r e a s e d p r o b a b i l i t y for e l e c t r o n c a p t u r e [2] r e s u l t i n g f r o m high c o l l i s i o n f r e q u e n c i e s . F o r c r y s t a l l i n e solids, the avoidance of s m a l l i m p a c t p a r a m e t e r c o l l i s i o n s and the lower e l e c t r o n d e n s i t y in open c r y s t a l d i r e c t i o n s may lead to a d e c r e a s e in c a p t u r e or loss c r o s s s e c tion for channeled p a r t i c l e s , and r e s u l t in lower [3-4] or higher [6] e q u i l i b r i u m charge s t a t e s than those o b s e r v e d for r a n d o m l y p e n e t r a t i n g particles. In this investigation, the e m e r g e n t charge state d i s t r i b u t i o n s of 60 MeV I - i o n s (initial charge 10+) p e n e t r a t i n g through thin Au single c r y s t a l s w e r e m e a s u r e d as a function of p a t h : length. After e m e r g e n c e f r o m the t a r g e t , the ions w e r e p a s s e d into an e l e c t r o s t a t i c a n a l y z e r fitted with a p o s i t i o n - s e n s i t i v e detector. The whole charge s p e c t r u m was r e c o r d e d s i m u l t a neously, e l i m i n a t i n g n o r m a l i z a t i o n p r o b l e m s . Charge s p e c t r a w e r e m e a s u r e d for the b e a m p e n e t r a t i n g through (100) p l a n a r c h a n n e l s of 650 A, 9 0 0 ~ and 1200/~ effective pathlength in an Au c r y s t a l in channeled and random d i r e c t i o n s (fig. 1). The v a r i a t i o n in pathlength was achieved by u s i n g different tilt angles with r e s p e c t to the n o r m a l with a 600 ~ thick (100) c r y s t a l . In the r a n d o m case, no difference i n the charge s p e c t r u m was found for 650~ and 1200/~ t h i c k n e s s , i n * Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. ** Present address: Kernforschungeanlage, Jttlich, W. Germany.
1.0
(IOO) Planar Channel Distributions L
60 MeV ~ZTI Through Au (+t0 Initial Charge State)
N
o
Random for Both 650 and
F Aq
~.J 200 z
0.t
"i~'~/
/
hU
\
/
ff
; I
i
I I iI
w
o.o~
"
/ !
! I
J 0.002
4Z~
t5
17 t9 Zt CHARGE STATE, q
23
25
27
Fig. 1. Charge state distributions of I ions emerging from Au crystals in random and channeled (100) directions. dicating e q u i l i b r i u m ; in the (100) p l a n a r channeling d i r e c t i o n , the charge state d i s t r i b u t i o n s w e r e c e n t e r e d at lower values and showed a m a r k e d dependence on t h i c k n e s s . With i n c r e a s i n g thickn e s s of the Au c r y s t a l , the charge d i s t r i b u t i o n s shifted to higher v a l u e s and b e c o m e wider. Identicai d i s t r i b u t i o n s w e r e obtained with 1250 ~ and 2500 A thick c r y s t a l s i m p l y i n g e q u i l i b r i u m . The 309
Volume 33A, number 5
PHYSICS LETTERS
o b s e r v e d d i s t r i b u t i o n s in the c a s e of 900, 1200 and 2500 ~ a r e not a m e r e s u p e r p o s t i o n of a p u r e l y ch an n el ed and a p u r e l y r a n d o m s p e c t r u m . Since e q u i l i b r i u m is r a p i d l y a t t a i n e d in the r a n d o m path, the i n c r e a s e in c h a r g e cannot be a c c o u n t e d f o r by an i n c r e a s e in the r a n d o m c o m ponent in the b e a m at g r e a t e r c r y s t a l t h i c k n e s s ; thus c h a r g e s t a t e e q u i l i b r i u m is r e a c h e d in pathlengths of about 1200~. T h i s is a m u c h g r e a t e r (> a f a c t o r of 20) t h i c k n e s s than that n e c e s s a r y to obtain c h a r g e e q u i l i b r i u m of fast ions in r a n d o m t a r g e t s [7, 8]. Since we a r e a p p r o a c h i n g e q u i l i b r i u m f r o m l o w e r c h a r g e s t a t e s , it is c l e a r that charge loss c r o s s sections are drastically r e duced. The c o n t r i b u t i o n to the e l e c t r o n l o s s c r o s s s e c t i o n by m o m e n t u m t r a n s f e r to the e l e c t r o n s in the ion w i l l d i m i n i s h e d b e c a u s e of the l a r g e i m p a c t p a r a m e t e r s and the l o w e r e l e c t r o n d e n s it y in the channel. In n er s h e l l e x c i t a t i o n (which m a y be followed by m u l t i p l e A u g e r ionization) has c r i t i c a l i m p a c t p a r a m e t e r s in the o r d e r of the c l a s s i c a l s h e l l r a d i i i n v o l v e d [9] and will t h e r e f o r e be n e g l i g i b l e f o r w e l l c h a n n e l e d p a r t i c l e s . It was shown by K e s s e l [10] that the diff e r e n t i a l e l e c t r o n l o s s c r o s s s e c t i o n s f o r iodine ions i n c r e a s e d with d e c r e a s i n g d i s t a n c e of c l o s e s t a p p r o a c h b et ween ion and t a r g e t atom. If only the l o s s c r o s s s e c t i o n s w e r e a f f e c t e d by the channeling p r o c e s s , a m u c h l o w e r e q u i -
310
16 November 1970
l i b r i u m c h a r g e d i s t r i b u t i o n would be e x p e c t e d ; d e c r e a s i n g ~e by a f a c t o r of 10 but keeping ~c constant would r e s u l t in a ~ 50% d e s c r e a s e in the m o s t p r o b a b l e c h a r g e . A c t u a l l y the d e c r e a s e in this d i f f e r e n c e is only 12% (2.5 c h a r g e units). The fact that the c h a r g e of the I - i o n in a channel is l o w e r than in a r a n d o m d i r e c t i o n s u g g e s t s that the d i f f e r e n t i a l c a p t u r e c r o s s s e c t i o n v a r i e s l e s s s t r o n g l y with i m p a c t p a r a m e t e r than the l o s s c r o s s section.
References [1] N. Bohr and J, Lindhard, Dan. Mat. Fyso Medd. 28 No. 7 (1954). [2] G. Ryding, H. D. Betz and A. Wittkower, Phys. Rev. Letters 24 (1970) 123. [3] H. O. Lutz, S. Datz, C. D. Moak, T. S. Noggle, L.D. Northcliffe, Bull. Am. Phys. Soc. 11 (1966} 177. [4] M. Kaminsky, l>roc. Int. Conf. on Mass spectroscopy, Kyoto, Japan, Sept. 1969. [5] T. Andersen, S. Datz, P. Hvelplund and G. SoCrensen Phys. Letters 33 A (1970) 121. [6] F.W. Martin, Phys. Rev. Letters 22 (1969) 329. [7] S. Datz, H.O. Lutz, L.B. Bridwell, C. D. Moak, H. D. Betz and L. D. Ellsworth, Phys. Rev., to be published. [8] Co D° Moak, H. O. Lutz, L. B. Bridwell, L. C. Northcliffe and S. Datz, Phys. Rev. Letters 18 (1967) 41. [9] H. F. Stein, H. O. Lutz, P.H. Moller, K. Sistemich and P. Armbruster, Phys. Rev. Letters 24 (1970) 701. [10] Q. C. Kessel, to be published.
PHYSICS L E T T E R S
NONEQUILIBRIUM
16 November 1970
CHARGE STATES OF 60 MeV IN Au SINGLE CRYSTALS*
I-IONS
CHANNELED
H. O. LUTZ **, S. DATZ, C.D. MOAK and T. S. NOGGLE Oak Ridge National Laboratory, Oak Ridge, Tennesse 37830, USA Received 29 September 1970
Iodine ions (60 MeV) penetrating Au crystals in (100) planar channeling directions require ca 1200 to reach equilibrium, implying a reduction of the charge changing cross sections. Differences in most probable charge of up to 3.6 units in 20 are observed.
The ionic charge of an e n e r g e t i c p a r t i c l e t r a v e r s i n g a solid or gaseous t a r g e t r e f l e c t s the b a l a n c e of the effective c r o s s s e c t i o n s for c a p t u r e and loss of e l e c t r o n s . Condensed t a r g e t s yield higher e q u i l i b r i u m charge s t a t e s ; this has b e e n a t t r i b u t e d to an i n c r e a s e d p r o b a b i l i t y for e l e c t r o n l o s s [1] or a d e c r e a s e d p r o b a b i l i t y for e l e c t r o n c a p t u r e [2] r e s u l t i n g f r o m high c o l l i s i o n f r e q u e n c i e s . F o r c r y s t a l l i n e solids, the avoidance of s m a l l i m p a c t p a r a m e t e r c o l l i s i o n s and the lower e l e c t r o n d e n s i t y in open c r y s t a l d i r e c t i o n s may lead to a d e c r e a s e in c a p t u r e or loss c r o s s s e c tion for channeled p a r t i c l e s , and r e s u l t in lower [3-4] or higher [6] e q u i l i b r i u m charge s t a t e s than those o b s e r v e d for r a n d o m l y p e n e t r a t i n g particles. In this investigation, the e m e r g e n t charge state d i s t r i b u t i o n s of 60 MeV I - i o n s (initial charge 10+) p e n e t r a t i n g through thin Au single c r y s t a l s w e r e m e a s u r e d as a function of p a t h : length. After e m e r g e n c e f r o m the t a r g e t , the ions w e r e p a s s e d into an e l e c t r o s t a t i c a n a l y z e r fitted with a p o s i t i o n - s e n s i t i v e detector. The whole charge s p e c t r u m was r e c o r d e d s i m u l t a neously, e l i m i n a t i n g n o r m a l i z a t i o n p r o b l e m s . Charge s p e c t r a w e r e m e a s u r e d for the b e a m p e n e t r a t i n g through (100) p l a n a r c h a n n e l s of 650 A, 9 0 0 ~ and 1200/~ effective pathlength in an Au c r y s t a l in channeled and random d i r e c t i o n s (fig. 1). The v a r i a t i o n in pathlength was achieved by u s i n g different tilt angles with r e s p e c t to the n o r m a l with a 600 ~ thick (100) c r y s t a l . In the r a n d o m case, no difference i n the charge s p e c t r u m was found for 650~ and 1200/~ t h i c k n e s s , i n * Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. ** Present address: Kernforschungeanlage, Jttlich, W. Germany.
1.0
(IOO) Planar Channel Distributions L
60 MeV ~ZTI Through Au (+t0 Initial Charge State)
N
o
Random for Both 650 and
F Aq
~.J 200 z
0.t
"i~'~/
/
hU
\
/
ff
; I
i
I I iI
w
o.o~
"
/ !
! I
J 0.002
4Z~
t5
17 t9 Zt CHARGE STATE, q
23
25
27
Fig. 1. Charge state distributions of I ions emerging from Au crystals in random and channeled (100) directions. dicating e q u i l i b r i u m ; in the (100) p l a n a r channeling d i r e c t i o n , the charge state d i s t r i b u t i o n s w e r e c e n t e r e d at lower values and showed a m a r k e d dependence on t h i c k n e s s . With i n c r e a s i n g thickn e s s of the Au c r y s t a l , the charge d i s t r i b u t i o n s shifted to higher v a l u e s and b e c o m e wider. Identicai d i s t r i b u t i o n s w e r e obtained with 1250 ~ and 2500 A thick c r y s t a l s i m p l y i n g e q u i l i b r i u m . The 309
Volume 33A, number 5
PHYSICS LETTERS
o b s e r v e d d i s t r i b u t i o n s in the c a s e of 900, 1200 and 2500 ~ a r e not a m e r e s u p e r p o s t i o n of a p u r e l y ch an n el ed and a p u r e l y r a n d o m s p e c t r u m . Since e q u i l i b r i u m is r a p i d l y a t t a i n e d in the r a n d o m path, the i n c r e a s e in c h a r g e cannot be a c c o u n t e d f o r by an i n c r e a s e in the r a n d o m c o m ponent in the b e a m at g r e a t e r c r y s t a l t h i c k n e s s ; thus c h a r g e s t a t e e q u i l i b r i u m is r e a c h e d in pathlengths of about 1200~. T h i s is a m u c h g r e a t e r (> a f a c t o r of 20) t h i c k n e s s than that n e c e s s a r y to obtain c h a r g e e q u i l i b r i u m of fast ions in r a n d o m t a r g e t s [7, 8]. Since we a r e a p p r o a c h i n g e q u i l i b r i u m f r o m l o w e r c h a r g e s t a t e s , it is c l e a r that charge loss c r o s s sections are drastically r e duced. The c o n t r i b u t i o n to the e l e c t r o n l o s s c r o s s s e c t i o n by m o m e n t u m t r a n s f e r to the e l e c t r o n s in the ion w i l l d i m i n i s h e d b e c a u s e of the l a r g e i m p a c t p a r a m e t e r s and the l o w e r e l e c t r o n d e n s it y in the channel. In n er s h e l l e x c i t a t i o n (which m a y be followed by m u l t i p l e A u g e r ionization) has c r i t i c a l i m p a c t p a r a m e t e r s in the o r d e r of the c l a s s i c a l s h e l l r a d i i i n v o l v e d [9] and will t h e r e f o r e be n e g l i g i b l e f o r w e l l c h a n n e l e d p a r t i c l e s . It was shown by K e s s e l [10] that the diff e r e n t i a l e l e c t r o n l o s s c r o s s s e c t i o n s f o r iodine ions i n c r e a s e d with d e c r e a s i n g d i s t a n c e of c l o s e s t a p p r o a c h b et ween ion and t a r g e t atom. If only the l o s s c r o s s s e c t i o n s w e r e a f f e c t e d by the channeling p r o c e s s , a m u c h l o w e r e q u i -
310
16 November 1970
l i b r i u m c h a r g e d i s t r i b u t i o n would be e x p e c t e d ; d e c r e a s i n g ~e by a f a c t o r of 10 but keeping ~c constant would r e s u l t in a ~ 50% d e s c r e a s e in the m o s t p r o b a b l e c h a r g e . A c t u a l l y the d e c r e a s e in this d i f f e r e n c e is only 12% (2.5 c h a r g e units). The fact that the c h a r g e of the I - i o n in a channel is l o w e r than in a r a n d o m d i r e c t i o n s u g g e s t s that the d i f f e r e n t i a l c a p t u r e c r o s s s e c t i o n v a r i e s l e s s s t r o n g l y with i m p a c t p a r a m e t e r than the l o s s c r o s s section.
References [1] N. Bohr and J, Lindhard, Dan. Mat. Fyso Medd. 28 No. 7 (1954). [2] G. Ryding, H. D. Betz and A. Wittkower, Phys. Rev. Letters 24 (1970) 123. [3] H. O. Lutz, S. Datz, C. D. Moak, T. S. Noggle, L.D. Northcliffe, Bull. Am. Phys. Soc. 11 (1966} 177. [4] M. Kaminsky, l>roc. Int. Conf. on Mass spectroscopy, Kyoto, Japan, Sept. 1969. [5] T. Andersen, S. Datz, P. Hvelplund and G. SoCrensen Phys. Letters 33 A (1970) 121. [6] F.W. Martin, Phys. Rev. Letters 22 (1969) 329. [7] S. Datz, H.O. Lutz, L.B. Bridwell, C. D. Moak, H. D. Betz and L. D. Ellsworth, Phys. Rev., to be published. [8] Co D° Moak, H. O. Lutz, L. B. Bridwell, L. C. Northcliffe and S. Datz, Phys. Rev. Letters 18 (1967) 41. [9] H. F. Stein, H. O. Lutz, P.H. Moller, K. Sistemich and P. Armbruster, Phys. Rev. Letters 24 (1970) 701. [10] Q. C. Kessel, to be published.