- Email: [email protected]
Atom-probe studies on surface segregation in the binary alloy, tungsten-molybdenum
]8]
Abstracts of 25th International Field Emission Symposium
J a p a n ; "[Department of M a t e r i a l s S c i ence, F a c u l t y of E n g i n e e r i n g , Univ e r s i t y of T o k y o , 7-3-1 H o n g o , Gunkyo-ku, T o k y o 113 J a p a n
FIM o b s e r v a t i o n s were m a d e of molybd e n u m alloys (Mo-0.03 at %Re-0.1 at %C, M o - 0 . 3 at %Re-0.2 at %C, Mo-3 at %Re-0.1 at %C). Many additional bright spots s c a t t e r e d on the usual m o l y b d e n u m image w e r e c h a r a c t e r i s t i c of the alloy images. The c o n c e n t r a t i o n of b r i g h t spots o b t a i n e d on (ll0) and {121} planes was about 10 times larger in the M o - 3 % R e alloy than in the M o - 0 . 3 % R e alloy, and also in M o - 0 . 3 % R e alloy than in M o - 0 . 0 3 % R e alloy, s u g g e s t i n g that these c h a r a c t e r i s t i c bright spots came from the r h e n i u m atoms. However, the c o n c e n t r a t i o n of bright spots in the M o - 0 . 3 % R e alloy images was about 0.5%, somewhat h i g h e r than that of the rhenium atoms themselves. This d i f f e r e n c e may imply the partial c o n t r i b u t i o n of carbon atoms to the b r i g h t spots. The d i s t r i b u t i o n of bright spots was also s t u d i e d in a c i r c u l a r a r e a on a (Ii0) plane. A series of images o b t a i n e d by s e q u e n t i a l field e v a p o r a t i o n gave a spatial distribution. Similar o b s e r v a t i o n s w e r e carried out on specimens a n n e a l e d at 1800°C for 1 hr. There was no n o t i c e a b l e d i f f e r e n c e in c o n c e n t r a t i o n and d i s t r i b u t i o n of the b r i g h t spots b e t w e e n these a s - r o l l e d and a n n e a l e d specimens.
F I E L D - I O N M I C R O S C O P I C STUDY ON P R E C I P I T A T I O N P R O C E S S E S IN F e - C r AND AI-Cu A L L O Y S Tetsuo
Abe,
Ken-ichi
Hirano
Department of Me£allurgy, Materials S c i e n c e and M e t a l P r o c e s s i n g , Faculty of E n g i n e e r i n g , Tohoku University, A o b a , A r a m a k i , S e n d a i , 980 Japan
The field-ion m i c r o s c o p e can provide us w l t h more direct i n f o r m a t i o n than can the t r a n s m i s s i o n electron m i c r o s c o p e on the atomic p r o c e s s e s of phase transformation, such as the spinodal d e c o m p o s i tion and formation of the G.P. zones. In F e - C r alloy systems, the spinodal d e c o m p o s i t i o n cannot be fully s t u d i e d by the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e b e c a u s e it cannot d i s t i n g u i s h the Ferich zones from the C r - r i c h zones b e c a u s e of the s i m i l a r i t y of Fe and Cr atoms in the atomic radii and the s c a t t e r i n g amp l i t u d e s for electrons. However, the f i e l d - i o n m i c r o s c o p e w i l l be a p o w e r f u l tool b e c a u s e it can p r o v i d e us w i t h images of i n d i v i d u a l atoms in w h i c h Fe
atoms can be d i s t i n g u i s h e d from Cr atoms. In some A1 alloys, the f o r m a t i o n process and the s t r u c t u r e of the G.P. zones have been p r o p o s e d b a s e d on the X-ray analysis, but the atomic s t r u c t u r e of the G.P. zones cannot be d i s p l a y e d clearly by the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e b e c a u s e of the e f f e c t of the strain field a r o u n d the G.P. zones. Here again the field-ion m i c r o s c o p e may be useful. We have s u c c e e d e d in o b s e r v i n g the m o d u l a t e d atomic a r r a n g e m e n t in Fe-45 at .% Cr alloy formed by the spinodal d e c o m p o s i t i o n and the G.P. zones (monolayer discs of Cu atoms) in AI-I.5 at .% Cu alloy.
C O M P O S I T I O N OF C A R B I D E IN S T A I N L E S S STEELS H.-O.
Andr6n,
PARTICLES
A. Henjered,
Department of P h y s i c s , v e r s i t y of T e c h n o l o g y , G~teborg, Sweden
H. N o r d 6 n Chalmers S - 4 0 2 20
Uni-
The creep s t r e n g t h of s t a i n l e s s steels c o n t a l n l n g M c - t y p e p a r t i c l e s is at p r e s e n t b e i n g investigated. In particular, the e f f e c t of a d d i n g two c a r b i d e - f o r m i n g e l e m e n t s to the steel is studied. As a part of this study, a t o m - p r o b e analyses of the carbide particles are b e l n g made. M c - t y p e p a r t i c l e s are n o n - s t o i c h i o m e t r i c in that some carbon is m i s s i n g (the s t a b i l i t y range of TiC is 32 to 49% C, to m e n t i o n one example). Some carbides form o r d e r e d s t r u c t u r e s at certain c o m p o s i t i o n s (e.g. VC0.84), and it is b e l i e v e d that o r d e r i n g c o n t r i b u t e s to s t a b i l i t y against p a r t i c l e c o a r s e n i n g during high t e m p e r a t u r e service. A n a l y z i n g these p a r t i c l e s in the atom-probe, we have found Ti, V and Nb to e v a p o r a t e from p a r t i c l e s doubly and triply charged (Nb also as Nb4+), but no m e t a l - c a r b o n complex ions of any kind have been detected. The analyses made so far show that small p a r t i c l e s contain c o n s i d e r a b l e amounts of c h r o m i u m (but little iron). The p a r t i c l e s were a n a l y z e d using a large a c c e p t a n c e angle w h i c h gives a greater y i e l d and thus a more accurate m e a s u r e of the composition. In this way e s t i m a t e s could also be made of the v a r i a t i o n in c o m p o s i t i o n w i t h i n the p a r t i c l e and at the p a r t i c l e - m a t r i x interface.
A T O M - P R O B E STUDIES ON S U R F A C E S E G R E G A T I O N IN THE B I N A R Y ALLOY, TUNGSTEN-MOLYBDENUM
382
Abstracts of 25th International Field Emission Symposium
M. L e i s c h Institut fur Angewandte Physik, T e c h n i c a l U n i v e r s i t y Graz, A u s t r i a In the past, e v i d e n c e has b e e n o b t a i n ed i n d i c a t i n g that the s u r f a c e c o m p o s i tion of a b i n a r y a l l o y can d i f f e r f r o m the b u l k c o m p o s i t i o n . Experimental work m a i n l y w i t h A u g e r s p e c t r o s c o p y • and recently atom-probe studies 2 have been done on this subject. In addition, there is an a b u n d a n c e of t h e o r e t i c a l work. 3 In this case an i m a g i n g a t o m - p r o b e has b e e n u s e d to a n a l y z e the top s u r f a c e layers. A probe d i a m e t e r of about 50 and e v a p o r a t i o n rates r a n g i n g from 1-5 events per p u l s e have been used. A q u a n t i t a t i v e a n a l y s i s of the TOFs p e c t r a of u n a n n e a l e d s p e c i m e n s give a s u r f a c e c o m p o s i t i o n close to the bulk composition. A f t e r h e a t t r e a t m e n t in situ, r e m a r k a b l e c h a n g e s in the a b u n d a n c e of t u n g s t e n and m o l y b d e n u m ions occur. The o b t a i n e d s u r f a c e c o m p o s i t i o n d e p e n d s on a n n e a l i n g t e m p e r a t u r e and the duration of the h e a t treatment. The p r e l i m inary results w i l l be d i s c u s s e d w i t h r e s p e c t to the cited works. 1. W. M. H. Sachtler, Le Vide 163-65, 19 (1973). 2. T. T. Tsong, Yee S. Ng, S. V. Krishnaswami, Appl. Phys. Lett. 32, 778 (1978). 3. F. L. Williams, D. Nason, S u r f a c e Sci. 45, 377 (1974).
ZONES
IN A L U M I N U M
ALLOYS
E. D. Boyes U n i v e r s i t y of C a m b r i d g e and U n i v e r sity of O x f o r d , U n i t e d K i n g d o m The t e c h n i q u e s 1,2 for i m a g i n g and a n a l y z i n g a l u m i n u m - b a s e d m a t e r i a l s in the F I M / A t o m - p r o b e w i l l be r e v i e w e d b r i e f l y and their a p p l i c a t i o n s to the study of s p e c i f i c m a t e r i a l s described. The i n i t i a l low t e m p e r a t u r e h o m o g e n eous p r e c i p i t a t i o n p r o d u c e s zones w h i c h may c o n t i n u e to c o n t a i n some aluminum, in, for example, AI-Cu, A l - Z n - M g and A I - M g - S i but not A I - S i alloys, and it has been s u g g e s t e d t h a t both the structure and m o r p h o l o g y of the zones may be a s e n s i t i v e f u n c t i o n of the alloy composition. The a d v a n t a g e s of the FIM are: (a) the s e n s i t i v i t y and r e l a t i v e l y simple i n t e r p r e t a t i o n of image c o n t r a s t features a few nm in size, (b) the reso l u t i o n of size m e a s u r e m e n t s , (c) simp l i f i e d m i c r o a n a l y s i s , p a r t i c u l a r l y for
m a t r i x e l e m e n t s in those p r e c i p i t a t e s w h i c h are m o r e r e f r a c t o r y than the matrix. The f i n e - s c a l e d i s t r i b u t i o n (10 i7 1019 per cc) is c o n v e n i e n t for F I M / A t o m probe. ~. E. D. Boyes, A. R. Waugh, P. J. Turner, P. F. Mills, M. J. Southon, 24th Int. FES, Oxford, 1977. 2. Ibid., Proc. EMAG 77, I n s t i t u t e of Physics, L o n d o n (1977), p. 343.
ATOM-PROBE
AN "IMAGING A T O M - P R O B E " FOR THE A N A L Y S I S OF E L E C T R O P L A T I N G S Michael Martinka,
S. Brooks M c l a n e
Physics Department, Pennsylvania S t a t e U n i v e r s i t y , U n i v e r s i t y Park, Pennsylvania 16802 This p a p e r d e s c r i b e s the d e s i g n of an " I m a g i n g A t o m - P r o b e : ''I-3 w h i c h w i l l disp l a y the s p a t i a l d i s t r i b u t i o n of a sele c t e d mass c o n s t i t u e n t of an e l e c t r o deposit. W i t h i n the mass s p e c t r o m e t e r ' s flight path, f i e l d - e v a p o r a t e d ions are r e t a r d e d and then p o s t - a c c e l e r a t e d to i n c r e a s e f l i g h t time and reduce a n g u l a r t i m i n g errors (flat 75 m m c h e v r o n detector), respectively. P a r t i c u l a r a t t e n t i o n is g i v e n to the w a v e s h a p e s of the e v a p o r a t i o n p u l s e and the d e t e c t o r g a t i n g pulse. The gate p u l s e is a p p l i e d to a s t a i n l e s s steel s u p p o r t ring at the p e r i m e t e r of the c h a n n e l plate, and p r o p a g a t e s t h r o u g h a c o n d u c t i v e c o a t i n g t o w a r d the c e n t e r of the d e t e c t o r surface. B. N a h r a t h , M. S h a k h a t r e and G. D e c k e r 4,5 h a v e simu l a t e d these t r a n s m i s s i o n p r o p e r t i e s by a 4 0 - e l e m e n t d i s t r i b u t e d RC system, and they o b t a i n e d a d e l a y time of 8 nanoseconds b e t w e e n the p e r i m e t e r and a onei n c h - d i a m e t e r c i r c l e about the center. This delay, t o g e t h e r w i t h the a t t e n u a tion and b r o a d e n i n g (Fig. 2, ref. 4), w o u l d limit the e x t e n t to w h i c h the d i s t r i b u t i o n of a s e l e c t e d mass s p e c i e s can be i m a g e d s i m u l t a n e o u s l y . We h a v e d i r e c t l y m e a s u r e d (i M e g ohm, 1.5 pf Probe) the t r a n s m i s s i o n p r o p e r t i e s of a 75 m m - d i a m e t e r I n c o n e l - c o a t e d , w i t h a r e s i s t a n c e of i0 ohms from p e r i m e t e r to center, c h a n n e l plate. Both 7 and 14 ns w i d e p u l s e s w i t h rise times of .5 ns, t e r m i n a t e d into 50 ohms, w e r e a p p l i e d to the c h a n n e l plate, and w a v e forms w e r e o b s e r v e d at the t e r m i n a t o r w i t h the plate not connected, the channel p l a t e p e r i m e t e r , and at the c e n t e r of the detector. The pulse at the
Abstracts of 25th International Field Emission Symposium
J a p a n ; "[Department of M a t e r i a l s S c i ence, F a c u l t y of E n g i n e e r i n g , Univ e r s i t y of T o k y o , 7-3-1 H o n g o , Gunkyo-ku, T o k y o 113 J a p a n
FIM o b s e r v a t i o n s were m a d e of molybd e n u m alloys (Mo-0.03 at %Re-0.1 at %C, M o - 0 . 3 at %Re-0.2 at %C, Mo-3 at %Re-0.1 at %C). Many additional bright spots s c a t t e r e d on the usual m o l y b d e n u m image w e r e c h a r a c t e r i s t i c of the alloy images. The c o n c e n t r a t i o n of b r i g h t spots o b t a i n e d on (ll0) and {121} planes was about 10 times larger in the M o - 3 % R e alloy than in the M o - 0 . 3 % R e alloy, and also in M o - 0 . 3 % R e alloy than in M o - 0 . 0 3 % R e alloy, s u g g e s t i n g that these c h a r a c t e r i s t i c bright spots came from the r h e n i u m atoms. However, the c o n c e n t r a t i o n of bright spots in the M o - 0 . 3 % R e alloy images was about 0.5%, somewhat h i g h e r than that of the rhenium atoms themselves. This d i f f e r e n c e may imply the partial c o n t r i b u t i o n of carbon atoms to the b r i g h t spots. The d i s t r i b u t i o n of bright spots was also s t u d i e d in a c i r c u l a r a r e a on a (Ii0) plane. A series of images o b t a i n e d by s e q u e n t i a l field e v a p o r a t i o n gave a spatial distribution. Similar o b s e r v a t i o n s w e r e carried out on specimens a n n e a l e d at 1800°C for 1 hr. There was no n o t i c e a b l e d i f f e r e n c e in c o n c e n t r a t i o n and d i s t r i b u t i o n of the b r i g h t spots b e t w e e n these a s - r o l l e d and a n n e a l e d specimens.
F I E L D - I O N M I C R O S C O P I C STUDY ON P R E C I P I T A T I O N P R O C E S S E S IN F e - C r AND AI-Cu A L L O Y S Tetsuo
Abe,
Ken-ichi
Hirano
Department of Me£allurgy, Materials S c i e n c e and M e t a l P r o c e s s i n g , Faculty of E n g i n e e r i n g , Tohoku University, A o b a , A r a m a k i , S e n d a i , 980 Japan
The field-ion m i c r o s c o p e can provide us w l t h more direct i n f o r m a t i o n than can the t r a n s m i s s i o n electron m i c r o s c o p e on the atomic p r o c e s s e s of phase transformation, such as the spinodal d e c o m p o s i tion and formation of the G.P. zones. In F e - C r alloy systems, the spinodal d e c o m p o s i t i o n cannot be fully s t u d i e d by the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e b e c a u s e it cannot d i s t i n g u i s h the Ferich zones from the C r - r i c h zones b e c a u s e of the s i m i l a r i t y of Fe and Cr atoms in the atomic radii and the s c a t t e r i n g amp l i t u d e s for electrons. However, the f i e l d - i o n m i c r o s c o p e w i l l be a p o w e r f u l tool b e c a u s e it can p r o v i d e us w i t h images of i n d i v i d u a l atoms in w h i c h Fe
atoms can be d i s t i n g u i s h e d from Cr atoms. In some A1 alloys, the f o r m a t i o n process and the s t r u c t u r e of the G.P. zones have been p r o p o s e d b a s e d on the X-ray analysis, but the atomic s t r u c t u r e of the G.P. zones cannot be d i s p l a y e d clearly by the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e b e c a u s e of the e f f e c t of the strain field a r o u n d the G.P. zones. Here again the field-ion m i c r o s c o p e may be useful. We have s u c c e e d e d in o b s e r v i n g the m o d u l a t e d atomic a r r a n g e m e n t in Fe-45 at .% Cr alloy formed by the spinodal d e c o m p o s i t i o n and the G.P. zones (monolayer discs of Cu atoms) in AI-I.5 at .% Cu alloy.
C O M P O S I T I O N OF C A R B I D E IN S T A I N L E S S STEELS H.-O.
Andr6n,
PARTICLES
A. Henjered,
Department of P h y s i c s , v e r s i t y of T e c h n o l o g y , G~teborg, Sweden
H. N o r d 6 n Chalmers S - 4 0 2 20
Uni-
The creep s t r e n g t h of s t a i n l e s s steels c o n t a l n l n g M c - t y p e p a r t i c l e s is at p r e s e n t b e i n g investigated. In particular, the e f f e c t of a d d i n g two c a r b i d e - f o r m i n g e l e m e n t s to the steel is studied. As a part of this study, a t o m - p r o b e analyses of the carbide particles are b e l n g made. M c - t y p e p a r t i c l e s are n o n - s t o i c h i o m e t r i c in that some carbon is m i s s i n g (the s t a b i l i t y range of TiC is 32 to 49% C, to m e n t i o n one example). Some carbides form o r d e r e d s t r u c t u r e s at certain c o m p o s i t i o n s (e.g. VC0.84), and it is b e l i e v e d that o r d e r i n g c o n t r i b u t e s to s t a b i l i t y against p a r t i c l e c o a r s e n i n g during high t e m p e r a t u r e service. A n a l y z i n g these p a r t i c l e s in the atom-probe, we have found Ti, V and Nb to e v a p o r a t e from p a r t i c l e s doubly and triply charged (Nb also as Nb4+), but no m e t a l - c a r b o n complex ions of any kind have been detected. The analyses made so far show that small p a r t i c l e s contain c o n s i d e r a b l e amounts of c h r o m i u m (but little iron). The p a r t i c l e s were a n a l y z e d using a large a c c e p t a n c e angle w h i c h gives a greater y i e l d and thus a more accurate m e a s u r e of the composition. In this way e s t i m a t e s could also be made of the v a r i a t i o n in c o m p o s i t i o n w i t h i n the p a r t i c l e and at the p a r t i c l e - m a t r i x interface.
A T O M - P R O B E STUDIES ON S U R F A C E S E G R E G A T I O N IN THE B I N A R Y ALLOY, TUNGSTEN-MOLYBDENUM
382
Abstracts of 25th International Field Emission Symposium
M. L e i s c h Institut fur Angewandte Physik, T e c h n i c a l U n i v e r s i t y Graz, A u s t r i a In the past, e v i d e n c e has b e e n o b t a i n ed i n d i c a t i n g that the s u r f a c e c o m p o s i tion of a b i n a r y a l l o y can d i f f e r f r o m the b u l k c o m p o s i t i o n . Experimental work m a i n l y w i t h A u g e r s p e c t r o s c o p y • and recently atom-probe studies 2 have been done on this subject. In addition, there is an a b u n d a n c e of t h e o r e t i c a l work. 3 In this case an i m a g i n g a t o m - p r o b e has b e e n u s e d to a n a l y z e the top s u r f a c e layers. A probe d i a m e t e r of about 50 and e v a p o r a t i o n rates r a n g i n g from 1-5 events per p u l s e have been used. A q u a n t i t a t i v e a n a l y s i s of the TOFs p e c t r a of u n a n n e a l e d s p e c i m e n s give a s u r f a c e c o m p o s i t i o n close to the bulk composition. A f t e r h e a t t r e a t m e n t in situ, r e m a r k a b l e c h a n g e s in the a b u n d a n c e of t u n g s t e n and m o l y b d e n u m ions occur. The o b t a i n e d s u r f a c e c o m p o s i t i o n d e p e n d s on a n n e a l i n g t e m p e r a t u r e and the duration of the h e a t treatment. The p r e l i m inary results w i l l be d i s c u s s e d w i t h r e s p e c t to the cited works. 1. W. M. H. Sachtler, Le Vide 163-65, 19 (1973). 2. T. T. Tsong, Yee S. Ng, S. V. Krishnaswami, Appl. Phys. Lett. 32, 778 (1978). 3. F. L. Williams, D. Nason, S u r f a c e Sci. 45, 377 (1974).
ZONES
IN A L U M I N U M
ALLOYS
E. D. Boyes U n i v e r s i t y of C a m b r i d g e and U n i v e r sity of O x f o r d , U n i t e d K i n g d o m The t e c h n i q u e s 1,2 for i m a g i n g and a n a l y z i n g a l u m i n u m - b a s e d m a t e r i a l s in the F I M / A t o m - p r o b e w i l l be r e v i e w e d b r i e f l y and their a p p l i c a t i o n s to the study of s p e c i f i c m a t e r i a l s described. The i n i t i a l low t e m p e r a t u r e h o m o g e n eous p r e c i p i t a t i o n p r o d u c e s zones w h i c h may c o n t i n u e to c o n t a i n some aluminum, in, for example, AI-Cu, A l - Z n - M g and A I - M g - S i but not A I - S i alloys, and it has been s u g g e s t e d t h a t both the structure and m o r p h o l o g y of the zones may be a s e n s i t i v e f u n c t i o n of the alloy composition. The a d v a n t a g e s of the FIM are: (a) the s e n s i t i v i t y and r e l a t i v e l y simple i n t e r p r e t a t i o n of image c o n t r a s t features a few nm in size, (b) the reso l u t i o n of size m e a s u r e m e n t s , (c) simp l i f i e d m i c r o a n a l y s i s , p a r t i c u l a r l y for
m a t r i x e l e m e n t s in those p r e c i p i t a t e s w h i c h are m o r e r e f r a c t o r y than the matrix. The f i n e - s c a l e d i s t r i b u t i o n (10 i7 1019 per cc) is c o n v e n i e n t for F I M / A t o m probe. ~. E. D. Boyes, A. R. Waugh, P. J. Turner, P. F. Mills, M. J. Southon, 24th Int. FES, Oxford, 1977. 2. Ibid., Proc. EMAG 77, I n s t i t u t e of Physics, L o n d o n (1977), p. 343.
ATOM-PROBE
AN "IMAGING A T O M - P R O B E " FOR THE A N A L Y S I S OF E L E C T R O P L A T I N G S Michael Martinka,
S. Brooks M c l a n e
Physics Department, Pennsylvania S t a t e U n i v e r s i t y , U n i v e r s i t y Park, Pennsylvania 16802 This p a p e r d e s c r i b e s the d e s i g n of an " I m a g i n g A t o m - P r o b e : ''I-3 w h i c h w i l l disp l a y the s p a t i a l d i s t r i b u t i o n of a sele c t e d mass c o n s t i t u e n t of an e l e c t r o deposit. W i t h i n the mass s p e c t r o m e t e r ' s flight path, f i e l d - e v a p o r a t e d ions are r e t a r d e d and then p o s t - a c c e l e r a t e d to i n c r e a s e f l i g h t time and reduce a n g u l a r t i m i n g errors (flat 75 m m c h e v r o n detector), respectively. P a r t i c u l a r a t t e n t i o n is g i v e n to the w a v e s h a p e s of the e v a p o r a t i o n p u l s e and the d e t e c t o r g a t i n g pulse. The gate p u l s e is a p p l i e d to a s t a i n l e s s steel s u p p o r t ring at the p e r i m e t e r of the c h a n n e l plate, and p r o p a g a t e s t h r o u g h a c o n d u c t i v e c o a t i n g t o w a r d the c e n t e r of the d e t e c t o r surface. B. N a h r a t h , M. S h a k h a t r e and G. D e c k e r 4,5 h a v e simu l a t e d these t r a n s m i s s i o n p r o p e r t i e s by a 4 0 - e l e m e n t d i s t r i b u t e d RC system, and they o b t a i n e d a d e l a y time of 8 nanoseconds b e t w e e n the p e r i m e t e r and a onei n c h - d i a m e t e r c i r c l e about the center. This delay, t o g e t h e r w i t h the a t t e n u a tion and b r o a d e n i n g (Fig. 2, ref. 4), w o u l d limit the e x t e n t to w h i c h the d i s t r i b u t i o n of a s e l e c t e d mass s p e c i e s can be i m a g e d s i m u l t a n e o u s l y . We h a v e d i r e c t l y m e a s u r e d (i M e g ohm, 1.5 pf Probe) the t r a n s m i s s i o n p r o p e r t i e s of a 75 m m - d i a m e t e r I n c o n e l - c o a t e d , w i t h a r e s i s t a n c e of i0 ohms from p e r i m e t e r to center, c h a n n e l plate. Both 7 and 14 ns w i d e p u l s e s w i t h rise times of .5 ns, t e r m i n a t e d into 50 ohms, w e r e a p p l i e d to the c h a n n e l plate, and w a v e forms w e r e o b s e r v e d at the t e r m i n a t o r w i t h the plate not connected, the channel p l a t e p e r i m e t e r , and at the c e n t e r of the detector. The pulse at the