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A comparison of high energy radiographic techniques for large diameter titanium billets
A comparison of high energy radiographic techniques for large diameter titanium billets F. J. S a t t l e r
The defect r e s o l u t i o n c a p a b i l i t i e s of a 25MeV b e t a t r o n and a 7 . 5 M e V l i n e a r a c c e l e r a t o r for the ndt of l a r g e d i a m e t e r t i t a n i u m b i l l e t s a r e c o m p a r e d . The effects of lead f r o n t and b a c k s c r e e n t h i c k n e s s e s , e x p o s u r e g e o m e t r y and f i l m v a r i a b l e s on the q u a l i t y of the r a d i o g r a p h i c i m a g e a r e d e s c r i b e d . The a p p l i c a b i l i t y of each u n i t is d i s c u s s e d in view of the r e s o l u t i o n achieved and the e c o n o m i c s of the r a d i o g r a p h i c procedures.
T i t a n i u m has b e e n u s e d e x t e n s i v e l y in both c o m m e r cial and m i l i t a r y a i r c r a f t e n g i n e s d u r i n g the p a s t decade b e c a u s e of i t s e x c e l l e n t s t r e n g t h - t o - w e i g h t r a t i o . F o r the c o m p r e s s o r s e c t i o n s of t h e s e e n g i n e s , t i t a n i u m b l a d e s , v a n e s and d i s c s a r e forged f r o m b i l l e t stock. F o r the l a r g e r e n g i n e s on a d v a n c e d a i r craft, c o m p r e s s o r d i s c s a r e forged f r o m b i l l e t stock of up to 18in (458mm) in d i a m e t e r . T h e s e l a r g e r d i a m e t e r b i l l e t s r e c e i v e l e s s work than s m a l l b i l l e t s d u r i n g f o r g i n g and t h e r e f o r e c o n t a i n a g e n e r a l l y coarser grained m i c r o s t r u c t u r e . This m i c r o s t r u c t u r e l i m i t s the d e t e c t i o n c a p a b i l i t y and r e l i a b i l i t y of finding s m a l l d e f e c t s when u s i n g s t a n d a r d u l t r a s o n i c i n s p e c t i o n t e c h n i q u e s for d e t e r m i n i n g b i l l e t q u a l i t y . In a m a c h i n e d - d i s c c o n f i g u r a t i o n , s e v e r a l ndt m e t h o d s a r e u s e d to d e t e c t defect c o n d i t i o n s . However, good cost c o n t r o l p r a c t i c e r e q u i r e s the l o c a t i o n of d e f e c t s in s t a r t i n g b i l l e t s r a t h e r than in f i n i s h e d p a r t s . F o r this r e a s o n , the United States A i r F o r c e M a t e r i a l s L a b o r a t o r y s p o n s o r e d p r o g r a m m e s to i m p r o v e the s e n s i t i v i t y of ndt in finding s m a l l d e f e c t s at the e a r l i e s t p o s s i b l e t i m e in the f a b r i c a t i o n s e q u e n c e ; n a m e l y , at the b i l l e t stage. A r e l i a b l e b i l l e t i n s p e c tion method was of p a r t i c u l a r i n t e r e s t for 17in d i a m e t e r (432mm) b i l l e t s u s e d in T F - 3 9 engine c o m pressor-disc forgings.
Mr S a t t l e r is with the M a t e r i a l s T e c h n o l o g y E q u i p m e n t G r o u p of TRW Inc, 23555 E u c l i d Avenue, C l e v e land, Ohio 44117, USA. His p a p e r was p r e s e n t e d at the AGARD m e e t i n g on ' A d v a n c e d t e c h n o l o g y for p r o d u c tion of a e r o s p a c e e n g i n e s ' held in London f r o m 6 - 1 0 A p r i l 1970.
E x p e r i m e n t a l work was p e r f o r m e d with both u l t r a s o n i c and r a d i o g r a p h i c i n s p e c t i o n methods, but only the r a d i o g r a p h i c work will be r e p o r t e d h e r e . The high e n e r g y r a d i a t i o n s o u r c e s u s e d for this e v a l u a t i o n w e r e a 7 . 5 M e V V a r i a n L i n e a r A c c e l e r a t o r and a 25MeV A l l i s - C h a l m e r s B e t a t r o n . The focal spot on the V a r i a n was s l i g h t l y l e s s than 1 . 5 m m . The b e t a t r o n d i f f e r s f r o m the l i n e a r a c c e l e r a t o r in the method of a c c e l e r a t i n g the e l e c t r o n s and s i n c e a n a r r o w x r a y b e a m i s e m i t t e d it m u s t be made m o r e h o m o g e n e o u s with an e x t e r n a l a b s o r p t i o n cone, the c o m p e n s a t o r . N o r m a l l y for i n d u s t r i a l work, a 30 p e r c e n t c o m p e n s a t o r is used; s i n c e the b e t a t r o n used in t h e s e e v a l u a t i o n s was a m e d i c a l unit, it c o n t a i n e d a 50 p e r cent c o m p e n s a t o r . T h i s m e d i c a l u n i t had a l o w e r output b e a m i n t e n s i t y than a c o m p a r a b l e i n d u s t r i a l u n i t but was m o r e u n i f o r m in its e n e r g y and i n t e n s i t y d i s t r i b u t i o n . The u n i t has an e x t r e m e l y s m a l l 0 . 3 m m focal spot. In o r d e r to e v a l u a t e the defect d e t e c t i o n c a p a b i l i t y of t h e s e two u n i t s , e x p e r i m e n t s w e r e p e r f o r m e d to d e t e r m i n e t h e following: 1
R e s o l u t i o n and i n t e n s i f i c a t i o n c h a r a c t e r i s t i c s of lead b a c k i n g s c r e e n s
2
R e s o l u t i o n and i n t e n s i f i c a t i o n c h a r a c t e r i s t i c s of lead f r o n t s c r e e n s
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R e s o l u t i o n o p t i m i z a t i o n with f r o n t and b a c k screen combinations
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R e s o l u t i o n o b t a i n a b l e with e n l a r g e m e n t techniques
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R e s o l u t i o n l i m i t s u s i n g s m a l l and f i n e grained x-ray films
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The r e s o l u t i o n was d e t e r m i n e d by o b s e r v i n g the s m a l l e s t 1T p e n e t r a m e t e r hole v i s i b l e when United States M i l i t a r y S t a n d a r d 453 t i t a n i u m p e n e t r a m e t e r s w e r e placed on the f r o n t of the b i l l e t s . Work by M i l l e r and T e n n e y 1 has e s t a b l i s h e d the r e s o l u t i o n of this type of p e n e t r a m e t e r hole to be a p p r o x i m a t e l y e q u i v a l e n t to the i m a g e o b t a i n e d for s p h e r i c a l c a v i t i e s having the s a m e hole d i a m e t e r ,
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A v a c u u m bag for f i l m c a s s e t t e s was u s e d for all e x p o s u r e s to a s s u r e adequate, u n i f o r m c o n t a c t b e t w e e n the lead s c r e e n s and the x - r a y film. S t a n d a r d c a r d b o a r d c a s s e t t e s w e r e used, except that the u s u a l lead b a c k i n g foils w e r e r e m o v e d . The x - r a y f i l m s for the b e t a t r o n e x p o s u r e s w e r e p r o c e s s e d m a n u a l l y while the l i n e a r a c c e l e r a t o r f i l m s w e r e p r o c e s s e d a u t o m a t i c a l l y . Work by P o l a n s k y and C r i s c u o l o 2 has shown that f i l m s p r o c e s s e d a u t o m a t i c a l l y tend to have h i g h e r g r a i n i n e s s due to the h i g h e r d e v e l o p e r t e m p e r a t u r e s . However, the a u t o m a t i c p r o c e s s o r u s e d to p r o c e s s the l i n e a r a c c e l e r a t o r f i l m s had a l o n g e r p r o c e s s i n g cycle and a l o w e r d e v e l o p e r t e m p e r a t u r e than u s u a l , which helped to l i m i t the g r a i n i n e s s .
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In o r d e r to obtain f i l m r e s p o n s e c u r v e s for both x - r a y g e n e r a t o r s , s e v e r a l r a d i o g r a p h s w e r e m a d e of a 10in (254mm) d i a m e t e r 6A1-4V t i t a n i u m a l l o y b i l l e t u s i n g the b e t a t r o n , and of a 17in (432mm) d i a m e t e r b i l l e t u s i n g the l i n e a r a c c e l e r a t o r . The data f r o m these radiographs provided exposure-density curves for E a s t m a n Kodak T y p e s AA and M i n d u s t r i a l x - r a y f i l m s . The l a r g e d i f f e r e n c e found b e t w e e n the e x p o s u r e r a t i o s of the two u n i t s is a f u n c t i o n of the b e a m e n e r g y d i f f e r e n c e s and the p r o b a b l e h i g h e r b a c k s c a t t e r i n t e n s i t y (of l o w e r e n e r g y ) in the l i n e a r accelerator radiographs.
F i g 1 F i l m d e n s i t y a s a f u n c t i o n of l e a d b a c k - s c r e e n thickness
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BACK SCREEN EXPOSURES AND RESOLUTION EVALUATION After the b a s i c f i h n c h a r a c t e r i s t i c c u r v e s w e r e made, the i n t e n s i f i c a t i o n c h a r a c t e r i s t i c s of the lead back s c r e e n s w e r e d e t e r m i n e d . F o r the b e t a t r o n e x p o s u r e s , a 6 0 m i l ( 1 . 5 m m ) lead f r o n t s c r e e n was used with v a r i o u s back s c r e e n t h i c k n e s s e s f r o m 0 to 120mil (0 to 3ram). A 3 0 m i l ( 0 . 7 5 m m ) f r o n t s c r e e n and the s a m e t h i c k n e s s r a n g e of b a c k s c r e e n s w e r e e v a l u a t e d for the l i n e a r a c c e l e r a t o r . An a p p r o p r i a t e e x p o s u r e was c h o s e n for each unit and was g i v e n to each of the l e a d - f i l m c o m b i n a t i o n s exposed with that unit. The v a r i a t i o n of the f i l m d e n s i t y as a f u n c t i o n of back s c r e e n t h i c k n e s s is shown in Fig 1 for the two u n i t s . F o r the b e t a t r o n , the f i l m d e n s i t y i n c r e a s e s s h a r p l y with the f i r s t 40rail ( l m m ) of back s c r e e n t h i c k n e s s and then r e m a i n s c o n s t a n t . The l i n e a r a c c e l e r a t o r c u r v e d i f f e r s f r o m the b e t a t r o n c u r v e by a l a r g e f a c t o r . The back s c r e e n i n t e n s i f i c a t i o n is v e r y low for the l i n e a r a c c e l e r a t o r . The i n t e n s i f i c a tion f a c t o r s for both u n i t s , plotted a s a f u n c t i o n of b a c k s c r e e n t h i c k n e s s in Fig 2, differ by a f a c t o r of 2 at the h i g h e r s c r e e n t h i c k n e s s e s . No d e g r a d a t i o n of the r a d i o g r a p h i c i m a g e s was noticed as a r e s u l t of b a c k s c a t t e r for the b e t a t r o n i m a g e s ; however, the s h a r p n e s s of the i m a g e s d e c r e a sed with i n c r e a s i n g back s c r e e n t h i c k n e s s . T h i s was p r e v i o u s l y noted by M i l l e r and Steely 3 in t h e i r b e t a t r o n i n v e s t i g a t i o n s . The opposite effect o c c u r r e d for the l i n e a r a c c e l e r a t o r i m a g e s . The r a d i o g r a p h s a p p e a r e d to have a high d e g r e e of s c a t t e r , r e d u c i n g the s h a r p n e s s and c o n t r a s t of the i m a g e s ; i m a g e
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F i g 2 R e l a t i v e i n t e n s i f i c a t i o n f a c t o r a s a f u n c t i o n of lead back-screen thickness s h a r p n e s s i n c r e a s e d with i n c r e a s i n g back s c r e e n t h i c k n e s s up to a p p r o x i m a t e l y 40rail ( l m m ) , and r e m a i n e d r e l a t i v e l y the s a m e for t h i c k e r back s c r e e n s . It was a p p a r e n t in the l i n e a r a c c e l e r a t o r r a d i o g r a p h s that a l a r g e a m o u n t of s c a t t e r e d r a d i a t i o n was p r e s e n t . The l o w e r b a c k s c r e e n t h i c k n e s s e s f i l t e r out l e s s of the b a c k s c a t t e r e d r a d i a t i o n than t h i c k e r s c r e e n s , thus allowing b a c k s c a t t e r to r e d u c e the defect r e s o l u t i o n . F r o m the b a c k s c r e e n s t u d i e s , it was d e t e r m i n e d that 0, 10, and 4 0 m i t (0, 0.25 and 1 . 0 r a m ) thick b a c k s c r e e n s should be i n c l u d e d in the e x p o s u r e g e o m e t r y and f r o n t s c r e e n c o m b i n a t i o n e v a l u a t i o n s u s i n g the b e t a t r o n . Since the s h a r p e s t i m a g e s w e r e obtained
with the 4 0 m i l ( l m m ) o r g r e a t e r b a c k s c r e e n t h i c k n e s s e s with the l i n e a r a c c e l e r a t o r , t h i s s c r e e n only was s e l e c t e d f o r f u r t h e r e v a l u a t i o n s u s i n g the l i n e a r a c c e l e r a t o r . The u s e of t h i c k e r b a c k s c r e e n s would not be e x p e c t e d to p r o v i d e any i m a g e i m p r o v e m e n t , s i n c e s e l f a b s o r p t i o n in the s c r e e n s would b e c o m e a f a c t o r a s shown in the l e v e l l i n g off of the i n t e n s i f i c a tion e f f e c t s in F i g 2.
F R ONT S C R E E N E X P O S U R E AND R E S O L U T I O N EVALUATION E x p o s u r e s w e r e s e l e c t e d to r e v e a l the i n t e n s i f i c a t i o n and r e s o l u t i o n c h a r a c t e r i s t i c s f o r s e v e r a l f r o n t s c r e e n t h i c k n e s s e s . L e a d s c r e e n s f r o m 5 to 1 2 0 m i l (0.12 to 3 . 0 m m ) t h i c k w e r e u s e d in th e c a s s e t t e with no b a c k i n g l e a d f o r the b e t a t r o n e x p o s u r e s . Since the l i n e a r a c c e l e r a t o r b a c k s c a t t e r p r o b l e m had to be m i n i m i z e d , a 40rail ( 1 . 0 m m ) t h i c k b a c k l e a d s c r e e n was u s e d f o r t h e s e e x p o s u r e s . A c o n s t a n t e x p o s u r e was g i v e n to e a c h of the l e a d - f i l m c o m b i n a t i o n s t h ro u g h the 17in (432mm) d i a m e t e r t i t a n i u m b i l l e t . F i l m d e n s i t y r e a d i n g s w e r e m e a s u r e d and a r e p l o t t e d in Fig 3 f o r both x - r a y g e n e r a t o r s . F r o m t h i s f i g u r e , the e f f e c t of f r o n t s c r e e n t h i c k n e s s on the i n t e n s i f i c a tion f a c t o r a p p e a r s r e l a t i v e l y u n i m p o r t a n t f o r t h e s e two units. Th e b e t a t r o n c u r v e d i f f e r s f r o m p r e d i c tions b a s e d on the b e t a t r o n s t u d i e s r e p o r t e d by M i l l e r and Steely3; in t h e i r work, the i n t e n s i f i c a t i o n f a c t o r r o s e v e r y s t e e p l y to about 4 0 m i l ( l m m ) of s c r e e n t h i c k n e s s , r e a c h e d a m a x i m u m at about 60rail ( 1 . 5 m m ) and d e c r e a s e d with f u r t h e r i n c r e a s e s in t h i c k n e s s . T h e l i n e a r a c c e l e r a t o r c u r v e in F i g 3 s h o w s a d e c r e a s e , but not a s g r e a t as t h e y r e p o r t e d . It is a p p a r ent that the r o u n d b i l l e t i t s e l f p r o v i d e s a d e f i n i t e s o u r c e of s c a t t e r and that the low s c r e e n t h i c k n e s s e s p r o v i d e an a l m o s t e q u a l a m o u n t of i n t e n s i f i c a t i o n s i n c e m o r e s c a t t e r e d r a d i a t i o n i n t e n s i t y is p a s s e d by the t h i n n e r s c r e e n s . E v e n t u a l l y the l a r g e r t h i c k n e s s e s
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120
F i g 4 R a d i o g r a p h i c r e s o l u t i o n a s a f u n c t i o n of l e a d front-screen thickness
of l e a d p r o v i d e m o r e f i l t e r i n g and r e d u c e the r a d i a tion e x p o s i n g the f i l m . T h i s e f f e c t is m o r e p r o n o u n c ed in the l i n e a r a c c e l e r a t o r r a d i o g r a p h s b e c a u s e of the l o w e r i n i t i a l r a d i a t i o n e n e r g i e s . In a s u b s e q u e n t s e q u e n c e of e x p o s u r e s , the e x p o s u r e t i m e s w e r e i n c r e a s e d f o r the b e t a t r o n f o r e a c h s c r e e n c o m b i n a t i o n to p r o d u c e a 2 . 2 f i l m d e n s i t y . T h e s e a d j u s t m e n t s w e r e n e c e s s a r y b e c a u s e of the r e l a t i v e l y low f i l m d e n s i t i e s in the i n i t i a l e x p o s u r e s , and so that the r a d i o g r a p h s of the b i l l e t s by both x - r a y g e n e r a t o r s could be e v a l u a t e d at the s a m e d e n s i t y with the s a m e a m o u n t of f i l m c o n t r a s t . Since the l i n e a r a c c e l e r a t o r r a d i o g r a p h s w e r e a l r e a d y in this r a n g e , no a d j u s t m e n t s w e r e n e c e s s a r y . In o r d e r to e v a l u a t e the r a d i o g r a p h s , the s m a l l e s t d e t e c t a b l e I T hole in MIL 453 r a d i o g r a p h i c p e n e t r a m e t e r s w as r e c o r d e d as the m i n i m u m r e s o l u t i o n f o r a p a r t i c u l a r r a d i o g r a p h . T he r e l a t i o n shown in F i g 4 i n d i c a t e s the m i n i m u m t h i c k n e s s and d i a m e t e r p e n e t r a m e t e r hole which w a s r e s o l v e d in the r a d i o g r a p h s m a d e with d i f f e r e n t f r o n t s c r e e n t h i c k n e s s e s . With the b e t a t r o n , the 6 0 m i l ( 1 . 5 m m ) f r o n t s c r e e n p r o d u c e d the b e s t c o m b i n a t i o n of s h a r p n e s s . The 40 and 8 0 m i l (1 and 2 m m ) c o m b i n a t i o n s p r o v i d e d a l m o s t e q u a l s e n s i t i v i t y . The l i n e a r a c c e l e r a t o r radiographs showed comparable s e n s i t i v i t y f o r low s c r e e n t h i c k n e s s e s , but the r e s o l u t i o n o b t a i n a b l e was not a s good as with the b e t a t r o n . It should be noted that an i n f i n i t e n u m b e r of p e n e t r a m e t e r s w as not a v a i l a b l e so that the r e p o r t e d r e s o l u tion is a l w a y s s o m e w h a t h i g h e r than the e x a c t l i m i t . As an e x a m p l e , e i t h e r the r e s o l u t i o n i s 80 or 100rail (2.0 or 2 . 5 m m ) with no p o s s i b l e v a l u e m e a s u r e d between. F o r the l i n e a r a c c e l e r a t o r r a d i o g r a p h s , although 100rail ( 2 . 5 r a m ) r e s o l u t i o n was o b t a i n e d o v e r a wide r a n g e of t h i c k n e s s e s , a 4 0 m i l ( 1 . 0 m m ) f r o n t s c r e e n p r o d u c e d the o p t i m i z e d i m a g e . The sh ap e of the
n o n - d e s t r u c t i v e t e s t i n g O c t o b e r 1970
359
b e t a t r o n f r o n t s c r e e n c u r v e in Fig 4 at l o w e r s c r e e n t h i c k n e s s e s is c a u s e d by the d e g r a d a t i o n of i m a g e q u a l i t y r e s u l t i n g f r o m s c a t t e r and, at the l a r g e r t h i c k n e s s e s , by the l a r g e r a r e a c o v e r e d by s e c o n d a r y r a d i a t i o n f r o m a t h i c k e r lead s c r e e n which d e c r e a s e s the s h a r p n e s s . F o r the l i n e a r a c c e l e r a t o r at the l a r g e r t h i c k n e s s e s , this l a t t e r effect is not as p r o n ounced b e c a u s e the t h i c k e r s c r e e n f i l t e r s s c a t t e r to i m p r o v e the r e s o l u t i o n .
2,S
100
80-
720
.o
_~60 O
41"5
SCREEN COMBINATIONS ¢-
B a s e d on the p r e v i o u s data for both f r o n t and back s c r e e n s u s i n g the b e t a t r o n , t h r e e f r o n t and t h r e e b a c k s c r e e n s w e r e c h o s e n for f u r t h e r e v a l u a t i o n s . The f r o n t s c r e e n s s e l e c t e d w e r e 40, 60, and 8 0 m i l (1, 1 . 5 and 2 m m ) thick, while the b a c k s c r e e n s w e r e 0, 10 and 4 0 m i l (0, 0.25 and 1 . 0 m m ) thick. Nine e x p o s u r e s with t h e s e s c r e e n c o m b i n a t i o n s w e r e made of the 17in (432mm) d i a m e t e r b i l l e t , followed by c o r r e c t i o n e x p o s u r e s for f i l m s not having d e n s i t i e s of 2 . 5 + 0 . 2 0 H&D d e n s i t y u n i t s . T h e s e r a d i o g r a p h s w e r e e v a l u a t e d and t h r e e p r e f e r r e d s c r e e n c o m b i n a t i o n s w e r e s e l e c t e d f o r the r e m a i n i n g e v a l u a t i o n s . A 6 0 m i l ( 1 . 5 m m ) lead f r o n t s c r e e n with no back s c r e e n c o m b i n a t i o n (ie 60-0 ( 1 . 5 - 0 ) ) was s e l e c t e d a s having the b e s t detail s h a r p n e s s and o v e r a l l b e s t r e s o l u t i o n ; an 80-10 ( 2 . 0 - 0 . 2 5 ) c o m b i n a t i o n showed i n t e r m e d i a t e s h a r p n e s s and c o n t r a s t ; while a n 80-40 ( 2 . 0 - 1 . 0 ) showed good c o n t r a s t but s o m e w h a t p o o r e r d e t a i l . As was noted, the 60-0 ( 1 . 5 - 0 ) c o m b i n a t i o n p r o d u c e d the b e s t s e n s i t i v i t y with the 80-10 ( 2 . 0 - 0 . 2 5 ) c o m b i n a t i o n a c l o s e second. The slight d i f f e r e n c e in s e n s i t i v i t y b e t w e e n t h e s e two lead s c r e e n c o m b i n a t i o n s was a c c o m p a n i e d by a o n e t h i r d r e d u c t i o n in e x p o s u r e t i m e for the 80-10 ( 2 . 0 0.25) c o m b i n a t i o n , r e s u l t i n g in l o w e r i n s p e c t i o n c o s t s without a l a r g e s a c r i f i c e in f i l m quality. F r o m the p r e v i o u s e v a l u a t i o n s on the l i n e a r a c c e l e r a t o r s c r e e n c o m b i n a t i o n s , the 4 0 - 4 0 ( 1 . 0 - 1 . 0 ) s c r e e n s p r o v i d e d o p t i m u m i m a g e s . Since t h e r e was no a p p r e c i a b l e d e t e r i o r a t i o n in s h a r p n e s s and d e t a i l as with the b e t a t r o n , t h i s c o m b i n a t i o n was s e l e c t e d for g e o m e t r i c e n l a r g e m e n t s and an a d d i t i o n a l 120-40 ( 3 . 0 - 1 . 0 ) c o m b i n a t i o n was s e l e c t e d for the fine g r a i n f i l m e v a l u a t i o n s which a r e d i s c u s s e d in the following p a r a g r a p h s . T h i s l a t t e r c o m b i n a t i o n was c h o s e n ~ o s e e if a thick f r o n t s c r e e n could r e d u c e unwanted scatter radiation.
EXPOSURE GEOMETRIC EVALUATIONS In o r d e r to d e t e r m i n e the effects of e x p o s u r e g e o m e t r y on i m a g e s e n s i t i v i t y , the 17in (432mm) b i l l e t was r a d i o g r a p h e d with t h r e e d i f f e r e n t e x p o s u r e g e o m e t r i e s and the t h r e e s e l e c t e d lead s c r e e n c o m b i n a t i o n s p r e v i o u s l y d e t e r m i n e d . The b i l l e t was c e n t r e d at 6ft ( 1 . 8 3 m ) while the c a s s e t t e c o n t a i n i n g Kodak AA f i l m was placed at 6ft 8 . 5 i n (2.04m), 9ft (2.7m) and 12ft (3.66m). T h e s e d i s t a n c e s w e r e s e l e c t e d to give a p p r o x i m a t e e n l a r g e m e n t s of 1.0, 1 . 5 and 2×. In p r a c t i c e , the a c t u a l e n l a r g e m e n t s a c h i e v e d will depend on the r e l a t i o n s h i p b e t w e e n the t a r g e t , defect and f i l m d i s t a n c e s , but the a v e r a g e e n l a r g e m e n t is n e a r to the d e s i r e d v a l u e s . The r e s u l t s of t h e s e t e s t s a r e shown in Fig 5 w h e r e r a d i o g r a p h i c r e s o l u t i o n is plotted as a function of the a c t u a l p e n e t r a m e t e r e n l a r g e m e n t m e a s u r e d in the r a d i o g r a p h s . The b e s t r e s o l u t i o n
360
n o n - d e s t r u c t i v e t e s t i n g O c t o b e r 1970
~1.0
Screen combinations
0 n,"
o a n
20-
0 1.0
t
60-0mil[l'5-0mm] 80-40mil [ 2-1mini 80-10rail [ 2 - 0 - 2 5 m m ]
0,5
I
I
1"5
2"0
t0
2"5
Actual penetrometer enlargement R a d i o g r a p h i c r e s o l u t i o n through 17in ( 4 3 2 m m ) of 6 A 1 - 4 V t i t a n i u m a s a function of e n l a r g e m e n t for E a s t m a n Kodak AA f i l m and t h r e e lead s c r e e n c o m b i n a t i o n s
was o b t a i n e d at the h i g h e s t m a g n i f i c a t i o n ; 4 0 m i l ( l m m ) r e s o l u t i o n r e s u l t e d with the d e t e c t i o n of the I T hole in the MIL 453, 2 . 0 i n ( 5 0 . 8 m m ) p e n e t r a m e t e r . The 60-0 ( 1 . 5 - 0 ) s c r e e n c o m b i n a t i o n p r o d u c e d the b e s t r e s u l t s with the 80-10 ( 2 . 0 - 0 . 2 5 ) c o m b i n a t i o n a c l o s e second. T a b l e 1 l i s t s the e x p o s u r e s to obtain a 2 . 5 H&D f i l m d e n s i t y for the 9 s c r e e n / e n l a r g e m e n t c o m b i n a t i o n s . In g e n e r a l , the l o n g e r e x p o s u r e t i m e s p r o d u c e d the b e s t r a d i o g r a p h i c r e s o l u t i o n s . The t i m e s l i s t e d a r e a p p r o x i m a t e s i n c e the b e t a t r o n output d e c r e a s e d with t i m e . A r e s o l u t i o n of 4 0 m i l ( l m m ) was obtained at 2 × in a p p r o x i m a t e l y 4 2 m i n for [he 80-10 ( 2 . 0 - 0 . 2 5 ) s c r e e n s , while 50mil ( 1 . 2 5 m m ) r e s o l u t i o n was obtained with the n o m i n a l 1 . 5 x e n l a r g e m e n t at 19rain with the 8 0 - 1 0 (2-0.25} s c r e e n s . With the 60-10 ( 1 . 5 - 0 ) s c r e e n s and an e x p o s u r e t i m e of l l m i n with no e n l a r g e m e n t s , 60rail ( 1 . 5 m m ) r e s o l u t i o n r e s u l t e d . Using the 8 0 - 4 0 (2-1) s c r e e n s , 80mil (2ram) r e s o l u t i o n r e s u l t e d for a 5 . 2 m i n e x p o s u r e . A c o m p a r a b l e s e t of e x p o s u r e s was p l a n n e d for the l i n e a r a c c e l e r a t o r in o r d e r to study the r e s o l u t i o n achieved with the s a m e e x p o s u r e g e o m e t r i e s . Howe v e r , a f t e r the i n i t i a l e x p o s u r e s w e r e made with the b i l l e t c e n t r e d at 6ft (1.83m) and the i n i t i a l e n l a r g e m e n t s made at the 9ft ( 2 . 7 4 m ) t a r g e t - t o - f i l m d i s t a n c e , it was e v i d e n t that e n l a r g e m e n t r a d i o g r a p h s were not p o s s i b l e b e c a u s e of the a m o u n t of s c a t t e r in the radiographs. Very little p e n e t r a m e t e r image detail was v i s i b l e in the r a d i o g r a p h s , although d e t a i l s of the c a s s e t t e s t r u c t u r e w e r e v i s i b l e . This i n d i c a t e d that r a d i a t i o n o t h e r than the p r i m a r y b e a m was f o r m i n g the i m a g e . On the r a d i o g r a p h s made i m m e d i a t e l y behind the b i l l e t at a 6ft 8 . 5 i n (2.04m) t a r g e t - t o - f i l m
T a b l e 1 E x p o s u r e t i m e a s a f u n c t i o n of f i l m s c r e e n c o m b i n a t i o n s and e n l a r g e m e n t s for 1 7 i n ( 4 3 2 m m ) of t i t a n i u m and A A f i l m * Screen combination
Average enlargement
Time min
Roentgens at I m
Target-film-distance (in) (m)
60-0 60-0 60-0 80-10 80-10 80-10 80-40 80-40 80-40
1.C 1.5 2.0 1.0 1.5 2.0 1.0 1.5 2.0
10.7 27.2 64.2 6.2 18.5 41.5 5.3 16.5 34.6
548 1390 3277 316 946 2118 272 843 1767
80.5 108 144 80.5 108 144 80.5 108 144
2.04 2.74 3.66 2.04 2.74 3.66 2.04 2.74 3.66
* 2.5 H&D film density
distance, the 120rail (3.0ram) penetrameter hole was resolved on AA film with 40mil (Imm) front and rear screens. This is in contrast to the 100rail (2.5mm) resolution achieved at the 9ft (2.74m) target-to-film distance illustrated previously in the front screen evaluations. From these two sources of data, it can be seen that the size of the focal spot plays a significant role in the image sharpness and, hence, the change in defect resolving capability with different target-to-film distances. The AA film exposure was 2,300rad, which at the I, 500rad/min intensity takes approximately 1.55min.
and would add greatly to exposure costs. Finer grained films provide better resolution capabilities but also raise exposure costs since their sensitivity to x-radiation is lower than the coarser grained films.
FILM
The use of finer grained films with the linear accelerator provided a large improvement in image sensitivity. When radiographs were made with Kodak type M film with the billet centred at 6ft (2.04m), 80mils (2.0ram) resolution resulted. The exposure for the 40M40 (1M1) combination (ie, using type M film) was
EVALUATIONS
The preceeding evaluations established the best techniques available with Eastman Kodak type AA film. For the betatron radiography, better resolution might have been produced with more enlargement. However, greater enlargement requires more film and more exposures to cover a 17in (432mm) diameter billet,
The use of finer grained films was evaluated with little success for the betatron. Since exposure times were excessive for Eastman Kodak type M industrial x-ray film, type T film was used. This film proved to be very sensitive to mottling when processed by hand and had to be processed automatically to minimize it. Using type T film, the radiographs did not show any better resolution than that obtained with the type AA film manually processed.
7380rads, which takes approximately 4.9min. With the billet centred at 8ft 3.5in (2.53m) and the film at
T a b l e 2 R e s o l u t i o n and e x p o s u r e t i m e s for 7 . 5 M e V l i n e a r a c c e l e r a t o r and 25MeV m e d i c a l b e t a t r o n for 17in ( 4 3 2 m m ) d i a m e t e r 6 A 1 - 4 V t i t a n i u m b i l l e t s Radiation Source
Resolution
Film*--Screen Combination
Linear Medical Industrial Accelerator Betatron Betatron** (mil) (ram) (mil)
(mm)
Exposure Time
(min)
Target-Object Distance From Billet C e n t r e (ft) (m)
Target-to-Film Distance (ft)
(m)
120
3.0
40AA40
1AA1
1. 55
6
1.83
6.71
2.04
100
2.5
40AA40
1AA1
1.95
8.29
2.53
9
2.74
80
2.0
40M40
1M1
4.90
6
1.83
6.71
2.04
60
1.5
40M40
1M1
6.60
8.29
2. 53
9
2.74
60
1.5
120M40
3M1
7.60
8.29
2.53
9
2.74
60
1.5
60AA0
1.5AA0
10.70
6
1.83
6.71
2.04
60
1.5
60AA0
1. 5AA0
6.42
6
1.83
6.71
2.04
80
2.0
80AA10
2AA0
6.20
6
1.83
6.71
2.04
80
2.0
80AA10
2AA0
3.72
6
1.83
6.71
2.04
80
2.0
80AA40
2AA1
5.30
6
1.83
6.71
2.04
80
2.0
80AA40
2AA1
3. 18
6
1.83
6.71
2.04
50
1.25
80AA10
2AA0.25
18.5
6
1.83
9
2.74
50
1.25
80AA10
2AA0.25
11.1
6
i. 83
9
2.74
40
1.0
60AA0
1.5AA0
64
40
1.0
60AA0
1.5AA0
38
6 6
I. 83 I. 83
12 12
3.66 3.66
40
1.0
80AA10
2AA0.25
41.5
6
I. 83
12
3.66
40
1.0
80AA10
2AA0.25
25
6
1.83
12
3.66
* Eastman Kodak Industrialx-ray film,types AAand M ** Estimated ~: 60 percent of medicalunitexposures
non-destructivetesting October 1970
361
9ft (2.74m), the r e s o l u t i o n obtained was 60rail ( 1 . 5 m m ) for the 120M40 (3M1) and 40M40 (1M1) s c r e e n c o m b i n a t i o n s , while the e x p o s u r e s w e r e l l 4 0 r a d s ( 7 . 6 m i n ) and 9 9 0 0 r a d s ( 6 . 6 m i n ) r e s p e c t i v e l y . The t h i c k e r f r o n t s c r e e n p r o v i d e d a s l i g h t i m p r o v e m e n t in i m a g e r e s o l u t i o n ; e v i d e n t l y it r e d u c e d the i n t e n s i t y of s c a t t e r e d r a d i a t i o n .
CONCLUSIONS The e v a l u a t i o n of the u s e of b e t a t r o n s and l i n e a r a c c e l e r a t o r s for i n s p e c t i o n of t i t a n i u m b i l l e t s has shown a wide r a n g e of defect s e n s i t i v i t y . F o r a p a r t i c u l a r x - r a y g e n e r a t o r , s e n s i t i v i t y is d e p e n d e n t on lead s c r e e n t h i c k n e s s e s , e x p o s u r e g e o m e t r y and f i l m grain size. Betatron radiographic techniques reported in this a r t i c l e w e r e developed u s i n g a m e d i c a l b e t a t r o n which has a l a r g e r c o m p e n s a t o r than that u s e d in i n d u s t r i a l u n i t s . The b e t a t r o n t e c h n i q u e t i m e s l i s t e d in T a b l e 1 can be r e d u c e d by 40 p e r c e n t with an i n d u s t r i a l unit. T a b l e 2 l i s t s the v a r i o u s r e s o l u t i o n s and e x p o s u r e t i m e s a c h i e v e d with v a r i o u s t e c h n i q u e s for both the l i n e a r a c c e l e r a t o r and m e d i c a l b e t a t r o n , and those a c h i e v a b l e with an i n d u s t r i a l b e t a t r o n . Although the r a d i a t i o n u n i t s for each x - r a y g e n e r a t o r a r e d i f f e r e n t , no c o n v e r s i o n was a t t e m p t e d for c o m p a r i s o n s . Instead, the a p p r o x i m a t e e x p o s u r e t i m e s a r e g i v e n for each u n i t u s i n g a p a r t i c u l a r x - r a y film. Since the i n i t i a l c o s t s , s e t t i n g - u p t i m e s and m a i n t e n a n c e c o s t s of t h e s e two u n i t s a r e a p p r o x i m a t e l y the s a m e , r e l a t i v e e x p o s u r e t i m e is a b a s i s for d e t e r m i n ing the cost of r a d i o g r a p h y . N e i t h e r u n i t h a s a c l e a r cut a d v a n t a g e when s e n s i t i v i t y and e x p o s u r e t i m e s a r e c o m p a r e d . With its h i g h e r output, the l i n e a r a c c e l e r a t o r p r o d u c e s s h o r t e r e x p o s u r e s on AA f i l m , but c a n n o t m a t c h the defect r e s o l u t i o n of the b e t a t r o n . The s a m e e x p o s u r e t i m e s r e s u l t when type M f i l m is used with the l i n e a r a c c e l e r a t o r to a c h i e v e the s a m e defect r e s o l u t i o n a s a c h i e v e d with the b e t a t r o n on AA f i l m . The l i n e a r a c c e l e r a t o r can a c h i e v e a m i n i m u m r e s o l u t i o n of 6 0 m i l ( 1 . 5 m m ) on the 17in (432mm) b i l l e t in 6 . 6 m i n c o m p a r e d to a p r o j e c t e d i n d u s t r i a l b e t a t r o n e x p o s u r e t i m e of 6 . 4 2 r a i n for the s a m e r e s o l u t i o n . On the o t h e r hand, the b e t a t r o n is
362
n o n - d e s t r u c t i v e t e s t i n g O c t o b e r 1970
c a p a b l e of p r o v i d i n g defect r e s o l u t i o n a s low as 40rail ( 1 . 0 m m ) , but at an e x p o s u r e t i m e of 25min. The b e t a t r o n a p p e a r s to have a slight a d v a n t a g e b e c a u s e of its v e r s a t i l i t y . It should be noted, however, that the l i n e a r a c c e l e r a t o r that was used in these i n v e s t i g a t i o n s was the f i r s t u n i t of its type. Many i m p r o v e m e n t s may be p o s s i b l e to l o w e r the s c a t t e r r a d i a t i o n of the l i n e a r a c c e l e r a t o r and to p e r m i t u s e of l o w e r t h i c k n e s s b a c k s c r e e n s and f a s t e r f i l m s for i m p r o v e d defect r e s o l u t i o n at s h o r t e r e x p o s u r e t i m e s .
ACKNOWLEDGEMENTS The a u t h o r is g r a t e f u l to Mr John K i r v e n and Dr R. K. L o e f f l e r of N o n d e s t r u c t i v e T e s t i n g Inc (Ohio, USA) for t h e i r help in p r o v i d i n g the b e t a t r o n e x p o s u r e s . Many t h a n k s m u s t go to Mr E. S. P r o c t o r of C o m b u s t i o n E n g i n e e r i n g ( T e n n e s s e e , USA) for p r o v i d i n g t i m e to u s e the l i n e a r a c c e l e r a t o r and to his staff for t h e i r e x c e l l e n t c o - o p e r a t i o n in m a k i n g the r a d i o g r a p h s . The a u t h o r a l s o w i s h e s to acknowledge the g u i d a n c e and a s s i s t a n c e of Mr J a m e s Hottoway-ef the United States A i r F o r c e M a t e r i a l s L a b o r a t o r y and to extend his g r a t i t u d e for the s u p p o r t of the A i r F o r c e Materials Laboratory.
REFERENCES 1
M i l l e r , N. C and T e n n e y , G. H., ' V a r i o u s p e n e t r a m e t e r types and t h e i r l i m i t a t i o n s ' , N o n d e s t r u c tive T e s t i n g (now M a t e r i a l s E v a l u a t i o n ) , Vol 11, No 2 (1953) p17
2
P o l a n s k y , D. and C r i s c u o l o , E. L., 'A method for the c l a s s i f i c a t i o n of i n d u s t r i a l x - r a y f i l m ' , P r o c e e d i n g s of the Fifth I n t e r n a t i o n a l C o n f e r e n c e on N o n d e s t r u c t i v e T e s t i n g , M o n t r e a l , Canada, (May 1967)
3
M i l l e r , N. C., and Steely, J. D., 'Some e x p e r i m e n tal f i n d i n g s and o p e r a t i n g p r a c t i c e s in b e t a t r o n r a d i o g r a p h y ' , N o n d e s t r u c t i v e T e s t i n g (now M a t e r i a l s E v a l u a t i o n ) , V o l 11, No 8 ( N o v e m b e r 1953) p35
The defect r e s o l u t i o n c a p a b i l i t i e s of a 25MeV b e t a t r o n and a 7 . 5 M e V l i n e a r a c c e l e r a t o r for the ndt of l a r g e d i a m e t e r t i t a n i u m b i l l e t s a r e c o m p a r e d . The effects of lead f r o n t and b a c k s c r e e n t h i c k n e s s e s , e x p o s u r e g e o m e t r y and f i l m v a r i a b l e s on the q u a l i t y of the r a d i o g r a p h i c i m a g e a r e d e s c r i b e d . The a p p l i c a b i l i t y of each u n i t is d i s c u s s e d in view of the r e s o l u t i o n achieved and the e c o n o m i c s of the r a d i o g r a p h i c procedures.
T i t a n i u m has b e e n u s e d e x t e n s i v e l y in both c o m m e r cial and m i l i t a r y a i r c r a f t e n g i n e s d u r i n g the p a s t decade b e c a u s e of i t s e x c e l l e n t s t r e n g t h - t o - w e i g h t r a t i o . F o r the c o m p r e s s o r s e c t i o n s of t h e s e e n g i n e s , t i t a n i u m b l a d e s , v a n e s and d i s c s a r e forged f r o m b i l l e t stock. F o r the l a r g e r e n g i n e s on a d v a n c e d a i r craft, c o m p r e s s o r d i s c s a r e forged f r o m b i l l e t stock of up to 18in (458mm) in d i a m e t e r . T h e s e l a r g e r d i a m e t e r b i l l e t s r e c e i v e l e s s work than s m a l l b i l l e t s d u r i n g f o r g i n g and t h e r e f o r e c o n t a i n a g e n e r a l l y coarser grained m i c r o s t r u c t u r e . This m i c r o s t r u c t u r e l i m i t s the d e t e c t i o n c a p a b i l i t y and r e l i a b i l i t y of finding s m a l l d e f e c t s when u s i n g s t a n d a r d u l t r a s o n i c i n s p e c t i o n t e c h n i q u e s for d e t e r m i n i n g b i l l e t q u a l i t y . In a m a c h i n e d - d i s c c o n f i g u r a t i o n , s e v e r a l ndt m e t h o d s a r e u s e d to d e t e c t defect c o n d i t i o n s . However, good cost c o n t r o l p r a c t i c e r e q u i r e s the l o c a t i o n of d e f e c t s in s t a r t i n g b i l l e t s r a t h e r than in f i n i s h e d p a r t s . F o r this r e a s o n , the United States A i r F o r c e M a t e r i a l s L a b o r a t o r y s p o n s o r e d p r o g r a m m e s to i m p r o v e the s e n s i t i v i t y of ndt in finding s m a l l d e f e c t s at the e a r l i e s t p o s s i b l e t i m e in the f a b r i c a t i o n s e q u e n c e ; n a m e l y , at the b i l l e t stage. A r e l i a b l e b i l l e t i n s p e c tion method was of p a r t i c u l a r i n t e r e s t for 17in d i a m e t e r (432mm) b i l l e t s u s e d in T F - 3 9 engine c o m pressor-disc forgings.
Mr S a t t l e r is with the M a t e r i a l s T e c h n o l o g y E q u i p m e n t G r o u p of TRW Inc, 23555 E u c l i d Avenue, C l e v e land, Ohio 44117, USA. His p a p e r was p r e s e n t e d at the AGARD m e e t i n g on ' A d v a n c e d t e c h n o l o g y for p r o d u c tion of a e r o s p a c e e n g i n e s ' held in London f r o m 6 - 1 0 A p r i l 1970.
E x p e r i m e n t a l work was p e r f o r m e d with both u l t r a s o n i c and r a d i o g r a p h i c i n s p e c t i o n methods, but only the r a d i o g r a p h i c work will be r e p o r t e d h e r e . The high e n e r g y r a d i a t i o n s o u r c e s u s e d for this e v a l u a t i o n w e r e a 7 . 5 M e V V a r i a n L i n e a r A c c e l e r a t o r and a 25MeV A l l i s - C h a l m e r s B e t a t r o n . The focal spot on the V a r i a n was s l i g h t l y l e s s than 1 . 5 m m . The b e t a t r o n d i f f e r s f r o m the l i n e a r a c c e l e r a t o r in the method of a c c e l e r a t i n g the e l e c t r o n s and s i n c e a n a r r o w x r a y b e a m i s e m i t t e d it m u s t be made m o r e h o m o g e n e o u s with an e x t e r n a l a b s o r p t i o n cone, the c o m p e n s a t o r . N o r m a l l y for i n d u s t r i a l work, a 30 p e r c e n t c o m p e n s a t o r is used; s i n c e the b e t a t r o n used in t h e s e e v a l u a t i o n s was a m e d i c a l unit, it c o n t a i n e d a 50 p e r cent c o m p e n s a t o r . T h i s m e d i c a l u n i t had a l o w e r output b e a m i n t e n s i t y than a c o m p a r a b l e i n d u s t r i a l u n i t but was m o r e u n i f o r m in its e n e r g y and i n t e n s i t y d i s t r i b u t i o n . The u n i t has an e x t r e m e l y s m a l l 0 . 3 m m focal spot. In o r d e r to e v a l u a t e the defect d e t e c t i o n c a p a b i l i t y of t h e s e two u n i t s , e x p e r i m e n t s w e r e p e r f o r m e d to d e t e r m i n e t h e following: 1
R e s o l u t i o n and i n t e n s i f i c a t i o n c h a r a c t e r i s t i c s of lead b a c k i n g s c r e e n s
2
R e s o l u t i o n and i n t e n s i f i c a t i o n c h a r a c t e r i s t i c s of lead f r o n t s c r e e n s
3
R e s o l u t i o n o p t i m i z a t i o n with f r o n t and b a c k screen combinations
4
R e s o l u t i o n o b t a i n a b l e with e n l a r g e m e n t techniques
5
R e s o l u t i o n l i m i t s u s i n g s m a l l and f i n e grained x-ray films
n o n - d e s t r u c t i v e t e s t i n g O c t o b e r 1970
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The r e s o l u t i o n was d e t e r m i n e d by o b s e r v i n g the s m a l l e s t 1T p e n e t r a m e t e r hole v i s i b l e when United States M i l i t a r y S t a n d a r d 453 t i t a n i u m p e n e t r a m e t e r s w e r e placed on the f r o n t of the b i l l e t s . Work by M i l l e r and T e n n e y 1 has e s t a b l i s h e d the r e s o l u t i o n of this type of p e n e t r a m e t e r hole to be a p p r o x i m a t e l y e q u i v a l e n t to the i m a g e o b t a i n e d for s p h e r i c a l c a v i t i e s having the s a m e hole d i a m e t e r ,
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A v a c u u m bag for f i l m c a s s e t t e s was u s e d for all e x p o s u r e s to a s s u r e adequate, u n i f o r m c o n t a c t b e t w e e n the lead s c r e e n s and the x - r a y film. S t a n d a r d c a r d b o a r d c a s s e t t e s w e r e used, except that the u s u a l lead b a c k i n g foils w e r e r e m o v e d . The x - r a y f i l m s for the b e t a t r o n e x p o s u r e s w e r e p r o c e s s e d m a n u a l l y while the l i n e a r a c c e l e r a t o r f i l m s w e r e p r o c e s s e d a u t o m a t i c a l l y . Work by P o l a n s k y and C r i s c u o l o 2 has shown that f i l m s p r o c e s s e d a u t o m a t i c a l l y tend to have h i g h e r g r a i n i n e s s due to the h i g h e r d e v e l o p e r t e m p e r a t u r e s . However, the a u t o m a t i c p r o c e s s o r u s e d to p r o c e s s the l i n e a r a c c e l e r a t o r f i l m s had a l o n g e r p r o c e s s i n g cycle and a l o w e r d e v e l o p e r t e m p e r a t u r e than u s u a l , which helped to l i m i t the g r a i n i n e s s .
~ 1.5 ~>"
In o r d e r to obtain f i l m r e s p o n s e c u r v e s for both x - r a y g e n e r a t o r s , s e v e r a l r a d i o g r a p h s w e r e m a d e of a 10in (254mm) d i a m e t e r 6A1-4V t i t a n i u m a l l o y b i l l e t u s i n g the b e t a t r o n , and of a 17in (432mm) d i a m e t e r b i l l e t u s i n g the l i n e a r a c c e l e r a t o r . The data f r o m these radiographs provided exposure-density curves for E a s t m a n Kodak T y p e s AA and M i n d u s t r i a l x - r a y f i l m s . The l a r g e d i f f e r e n c e found b e t w e e n the e x p o s u r e r a t i o s of the two u n i t s is a f u n c t i o n of the b e a m e n e r g y d i f f e r e n c e s and the p r o b a b l e h i g h e r b a c k s c a t t e r i n t e n s i t y (of l o w e r e n e r g y ) in the l i n e a r accelerator radiographs.
F i g 1 F i l m d e n s i t y a s a f u n c t i o n of l e a d b a c k - s c r e e n thickness
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BACK SCREEN EXPOSURES AND RESOLUTION EVALUATION After the b a s i c f i h n c h a r a c t e r i s t i c c u r v e s w e r e made, the i n t e n s i f i c a t i o n c h a r a c t e r i s t i c s of the lead back s c r e e n s w e r e d e t e r m i n e d . F o r the b e t a t r o n e x p o s u r e s , a 6 0 m i l ( 1 . 5 m m ) lead f r o n t s c r e e n was used with v a r i o u s back s c r e e n t h i c k n e s s e s f r o m 0 to 120mil (0 to 3ram). A 3 0 m i l ( 0 . 7 5 m m ) f r o n t s c r e e n and the s a m e t h i c k n e s s r a n g e of b a c k s c r e e n s w e r e e v a l u a t e d for the l i n e a r a c c e l e r a t o r . An a p p r o p r i a t e e x p o s u r e was c h o s e n for each unit and was g i v e n to each of the l e a d - f i l m c o m b i n a t i o n s exposed with that unit. The v a r i a t i o n of the f i l m d e n s i t y as a f u n c t i o n of back s c r e e n t h i c k n e s s is shown in Fig 1 for the two u n i t s . F o r the b e t a t r o n , the f i l m d e n s i t y i n c r e a s e s s h a r p l y with the f i r s t 40rail ( l m m ) of back s c r e e n t h i c k n e s s and then r e m a i n s c o n s t a n t . The l i n e a r a c c e l e r a t o r c u r v e d i f f e r s f r o m the b e t a t r o n c u r v e by a l a r g e f a c t o r . The back s c r e e n i n t e n s i f i c a t i o n is v e r y low for the l i n e a r a c c e l e r a t o r . The i n t e n s i f i c a tion f a c t o r s for both u n i t s , plotted a s a f u n c t i o n of b a c k s c r e e n t h i c k n e s s in Fig 2, differ by a f a c t o r of 2 at the h i g h e r s c r e e n t h i c k n e s s e s . No d e g r a d a t i o n of the r a d i o g r a p h i c i m a g e s was noticed as a r e s u l t of b a c k s c a t t e r for the b e t a t r o n i m a g e s ; however, the s h a r p n e s s of the i m a g e s d e c r e a sed with i n c r e a s i n g back s c r e e n t h i c k n e s s . T h i s was p r e v i o u s l y noted by M i l l e r and Steely 3 in t h e i r b e t a t r o n i n v e s t i g a t i o n s . The opposite effect o c c u r r e d for the l i n e a r a c c e l e r a t o r i m a g e s . The r a d i o g r a p h s a p p e a r e d to have a high d e g r e e of s c a t t e r , r e d u c i n g the s h a r p n e s s and c o n t r a s t of the i m a g e s ; i m a g e
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F i g 2 R e l a t i v e i n t e n s i f i c a t i o n f a c t o r a s a f u n c t i o n of lead back-screen thickness s h a r p n e s s i n c r e a s e d with i n c r e a s i n g back s c r e e n t h i c k n e s s up to a p p r o x i m a t e l y 40rail ( l m m ) , and r e m a i n e d r e l a t i v e l y the s a m e for t h i c k e r back s c r e e n s . It was a p p a r e n t in the l i n e a r a c c e l e r a t o r r a d i o g r a p h s that a l a r g e a m o u n t of s c a t t e r e d r a d i a t i o n was p r e s e n t . The l o w e r b a c k s c r e e n t h i c k n e s s e s f i l t e r out l e s s of the b a c k s c a t t e r e d r a d i a t i o n than t h i c k e r s c r e e n s , thus allowing b a c k s c a t t e r to r e d u c e the defect r e s o l u t i o n . F r o m the b a c k s c r e e n s t u d i e s , it was d e t e r m i n e d that 0, 10, and 4 0 m i t (0, 0.25 and 1 . 0 r a m ) thick b a c k s c r e e n s should be i n c l u d e d in the e x p o s u r e g e o m e t r y and f r o n t s c r e e n c o m b i n a t i o n e v a l u a t i o n s u s i n g the b e t a t r o n . Since the s h a r p e s t i m a g e s w e r e obtained
with the 4 0 m i l ( l m m ) o r g r e a t e r b a c k s c r e e n t h i c k n e s s e s with the l i n e a r a c c e l e r a t o r , t h i s s c r e e n only was s e l e c t e d f o r f u r t h e r e v a l u a t i o n s u s i n g the l i n e a r a c c e l e r a t o r . The u s e of t h i c k e r b a c k s c r e e n s would not be e x p e c t e d to p r o v i d e any i m a g e i m p r o v e m e n t , s i n c e s e l f a b s o r p t i o n in the s c r e e n s would b e c o m e a f a c t o r a s shown in the l e v e l l i n g off of the i n t e n s i f i c a tion e f f e c t s in F i g 2.
F R ONT S C R E E N E X P O S U R E AND R E S O L U T I O N EVALUATION E x p o s u r e s w e r e s e l e c t e d to r e v e a l the i n t e n s i f i c a t i o n and r e s o l u t i o n c h a r a c t e r i s t i c s f o r s e v e r a l f r o n t s c r e e n t h i c k n e s s e s . L e a d s c r e e n s f r o m 5 to 1 2 0 m i l (0.12 to 3 . 0 m m ) t h i c k w e r e u s e d in th e c a s s e t t e with no b a c k i n g l e a d f o r the b e t a t r o n e x p o s u r e s . Since the l i n e a r a c c e l e r a t o r b a c k s c a t t e r p r o b l e m had to be m i n i m i z e d , a 40rail ( 1 . 0 m m ) t h i c k b a c k l e a d s c r e e n was u s e d f o r t h e s e e x p o s u r e s . A c o n s t a n t e x p o s u r e was g i v e n to e a c h of the l e a d - f i l m c o m b i n a t i o n s t h ro u g h the 17in (432mm) d i a m e t e r t i t a n i u m b i l l e t . F i l m d e n s i t y r e a d i n g s w e r e m e a s u r e d and a r e p l o t t e d in Fig 3 f o r both x - r a y g e n e r a t o r s . F r o m t h i s f i g u r e , the e f f e c t of f r o n t s c r e e n t h i c k n e s s on the i n t e n s i f i c a tion f a c t o r a p p e a r s r e l a t i v e l y u n i m p o r t a n t f o r t h e s e two units. Th e b e t a t r o n c u r v e d i f f e r s f r o m p r e d i c tions b a s e d on the b e t a t r o n s t u d i e s r e p o r t e d by M i l l e r and Steely3; in t h e i r work, the i n t e n s i f i c a t i o n f a c t o r r o s e v e r y s t e e p l y to about 4 0 m i l ( l m m ) of s c r e e n t h i c k n e s s , r e a c h e d a m a x i m u m at about 60rail ( 1 . 5 m m ) and d e c r e a s e d with f u r t h e r i n c r e a s e s in t h i c k n e s s . T h e l i n e a r a c c e l e r a t o r c u r v e in F i g 3 s h o w s a d e c r e a s e , but not a s g r e a t as t h e y r e p o r t e d . It is a p p a r ent that the r o u n d b i l l e t i t s e l f p r o v i d e s a d e f i n i t e s o u r c e of s c a t t e r and that the low s c r e e n t h i c k n e s s e s p r o v i d e an a l m o s t e q u a l a m o u n t of i n t e n s i f i c a t i o n s i n c e m o r e s c a t t e r e d r a d i a t i o n i n t e n s i t y is p a s s e d by the t h i n n e r s c r e e n s . E v e n t u a l l y the l a r g e r t h i c k n e s s e s
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.m
~. 6C o "- ~0 e,-
o
Betot ran
A
L i n e a r accelerator
2O
0 0
1"0
0.5
'20 Lead
"
/,0 front
'
'
'
60 80 100 screen thickness[mill
0
120
F i g 4 R a d i o g r a p h i c r e s o l u t i o n a s a f u n c t i o n of l e a d front-screen thickness
of l e a d p r o v i d e m o r e f i l t e r i n g and r e d u c e the r a d i a tion e x p o s i n g the f i l m . T h i s e f f e c t is m o r e p r o n o u n c ed in the l i n e a r a c c e l e r a t o r r a d i o g r a p h s b e c a u s e of the l o w e r i n i t i a l r a d i a t i o n e n e r g i e s . In a s u b s e q u e n t s e q u e n c e of e x p o s u r e s , the e x p o s u r e t i m e s w e r e i n c r e a s e d f o r the b e t a t r o n f o r e a c h s c r e e n c o m b i n a t i o n to p r o d u c e a 2 . 2 f i l m d e n s i t y . T h e s e a d j u s t m e n t s w e r e n e c e s s a r y b e c a u s e of the r e l a t i v e l y low f i l m d e n s i t i e s in the i n i t i a l e x p o s u r e s , and so that the r a d i o g r a p h s of the b i l l e t s by both x - r a y g e n e r a t o r s could be e v a l u a t e d at the s a m e d e n s i t y with the s a m e a m o u n t of f i l m c o n t r a s t . Since the l i n e a r a c c e l e r a t o r r a d i o g r a p h s w e r e a l r e a d y in this r a n g e , no a d j u s t m e n t s w e r e n e c e s s a r y . In o r d e r to e v a l u a t e the r a d i o g r a p h s , the s m a l l e s t d e t e c t a b l e I T hole in MIL 453 r a d i o g r a p h i c p e n e t r a m e t e r s w as r e c o r d e d as the m i n i m u m r e s o l u t i o n f o r a p a r t i c u l a r r a d i o g r a p h . T he r e l a t i o n shown in F i g 4 i n d i c a t e s the m i n i m u m t h i c k n e s s and d i a m e t e r p e n e t r a m e t e r hole which w a s r e s o l v e d in the r a d i o g r a p h s m a d e with d i f f e r e n t f r o n t s c r e e n t h i c k n e s s e s . With the b e t a t r o n , the 6 0 m i l ( 1 . 5 m m ) f r o n t s c r e e n p r o d u c e d the b e s t c o m b i n a t i o n of s h a r p n e s s . The 40 and 8 0 m i l (1 and 2 m m ) c o m b i n a t i o n s p r o v i d e d a l m o s t e q u a l s e n s i t i v i t y . The l i n e a r a c c e l e r a t o r radiographs showed comparable s e n s i t i v i t y f o r low s c r e e n t h i c k n e s s e s , but the r e s o l u t i o n o b t a i n a b l e was not a s good as with the b e t a t r o n . It should be noted that an i n f i n i t e n u m b e r of p e n e t r a m e t e r s w as not a v a i l a b l e so that the r e p o r t e d r e s o l u tion is a l w a y s s o m e w h a t h i g h e r than the e x a c t l i m i t . As an e x a m p l e , e i t h e r the r e s o l u t i o n i s 80 or 100rail (2.0 or 2 . 5 m m ) with no p o s s i b l e v a l u e m e a s u r e d between. F o r the l i n e a r a c c e l e r a t o r r a d i o g r a p h s , although 100rail ( 2 . 5 r a m ) r e s o l u t i o n was o b t a i n e d o v e r a wide r a n g e of t h i c k n e s s e s , a 4 0 m i l ( 1 . 0 m m ) f r o n t s c r e e n p r o d u c e d the o p t i m i z e d i m a g e . The sh ap e of the
n o n - d e s t r u c t i v e t e s t i n g O c t o b e r 1970
359
b e t a t r o n f r o n t s c r e e n c u r v e in Fig 4 at l o w e r s c r e e n t h i c k n e s s e s is c a u s e d by the d e g r a d a t i o n of i m a g e q u a l i t y r e s u l t i n g f r o m s c a t t e r and, at the l a r g e r t h i c k n e s s e s , by the l a r g e r a r e a c o v e r e d by s e c o n d a r y r a d i a t i o n f r o m a t h i c k e r lead s c r e e n which d e c r e a s e s the s h a r p n e s s . F o r the l i n e a r a c c e l e r a t o r at the l a r g e r t h i c k n e s s e s , this l a t t e r effect is not as p r o n ounced b e c a u s e the t h i c k e r s c r e e n f i l t e r s s c a t t e r to i m p r o v e the r e s o l u t i o n .
2,S
100
80-
720
.o
_~60 O
41"5
SCREEN COMBINATIONS ¢-
B a s e d on the p r e v i o u s data for both f r o n t and back s c r e e n s u s i n g the b e t a t r o n , t h r e e f r o n t and t h r e e b a c k s c r e e n s w e r e c h o s e n for f u r t h e r e v a l u a t i o n s . The f r o n t s c r e e n s s e l e c t e d w e r e 40, 60, and 8 0 m i l (1, 1 . 5 and 2 m m ) thick, while the b a c k s c r e e n s w e r e 0, 10 and 4 0 m i l (0, 0.25 and 1 . 0 m m ) thick. Nine e x p o s u r e s with t h e s e s c r e e n c o m b i n a t i o n s w e r e made of the 17in (432mm) d i a m e t e r b i l l e t , followed by c o r r e c t i o n e x p o s u r e s for f i l m s not having d e n s i t i e s of 2 . 5 + 0 . 2 0 H&D d e n s i t y u n i t s . T h e s e r a d i o g r a p h s w e r e e v a l u a t e d and t h r e e p r e f e r r e d s c r e e n c o m b i n a t i o n s w e r e s e l e c t e d f o r the r e m a i n i n g e v a l u a t i o n s . A 6 0 m i l ( 1 . 5 m m ) lead f r o n t s c r e e n with no back s c r e e n c o m b i n a t i o n (ie 60-0 ( 1 . 5 - 0 ) ) was s e l e c t e d a s having the b e s t detail s h a r p n e s s and o v e r a l l b e s t r e s o l u t i o n ; an 80-10 ( 2 . 0 - 0 . 2 5 ) c o m b i n a t i o n showed i n t e r m e d i a t e s h a r p n e s s and c o n t r a s t ; while a n 80-40 ( 2 . 0 - 1 . 0 ) showed good c o n t r a s t but s o m e w h a t p o o r e r d e t a i l . As was noted, the 60-0 ( 1 . 5 - 0 ) c o m b i n a t i o n p r o d u c e d the b e s t s e n s i t i v i t y with the 80-10 ( 2 . 0 - 0 . 2 5 ) c o m b i n a t i o n a c l o s e second. The slight d i f f e r e n c e in s e n s i t i v i t y b e t w e e n t h e s e two lead s c r e e n c o m b i n a t i o n s was a c c o m p a n i e d by a o n e t h i r d r e d u c t i o n in e x p o s u r e t i m e for the 80-10 ( 2 . 0 0.25) c o m b i n a t i o n , r e s u l t i n g in l o w e r i n s p e c t i o n c o s t s without a l a r g e s a c r i f i c e in f i l m quality. F r o m the p r e v i o u s e v a l u a t i o n s on the l i n e a r a c c e l e r a t o r s c r e e n c o m b i n a t i o n s , the 4 0 - 4 0 ( 1 . 0 - 1 . 0 ) s c r e e n s p r o v i d e d o p t i m u m i m a g e s . Since t h e r e was no a p p r e c i a b l e d e t e r i o r a t i o n in s h a r p n e s s and d e t a i l as with the b e t a t r o n , t h i s c o m b i n a t i o n was s e l e c t e d for g e o m e t r i c e n l a r g e m e n t s and an a d d i t i o n a l 120-40 ( 3 . 0 - 1 . 0 ) c o m b i n a t i o n was s e l e c t e d for the fine g r a i n f i l m e v a l u a t i o n s which a r e d i s c u s s e d in the following p a r a g r a p h s . T h i s l a t t e r c o m b i n a t i o n was c h o s e n ~ o s e e if a thick f r o n t s c r e e n could r e d u c e unwanted scatter radiation.
EXPOSURE GEOMETRIC EVALUATIONS In o r d e r to d e t e r m i n e the effects of e x p o s u r e g e o m e t r y on i m a g e s e n s i t i v i t y , the 17in (432mm) b i l l e t was r a d i o g r a p h e d with t h r e e d i f f e r e n t e x p o s u r e g e o m e t r i e s and the t h r e e s e l e c t e d lead s c r e e n c o m b i n a t i o n s p r e v i o u s l y d e t e r m i n e d . The b i l l e t was c e n t r e d at 6ft ( 1 . 8 3 m ) while the c a s s e t t e c o n t a i n i n g Kodak AA f i l m was placed at 6ft 8 . 5 i n (2.04m), 9ft (2.7m) and 12ft (3.66m). T h e s e d i s t a n c e s w e r e s e l e c t e d to give a p p r o x i m a t e e n l a r g e m e n t s of 1.0, 1 . 5 and 2×. In p r a c t i c e , the a c t u a l e n l a r g e m e n t s a c h i e v e d will depend on the r e l a t i o n s h i p b e t w e e n the t a r g e t , defect and f i l m d i s t a n c e s , but the a v e r a g e e n l a r g e m e n t is n e a r to the d e s i r e d v a l u e s . The r e s u l t s of t h e s e t e s t s a r e shown in Fig 5 w h e r e r a d i o g r a p h i c r e s o l u t i o n is plotted as a function of the a c t u a l p e n e t r a m e t e r e n l a r g e m e n t m e a s u r e d in the r a d i o g r a p h s . The b e s t r e s o l u t i o n
360
n o n - d e s t r u c t i v e t e s t i n g O c t o b e r 1970
~1.0
Screen combinations
0 n,"
o a n
20-
0 1.0
t
60-0mil[l'5-0mm] 80-40mil [ 2-1mini 80-10rail [ 2 - 0 - 2 5 m m ]
0,5
I
I
1"5
2"0
t0
2"5
Actual penetrometer enlargement R a d i o g r a p h i c r e s o l u t i o n through 17in ( 4 3 2 m m ) of 6 A 1 - 4 V t i t a n i u m a s a function of e n l a r g e m e n t for E a s t m a n Kodak AA f i l m and t h r e e lead s c r e e n c o m b i n a t i o n s
was o b t a i n e d at the h i g h e s t m a g n i f i c a t i o n ; 4 0 m i l ( l m m ) r e s o l u t i o n r e s u l t e d with the d e t e c t i o n of the I T hole in the MIL 453, 2 . 0 i n ( 5 0 . 8 m m ) p e n e t r a m e t e r . The 60-0 ( 1 . 5 - 0 ) s c r e e n c o m b i n a t i o n p r o d u c e d the b e s t r e s u l t s with the 80-10 ( 2 . 0 - 0 . 2 5 ) c o m b i n a t i o n a c l o s e second. T a b l e 1 l i s t s the e x p o s u r e s to obtain a 2 . 5 H&D f i l m d e n s i t y for the 9 s c r e e n / e n l a r g e m e n t c o m b i n a t i o n s . In g e n e r a l , the l o n g e r e x p o s u r e t i m e s p r o d u c e d the b e s t r a d i o g r a p h i c r e s o l u t i o n s . The t i m e s l i s t e d a r e a p p r o x i m a t e s i n c e the b e t a t r o n output d e c r e a s e d with t i m e . A r e s o l u t i o n of 4 0 m i l ( l m m ) was obtained at 2 × in a p p r o x i m a t e l y 4 2 m i n for [he 80-10 ( 2 . 0 - 0 . 2 5 ) s c r e e n s , while 50mil ( 1 . 2 5 m m ) r e s o l u t i o n was obtained with the n o m i n a l 1 . 5 x e n l a r g e m e n t at 19rain with the 8 0 - 1 0 (2-0.25} s c r e e n s . With the 60-10 ( 1 . 5 - 0 ) s c r e e n s and an e x p o s u r e t i m e of l l m i n with no e n l a r g e m e n t s , 60rail ( 1 . 5 m m ) r e s o l u t i o n r e s u l t e d . Using the 8 0 - 4 0 (2-1) s c r e e n s , 80mil (2ram) r e s o l u t i o n r e s u l t e d for a 5 . 2 m i n e x p o s u r e . A c o m p a r a b l e s e t of e x p o s u r e s was p l a n n e d for the l i n e a r a c c e l e r a t o r in o r d e r to study the r e s o l u t i o n achieved with the s a m e e x p o s u r e g e o m e t r i e s . Howe v e r , a f t e r the i n i t i a l e x p o s u r e s w e r e made with the b i l l e t c e n t r e d at 6ft (1.83m) and the i n i t i a l e n l a r g e m e n t s made at the 9ft ( 2 . 7 4 m ) t a r g e t - t o - f i l m d i s t a n c e , it was e v i d e n t that e n l a r g e m e n t r a d i o g r a p h s were not p o s s i b l e b e c a u s e of the a m o u n t of s c a t t e r in the radiographs. Very little p e n e t r a m e t e r image detail was v i s i b l e in the r a d i o g r a p h s , although d e t a i l s of the c a s s e t t e s t r u c t u r e w e r e v i s i b l e . This i n d i c a t e d that r a d i a t i o n o t h e r than the p r i m a r y b e a m was f o r m i n g the i m a g e . On the r a d i o g r a p h s made i m m e d i a t e l y behind the b i l l e t at a 6ft 8 . 5 i n (2.04m) t a r g e t - t o - f i l m
T a b l e 1 E x p o s u r e t i m e a s a f u n c t i o n of f i l m s c r e e n c o m b i n a t i o n s and e n l a r g e m e n t s for 1 7 i n ( 4 3 2 m m ) of t i t a n i u m and A A f i l m * Screen combination
Average enlargement
Time min
Roentgens at I m
Target-film-distance (in) (m)
60-0 60-0 60-0 80-10 80-10 80-10 80-40 80-40 80-40
1.C 1.5 2.0 1.0 1.5 2.0 1.0 1.5 2.0
10.7 27.2 64.2 6.2 18.5 41.5 5.3 16.5 34.6
548 1390 3277 316 946 2118 272 843 1767
80.5 108 144 80.5 108 144 80.5 108 144
2.04 2.74 3.66 2.04 2.74 3.66 2.04 2.74 3.66
* 2.5 H&D film density
distance, the 120rail (3.0ram) penetrameter hole was resolved on AA film with 40mil (Imm) front and rear screens. This is in contrast to the 100rail (2.5mm) resolution achieved at the 9ft (2.74m) target-to-film distance illustrated previously in the front screen evaluations. From these two sources of data, it can be seen that the size of the focal spot plays a significant role in the image sharpness and, hence, the change in defect resolving capability with different target-to-film distances. The AA film exposure was 2,300rad, which at the I, 500rad/min intensity takes approximately 1.55min.
and would add greatly to exposure costs. Finer grained films provide better resolution capabilities but also raise exposure costs since their sensitivity to x-radiation is lower than the coarser grained films.
FILM
The use of finer grained films with the linear accelerator provided a large improvement in image sensitivity. When radiographs were made with Kodak type M film with the billet centred at 6ft (2.04m), 80mils (2.0ram) resolution resulted. The exposure for the 40M40 (1M1) combination (ie, using type M film) was
EVALUATIONS
The preceeding evaluations established the best techniques available with Eastman Kodak type AA film. For the betatron radiography, better resolution might have been produced with more enlargement. However, greater enlargement requires more film and more exposures to cover a 17in (432mm) diameter billet,
The use of finer grained films was evaluated with little success for the betatron. Since exposure times were excessive for Eastman Kodak type M industrial x-ray film, type T film was used. This film proved to be very sensitive to mottling when processed by hand and had to be processed automatically to minimize it. Using type T film, the radiographs did not show any better resolution than that obtained with the type AA film manually processed.
7380rads, which takes approximately 4.9min. With the billet centred at 8ft 3.5in (2.53m) and the film at
T a b l e 2 R e s o l u t i o n and e x p o s u r e t i m e s for 7 . 5 M e V l i n e a r a c c e l e r a t o r and 25MeV m e d i c a l b e t a t r o n for 17in ( 4 3 2 m m ) d i a m e t e r 6 A 1 - 4 V t i t a n i u m b i l l e t s Radiation Source
Resolution
Film*--Screen Combination
Linear Medical Industrial Accelerator Betatron Betatron** (mil) (ram) (mil)
(mm)
Exposure Time
(min)
Target-Object Distance From Billet C e n t r e (ft) (m)
Target-to-Film Distance (ft)
(m)
120
3.0
40AA40
1AA1
1. 55
6
1.83
6.71
2.04
100
2.5
40AA40
1AA1
1.95
8.29
2.53
9
2.74
80
2.0
40M40
1M1
4.90
6
1.83
6.71
2.04
60
1.5
40M40
1M1
6.60
8.29
2. 53
9
2.74
60
1.5
120M40
3M1
7.60
8.29
2.53
9
2.74
60
1.5
60AA0
1.5AA0
10.70
6
1.83
6.71
2.04
60
1.5
60AA0
1. 5AA0
6.42
6
1.83
6.71
2.04
80
2.0
80AA10
2AA0
6.20
6
1.83
6.71
2.04
80
2.0
80AA10
2AA0
3.72
6
1.83
6.71
2.04
80
2.0
80AA40
2AA1
5.30
6
1.83
6.71
2.04
80
2.0
80AA40
2AA1
3. 18
6
1.83
6.71
2.04
50
1.25
80AA10
2AA0.25
18.5
6
1.83
9
2.74
50
1.25
80AA10
2AA0.25
11.1
6
i. 83
9
2.74
40
1.0
60AA0
1.5AA0
64
40
1.0
60AA0
1.5AA0
38
6 6
I. 83 I. 83
12 12
3.66 3.66
40
1.0
80AA10
2AA0.25
41.5
6
I. 83
12
3.66
40
1.0
80AA10
2AA0.25
25
6
1.83
12
3.66
* Eastman Kodak Industrialx-ray film,types AAand M ** Estimated ~: 60 percent of medicalunitexposures
non-destructivetesting October 1970
361
9ft (2.74m), the r e s o l u t i o n obtained was 60rail ( 1 . 5 m m ) for the 120M40 (3M1) and 40M40 (1M1) s c r e e n c o m b i n a t i o n s , while the e x p o s u r e s w e r e l l 4 0 r a d s ( 7 . 6 m i n ) and 9 9 0 0 r a d s ( 6 . 6 m i n ) r e s p e c t i v e l y . The t h i c k e r f r o n t s c r e e n p r o v i d e d a s l i g h t i m p r o v e m e n t in i m a g e r e s o l u t i o n ; e v i d e n t l y it r e d u c e d the i n t e n s i t y of s c a t t e r e d r a d i a t i o n .
CONCLUSIONS The e v a l u a t i o n of the u s e of b e t a t r o n s and l i n e a r a c c e l e r a t o r s for i n s p e c t i o n of t i t a n i u m b i l l e t s has shown a wide r a n g e of defect s e n s i t i v i t y . F o r a p a r t i c u l a r x - r a y g e n e r a t o r , s e n s i t i v i t y is d e p e n d e n t on lead s c r e e n t h i c k n e s s e s , e x p o s u r e g e o m e t r y and f i l m grain size. Betatron radiographic techniques reported in this a r t i c l e w e r e developed u s i n g a m e d i c a l b e t a t r o n which has a l a r g e r c o m p e n s a t o r than that u s e d in i n d u s t r i a l u n i t s . The b e t a t r o n t e c h n i q u e t i m e s l i s t e d in T a b l e 1 can be r e d u c e d by 40 p e r c e n t with an i n d u s t r i a l unit. T a b l e 2 l i s t s the v a r i o u s r e s o l u t i o n s and e x p o s u r e t i m e s a c h i e v e d with v a r i o u s t e c h n i q u e s for both the l i n e a r a c c e l e r a t o r and m e d i c a l b e t a t r o n , and those a c h i e v a b l e with an i n d u s t r i a l b e t a t r o n . Although the r a d i a t i o n u n i t s for each x - r a y g e n e r a t o r a r e d i f f e r e n t , no c o n v e r s i o n was a t t e m p t e d for c o m p a r i s o n s . Instead, the a p p r o x i m a t e e x p o s u r e t i m e s a r e g i v e n for each u n i t u s i n g a p a r t i c u l a r x - r a y film. Since the i n i t i a l c o s t s , s e t t i n g - u p t i m e s and m a i n t e n a n c e c o s t s of t h e s e two u n i t s a r e a p p r o x i m a t e l y the s a m e , r e l a t i v e e x p o s u r e t i m e is a b a s i s for d e t e r m i n ing the cost of r a d i o g r a p h y . N e i t h e r u n i t h a s a c l e a r cut a d v a n t a g e when s e n s i t i v i t y and e x p o s u r e t i m e s a r e c o m p a r e d . With its h i g h e r output, the l i n e a r a c c e l e r a t o r p r o d u c e s s h o r t e r e x p o s u r e s on AA f i l m , but c a n n o t m a t c h the defect r e s o l u t i o n of the b e t a t r o n . The s a m e e x p o s u r e t i m e s r e s u l t when type M f i l m is used with the l i n e a r a c c e l e r a t o r to a c h i e v e the s a m e defect r e s o l u t i o n a s a c h i e v e d with the b e t a t r o n on AA f i l m . The l i n e a r a c c e l e r a t o r can a c h i e v e a m i n i m u m r e s o l u t i o n of 6 0 m i l ( 1 . 5 m m ) on the 17in (432mm) b i l l e t in 6 . 6 m i n c o m p a r e d to a p r o j e c t e d i n d u s t r i a l b e t a t r o n e x p o s u r e t i m e of 6 . 4 2 r a i n for the s a m e r e s o l u t i o n . On the o t h e r hand, the b e t a t r o n is
362
n o n - d e s t r u c t i v e t e s t i n g O c t o b e r 1970
c a p a b l e of p r o v i d i n g defect r e s o l u t i o n a s low as 40rail ( 1 . 0 m m ) , but at an e x p o s u r e t i m e of 25min. The b e t a t r o n a p p e a r s to have a slight a d v a n t a g e b e c a u s e of its v e r s a t i l i t y . It should be noted, however, that the l i n e a r a c c e l e r a t o r that was used in these i n v e s t i g a t i o n s was the f i r s t u n i t of its type. Many i m p r o v e m e n t s may be p o s s i b l e to l o w e r the s c a t t e r r a d i a t i o n of the l i n e a r a c c e l e r a t o r and to p e r m i t u s e of l o w e r t h i c k n e s s b a c k s c r e e n s and f a s t e r f i l m s for i m p r o v e d defect r e s o l u t i o n at s h o r t e r e x p o s u r e t i m e s .
ACKNOWLEDGEMENTS The a u t h o r is g r a t e f u l to Mr John K i r v e n and Dr R. K. L o e f f l e r of N o n d e s t r u c t i v e T e s t i n g Inc (Ohio, USA) for t h e i r help in p r o v i d i n g the b e t a t r o n e x p o s u r e s . Many t h a n k s m u s t go to Mr E. S. P r o c t o r of C o m b u s t i o n E n g i n e e r i n g ( T e n n e s s e e , USA) for p r o v i d i n g t i m e to u s e the l i n e a r a c c e l e r a t o r and to his staff for t h e i r e x c e l l e n t c o - o p e r a t i o n in m a k i n g the r a d i o g r a p h s . The a u t h o r a l s o w i s h e s to acknowledge the g u i d a n c e and a s s i s t a n c e of Mr J a m e s Hottoway-ef the United States A i r F o r c e M a t e r i a l s L a b o r a t o r y and to extend his g r a t i t u d e for the s u p p o r t of the A i r F o r c e Materials Laboratory.
REFERENCES 1
M i l l e r , N. C and T e n n e y , G. H., ' V a r i o u s p e n e t r a m e t e r types and t h e i r l i m i t a t i o n s ' , N o n d e s t r u c tive T e s t i n g (now M a t e r i a l s E v a l u a t i o n ) , Vol 11, No 2 (1953) p17
2
P o l a n s k y , D. and C r i s c u o l o , E. L., 'A method for the c l a s s i f i c a t i o n of i n d u s t r i a l x - r a y f i l m ' , P r o c e e d i n g s of the Fifth I n t e r n a t i o n a l C o n f e r e n c e on N o n d e s t r u c t i v e T e s t i n g , M o n t r e a l , Canada, (May 1967)
3
M i l l e r , N. C., and Steely, J. D., 'Some e x p e r i m e n tal f i n d i n g s and o p e r a t i n g p r a c t i c e s in b e t a t r o n r a d i o g r a p h y ' , N o n d e s t r u c t i v e T e s t i n g (now M a t e r i a l s E v a l u a t i o n ) , V o l 11, No 8 ( N o v e m b e r 1953) p35