A comparison of bulk and surface hydrogen concentration in NBS hydrogen-in-titanium standards
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A Comparison of Bulk and Surface Hydrogen Concentration in NBS Hydrogen-in-Titanium Standards G.M. PADAWER, D.J. ADLER
i z e d h y d r o g e n - r i c h r e g i o n s , e i t h e r at s u r f a c e s o r within the bulk, are envisioned. Means for detecting t h e s e l o c a l i z e d r e g i o n s a r e l i m i t e d a n d d e s t r u c t i v e . 9'1~ In t h e c o u r s e o f t h e d e v e l o p m e n t of a l i t h i u m m i c r o p r o b e t e c h n i q u e f o r d e t e c t i n g t h e d i s t r i b u t i o n of h y d r o g e n in t h e v i c i n i t y o f s u r f a c e s , d i s c r e p a n c i e s w e r e o b served between the microprobe results and those v a l u e s o b t a i n e d by b u l k - a n a l y s i s m e a s u r e m e n t s . 1~''2 This note presents results that resolve the discrepancies. The lithium microprobe technique utilizes the prod u c t i o n of n u c l e a r e a p t u r e - y r a d i a t i o n t h a t i s e m i t t e d when accelerated lithium ions interact with hydrogen a t o m s l o c a t e d w i t h i n t h e f i r s t few m i c r o n s of a s p e c i m e n s u r f a c e . L i t h i u m (TLi) i o n s a t a k i n e t i c e n e r g y of 3.071 m e v r e a c t w i t h h y d r o g e n n u c l e i to f o r m t h e f i f t h e x c i t e d s t a t e o f aBe, w h i c h d e c a y s to e i t h e r t h e g r o u n d s t a t e o r t h e f i r s t e x c i t e d s t a t e by e m i t t i n g 17.64 o r 14.74 m e v y r a y s , r e s p e c t i v e l y . T h e n u m b e r o f y r a y s d e t e c t e d in a N a I (T1) s c i n t i l l a t o r i s d i r e c t l y p r o p o r t i o n a l to t h e n u m b e r o f h y d r o g e n n u c l e i a t t h e p a r t i c ular depth probed. This depth can be determined acc u r a t e l y f r o m b o t h t h e b o m b a r d i n g e n e r g y of t h e l i t h i u m i o n s a n d t h e s l o w i n g - d o w n p r o p e r t y of t h e t a r g e t m a terial. Several microns penetration with 0.1p depth r e s o l u t i o n c a n b e m e a s u r e d to a s e n s i t i v i t y o f 11 p p m . A b e a m d i a m e t e r o f a p p r o x i m a t e l y 2.5 m m w a s u s e d in t h e s e e x p e r i m e n t s . D e t a i l s of t h i s t e c h n i q u e h a v e b e e n r e p o r t e d . 11, lz A m o n g s a m p l e s e x a m i n e d d u r i n g t h e d e v e l o p m e n t of t h e l i t h i u m m i c r o p r o b e w e r e t h r e e N a t i o n a l B u r e a u of S t a n d a r d s h y d r o g e n - i n - t i t a n i u m s t a n d a r d s . 13 T y p i c a l s a m p l e s w e r e 6 b y 6 by 1.3 m m . A s s h o w n in T a b l e I, r e s u l t s of i n - h o u s e b u l k - h y d r o g e n v a c u u m - f u s i o n a n a l y s i s a r e in a g r e e m e n t w i t h t h e NBS c e r t i f i e d v a l u e s . H o w e v e r , Lithium m i c r o p r o b e r e s u l t s a r e i n c o n s i s t e n t w i t h t h e s e v a l u e s : NBS 352, w i t h t h e l o w e s t b u l k c o n centration, has the highest concentration as measured by the m i c r o p r o b e , and the r e v e r s e is o b s e r v
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