Analytical determination of the potassium content of single bubbles in annealed tungsten wires
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AND
JULIUS
STARK
It is w e l l - k n o w n that in s e v e r e l y d r a w n K-A1-Si doped t u n g s t e n w i r e s , rows of n e a r l y e q u i a x i a l s t r e s s f r e e second p h a s e s a r e f o r m e d d u r i n g r e c r y s t a l l i z a tion. In thinned foils, at low t e m p e r a t u r e s , those second p h a s e s produce in many c a s e s s e l e c t e d a r e a d i f f r a c t i o n p a t t e r n s , s u g g e s t i n g the p r e s e n c e of c r y s t a l l i n e p o t a s s i u m . *,2 T h e m o s t d i r e c t v e r i f i c a t i o n of the p r e s e n c e of those c r y s t a l l i n e p o t a s s i u m second p h a s e s in a quenched t u n g s t e n m a t r i x could be r e n d e r e d by s i m u l t a n e o u s m i c r o a n a l y s i s and s e l e c t e d a r e a d i f f r a c t i o n in thinned f o i l s . Although some a t t e m p t s have b e e n made in this d i r e c t i o n , none of t h e m p r o v e d to be s u c c e s s f u l . 3'4 The v e r y s m a l l quantity of p o t a s s i u m (~10 -*7 g) b e i n g p r e s ent in a single bubble cannot be m e a s u r e d even by m e a n s of the l a t e s t c o m m e r c i a l a n a l y t i c a l e l e c t r o n m i c r o s c o p e s equipped with e n e r g y - d i s p e r s i v e X - r a y s p e c t r o m e t e r s s i n c e they allow the detection of e l e m e n t s with an a t o m i c n u m b e r Z > 10 in the said q u a n t i t i e s s o l e l y i n a m a t r i x with Z < 10 and as long as the n a t u r e of the p r o b l e m does not r e q u i r e a c c e l e r a t i n g voltages or above 100 kV. While developing an e x p e r i m e n t a l s e t - u p allowing the m e a s u r e m e n t of the p o t a s s i u m content of a s i n g l e b u b ble, one has to b e a r in mind the following points. Reed s and Gedcke 6 have shown that w a v e l e n g t h - d i s p e r s i v e X - r a y s p e c t r o m e t e r s p r e s e n t an advantage over e n e r g y d i s p e r s i v e ones when s m a l l a m o u n t s of e l e m e n t s have to be q u a n t i t a t i v e l y m e a s u r e d in a m a t r i x with high a t o m i c n u m b e r . That advantage c o m e s f r o m a b e t t e r e n e r g y r e s o l u t i o n and a b e t t e r s i g n a l - t o - n o i s e r a t i o of the w a v e l e n g t h - d i s p e r s i v e m e a s u r e m e n t . In o r d e r to i m p r o v e the detection l i m i t of the w a v e l e n g t h - d i s p e r s i v e a n a l y s i s , the spot s i z e of the e l e c t r o n p r o b e with high i n t e n s i t y has to be r e d u c e d in a way that r e t a i n s the p o s s i b i l i t y of the s i m u l t a n e o u s t r a n s m i s sion e l e c t r o n m i c r o s c o p i c i n v e s t i g a t i o n . An i n t e n s e e l e c t r o n p r o b e with a d i a m e t e r of about 0.1 to 0.2 tan was achieved by Cooke and D u n c u m b e 7 i n a TEM by putting a m i n i l e n s 8 a f t e r the second c o n d e n s e r lens d i r e c t l y above the object. A s i m i l a r s o l u t i o n was des c r i b e d by Louis and
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