INSTRUMENTATION, ELECTRON OPTICS AND X-RAY SPECTROSCOPY

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INSTRUMENTATION, ELECTRON OPTICS AND X-RAY SPECTROSCOPY

T. Mulvey

To cite this version:

T. Mulvey. INSTRUMENTATION, ELECTRON OPTICS AND X-RAY SPECTROSCOPY. Journal

de Physique Colloques, 1984, 45 (C2), pp.C2-149-C2-154. �10.1051/jphyscol:1984233�. �jpa-00223946�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n°2, Tome 45, février 1984 page C2-149

INSTRUMENTATION, ELECTRON OPTICS AND X-RAY SPECTROSCOPY

T. Mulvey

Department of Physios, The University of Aston in Birmingham, B4 7ET, U.K.

Résumé - Les développements récents dans le champ d'instrumentation du micro- analyseur à rayons X sont revus, ainsi que les canons électroniques, les systèmes des lentilles et les détecteurs des électrons rétrodiffusés (Z contraste). Les amé- liorations souhaitables pour l'analyse des échantillons minces à l'aide de la spec- trométrie à dispersion en énergie sont aussi discutées.

Abstract - Recent instrumental developments in the field of electron probe instrumentation are reviewed. These include electron guns, lens systems and back-scattered electron (Z contrast) detectors. Desirable changes in instrumental design are discussed for the analysis of thin specimens by the use of energy dispersive spectrometers.

1 - INTRODUCTION

The present symposium is concerned with recent developments in all aspects of electron instrumentation of electron probe X-ray microanalysis. It is perhaps interesting to note that the initial stimulus to develop the electron probe micro- analyser was the early discussions between Castaing and Guinier in the period 1947-1948 as to whether it was possible to analyse by X-ray spectrometry the fine detail visible at that time in thin metallic specimens in the electron microscope.

It was of course necessary to begin with the more modest resolution set by the light microscope. After a period of some thirty years of development, microanalysis of this type has now reached a high stage of technological development especially with the recent introduction of microprocessor and mini-computer control. Nevertheless, further development is still possible, especially in the choice and control of X-ray spectrometers. On the other hand, recent progress in atomic resolution transmission electron microscopy (TEH) and the development of high resolution analytical scanning transmission microscopes (STEM) has made it desirable to review the instrumental techniques used in X-ray analysis of thin specimens at high spatial resolution

( = 2 nm) .

These include, among others, the elimination from the detector of all X-rays not emanating from the specimen area under investigation and the handling of the greatly increased amount of analytical data produced by the modern analytical STEM. In addition, it is desirable to be able to correlate the analytical information from the characteristic X-ray emission with that from Auger and energy loss spectroscopy and perhaps from convergent beam diffraction, which combines the high spatial reso- lution of electron probe analysis with crystallographic information not available with any other X-ray diffraction technique,

The increasing computer automation that is available today also calls for an

approach to instrumentation that eliminates, as far as possible, intervention by the operator in setting up and aligning the instrument and the recognition of

instrumental artefacts arising during the analysis.

Improved methods of detection of light elements in low concentrations and the increased ability to examine thin surface layers have recently emphasized the need to improve, by a few orders of magnitude, the traditional poor vacuum ( - 10 mbar)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984233

JOURNAL DE PHYSIQUE

of many X-ray microanalysers. Here t h e i n t r o d u c t i o n of turbo-molecular pumps, t h e a v a i l a b i l i t y of improved d i f f u s i o n pump o i l s , and t h e i n s e r t i o n of e f f i c i e n t t r a p s i n backing pump l i n e s has done much t o reduce t h e hydrocarbon c o n t e n t of t h e r e s i d u a l g a s e s i n t h e system. Such improved vacuum systems a l s o make it p o s s i b l e t o r e p l a c e t h e t r a d i t i o n a l t u n g s t e n f i l a m e n t by, f o r example, a lanthanum hexaboride cathode with a s u b s t a n t i a l improvement b o t h i n e l e c t r o n o p t i c a l b r i g h t n e s s and f i l a m e n t l i f e . STEM i n s t r u m e n t s need f i e l d emission cathodes and hence w i l l be designed f o r a vacuum p r e s s u r e i n t h e r e g i o n of lo-'' mbar. Such guns were

o r i g i n a l l y designed f o r t h e p r o d u c t i o n of nanometre probes and so a r e n o t e f f i c i e n t f o r producing an adequate c u r r e n t i n t h e l a r g e r probes needed f o r a n a l y t i c a l work.

F u r t h e r development i s however p o s s i b l e .

A

v a l u a b l e a n c i l l a r y development i s t h e high energy b a c k s c a t t e r e d e l e c t r o n o r

c o n t r a s t ' d e t e c t o r whose o u t p u t i s p r o p o r t i o n a l t o t h e average atomic number of of t h e specimen. When made i n t h e form o f f o u r a d j a c e n t q u a d r a n t s , s o a s t o e l i m i n a t e t o p o g r a p h i c a l c o n t r a s t , a r a p i d p r e l i m i n a r y a n a l y s i s can be made of t h e average atomic number of t h e main c o n s t i t u e n t s of t h e specimen.

2 - ELECTRON GUNS

The c u r r e n t d e n s i t y

of emission from t h e h e a t e d cathode wire i s given by t h e Richardson e q u a t i o n

(1)

^A

^{exp (-e} @ /k T) ,

where

i s a c o n s t a n t f o r a given e m i t t i n g m a t e r i a l , T i s t h e a b s o l u t e temperature, k i s Boltzmann's c o n s t a n t and @ i s t h e 'work f u n c t i o n ' o f t h e m a t e r i a l expressed i n

' e l e c t r o n v o l t s ' . The maximum b r i g h t n e s s ( R i c h t s t r a h l w e r t ) B of t h e r e s u l t i n g e l e c t r o n beam a f t e r a c c e l e r a t i o n through a p o t e n t i a l d i f f e r e n c e V i s given approxi- mately by

The work f u n c t i o n of t u n g s t e n i s i n c o n v e n i e n t l y l a r g e , approximately 4.5 eV, b u t t h i s can be reduced by t h e Schottky e f f e c t , by which a s t r o n g e l e c t r i c f i e l d of s t r e n g t h E a t t h e cathode s u r f a c e enhances t h e emission c u r r e n t d e n s i t y

by a f a c t o r exp(0.44 E / T ) . f Here

i s measured i n v o l t s / m e t r e . This f i e l d can be most c o n v e n i e n t l y produced by forming a f i n e p o i n t on t h e e m i t t i n g s u r f a c e . Thus Van d e r Mast e t a l . /1/ succeeded i n producing a l a s e r - h e a t e d Schottky thermionic source t h a t i s comparable i n performance w i t h a f i e l d emission e l e c t r o n gun, b u t does n o t need a p a r t i c u l a r l y good vacuum. I n a d d i t i o n it i s c a p a b l e of d e l i v e r i n g t h e l a r g e r probe c u r r e n t s t h a t a r e needed f o r a n a l y t i c a l work. I n t h i s source ( F i g . 1) a t h i n t u n g s t e n wire of some 1 0 w i n diameter i s h e a t e d l o c a l l y a t i t s t i p t o a

temperature n e a r i t s melting p o i n t . An e l e c t r o d e a t a p o s i t i v e p o t e n t i a l p o s i t i o n e d near t h e source c r e a t e s t h e e l e c t r i c f i e l d needed f o r enhanced Schottky emission.

Under t h e i n f l u e n c e of t h i s f i e l d and s u r f a c e t e n s i o n a s t a b l e e m i t t e r of f i x e d small r a d i u s of c u r v a t u r e i s produced. A s t h e cathode wire i s evaporated t h e wire i s f e d i n t o t h e l a s e r focus t o p r o v i d e t h e necessary replacement. The measured b r i g h t n e s s of t h i s s o u r c e i s i n t h e r e g i o n of 8

10' A/cm2/sterad. a t 25 kV, comparable with t h a t of a f i e l d emission gun. This e l e c t r o n gun i s n o t y e t

a v a i l a b l e commercially b u t i t s performance i n d i c a t e s t h a t expremely high b r i g h t n e s s can be produced i n e l e c t r o n guns w i t h t u n g s t e n f i l a m e n t s /2,3/ i f t e c h n o l o g i c a l problems can be s o l v e d .

should perhaps b e p o i n t e d o u t t h a t t h e gun b r i g h t n e s s a s measured under o p e r a t i n g c o n d i t i o n s is n o t n e c e s s a r i l y t h e maximum b r i g h t n e s s a s d e f i n e d i n Equation ( 2 ) b u t r a t h e r a 'mean b r i g h t n e s s ' /4/ a s l i m i t e d , f o r example, by t h e t o t a l cathode c u r r e n t t h a t may be drawn from t h e power supply o r a p a r t i c u l a r arrangement of l e n s a p e r t u r e s . I n g e n e r a l t h e b r i g h t n e s s of an e l e c t r o n gun

improves a s t h e work f u n c t i o n and r a d i u s of t h e cathode i s reduced /5/, a s t h e

t o t a l c u r r e n t i s i n c r e a s e d and a s t h e vacuum i s made b e t t e r . These requirements a r e

n o t e a s i l y s a t i s f i e d simultaneously and economically i n commercial equipment.

p r e s e n t t h e t r e n d is towards t h e u s e of lanthanum hexaboride a s a s u b s t i t u t e f o r t h e

t u n g s t e n cathode.

comprehensive review of t h e u n d e r l y i n g t h e o r y and measurement

WEHNELT

lUNGSTEN W I R E F E E D MECHANISM

ANODE /I

HIGH B R I G H T N E S S ELECTRON BEAM

I

FOCUSED L A S E R BEAM

MODULATOR

S H I E L D

LABs ROD

HEAT S I N K

Fig.1 Schottky-assisted thermionic emission cathode with laser

Fig.2 Schematic diagram of LaB6 rod cathode

heating

G R A P H I T E BLOCK

Fig.3 Schematic diagram of mechanical clamping of LaB6 crystal between graphite blocks

CARBON ARCH

LAB6 CRYSTAL\

@

CARBON ARCH' RHENIUM F I L A M E N T

B A S E

Fig.4 Schematic diagram of LaB6 crystal sintered to two carbon arches

Fig.5 LaB6 crystal arc-bonded to

rhenium wire filament

C2-152 JOURNAL DE PHYSIQUE

of t h e performance of t r i o d e thermionic guns h a s r e c e n t l y been given by Lauer / 6 / .

3 - LANTHANUM HEXABORIDE CATHODES

Lanthanum hexaboride was suggested a s a cathode m a t e r i a l by L a f f e r t y / 7 / more t h a n t h i r t y y e a r s ago. LaB6 has a low work f u n c t i o n

2 . 2 eV) and is capable of producing a c u r r e n t d e n s i t y i n e x c e s s of 50 A/cm2. I n g e n e r a l LaB6 cathodes can produce an o r d e r of magnitude i n c r e a s e i n b r i g h t n e s s and l i f e compared with a c o n v e n t i o n a l t u n g s t e n f i l a m e n t . The c h i e f d i f f i c u l t i e s l i e i n t h e f a c t t h a t LaB6 is chemically r e a c t i v e and t h e r e f o r e d i f f i c u l t t o mount i n a cathode block. I n a d d i t i o n it r e q u i r e s a c o n s i d e r a b l y b e t t e r vacuum (lo-' mbar) t h a n i s u s u a l l y a a v a i l a b l e i n an X-ray microanalyzer i f s t a b l e o p e r a t i o n i s t o be achieved. Moreover, e x i s t i n g f i l a m e n t c u r r e n t s u p p l i e s a r e n o t u s u a l l y designed s p e c i f i c a l l y f o r t h e s p e c i a l requirements of such c a t h o d e s . The t e c h n i c a l and s c i e n t i f i c problems a s s o c i a t e d with t h e c o n s t r u c t i o n of LaB6 cathodes have r e c e n t l y been reviewed by Crawford /8/ t o g e t h e r w i t h an e x t e n s i v e b i b l i o g r a p h y . The f i r s t cathode

c o n s t r u c t i o n t h a t succeeded i n producing a h i g h e r c u r r e n t d e n s i t y t h a n t h a t of c o n v e n t i o n a l t u n g s t e n cathodes was t h a t of Broers /9/ shown s c h e m a t i c a l l y i n Figure 2 . A long rod of s i n t e r e d o r s i n g l e c r y s t a l LaB6 i s supported i n a cooled s u p p o r t and heated by r a d i a t i o n and e l e c t r o n bombardment h e a t i n g . Thus t h e h o t LaB6 i s n o t i n c o n t a c t with any o t h e r m a t e r i a l . Considerable development h a s taken p l a c e

/lo, 11, 1 2 / i n t h e l a s t few y e a r s , r e s u l t i n g i n an optimised d e s i g n . The c h i e f advantage of t h i s d e s i g n i s t h a t t h e cathode rod i s r i g i d l y mounted and does n o t r e a c t chemically with t h e mounting block. The l a r g e volume of LaB6 a l s o e n s u r e s a long l i f e . E i t h e r p o l y c r y s t a l l i n e o r s i n g l e c r y s t a l r o d s can be employed. Poly- c r y s t a l l i n e m a t e r i a l can y i e l d t h e d e s i r e d t e n f o l d i n c r e a s e of b r i g h t n e s s over t h a t of t u n g s t e n . S i n g l e c r y s t a l cathodes appear t o achieve high b r i g h t n e s s more c o n s i s t e n t l y /3/. The disadvantage of t h i s d e s i g n i s t h a t t h e h e a t i n g arrangement is complicated b o t h mechanically and e l e c t r i c a l l y and consumes c o n s i d e r a b l y more power, (10-30 w a t t s depending on t h e d e s i g n ) , t h a n t h a t of o t h e r p o s s i b l e arrange- ments. This i n t u r n may cause o u t g a s s i n g problems i n t h e cathode r e g i o n . I d e a l l y t h e e l e c t r o n o p t i c a l column should be designed from t h e beginning t o a c c e p t t h i s type of gun. Where t h i s has been done, a s f o r example i n t h e Cambridge high

r e s o l u t i o n high v o l t a g e microscope /14/ t h e expected t e n f o l d improvement of e l e c t r o n - o p t i c a l b r i g h t n e s s of lanthanum hexaboride h a s been achieved.

Since h e a t e d LaB6 does n o t r e a c t a p p r e c i a b l y with h o t g r a p h i t e it i s p o s s i b l e t o clamp a s m a l l s i n g l e c r y s t a l between two g r a p h i t e blocks a s shown s c h e m a t i c a l l y i n Figure 3/15, 16, 17/. Heating c u r r e n t i s passed through t h e v i t r e o u s carbon b l o c k s of low thermal and e l e c t r i c a l c o n d u c t i v i t y and through t h e base of t h e s i n g l e c r y s t a l f i l a m e n t . The power requirements a r e modest (2-3 w a t t s ) , comparable w i t h t h a t of a c o n v e n t i o n a l t u n g s t e n f i l a m e n t . Considerable c a r e must be taken with t h e mechanical and thermal d e s i g n of t h e c r y s t a l h o l d e r and w i t h t h e c h o i c e of m a t e r i a l s . Such a cathode assembly has t h e advantage t h a t it u s e s o n l y a small volume of

expensive LaB6 s i n g l e c r y s t a l m a t e r i a l . I n a d d i t i o n it can be made a s a d i r e c t replacement f o r a s t a n d a r d t u n g s t e n cathode. However c a r e must be taken with t h e e l e c t r i c a l s u p p l i e s t o avoid a c c i d e n t a l o v e r h e a t i n g of t h e c r y s t a l . For example, t h e h o t r e s i s t a n c e of t h e g r a p h i t e b l o c k s i s lower, perhaps by one t h i r d , t h a n t h e room temperature r e s i s t a n c e . I t would be an advantage i f t h e v o l t a g e a c r o s s t h e blocks and t h e c u r r e n t through them could be measured i n o r d e r t o c o n t r o l

e l e c t r o n i c a l l y t h e power r e a c h i n g t h e f i l a m e n t . Such a c o n t r o l system, o r even t h e d i r e c t s e n s i n g of t h e temperature of t h e e m i t t i n g t i p would be of c o n s i d e r a b l e b e n e f i t f o r a l l t y p e s of cathodes. However, it would r e q u i r e e x t r a w i r e s i n t h e c o r e of t h e high v o l t a g e c a b l e . This may w e l l become n e c e s s a r y i n f u t u r e when f u l l y automatic o p e r a t i o n of X-ray microanalysers i s r e q u i r e d i n c l u d i n g s w i t c h i n g on, and alignment of t h e column.

A d e s i g n /8/ which a l l o w s an LaB6 c r y s t a l t o be mounted on a s t a n d a r d t u n g s t e n f i l a m e n t b a s e i s shown s c h e m a t i c a l l y i n F i g u r e 4 . An a c c u r a t e l y machined block of LaB6 i s mounted between two p a r a l l e l arched carbon s t r i p s . The mechanical j o i n t i s made by h e a t i n g t h e carbon s o a s t o a l l o w l o c a l i n t e r d i f f u s i o n of carbon and LaB6.

I n t h i s design t h e s k i l l of t h e manufacturer e l i m i n a t e s t h e need f o r a complicated

mechanical clamping device. The power requirements a r e a g a i n comparable with t h a t of t h e t u n g s t e n f i l a m e n t t h a t it r e p l a c e s . The main disadvantage of t h i s d e s i g n i s i t s fragility /8/ and t h e absence of any means of a d j u s t i n g t h e o r i e n t a t i o n of t h e c r y s t a l , should t h i s become necessary a f t e r t h e i n i t i a l a c t i v a t i o n o p e r a t i o n . I t i s interesting t h a t t h a t LaB6 can now be succeSSfully arc-bonded t o a rhenium wire /18/

a t a temperature of 2000' C. A d . c . a r c i s s t r u c k between t h e LaB6 and t h e rhenium wire i n an argon atmosphere of 10-100 mbar. The r e s u l t i n g j o i n t i s subse- q u e n t l y s t a b l e , a t l e a s t f o r temperatures up t o 1550° C. The form of t h i s cathode c o n s t r u c t i o n i s shown s c h e m a t i c a l l y i n F i g u r e 5 . This cathode, mounted on a s t a n d a r d t u n g s t e n f i l a m e n t b a s e , c l o s e l y resembles a p o i n t e d t u n g s t e n cathode i n appearance and has s i m i l a r e l e c t r o n o p t i c a l p r o p e r t i e s b u t with h i g h e r b r i g h t n e s s and longer l i f e , assuming t h a t t h e vacuum p r e s s u r e i s s u f f i c i e n t l y low.

A m o r e r e c e n t l y d e v e l o p e d method of c o n s t r u c t i o n i s t o embed a s m a l l s i n g l e c r y s t a l of LaB6 i n t o a g l a s s y carbon f i l a m e n t /19/. A h e a t e r power of some 7 w a t t s i s s u f f i c i e n t t o produce a t i p temperature of about 1600' C. The a u t h o r s have c a r r i e d o u t c a r e f u l measurements with t h i s system on t h e p o i s o n i n g e f f e c t s of oxygen, water vapour and hydrocarbons andconclude t h a t p r e v i o u s a u t h o r s may have underestimated t h e e f f e c t s of r e s i d u a l vapours and g a s e s on t h e chemical s t a b i l i t y of t h e LaB6 s u r f a c e and t h e trme r e q u i r e d t o overcome p o i s o n i n g e f f e c t s i f t h e vacuum i s poor.

These i d e a s have been s t r e n g t h e n e d by t h e work of Sewell and Ramachandran /20/ who have i n v e s t i g a t e d t h e e f f e c t s of t h e v i t r e o u s f i l m s t h a t form t h e Wehnelt c y l i n d e r a p e r t u r e when o p e r a t i n g LaB6 s o u r c e s i n t h e 1700' C - 1800' C range. Such f i l m s l e a d t o unacceptable beam i n s t a b i l i t i e s . It seems l i k e l y t h a t t h e s e f i l m s a r e n o t caused by d i r e c t e v a p o r a t i o n of LaB6 b u t a r e o x i d a t i o n p r o d u c t s caused by i o n bombardment following i o n i z a t i o n of t h e r e s i d u a l g a s e s . To e l i m i n a t e t h e s e e f f e c t s f o r s o u r c e s o p e r a t i n g above 1600'

a p r e s s u r e of lo-' mbar o r b e t t e r i s necessary.

W e may conclude t h e r e f o r e , t h a t t h e r e i s now s u f f i c i e n t s c i e n t i f i c and t e c h n o l o g i c a l information a v a i l a b l e t o e n a b l e michoanalyzers t o be adapted f o r o p e r a t i o n , a t c h o i c e , with e i t h e r s i n g l e c r y s t a l LaB6 cathodes o r perhaps p o i n t e d t u n g s t e n cathodes.

4 - FIELD EMISSION GUNS

Of a l l e l e c t r o n guns, t h e f i e l d emission gun has t h e h i g h e s t b r i g h t n e s s and t h e lowest energy s p r e a d . Because of i t s small e f f e c t i v e source s i z e (10-15 nm) and t h e a b e r r a t i o n s of t h e f i r s t a c c e l e r a t i n g e l e c t r o d e , it i s s u p e r i o r t o o t h e r s o u r c e s only f o r e l e c t r o n probes of t h i s o r d e r of magnitude. For a n a l y t i c a l i n s t r u m e n t s it i s n e c e s s a r y t o improve p r e s e n t d e s i g n s s o t h a t a corresponding s u p e r i o r i t y should be p o s s i b l e f o r much l a r g e r probe s i z e s . The whole s u b j e c t h a s r e c e n t l y been reviewed i n d e t a i l by Kasper /21/ and van d e r Mast /22/, b o t h a r t i c l e s c o n t a i n i n g e x t e n s i v e b i b l i o g r a p h i e s .

i s perhaps s u f f i c i e ~ ~ t t o p i n - p o i n t important a r e a s f o r f u r t h e r development. Apart from t h e d i f f i c u l t y of m a i n t a i n i n g t h e b e s t p o s s i b l e vacuum, ( b e t t e r t h a n lo-'' mbar), t h e c h i e f problem, a s i n t h e r m i o n i c cathodes i s t o lower t h e work f u n c t i o n i n a s t a b l e manner. This may be achieved i n p r a c t i c e with e i t h e r oxygen p r o c e s s i n g /23/ o r w i t h zirconium oxide p r o c e s s i n g /24/. The n e x t s t e p i s t o reduce t h e e f f e c t i v e a b e r r a t i o n of t h e a c c e l e r a t i n g system by immersing t h e t i p i n a s t r o n g magnetic f i e l d ( s e e / 2 2 / ) . These a r e now mainly t e c h n o l o g i c a l problems perhaps i n v o l v i n g some compromise between t h e i d e a l arrangement of t h e l e n s and t h e s t r i n g e n t requirements of t h e vacuum system. I t i s p o s s i b l e t h a t a t l e a s t some of t h e s e problems can be solved by t h e computer a s s i s t e d p r o c e s s i n g of t h e a c t i v a t i o n of t h e e m i t t i n g t i p , and of any 'remoulding' o p e r a t i o n t h a t may be necessary t o produce t h e optimum shape and r a d i u s of t h e t i p .

5 - IMPROVED '2-CONTRAST' DETECTORS

The s t a n d a r d Everhart-Thornley s c i n t i l l a t i o n d e t e c t o r /25/ i s an e f f i c i e n t wide-band d e t e c t o r b u t u n t i l r e c e n t l y /26/ n o t analysed i n detail+++New s c i n t i l l a t i o n

c r y s t a l s , e s p e c i a l l y y t t r i u m aluminium g a r n e t (YAG - Ce ) / 2 7 / e n a b l e t h i s

d e t e c t o r t o be used a l s o a s a high energy b a c k s c a t t e r

c o n s t r a s t ) d e t e c t o r /28/ a t

TV r a t e s . Since t h i s c r y s t a l can be machined it i s p o s s i b l e t o use it i n t h e

c o n s t r u c t i o n of quadrant d e t e c t o r s of t h e Volbert and Reimer /29/ type i n which

C2-154 JOURNAL DE PHYSIQUE

topographical and atomic number contrast can be displayed separately at TV rates.

Such a detector represents a substantial improvement over all previous detectors.

6 - X-RAY SPECTRA FROM THIN FILM SPECIMENS

For the highest spatial resolution, thin film specimens must be used. Because of the low counting rates energy dispersive spectrometers are generally employed. To extract the maximum information from such a specimen it is essential that the X-rays passing into the detector should originate at the point under the probe and not from elsewhere. The design principles of eliminating such spurious radiation have recently been established by Nicholson et a1 /30/. These mainly concern the design of components in the vicinity of the specimen stage. Such refinements allow areas of about 2 nm in diameter to be analysed.

7 - ACKNOWLEDGEMENTS

The author would like to thank Dr R Autrata and his colleagues from the Institute of Scientific Instruments of the Czech Academy of Sciences at Brno for stimulating discussions on YAG crystals and Ing Salyk of the same Institute and Dr C Crawford of Kimball Physics USA for valuable discussions on LaB6 cathodes.

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