96 



The Electron Microscope 



level of the amplifier. The first condition for this, according to 

 equation ( 10) of chapter 2, is an electron source with high 

 current density. After some experiments with thermionic 

 cathodes the R.C.A. tried to use cold fine-etched tungsten points 

 which by field emission can yield currents of the enormous 

 density of 10,000 amp/cm-, or even more. But these proved too 

 unstable, and they had to return to incandescent tungsten 

 cathodes. 



4TM L*. UMS 



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UMITIN« APEBTUftC 



LE0CNO 



INCIOCNT CLCCTBON 

 BCAM 



9CCON0ABY CLECTtON 



BIAM — 



LISMT BCAM -' — 



8PCCIMEN 



PHOTO CATHOeC 

 -^ I or CLCCTROM 



MULTIPLICa 



••D ca.iXNa 



4CANNIN0 ocrLccnoN 



FIELD 



t ND L*. tCN* 



LIMITING APCBTUQC 



tST L*. LtNS 

 UMITMC APCBTuaC 



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Fig. 35. Ray diagram of R.C.A. scanning electron microscope 



A first electrostatic lens produces a cross-over which has been 

 discussed in connection with figure 13. The cross-over serves 

 as electron source for a second electrostatic lens which produces 

 a strongly reduced image of it. An aperture cuts out rays beyond 

 a certain angle, to reduce the spherical aberration. This is fol- 

 lowed by a magnetic scanning field, similar to that used in 

 television tubes, and by a third lens system which reproduces the 

 first image on the specimen, with approximately unit magnifica- 



