x-KAY mk:koscopy 



FILM 



CMR 



PMR 



Fig. 1. Comparison of contact microradiog- 

 raphy, CMR, and projection microradiography, 

 PMR. (From V. E. Cosslett and W. C. Nixon,T/ie 

 Times Science Review, London, 18, 10, 1955.) 



FILM OR FLUORESCENT SCREEN 

 X-RAY BEAUR^ I // .<;aMERA 



STAGE 



TARGE 



OBJECTIVE 

 LENS 



PUMPS 



ELECTRON GUN|]q£^ 



CONDENSER 

 LENS 

 .ELECTRON BEAM 

 IN VACUUM 



Fig. 2. Essential parts of a Projection X-ray 

 Microscope. 



the specimen is placed almost in contact with 

 the photographic recording film. All of the 

 enlargement is obtained optically with a 

 light microscope from the original x-ray 

 negative. This method is simple, quick and 

 inexpensive and has been used by many 

 workers, both in biology and metallurgy, 

 since the original discovery of x-rays. With 

 projection microradiography (PMR) the 

 difficulty of focusing x-rays is circumvented 

 by using a special x-ray tube to form a point 

 source of radiation, and initial x-ray enlarge- 

 ment is produced by simple geometrical pro- 

 jection of the image of a specimen close to 

 the source. The production of a magnified 

 image using x-rays has encouraged the use 

 of the term "x-ray microscope," quaUfied by 



the word "projection," although no x-ray 

 lenses or mirrors are needed. 



This form of x-ray microscope is seen in 

 detail in Fig. 2. An electron gun (hot tung- 

 sten filament, biasing cap and anode at earth 

 potential) forms a narrow beam of electrons 

 accelerated to 5 to 20 kV. The space trav- 

 ersed by the electrons is evacuated as in an 

 electron microscope and the electron beam 

 is focused by two magnetic lenses placed 

 outside of the non-magnetic vacuum cham- 

 ber. The objective lens with pole pieces re- 

 duces the size of the electron source and the 

 condenser lens, although weaker, conven- 

 iently determines the amount of reduction 

 over-all. The minute electron beam, from a 

 few microns down to 0.1 micron or less, 

 strikes the thin metal foil target that also 

 forms the vacuum wall of the upper end of 

 the x-ray tube. The x-rays generated in this 

 way come from a spot similar to the electron 

 beam size and are used to form an x-ray 

 projected image of the specimen on the film 

 or fluorescent screen. The specimen is held 

 in a stage with three degrees of freedom for 

 scanning the field of view in two directions 

 and changing the magnification with the 

 third. The camera for recording the x-ray 

 image is merely a light tight box holding a 

 cassette so that the plate can be exposed to 

 the x-ray beam. 



The x-ray target and electron beam are 

 shown schematically in Fig. 3 to demonstrate 

 the method of image formation in the projec- 

 tion x-ray microscope. The electron beam, 

 coming from the left and focused by the ob- 

 jective lens, is enlarged by the lens aberra- 

 tions to a size 5, even if the total reduction 

 in beam size would be much smaller than 

 this. The semi-angular aperture of the lens, 

 a, determines the value of 5 and also the 

 total beam current striking the target. A 

 strong lens with the lowest aberration is 

 used for the final beam reduction although 

 this means a short working distance and 

 consequent placing of the target in the pole 

 piece gap. The exact opposite arrangement is 



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