PROJECTION MICROSCOPY 



lower x-ray intensity and thus necessitating heat dissipation in the target is not a prob- 



a longer exposure time. And the combined lem (15). As the source size becomes smaller, 



electron optical and thermal effect is more the heat flow becomes more favorable. The 



serious. admissible specific load may be increased 



(b) Thermal stability. The electron spot on proportionally with the reciprocal value of 

 the target is not the image of the cathode, the spot diameter. So, in a spot size of 1 n, 

 but that of the crossover which has a more a load as high as 10^ W/cm^ can be tolerated, 

 uniform intensity distribution and a smaller However, with the conventional hot cathode 

 size. The apparent position of this crossover this is hardly realizable when low voltages 

 depends greatly on the geometry of the are used. 



cathode, Wehnelt cylinder and anode as these 



elements act as an immersion lens. Description of a Commercial Projection 



Temperature changes effect the optical X-ray Microscope 



properties and thus the position of the cross- In comparison with the contact tube, the 



over. This effect is especially serious when it projection x-ray unit is a rather compHcated 



is accompanied with a disalignment of the electron optical and mechanical device. First 



immersion lens. Bearing in mind that the it must be continuously pumped to maintain 



cathode is at a high temperature and the the reciuired vacuum. (For ultrasoft x-ray 



surroundings have a relatively large mass, work, the contact tube cannot be made as a 



the temperature may still change consider- sealed off tube either.) Furthermore, it must 



ably during an exposure of more than 20 contain a highly stabilized anode voltage 



minutes. For high-resolution work, refocus- and lens current supply and an accurately 



ing before each exposure is necessary, even machined specimen and target stage. As the 



after several hours of continuous use. Pre- requirements are similar to those of an elec- 



heating the surrounding parts may reduce tron microscope some manufacturers provide 



this effect. A more effective solution will be adaptors which allow the use of the electron 



to use an illuminated pinhole aperture as microscope as an x-ray microscope. It is 



the electron source. This will, however, need encouraging to notice that complete com- 



one extra lens. mercial projection units are also available 



(c) Mechanical stability. Mechanical sta- now. 



bility may be achieved by appropriately Recently the Electron Microscope Divi- 



constructing and mounting the different sion of the Technical Physics Department, 



parts. The most serious error results from T.N.O. and T.H. at Delft, Holland, intro- 



relative movement between x-ray source duced its commercial microscope, which has 



and specimen during exposure, therefore, some special features. This instrument, a 



these parts need special attention. picture of which is shown in Fig. 7 and a 



Other limitations. The resolution limita- simplified cross section in Fig. 8, will briefly 



tion due to diffraction is discussed by Henke be described. The principal parts, i.e., the 



in this volume (p. 677). Another limitation electron source, electron lenses, target and 



of the resolution may be due to the depth of specimen stage and the camera may easily 



penetration and diffusion of the electron in be recognized. The electron reflection focus- 



the target. This effect can be reduced either sing method, mentioned previously, is used 



by using a lower voltage or a very thin tar- here. A simple but effective mirror optic in 



get (13). The effect of carbon contamination combination with a concentric corrective 



proves to be negUgible when the specific load lens and a 5 X ocular are used for observing 



is high enough (14). the focusing screen. The total magnification 



In contrast to conventional x-ray tubes, of this viewer system is 25 X. 



665 



