ULTRASOFT X-RAY MICROSCOPY 



Pinhole ~^~^~-~S~~ 



Fig. 1. The Buerger Two-Wave Microscope showing atomic arrangement of FeS2 , marcasite, on 

 the screen. 



patterns to give the structure pattern. The 

 result for marcasite, FeS2 , is shown in Fig. 

 1. 



Thus the good features of x-rays, with 

 wavelengths short enough to be compatible 

 with atomic dimensions, are combined with 

 the scaling up by substitution of visible light 

 of the whole optical image-forming process, 

 so that the image is magnified by a factor of 

 the ciuotient of the wavelength of visible 

 light to that of x-rays, or 10^ diameters. 



REFERENCES 



1. Buerger, M. J., J. A^-pl. Phijs., 21, 909 (1950). 



2. Clark, G. L., "Applied X-rays," 4th ed., Mc- 



Graw-Hill Book Company, N. Y., pp. 460- 

 462, 1955. 



G. L. Clark 



ULTRASOFT X-RAY MICROSCOPY 



X-ray microscopy has essentially the same 

 resolution limit as does light microscopy and 

 it is usually the slower and less convenient 

 of the two methods. Nevertheless there are 

 important possible advantages of x-ray mi- 

 croscopy which arise mainly through the 

 basic differences in the optics of image for- 

 mation and in the image contrast mecha- 

 nism. 



The simple method utilized in the projec- 



tion of x-ray images is illustrated in Fig. 1. 

 Here it is seen that all planes of the three- 

 dimensional object are imaged, with equal 

 sharpness, into an essentially two dimen- 

 sional plane — usually a thin photographic 

 emulsion which becomes the "microradio- 

 gram." Such complete depth of field can 

 only be approximately realized with light 

 microscopy through the use of very small 

 objective apertures and with a consequent 

 sacrifice of resolution due to diffraction. 

 Sharp projection images, relatively free of 

 Fresnel diffraction blurring, are possible only 

 with the shorter wavelength x-radiations. 

 This two dimensional microradiogram 

 "model" of the object is often more ame- 

 nable to high resolution measurement of 

 structural detail and to quantitative stereo- 

 graphic analysis of thickness or depth di- 

 mensions. 



Unlike light images, x-ray images have 

 contrast which is due mainly to photo-elec- 

 tric absorption of the x-radiation by the 

 sample; the complicating effects of refrac- 

 tion, diffraction and reflection are either 

 small or completely negligible. This leads to 

 a relatively unambiguous interpretation of 

 contrast and often permits quantitative 

 microabsorption analysis. X-ray absorption 

 analysis complements that by light absorp- 



675 



