X-RAY MICROSCOPY 



Fig. 9. Close up of the target-, aperture- and specimen holder. 



The required storage time can easily be 

 achieved by using a photographic fihii. In 

 the wavelength region normally used for 

 projection microscopy, the quantum yield, 

 i.e., the number of developed silver grains 

 per absorbed x-ray quantum, is very close 

 to 1. This results in a linear relation between 

 film density and exposure. Contrast is pro- 

 portional to the density, so for full visual 

 information transfer a contrast correction 

 may be necessary (17). 



Although the depth of field of the projec- 

 tion microscope is large, a thick specimen 

 may obscure important detail and further- 

 more it needs a longer exposure time. So 

 the thickness will be determined by the fact 

 whether we want to see the absorption dis- 

 tribution in a plane or in a volume. The last 

 one is made possible by stereomicroscopy. 



As the anode voltage and target material 

 determine the spectral distribution of the 

 x-ray source, they must be adapted to the 

 specimen to give an adequate contrast. 

 When using monochromatic radiations of 

 different wavelengths a qualitative chemical 

 analysis can be carried out (18). The fact 

 that the specimen is spatially separated from 

 both film and target allows us to make a 



series of exposures with different wave- 

 lengths while still projecting the specimen 

 at exactly the same angle. This fixed source- 

 to-specimen position is necessary to get an 

 unambiguous relation between the specimen 

 and its projected image. 



A direct consequence of the large depth 

 of field is the fact that the magnification of 

 the image is not well defined. It may vary 

 widely over the different parts of the speci- 

 men, especially when the thickness of the 

 specimen is comparable with its distance to 

 the source (see Fig. 2). 



For thin specimens, the magnification can 

 be determined in different ways similar to 

 those used in light microscopy. A method to 

 determine the magnification of the various 

 parts of a thick specimen will be discussed 

 in the next section about stereomicroscopy. 

 As in projection x-ray microscopy the use 

 of long wavelengths is very limited by the 

 intensity, the contrast for thin organic sam- 

 ples is very poor. If the special features of 

 this type of microscope are desirable for such 

 specimen, contrast can be improved by an 

 appropriate treatment. In many cases this 

 kind of preparation and staining technique 

 can be a modification of existing techniques 



668 



