X-RAY MICROSCOPY 



100 microns), the maximum allowable 

 penumbra usually determines the minimum 

 source-to-specimen distance. However, if a 

 variable focus microbeam x-ray source is 

 available the optimum choice of source 

 diameter and source-to-specimen distance 

 will be determined by both the desired 

 resolution and width of field. Since the 

 maximum permissible loading on microfocus 

 tubes increases approximately inversely as 

 the source diameter (9, 10) , it is an advantage 

 in speed to use the closest source to specimen 

 distance that will illuminate the entire speci- 

 men evenly, and the largest source diameter 

 which, at that distance does not cause 

 penumbral blurring above the desired mini- 

 mum resolving distance. 



For example, referring to Figure 1, if we 

 have a specimen of thickness h, located a 

 distance a from a source of x-rays of diameter 

 s, then the maximum possible penumbral 

 width P will be given by P = (& + c)s/a, 

 where c is the thickness of the recording 

 material. This penumbra may be made 

 arbitrarily small by decreasing s or increasing 

 a, but only with a corresponding decrease 

 in intensity. It is also obvious that the 

 desired minimum geometric resolving dis- 

 tance for a given specimen and recording 

 material thickness determines only the ratio 



Fig. 1. Geometry of contact image (see text). 



s/a and not the scale. Because of the afore- 

 mentioned inverse relation of source loading 

 to source diameter the maximum intensity 

 at the detector for a constant ratio s/a is 

 obtained as s and a are made as small as 

 possible. However, under these conditions 

 the area of the specimen which is evenly 

 illuminated also becomes small. In practice, 

 a certain width of field is desired for a given 

 specimen and this determines the smallest 

 useful values of s and a. In general, the 

 useful width of field is about 3^ a, although 

 a wider field can be imaged adequately for 

 qualitative work if the lower intensity at 

 the margins can be tolerated. 



It should be emphasized that if exposure 

 time is not an important consideration, and 

 if good specimen contrast can be obtained 

 using wavelengths of 2.5 A or shorter, a 

 standard diffraction tube with millimeter 

 focal spot size, properly arranged, can give 

 results equal in quality to those of much 

 more complicated systems. For thin sections 

 of biological material, on the other hand, 

 the use of ultrasoft x-rays is a necessity for 

 good contrast, and therefore special effort 

 must be made to achieve adequate intensity. 

 For this reason, at wave lengths of about 5 A 

 and longer, microfocus tubes are a distinct 

 advantage. 



Selection and Production of Proper 

 X-ray Spectrum 



The determination of a suitable wave- 

 length for a given problem is much less 

 difficult than producing the desired spectrum 

 at a useful intensity. For qualitative observa- 

 tion it is normally sufficient to choose an 

 average wavelength which will be neither 

 too strongly absorbed nor too strongly 

 transmitted by the specimen. For maximum 

 accuracy one may use the criterion that the 

 wave length should be chosen so that 1/m is 

 equal to the specimen thickness (11). 



For quantitative measurements of ab- 

 sorption, the production of sufficiently 

 monochromatic radiation is more difficult. 



562 



