X-RAY MICROSCOPY 



Fig. 7. Silver grid, 1500 mesh/inch, 3 micron 

 bars, white Fresnel Fringe (From W. C. Nixon, 

 Nature, 175, 1078, 1955.) Magnification given by 2 

 micron bar. 



Fig. 8. Bean section, high magnification using 

 thin target. Magnification given by 10 micron bar. 



The X2 method of Le Poole and Ong is 

 another way of getting a sHght gain in reso- 

 lution if the film grain size and the x-ray 

 source size are similar. The exact improve- 

 ment is still arguable but the results they 

 show are very good and the gold-shadowed 

 and unshadowed bull spermatozoa taken in 

 this way are superior to those obtained with 

 the thin metal targets used for Figs. 7 and 8. 



Other projection methods are outlined by 

 Rovinsky where in one case a pin-hole pro- 

 duces the small x-ray source and in the 

 other a fine tungsten point acts as the x-ray 

 target. The resolution and exposure times are 

 similar to the more standard methods and 



these variations on the basic theme may lead 

 to improvements in the future, although not 

 offering better resolution at present. 



Microanalysis 



X-ray microscopy in common with other 

 methods of micro.scopy is intended to supply 

 information on the microscale. With the 

 electron microscope the scattering of elec- 

 trons to produce the contrast in the image 

 does not bear a simple relationship to the 

 elements of the specimen. Selected area micro 

 electron diffraction must be employed to 

 identify the crystalline components of the 

 object, from an area of one micron square or 

 slightly less. A similar method can be used 

 with the projection x-ray microscope where 

 selected area x-ray diffraction can be used 

 for areas down to a few microns in size. Lo- 

 calization of the area is done from the x-ray 

 image seen on the fluorescent screen; an 

 aperture is brought into the beam so that a 

 narrow pencil of rays strikes the specimen 

 and the broader diffracted beam is recorded 

 on fast x-ray film in place of the slower emul- 

 sions used for microscopy. This method 

 allows the identification of complicated crys- 

 talline compounds containing many sepa- 

 rate elements. 



In addition to this method, analogous to 

 electron diffraction, it is also possible to per- 

 form x-ray absorption and emission micro- 

 analysis in a manner that has no parallel in 

 electron microscopy. For the absorption 

 method the techniques developed by Prof. 

 Engstrom and his colleagues can be applied 

 to projection x-ray microscopy with the ad- 

 vantage of using an x-ray enlarged image for 

 analysis. If this is recorded photographically 

 the final accuracy with the present x-ray 

 microscope resolution is similar to that of 

 the contact method but it is more difficult 

 to produce monochromatic radiation with 

 the projection tube. The enlarged image does 

 allow the use of radiation counter recording 

 and this may speed up the whole process 

 and make it more automatic. A more power- 



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