POINT PROJECTION X-RAY MICROSCOPY 



mouse head, both new born and embryonic, 

 show the developing tooth buds in situ, and 

 the spatial arrangement is seen with a stere- 

 ographic view. 



Other metallurgical, mineralogical, chemi- 

 cal and industrial applications may be found 

 in the publications listed at the end of the 

 article. 



Resolution 



Electron penetration in the target varies 

 roughly as the square of the kilovoltage of 

 the electron beam and reducing the voltage 

 from 10 kV to 2 kV would imply an improve- 

 ment of 25 X in resolution. In addition the 

 x-rays generated at this lower voltage would 

 be more easily absorbed by the thinner speci- 

 mens and sufficient contrast would be ex- 

 pected even for much finer detail. However, 

 the x-ray intensity falls ofT very rapidly as 

 the kilovoltage is lowered due to less efficient 

 electron gun operation, less efficient x-ray 

 generation, more absorption in the target 

 and in the x-ray path to the photographic 

 plate. The ideal solution is some form of soft 

 x-ray image intensifier but until that time it 

 is necessary to use the higher kilovoltage and 

 overcome the electron limit in some other 

 way. 



A thin layer of evaporated metal on a 

 thicker beryllium backing will not give an 

 improvement because x-rays are generated 

 in the beryllium by the electrons that pass 

 through the thin metal layer and this back- 

 ground fogging obscures the improved resolu- 

 tion. The alternative is to use a thin foil with 

 no backing at all and this has been done for 

 Figs. 7 and 8. In Fig. 7 the same type of test 

 object as used previously by all three meth- 

 ods of x-ray microscopy is shown at a higher 

 magnification and with a resolution that is 

 estimated to be close to 0.1 micron. A faint 

 white Fresnel diffraction fringe of this width 

 can be seen around the grid bars. An actual 

 specimen as opposed to a test object is shown 

 in Fig. 8. This is a bean section, also taken 



Fig. 6. Frozen dried new born mouse foot. 

 (From V. M. Mosely and R. W. G. Wyckoff, un- 

 published.) 



with a thin metal target and the same fringe 

 can be seen around this specimen as well. 



These photographs represent about the 

 best that can be done with the static x-ray 

 projection microscope until some means of 

 increasing x-ray intensity is found by any or 

 all of the possible approaches including image 

 intensifiers, field emission electron sources 

 and better electron lenses corrected for spher- 

 ical aberration. 



Other Methods of Point Source X-Ray 

 Microscopy 



The difficulty of finding x-ray fringes was 

 shown in Fig. 4 and the small size even at 

 high resolution is seen in Figs. 7 and 8. This 

 makes the practice of Gabor diffraction mi- 

 croscopy with x-rays very difficult if not im- 

 possible since it is necessary to have many 

 fringes in the hologram if there is to be any 

 information in the reconstruction. Baez and 

 El-Sum show other difficulties as well, such 

 as monochromatizing the x-ray beam, using 

 opaque specimens and producing a mathe- 

 matical point source. 



651 



