X-RAY MICROSCOPY 



lO 

 5 



I 

 FRINGE 



OS 

 WIDTH _ 



IN ;j 



O.I 



Q05 



O.OI 



O.I 



0.5 I 

 PLATE 



5 lO 

 DISTANCE 



50 lOO 

 IN CM 



Fig. 4. Minimum plate distance from x-ray 

 source to observe a fringe of width d, allowing 

 lOX photographic enlargement. (From W. C. 

 Nixon, Proc. Roy. Soc, A232, 475, 1955.) 



Fig. 5. Section of dog skull showing the diploe 

 veins. (From C. G. Hewes, W. C. Nixon, A. V. 

 Baez and O. F. Kampmeir, Science, 124, 129, 1956.) 



X-ray negative, and the actual values will 

 determine the exact result. However, the 

 values shown give the order of magnitude of 

 the effect. 



It is seen that for a fringe of one micron 

 width, the plate must be 10 cm from the 

 x-ray source for that fringe to be visible in 

 the final image. This distance is longer than 

 usually used and the exposure time would be 

 longer as well. In fact, with one micron reso- 

 lution no one micron fringes have been seen 

 due to Fresnel diffraction for this reason. As 



the resolution improves to 0.1 micron the 

 plate distance falls to 1 cm and this is the 

 distance most used in practice and indeed 

 fringes of this size have been seen. 



These then are the main limitations on the 

 performance of the projection x-ray micro- 

 scope. All attempts at making an x-ray mi- 

 croscope of this type quickly lead to a resolu- 

 tion "less than one micron" and equal to the 

 penetration limit for the kilovoltage and tar- 

 get metal used. The different types of lenses 

 that focus and reduce the electron beam 

 merely affect the number of electrons in the 

 final spot and thus the exposure time, with 

 the poorer lens giving a longer exposure but 

 at the same resolution. This implies enough 

 intensity on the fluorescent screen to focus 

 the microscope in the first place. Perform- 

 ance can only be judged if all comparison 

 figures are given such as spot size, beam cur- 

 rent at the target, kV, exposure time, plate 

 distance and specimen. 



Results 



An x-ray micrograph of a 5-mm section 

 of dog skull is shown in Fig. 5, taken with 

 Dr. Baez' x-ray microscope at the University 

 of Redlands, California. The blood \'essels 

 within the bone have been injected with a 

 blue vinyl plastic and detail of the diploe 

 veins is shown. This single 5 minute exposure 

 presents the same information as would be 

 obtained after some 25 hours of normal op- 

 tical sectioning and examination. A detailed 

 examination would take several months by 

 normal methods, but a few 5-minute expo- 

 sures with the x-ray microscope. 



Another example is the frozen-dried foot 

 of a newborn mouse seen in Fig. 6, and taken 

 by Mosley and Wyckoff at the National In- 

 stitutes of Health, Bethesda, Maryland. In 

 this case a study of developing bone and 

 teeth is aided by the penetration possible 

 with the x-ray microscope and freeze-drying 

 is used both for fixation and removal of the 

 water that would obscure the detail of the 

 bone and tissue. Similar photographs of the 



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