X-RAY !MICROSCOPY 



Fig. 7. A print taken directly from a micro- 

 radiogram of 24 embryonic rat head sections illus- 

 trating the relatively large uniform field available 

 for an exposure wedge, multiple samples and con- 

 trols. 



25 



20 



.15 



.10 



.05 



.00 



5 10 15 20 25 30 35 40 



X (A) 



Fig. 8. The transmission characteristic for a 

 typical filter as used for isolating the Cr-L(21.7 A) 

 and 0-K(23.7 A) radiations. One kilovolt excita- 

 tion of these characteristic wavelengths places the 

 relatively sharp peak of the continuous radiation 

 within the absorption band, 12 to 18 A. 



care and control in photographic develop- 

 ment as required by separate emulsion cali- 

 bration exposures. (This same requirement 

 for a large uniform field obtains also for the 

 conventional step-wedge of foils of a material 

 of known thickness and of similar absorption 

 characteristics as that of the sample.) In or- 

 der to achieve this large angular field for the 

 x-ray source without the use of excessively 

 large, thin-foil window areas, a "line" source 

 is employed as shown in Fig. 5, consisting 

 of a water-cooled anode tube. The window is 

 of slit geometry, the long dimension being 



normal to the direction of the anode so that 

 effectively a "point" area of the anode is 

 "seen" at any given position on the micro- 

 radiographic plate. With the instrument 

 described here, 2 by 3 inch glass microradio- 

 graphic plates are exposed with a good uni- 

 formity of field area for the exposure wedge 

 and with four square inches of area for the 

 mounting of multiple samples and controls 

 (see Fig. 7). 



In order to gain high specific intensity 

 from the target, the tube has been designed 

 for large anode power — up to about 1000 ma 

 beam currents for anode voltages in the 

 range of 500 to 2000 volts. 



Almniniun, copper and iron anode tubes 

 are used for Al-K (8.3A) Cu-L(13.3 A) and 

 Fe-L(17.6 A) respectively, vising filter ma- 

 terials of the same metal as the target. A 

 copper anode is used, plated with chromium 

 for Cr-L(21.7 A), oxidized for 0-K(23.7 A), 

 vacuum evaporation coated with titanium 

 for Ti-L(27.4 A), and painted with aquadag 

 for C-K(44 A). 



In general, relatively heavy filtering and 

 low-excitation voltages are used in order to 

 insure monochromaticity. For example a 

 typical transmission curve for a chromium 

 filter is as shown in Fig. 8. For chromium 

 radiation, one kilovolt excitation is used, 

 placing the relatively sharp peak of the con- 

 tinuous radiation well within the 12 to 18 

 angstrom absorption band which is illus- 

 trated here. The LS radiation is repressed by 

 the effect of its wavelength lying between 

 those of the critical L II and L III absorption 

 edges. To estimate the effectiveness of a 

 given filter and to determine the maximum 

 tube voltage permissible, it has been con- 

 venient to observe the pulse height distribu- 

 tion appropriately displayed on an oscillo- 

 scope which is connected directly to the 

 preamplifier of a flow-type proportional 

 counter. A pulse height distribution curve for 

 the 0-K radiation from a copper oxide tar- 

 get at 1 kV potential and filtered by a chro- 

 mium foil filter is illustrated in Fig. 9. 



684 



