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RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



sources of radiation in a sample, each one will produce a circular area of 

 darkening in the emulsion. If these areas overlap, then it becomes impos- 

 sible to interpret from the autoradiogram that there were two distinct 

 sources. The greater the resolution, the closer together are the areas of 

 darkening that can still be distinguished from each other. Since the pri- 

 mary objective is the correlation of areas of darkening wdth localization of 

 radioactivity in the sample, it is clear that increased resolution is always a 

 major goal, especially for studies on the cellular level. In general, the 

 following conditions promote increased resolution: radiation of low 

 energy; thin specimens; thin emulsions; minimum of scattering from film 

 backing and slide; fine-grained emulsion; and close contact between 

 specimen and emulsion, which is perhaps the most important variable. 

 These factors form the basis for many of the techniques that have been 

 devised and are discussed subsequently from a practical point of view. 



The emulsion to be employed should be fast enough so that long expo- 

 sure times or high tissue levels of radioactivity will not be required; on 

 the other hand, the grain size must not be so large that the resolution is 

 inadeciuate. There should be a low background fog, especially when 

 individual grains or tracks are to be observed. Commercial films are 

 available for autoradiographic studies which offer a balance between these 

 factors and permit a choice dependent upon experimental requirements. 

 Some of these films are described subjectively in Table 7-1, and recom- 

 mendations are given for their use. 



Table 7-1. Film for Autoradiography 



[From Patrick J. Fitzgerald, Eva Simmol, Jerry Weinstein, and Cynthia Martin, 

 Radioautography: Theory, Technic, and Applications, Lab. Invest., 2: 181-222 (1953), 

 (Paul B. Hoeber, Inc., publisher).] 



