382 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 14 



alpha particle since the former loses energy more slowly. As a result, an 

 image produced by a point source of a beta emitter will be quite large; for 

 example, from a point source of beta particles with an average path length of 

 2,000 n, a symmetrical image approximately 4 mm in diameter is produced. 



These circumstances are unfortunate for they limit the resolution of beta 

 autographs and decrease the possibility of determining with certainty the 

 origin of beta radiation from tissue samples. It is especially unfortunate 

 since many of the isotopes most useful in biological studies, such as carbon, 

 sulfur, sodium, and iodine, are beta emitters. 



The alpha emitters are heavy metals, and their present importance falls 

 primarily in the realm of health protection of workers in atomic-energy 

 plants and in the effects of the atomic bomb on plant and animal life. 



14.2. Techniques for Preparing Radioautographs. A radioautograph of a 

 large sample of tissues containing radioactive material, such as half a kidney 

 or a smooth bone surface, is relatively simple to produce. The block of 

 tissue is held in firm contact with the film for the desired length of time, and 

 the film is then developed, after which, areas of blackening are easily cor- 

 related with regions of the tissue block. The problem of more accurate 

 localization to particular structures in the tissue is more complex. In this 

 type of radioautograph the following factors are of particular importance: 

 the resolution or separation of darkened areas and their restriction to an 

 area the same as that of the source should be as great as possible in order 

 that localization may be accurate; the sensitivity must be adequate to detect 

 even small amounts of radioactive material; and, for the purpose of com- 

 paring concentrations of material in various areas, it is necessary that the 

 range of response be as great as possible. These factors may be controlled, 

 in so far as they are independent of the type of radiation, by manipulation of 

 the various conditions discussed below. 



The most important aspects to be considered are contact between emulsion 

 and specimen during exposure and alignment of radioautograph and specimen 

 after development. Contact and alignment may be secured (1) by mounting 

 the section and emulsion on separate supports such as microscope slides, 

 (2) by spreading liquid emulsion or stripping film over the section, or (3) by 

 mounting the section on the emulsion. 



In the first method (1) the tissue is prepared by the usual histological 

 techniques, and one or more sections are mounted on a microscope slide or 

 cover slip. Factors such as solubility of the radioactive material and section 

 thickness, discussed in detail below, must be carefully controlled for best 

 results. The slide holding the unstained section is then placed over the 

 proper type of film, section and emulsion together, and firm, even pressure is 

 applied for the duration of the exposure. After developing the film and 

 staining the section the two may be easily compared for evidence of gross 



