CHAPTER 14 

 THE RADIOAUTOGRAPH 



Patricia P. Weymouth 



14.1. Introduction. Established methods for analyses of radioactivity in 

 biological specimens in which the activity of a suitable aliquot of treated 

 tissue is determined will give valuable information concerning gross dis- 

 tribution of the material under study. However, the local distribution of 

 active material in particular parts of an organ and in special cells of a tissue 

 can be studied most effectively by the technique of radioautography. This 

 technique, as the name implies, provides a self-photograph of a radioactive 

 substance as a result of the action of alpha or beta particles on a photographic 

 emulsion (gamma radiation will not be considered since it is of limited 

 importance). 



The usual photograph, taken with a camera, is the result of activation 

 of silver bromide grains by photons; the energy state of the silver bromide is 

 changed so that at some later time the silver may be easily reduced. Pre- 

 sumably, the action of alpha and beta particles is qualitatively similar to 

 that of photons but quantitatively different because of the physical char- 

 acteristics of the radiations. The alpha particle has a mass about 7,400 

 times that of the beta particle, and when both particles are of equal energy 

 the beta particle will travel at a much greater speed than the alpha particle. 

 When the alpha particle passes through an emulsion containing silver bromide 

 grains, it proceeds in a straight line giving up energy to the grains along its 

 path until it is brought to rest and neutralized. A radioautograph produced 

 from an alpha source will show distinct tracks of closely spaced grains. The 

 tracks, each produced by a single alpha particle, proceed in a straight line, 

 lie in random directions, and end abruptly. In a picture from a beta source 

 the situation is somewhat different, for in this case the velocity of the beta 

 particle compared with that of an alpha particle of equal energy is much 

 greater; the grains affected by a single particle are more widely spaced since 

 the beta particle passes many grains too rapidly to induce an image on them. 

 Furthermore, because of the small mass of the beta particle, it is easily 

 deflected by atoms in its path and will not travel in a straight line. From 

 this discussion, it will be recognized that with a beta source the picture 

 obtained will consist of a random distribution of developed grains, no two 

 of which can be ascribed to any one beta particle. In addition, the length 

 of path of the beta particle through material is much greater than that of an 



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