Sec. 14.3] THE RADIO AUTOGRAPH 385 



14.3. Radioautographic Emulsions. Photographic emulsions available for 

 radioautography may be divided into four general groups according to the 

 application for which they are intended: those used with visible light, x-ray, 

 spectroscopic, and special particle plates. For alpha irradiation the very 

 fine-grained emulsions of the last two groups are most useful, especially the 

 "alpha-particle" film. For beta radiation, however, these plates are not 

 sensitive enough. X-ray or dental film is very sensitive to beta particles, 

 but the grains are large and irregular and hence definition is poor. The 

 somewhat finer grained lantern-slide plates are a suitable compromise. In 

 general, with a small grain size, the probability of a relatively slow and heavy 

 alpha particle hitting and imposing a latent image on a grain is greater than 

 that of a faster beta particle. The probability of a hit with a beta particle, 

 and therefore the sensitivity, increases with increasing grain size, but the 

 resolution diminishes. 



As charged particles pass through the emulsion they will be scattered by 

 the atoms of the emulsion. Hence with thicker emulsions, the scattering 

 will be greater and the resolution consequently, diminished. The backing or 

 film support also contributes to this effect especially in the case of beta 

 particles of high energy. The usual thickness of x-ray film and lantern-slide 

 plates is approximately 100 fx, and alpha-particle plates have an emulsion 

 thickness of about 40 p.. A thinner film may be obtained by using liquid 

 emulsion or by the use of plates recently developed by the Eastman Kodak 

 Company which have emulsion thicknesses of 5 to 10 ;u, which are available 

 under the designation NTP. The use of liquid emulsion or stripping film 

 eliminates scattering due to film backing. On the other hand, if the number 

 of grains is increased by increasing the emulsion thickness, the probability 

 of a beta particle imposing a latent image increases. Therefore when sen- 

 sitivity and range of response are the desiderata, a thick emulsion should be 

 used. 



Exposure and development are the remaining conditions to be considered 

 in the preparation of a radioautograph. The time of exposure will depend 

 to a great extent on the amount of radiation present in the samples to be 

 analyzed. This may be determined prior to preparation of the radioauto- 

 graph by determining the activity of one or of a few sections by a suitable 

 counter. In general, a satisfactory beta autograph may be obtained with 

 about 10 7 particles per square centimeter, whereas 2 X 10 6 alpha particles 

 will be sufficient. However, less total radiation is often sufficient if local 

 concentrations are high. This is also true in the case of alpha autographs 

 made for the study of tracks rather than film blackening. When possible, 

 a number of plates should be exposed and developed at various intervals to 

 enable selection of the most suitable exposure time. In the case of beta 

 autographs made on lantern-slide emulsions, a prior run with the more 



