386 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 14 



sensitive and faster x-ray film will indicate the proper exposure time. Where 

 other factors permit, exposure time should be less than 2 to 3 half-lives of 

 the radioisotope since the latent image fades with time, a phenomenon that 

 may be slowed by storing the setup at low temperature. Although a longer 

 exposure time increases the range, of response, the resolution is thereby 

 decreased and the possibility of background fog from stray radiation is 

 increased. 



Background fog is a serious problem in cases of low-intensity radiation 

 and in attempting to estimate the quantity of radioactive material by 

 numbers of developed grains. It may be controlled to some extent by careful 

 selection of emulsion and, in some cases, by overexposure and underdevelop- 

 ment. A permanganate wash during development also diminishes back- 

 ground, but care must be taken since it also decreases image intensity. 



In summary, the conditions affecting the three desirable factors are as 

 follows: (1) Sensitivity is increased by use of a thick emulsion of large grain 

 size, by prolonged exposure, and by reduction of preparation time to a 

 minimum. (2) Resolution is improved by careful selection of the proper 

 emulsion for a particular type of irradiation, by use of a thin emulsion with 

 small grain size, by keeping exposure time to a minimum, by decreasing back- 

 ground fog by overexposure and underdevelopment, and through the use of 

 proper chemicals. (3) Adequate range of response is dependent on use of 

 thick emulsions, complete development, and comparison of radioautographs 

 made with various exposure times. Other conditions that are of general 

 value in obtaining good radioautographs include the following: the distance 

 between the sample to be studied and the emulsion must be reduced to a 

 minimum; the radioactive material must not be appreciably soluble in any 

 of the reagents used prior to exposure of emulsion to section; the stain used 

 for the tissue must yield good differentiation, i.e., if applied after develop- 

 ment it should not obscure or fade the image, and if applied before exposure it 

 should not be washed out by any of the reagents used for development. 



References 1, 2, and 3 are excellent reviews with extensive bibliographies, 

 but a number of the more recent references dealing with the general subject 

 of radioautography will be given here. The physical principles are dis- 

 cussed in papers 4 and 5. The technique of making radioautographs with 

 section and emulsion supported individually and realigned after separate 

 staining and developing is described in 4 and 6. Articles 7 and 8 deal with 

 preparation of liquid emulsions and methods of spreading these on sections. 

 The method of floating section onto emulsion is detailed in references 9 and 10. 



REFERENCES FOR CHAP. 14 



1. Axelrod, D. J., and J. G. Hamilton: University of California Radiation Laboratory 

 Report BP 111. 



