126 S. R. Pelc 



X-ray emulsions as well as for special stripping film. Lamerton 

 (1950) fomid values of 0-5 to 0-7 for ^^F and 1-5 for ^^S on 

 special stripping film for autoradiography. These discrepan- 

 cies are not surprising in view of the difficulties of absolute 

 measurement of radioactive substances, reproducible develop- 

 ment and grain counts under the microscope. For the purpose 

 of preliminary calculations, a value of one grain per incident 

 electron can be assumed. 



To observe the ARG we require a minimum grain density 

 above background which is dependent on the magnification. 

 At one extreme are autoradiographs of large objects, such as 

 leaves, etc., which may be viewed directly or with low magni- 

 fications. Resolving powers of 50/x-lOO/x are sufficient for 

 this purpose, and the work can conveniently be done with 

 X-ray film pressed against the specimen. At the other 

 extreme are autoradiographs of high resolving power viewed 

 at high magnifications for which we use fine grained emulsions 

 and more specialized techniques. 



When only macroscopic detail is wanted we observe the 

 optical density of the autoradiograph, while with high magni- 

 fications the number of grains per unit area is of importance. 

 Various estimates of the number of electrons necessary to 

 give a visible autoradiograph on X-ray film range from 2 x 10^ 

 to 10' electrons per sq. cm. It w^as found in recent experi- 

 ments (Howard and Pelc, 1950) that concentrations of 10 

 grains per lOO/x^ (above background) can be observed using 

 Kodak special stripping film for autoradiography, for low 

 background. This is equal to 10' grains per sq. cm. It should 

 be stressed that 10 grains per lOO/x^ will not result in a visible 

 blackening of the film and that they are observable onlv 

 under favourable conditions. For a general calculation, it 

 will be prudent to assume a minimum of 2 x 10^ grains per 

 sq. cm. for X-ray film and 10' grains per sq. cm. for special 

 stripping film. 



The thickness of the specimen determines the volume of 

 material available, and the necessary concentration of radio- 

 active material will be inversely proportional to the thickness. 



