Radiation Dose in Autoradiography 127 



For work with X-ray film, sections up to 20/z thickness can 

 be used, whilst for high resolution not more than 5/^ thickness 

 can be tolerated. Special conditions are presented in smears 

 and squashes where the thickness of the specimen can usually 

 not be measured. The area taken up by one cell nucleus in 

 the final preparation can be measured and from this the 

 actual volume per unit area calculated. 



Autoradiographs are usually used for the determination of 

 the concentration in certain parts of the material, under 

 investigation, and consequently the minimum concentration 

 per ml. of tissue will be proportional to the ratio of the radio- 

 active volume to the total volume of tissue, e.g. for a study of 

 deoxyribonucleic acid labelled with ^^P in cell nuclei the 

 ratio of the volume taken up by the nuclei to the total 

 volume. (See Fig. 1 in the paper by A. Howard and S. R. 

 Pelc in this volume.) This ratio has to be determined with 

 great caution when the size of the object is of the same order 

 as the resolving powxr. Thus, sparsely distributed cell nuclei 

 of 10/x diameter in autoradiographs of 100/x resolving power 

 have to make developable sufficient grains in an area of 

 approximately 7,500/x2. jj^ high resolution work a similar 

 limitation appears when the area in question is smaller than 

 approximately 100^^. While 10 grains per lOO/x^ can be 

 regarded as an autoradiograph, one grain per lOft^ can not; 

 we may regard an accumulation of 10 grains above back- 

 ground per single object as the minimum when this object 

 covers an area of less than lOO/x^. 



Thus if we need n grains per sq. cm. to obtain an observable 

 autoradiograph, the specimen must contain nf/0 • 375 radio- 

 active atoms per sq. cm., where f is the ratio of radioactive 

 volume to non-radioactive volume. This concentration must, 

 of course, be present after extraction of all the labelled com- 

 pounds which are not under investigation. 



The volume per sq. cm. of specimen will be equal to the 

 thickness (8), and the number of radioactive atoms per ml. 

 of tissue must therefore be nf/0 -3758. One fxC of radioactive 

 material contains 4 • 64 x 10^ X H atoms, where H is its half-life 



