86 



APPLIED RADIOACTIVITY 



that all eggs were 1 cm from the radiating source. The eggs were ex- 

 posed to gamma radiations filtered by 0.5 mm of Pt and 4 mm of Bake- 

 lite. They were tested for survival after exposures varying from 1 to 

 55 mc-hr. The results are shown in Fig. 11-12. 



50 



100 



150 



200 



250 



Fig. 11-12. Survival of Drosophila eggs after exposure to gamma rays in millicurie- 

 hours. The x-ray survival curve is shown superimposed on the gamma-ray survival 

 data. Fifty per cent of the eggs survive when exposed to a dose of 38 mc-hr. (By 

 courtesy of C. Packard.) 



If the x-ray survival data are superimposed on the gamma-ray data, 

 it is found that a single curve fits the two sets of data equally well. 

 Figure 11-12 shows how well the x-ray survival curve fits the gamma- 

 ray data. 



These composite data show that 50 per cent of the eggs survive an 

 exposure of 38 mc-hr and also 190 roentgens. These two doses are 

 therefore biologically equivalent, if Drosophila eggs are acceptable as a 

 standard test substance and if the response is independent of the wave- 

 length of the radiant energy. 



The millicurie-hour is therefore equal to 5 " biological " roentgens 

 (1 mc-hr = 5 br). 



Other recent attempts to measure x-rays and gamma rays in the same 

 biological unit were made by Braun [1930]. Using Ascaris eggs, he 

 obtained 1 mc-hr = 5.3 br. Henshaw and Francis [1936], using Droso- 

 phila eggs, obtained 1 mc-hr == 5.4 br. 



