150 RADIATION BIOLOGY 



As a consequence of Eq. (3), the dose D for any biological object will be 



Z) = 83.4 X roentgens • e„ • e^/e^ ergs/g (4) 



In practice, whenever e„ can he considered equal to unity at the point of 

 interest, the dose may be expressed simply as 



D = 83.4 X roentgens X ec/e", ergs/g (5) 



The ratio e^/e^ is calculable for the case of moderately energetic mono- 

 chromatic X rays, since the conversion of photon energy to electron 

 energy takes place by means of the well-known Compton and photoelec- 

 tric processes. Figure 2-1 shows, for instance, the relation 83.4 X ^c/^l 

 for "wet tissue" as a function of photon energy (Lea, 1946). 



200 400 



1000 



^ 2 4 6 8 10 20 40 60 100 



PHOTON ENERGY, kev 

 Fig. 2-1. Energy absorption per roentgen in "wet" tissue as a function of photon 

 energy. {Lea, 1946.) 



It should be remembered, however, that to achieve good dosimetry the 

 experimenter must evaluate e„ at the point of interest in the biological 

 material or its equivalent mock-up; this point will be reconsidered under 

 the section of practical problems of "external" dosimetry. 



EXPERIMENTAL REALIZATION OF THE ROENTGEN 



It is evident from Fig. 2-1 that the roentgen follows closely the energy 

 absorption in most biological materials within the X-ray region indicated 

 and that the lack of monochromaticity present in radiations available 

 from most X-ray apparatus does not raise uncertainties of serious con- 

 sequences in biological work. It is not surprising, therefore, that since 

 its adoption in 1928 the roentgen has proved to be of great value in 

 radiological work and that it has been Retained by the International Com- 

 mission on Radiological Units (London, 1950) as the unit of dose in the 

 range of photon energies up to 3 Mev (Com. on Stand. X-ray Meas., 



