XV. ELECTRONS, NEUTRONS, A.ND ALPHA PARTICLES 519 



be mediated through radiochemical change either within the structure 

 or in the immediately surrounding fluid. In such cases differences 

 in radiochemical efficiency as between one radiation and another 

 must contribute to the over-all efficiency of the radiation in causing 

 the damage under consideration; indeed, the similarity between the 

 curves relating biological efficiency to ion density for chromosome 

 damage, and some other effects, in widely different organisms, may 

 well result from the fact that these particular biological effects result 

 from the same, or closely related radiochemical changes, brought 

 about with characteristic efficiency by each type of ionizing radiation. 



C. SOURCES OF IONIZING RADIATION FOR RESEARCH 



From what has been said in the previous section, it follows that 

 the quantitative interpretation of anj^ particular radiobiological ex- 

 posure will be simplified if the ionizing particles to which the cells are 

 exposed all have the same speed. On account of the limited range of 

 a particles and protons, and of electrons of less than about 100 e.kv. 

 of energy, this is in general possible only if the biological specimen is 

 a unicellular organism that may be exposed in a thin film or on the 

 surface of a culture medium. Cells irradiated within a mass of tis- 

 sue, whether by a or j3 radiation from radioactive material present in 

 the tissue or by an external source of X rays, 7 rays, or neutrons, are 

 inevitably exposed to inhomogeneous ionizing particles. In this 

 section we shall present data as to the velocity distribution among 

 the ionizing particles from which the mean linear ion density of the 

 radiation may be estimated, as well as of the intensity and penetrat- 

 ing power of the radiation from available sources. 



1. Electrons 



Electrons of Uniform Energy. The only natural sources that 

 emit electrons of constant energy are a few I'adioactive isotopes such 

 as Te^^^ that undergo a nuclear isomeric transformation. The charge 

 and mass of the nucleus are unchanged and no particle is emitted from 

 the nucleus but only energy in the form of a monochromatic 7 ray, 

 which in a proportion of disintegrations may undergo "internal con- 

 version," i.e., the whole energy of the quantum is transferred to an 

 orbital electron of the disintegrating atom, which is therefore emitted 

 with an energy that is constant and equal to the difference between 

 the energy of the quantum and the orbital energy of the electron. It 



