XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 557 



ill Escherichia coli, Sect. A6) the ions are the effective agent. On a 

 number of occasions it has been emphasized that excitation should 

 not be overlooked. 



From the considerations advanced in this section, it is clear that 

 the quantity we are able to estimate by the appropriate use of ioniza- 

 tion chambers is not the amount of ionization produced in the tissue 

 but the total energy in all forms — ionization, excitation, chemical 

 change, and thermal agitation — transferred by the ionizing particles 

 to the tissue. So far no one has even demonstrated that the ions 

 H2O+ and H20~ are in fact formed by the irradiation of liquid water, 

 though on theoretical grounds there can be no doubt as to their forma- 

 tion. These ions are demonstrable in the gaseous phase and the 

 minimum energy of an electron capable of ionizing water vapor is 

 known to be 13 e.v. The presence of water vapor in ionization 

 chambers is usually studiously avoided for the sake of maintaining 

 the essential high grade insulation between collecting electrode and 

 earth so that until recently no measurements had been made of TrH20, 

 the mean energy expended in the production of a pair of ions in water 

 vapor. Appleyard (102) finds TF water vapor/ TFair = 0.88 ± 0.02 

 for a particles of 4.5 m.e.v. initial energy. Since the value in other 

 gases is usually about double the ionization potential, the value of 

 ^HzO for electrons would be expected to be about 26 e.v. 



It would seem that, whereas dose may appropriately be specified 

 in some quantity such as roentgens, equivalent roentgens, or energy 

 units directly related to ergs of energy absorbed per gram of tissue, 

 it is useful to recognize that there are probably formed about two 

 ions in each cubic micron of tissue exposed to 1 equivalent roentgen 

 of any kind of ionizing radiation. 



References 



1 . Lea, D. E., Actions of Radiations on Living Cells. Cambridge Univ. 



Press, Cambridge, 1946. 



S. Kotval, J. P., and L. H. Gray, /. Genetics, 48, 135 (1947). 



3. Figge, F. H. J., Science, 105, 323 (1947). 



4. The Plutonium Project, Radiology, 49, 269 (1947). 



5 . Henshaw, P., /. N^at. Cancer Inst., 1, 789 (1941). 



6. Raper, J. R., Radiology, 49, 314 (1947). 



7. Henshaw, P., E. F. Riley, and G. E. Stapieton, Radiology, 49, 349 (1947). 



8. Lorenz, E., W. E. Heston, A. B. Eschenbrenner, and M. K. Deringer, 



Radiology, 49, 274 (1947). 



