176 Applied Biophysics 



may be possible, in a gas at any rate, continuously to remove 

 the ions to the two electrodes almost as fast as they are produced 

 by the ionizing radiation, before any appreciable recombination 

 can take place. The electric current in such circumstances is 

 called the "saturation current" and, in most cases arising in 

 practice, it is very minute. 



Examples of ionizing radiations are the electromagnetic type 

 as in X-rays, and the gamma rays from radioactive substances, 

 the swift electrons in cathode rays and the beta rays from 

 radioactive substances, protons, alpha particles, etc., the neu- 

 trons, all of which have a similar ultimate mode of action in 

 biology. 



The necessity for some system of measurement of radiation 

 in biological and therapeutic studies need hardly be emphasized, 

 but in practice it has proved an exacting pursuit, aptly illustrat- 

 ing Kelvin's historical remark that no phenomenon can be 

 understood till it can be measured and expressed in numerical 

 terms. The difficulties lie in deciding on, and realizing prac- 

 tically, a suitable measure of "amount" of radiation, and arise 

 partly from that common feature of the radiations which is most 

 obvious, namely, their power of penetrating matter, and partly 

 from the very small amounts of energy involved. For example, 

 the total amount of energy communicated to the tissues in a 

 typical complete therapeutic treatment would suffice only to 

 augment the temperature of the mass by about one hundredth 

 of a degree Centigrade. 



To keep our discussion to a reasonable length, it will be 

 necessary to confine ourselves to what is by far the most 

 important method in this branch of radiation measurement, the 

 ionization method, and to concentrate on the principles involved, 

 omitting detailed descriptions of techniques. In an adequate 

 historical account, considerable interest would attach to the 

 photographic method of measurement,* but here, we merely 

 remark in passing that it has been developed as a precision 

 technique only in certain rather restricted fields, though it 



* Some of the earliest dosimetry was done by finding the time required to 

 photograph a hand! 



