The Measurement of Radiation 191 



they ionize directly, and in a qualitatively similar manner. Such 

 radiations include electrons (beta particles), and the whole range 

 of swiftly moving atomic nuclei, best-known of which are the 

 helium nuclei or alpha particles, emitted by natural radioactive 

 substances. Of these, electrons are practically the only kind 

 of radiation used as an external beam, and even these not 

 widely. But with the development of the betatron for producing 

 very intense beams of high-energy electrons, the therapeutic 

 applications may well be extended. 



Since X- and gamma radiations produce their effects via the 

 intermediary or secondary electrons, it is clear that the rontgen 

 unit may legitimately be used for expressing dose in the case 

 of a primary beam of electrons. A measurement of the ionization 

 per unit volume of air gives the dose directly in rontgens.* This 

 concept is also satisfactory for any other directly-ionizing radia- 

 tion. The ionization in a "thimble'' chamber is now independent 

 of the nature of the walls, provided the primary radiation is not 

 appreciably attenuated or reflected by them. Thus the dose rate 

 of the primary beta radiation from ''unscreened" radium plaques 

 has been measured in rontgens. 



In some cases, the radioactive substances are dispersed 

 throughout the biological material. For example, radioactive 

 prosphorus is used therapeutically for leukemia, and biological 

 specimens have been immersed in an aqueous solution of radon. 

 For such cases, slightly different concepts are appropriate, for 

 the radiation is usually absorbed completely within the medium. 

 Thus, knowing the total quantity of radioactive substance intro- 

 duced, and the total energy emitted by each disintegrating atom, 

 the quantity of energy communicated to the medium is known, 

 i.e., the fundamental biological quantity is known at the outset. 

 It merely remains to compare this true energy absorption (de- 

 termined solely by the radioactive substance and entirely inde- 

 pendent of the medium in which the substance finds itself) with 

 that which is produced by other radiations in order to express 

 it in ''equivalent rontgens." This involves the adoption of some 

 convention. 



* The true energy absorption for a medium of specified atomic make-up could 

 be calculated from this rontgen dose. 



