556 L. H. GRAY 



The same formula maj^ be used for a source that emits a single a 

 particle at each disintegration if the full energy of the a particle is 

 substituted for E^. E^ has been evaluated {55) for a considerable 

 number of radioelements so that if C, the concentration of element in 

 microcuries per gram is kno^vn, the dose rate in the tissues may be 

 evaluated. 



Unfortunately, there is still much disagreement among different 

 laboratories in the estimation of the strength of a given source, and 

 the method of estimating source strength by jS-7 coincidences is not 

 applicable to one of the commonest elements, P^-, because this iso- 

 tope emits no 7 rays. It is therefore worth noting that if a radio- 

 element is uniformly incorporated in the material of the wall of an 

 ionization chamber the ionization recorded is related to the energy 

 absorbed per gram of wall material in exactly the same way as X-ray 

 ionization and neutron ionization are related to the absorption of pri- 

 mary radiation energy in the chamber wall material. As is the case 

 with X rays it is almost always possible to choose the size of chamber 

 and the gas pressure so that air maj^ be used, and the necessity' for 

 special gas fillings avoided. Failla {^9, p. 637) has investigated the 

 possibiUty of using the "extrapolation ionization chamber" for the 

 measurement of the concentration of radioactive material in solu- 

 tions, and Gray {101) has measured the rate of emission of /3-ray 

 energy by a number of isotopes by cavity-type ionization chambers. 

 The ionization method may well prove to be one of the most satis- 

 factory methods of estimating dose in those cases in which uncertainty 

 exists concerning the form of the jS-ray spectrum, for estimating con- 

 centration of radioactive material, and even for determination of the 

 absolute strength of sources, in millicuries, by the appUcation of equa- 

 tion (26) in those cases in which the form of the /3-ray spectrum has 

 been well established by /3-ray spectroscopy. 



5. Ionization in Tissue 



In the earUer sections of this chapter we frequently spoke of the 

 number of ions formed in tissue by a given dose of radiation and of 

 the number of ions per micron of track. This was done because it is 

 very helpful to our understanding of the biological effects of radia- 

 tion to visualize the approximate position of the ions, and because there 

 are experimental grounds for believing that at least in some cases 

 (c/. the discussion of the production of mutations to phage resistance 



