406 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



It was in effect stated above that energy absorbed and ionization produced 

 per unit volume are absolute measures of tissue dose. While this is strictly 

 true within the limits of the definitions of the various dose units, there still 

 remain important uncertainties in the actual measurement of ionization under 

 certain conditions and in both the physical and biological significance of the 

 energy absorbed. 



The first difficulty is encountered only in measurements of ionization 

 produced by heavily ionizing particles such as alpha particles and fission 

 fragments. The ionization produced by these particles is extremely intense 

 since their entire energy is spent in ionizing the relatively small volume of 

 tissue enclosing the short path of each particle. The resulting high ion 

 density presents a difficulty in measuring dose with an ionization chamber 

 in that partial recombination tends to take place among ions of opposite 

 charge before they have been separated and swept out of the gas volume of 

 the ionization chamber by the electrostatic collecting field. It is likely that 

 something less than the total ionization produced by multiply-charged 

 particles is actually detected, whereas, for beta particles whose ionization 

 is more dispersed, this is not the case except at very high pressure (> 10 atm). 



The local high density of ionization caused by these radiations also leads 

 to more profound biological and clinical effects than are observed for the 

 same energy absorption or ionization produced per cubic centimeter of tissue 

 exposed to beta particles and gamma rays. Greater biological effectiveness 

 of a similar magnitude is also found for protons, deuterons, and for fast and 

 slow neutrons. Indeed, there is reason to believe that for the same dose the 

 biological effectiveness of beta particles varies with energy, although to a 

 lesser extent than the difference between beta particles and protons or alpha 

 particles. It is also well known that even for electromagnetic radiation 

 clinical effects vary with the energy composition of x-ray beam. At the 

 present time these effects can only be lumped under the term relative biologi- 

 cal effectiveness (R.B.E.) which expresses the ratio of gamma-ray dose to 

 the dose that is required to produce the same biological effect by the radiation 

 in question. The evaluation of this factor for various radiations is not easy 

 nor can it be extrapolated with certainty from one tissue to another. This 

 is due primarily to the difficulty in evaluating biological effects quantitatively 

 and because of the striking variation in the magnitude of the effects produced 

 in different tissues and animals. The R.B.E. of protons, deuterons, alpha 

 particles, and neutrons has, however, been extensively investigated in certain 

 species of animals, notably the mouse and rat. For these mammals, and 

 possibly also for man, the R.B.E. of protons, deuterons, and neutrons is 

 about 4, while for alpha particles it has a value of about 10, i.e., 0.1 rep of 

 alpha particles produces about the same biological effect as 1 r of x-rays 

 (x-rays not photoelectrically converted). Considerable caution must be 



