IONIZATION AND BIOLOGICAL EFFECTS 117 



enable the investigator or others to make a rough estimate of the tissue 

 ionization (amount and distribution) at a subsequent time, when more 

 information may be available. Strictly quantitative experiments are 

 more exacting. Since it is simpler to insure a substantially uniform 

 distribution of radiation within the material than to determine its effec- 

 tive intensity from point to point, the experiment should be planned 

 with this idea in mind. A given object is irradiated more uniformly: 

 (a) the longer the distance from the source at which it is placed (this 

 follows from the inverse square law, as already explained) ; (6) the more 

 penetrating and homogeneous the radiation used; (c) the smaller the 

 dimensions of the object to be irradiated, particularly the thickness 

 traversed by the rays. When radium is employed as the source of radi- 

 ation, the amount available is usually too small to permit its use at long 

 distances. It is then especially important to use a thin layer of the 

 material to be treated. Better distribution of radiation may be obtained 

 by placing the test object between two equal sources suitably placed. 

 In the case of gamma rays one may obtain a fairly close estimate of the 

 distribution of radiation by means of the inverse square law alone, and 

 from that decide on the distance at which the material should be placed. 

 The determination of the ionization produced in a biological material 

 is, in general, a difficult matter, because all the necessary information is 

 not readily available. The biologist should exercise great care in so 

 doing, and should preferably consult a physicist who is familiar with the 

 subject. In addition, he should describe the process by which the tissue 

 ionization was arrived at and give all the primary factors of the irradi- 

 ation (voltage, milliamperes, distance, etc.). The desirability — indeed 

 the necessity — of stating the effective intensity of radiation at the beam 

 cross section where the material was placed is obvious. However, it 

 should be remembered that the ionization chamber by which this is 

 measured, may influence the result markedly. In this connection it is 

 advisable to state the type of instrument used, also how, by whom, and 

 when it was calibrated. 



EFFECTIVE QUANTITY OF RADIATION 



This brings us to the next topic to be discussed here, but a brief 

 digression is necessary to introduce the subject properly. The inter- 

 national unit of X-radiation — the roentgen — is a unit of "quantity" of 

 radiation. Since nothing has been said about this so far, some explan- 

 atory remarks are in order. The distinction between true and effective 

 intensity of radiation, which has already been brought out, is also 

 involved in this case. A source which emits radiation at a constant rate 

 delivers energy to a certain point in space at a constant rate, and therefore 



