IONIZATION AND BIOLOGICAL EFFECTS 119 



The fact that the roentgen is a unit of quantity of radiation — albeit 

 in a restricted sense — has tended to emphasize unduly this factor. One 

 often finds the statement that the quantity of radiation (usually referred 

 to as the dose) administered to a certain patient or organism was so 

 many roentgens, omitting any reference to the duration of the treatment. 

 The same quantity of radiation may be administered, of course, in a short 

 or long time, and the biological effect in general is not independent of the 

 duration of the irradiation. For this reason it is necessary to state also 

 the time during which a certain quantity of radiation was delivered to the 

 test object. From this point of view it is preferable to think of (effective) 

 intensity of radiation, expressed in roentgens per minute, as the prime 

 factor. Then a treatment may be said to have been given by using an 

 intensity of radiation of so many roentgens per min. for a certain number 

 of minutes. By this notation less confusion is apt to occur also in another 

 sense. Quantity of radiation as defined here refers only to 1 cm.^ of the 

 beam cross section (or 1 cm.^ of air according to the definition of the 

 roentgen^i). Therefore, the quantity of radiation expressed in roentgens 

 in the specification of a radiation treatment does not represent the total 

 quantity of radiation falling on the entire object, but the quantity 

 delivered per square centimeter. This restriction is visualized more 

 readily when one speaks of the exposure of a living organism to a certain 

 intensity of radiation for a certain length of time. 



In view of the numerous restrictions and qualifications which must be 

 borne in mind when the foregoing terminology is employed, it is not 

 surprising that considerable confusion has resulted therefrom. In this 

 paper the factor w^hich has been emphasized most strongly is the 

 ionization in the biological material itself, since, so far as we know at 

 present, this is the primary factor which controls, directly or indirectly, 



21 The effective intensity of radiation at a certain point in a beam need not be, and 

 in general is not, determined by measuring the rate of ion production in exactly 

 1 cm.^ of air. The question then arises as to whether the same result is obtained 

 irrespective of the shape and size of the volume in which the ionization is measured. 

 In the case of the standard chamber (cf. L. S. Taylor) this is very nearly so, provided 

 the radiation is so penetrating that the decrease of intensity due to the thickness of 

 air traversed in the volume is negligible, and the intensity throughout the cross section 

 of the beam which is utiUzed is practically constant. In the case of the closed chamber, 

 the thickness, at least, of the air volume should be small in comparison to the distance 

 from the source at which the measurement is made, in order to avoid too great a differ- 

 ence between the intensity of radiation at the point of incidence and that at the point 

 of emergence. To minimize further errors due to the variation of intensity with 

 distance, the small ionization chamber should be placed with its center at the point 

 for which the measurement is desired. (It might be mentioned that the effective 

 center of an ionization chamber is not its geometrical center. However, the factors 

 which determine the effective center are so complex that the only practical thing 

 to do is to use its geometrical center for this purpose.) 



