110 BIOLOGICAL EFFECTS OF RADIATION 



EFFECTIVE INTENSITY OF RADIATION 



We have seen how two sources of monochromatic radiation of widely 

 different quality (ordinary X-rays and gamma rays), which deliver the 

 same intensity of radiation at the point where the biological material is 

 placed, do not produce the same ionization in the material. It has been 

 explained that this results from the fact that the more penetrating radia- 

 tion passes through the material much more easily and leaves behind 

 very little energy in the form of ionization. Exactly the same thing 

 happens in the ionization chamber (cf. L. S. Taylor, Paper II) of a meas- 

 uring instrument. The ionization produced therein represents a certain 

 quantity of energy which was derived from the beam of radiation, but it 

 tells us nothing about the total amount of radiant energy which was 

 flowing through the chamber at the time of the measurement. In other 

 words, the reading of the instrument per se gives us no indication of the 

 intensity of the radiation. Nevertheless, in the art this is what is meant 

 by the intensity of a beam of X-rays. In order to avoid confusion we 

 shall refer to it here as the effective intensity. The distinction between 

 the two must be kept clearly in mind. To reiterate, the true intensity 

 of radiation at a given point in the beam represents the amount of radiant 

 energy passing through a surface of unit area perpendicular to the direc- 

 tion of travel of the photons at the point in question per unit time. On 

 the other hand, the effective intensity of radiation at a given point in the 

 beam represents that part of the radiant energy passing through 1 cm.' 

 of air in the ionization chamber per unit time which is required to pro- 

 duce the observed number of ion pairs per cubic centimeter of air per 

 unit time at the point in question. We have called this the "effective" 

 intensity because it is the attribute of a beam of radiation which deter- 

 mines the degree of ionization (that is, the number of ion pairs per cubic 

 centimeter per second) produced in air. In view of the similarity between 

 the atomic numbers of the principal components of air and living tissues, 

 this is also the attribute of a beam of radiation which is of chief interest 

 to us. Accordingly in practice we need not concern ourselves with the 

 true intensity of radiation. For this reason and the fact that ionization 

 measurements always determine something more or less closely related 

 to the effective intensity, it is customary to refer to it simply as the 

 "intensity." This would cause no trouble if its significance were always 

 borne in mind. Unfortunately, however, the two meanings of the term 

 are often confused. Perhaps a simple analogy in the realm of visible 

 light, with which we are more familiar, will serve to clarify our ideas. 

 We may think of the intensity of light as the factor which determines 

 how easily we can read a printed page. Projecting on the paper, suc- 

 cessively, lights of different colors ranging from red to violet, we may find 



