Sec. 16.1] INTERNAL DOSIMETRY 407 



exercised in extrapolating these values to other animals or to specific tissue 

 since, as has been illustrated by extensive experiments with neutrons, values 

 of R.B.E. varying from 0.8 to as much as 30, depending upon the neutron 

 energy and substance irradiated, have been reported by various investigators. 



The precise physical significance of the absorbed energy is not as yet 

 entirely clear. The greater part of the energy absorbed from incident radia- 

 tion is perhaps utilized in the production of ions in the medium, but there is 

 also reason to believe that an appreciable fraction of the absorbed energy is 

 taken up by processes in which no ions are formed, such as excitation of atoms 

 and molecules, decomposition of complex molecules, and the formation of 

 certain substances as in water exposed to x-rays. Although the influence of 

 these processes is not yet well known, they do not directly affect the practical 

 problems of dosimetry since the term dose as used in radiobiology means 

 essentially a measure of the energy absorbed without regard to the manner 

 in which it is absorbed. Ionization, therefore, serves as an empirical measure 

 of energy absorbed. 



The empirical determination of dose received by tissue exposed to radiation 

 requires some means for ascertaining the density of ionization within the 

 tissue. This is made possible by measurement of the ionization produced 

 in the gas of a small cavity ionization chamber placed at the point under 

 consideration. A discussion of the principles of the cavity chamber is given 

 in Sec. 12.2, based on the detailed description of the theory and application 

 of such chambers to gamma rays and neutrons reported by Gray [12,18]. 

 In brief, it consists of a small gas-filled cavity with linear dimensions that 

 are small compared to the range of the secondary beta particles in the gas. 

 If the cavity walls are tissuelike in atomic composition or, if differing from 

 tissue, they are made extremely thin and surrounded with tissue, the energy 

 absorbed in the tissue per cubic centimeter of tissue at the point where the 

 cavity is placed is related to the observed gas ionization by the simple 

 relation 



E = JWS ergs/cc/sec 



where / = number of ion pairs formed in gas per cc per sec 



II" = average energy absorbed to form one ion pair in gas (about 32.5 ev 



for electrons in air) 

 5 = stopping power of wall material (tissue) relative to the gas 

 The factor S is given by the ratio N t B t /N u B g , where B t and B u are the stop- 

 ping numbers of tissue and gas, respectively, computed from Bethe's stopping 

 formula, and A T f and A r y are the respective numbers of atoms per unit volume 

 of tissue and chamber gas. If the atomic composition of the chamber gas 

 is made similar to that of tissue, then B t = B fl and the factor S is simply the 

 ratio of the number of atoms per unit volume of tissue to the number per unit 



