INFLUENCE OF LINEAR ENERGY TRANSFER 319 



set in motion by X rays is based upon ionization measurements in air. 

 The accepted unit of air ionization is the roentgen (r), which was defined 

 in 1927 for X rays and redefined in 1937 for both X rays and 7 rays by 

 international committees. According to the definition of 1937,- "the 

 roentgen shall be the quantity of X or 7 radiation such that the associated 

 corpuscular emission per 0.001293 g of air produces, in air, ions carrying 

 1 esu of ciuantity of electricity of either sign." This corresponds to 

 2.08 X 10^ ion pairs per 0.001293 g of air, or 1.61 X lO^^ ion pairs per 

 gram. Since the average energy absorbed by air, per ion pair produced 

 by electrons, is 32.5 electron volts, the energy absorbed per gram of air 

 per roentgen is 5.24 X 10^^ electron volts (83.8 ergs). From this value 

 the energy absorbed per gram of tissue from a given photon beam of 

 known spectrum can be calculated if the atomic composition of the 

 tissue is known.* 



Beta Rays. In the pertinent |8-ray investigations the source of radia- 

 tion has been P*^ or radium (sometimes radon) in equilibrium with its 

 /3-emitting decay products. 



Three methods of dosage estimation have been used. In one the ion- 

 ization per unit mass of air is directly measured by means of special small 

 ionization chambers with thin walls containing only elements of low 

 atomic number (Crabtree and Gray, 1939; Zimmer et al., 1937). The 

 energy absorbed in tissue is then calculated from the energy absorbed in 

 air and the stopping power of tissue relative to air. 



The second method (Lea et al., 1936) is not so direct. First, the 

 number of ^ particles traversing a small volume of air at the position of 

 the biological material is calculated from the /3 emission and the geo- 

 metrical relationships between the radon source and the irradiated 

 material. This calculated number of 13 particles is then multiplied by 

 an estimated average value for the ionization per 5 track in the selected 

 small volume of air. From the product of these two quantities the 

 ionization per unit volume of air is calculated, and transition from ioniza- 

 tion in air to energy absorbed in tissue is then made as above. 



The third method (Raper et al., 1951) is similar to the first except that 

 the ionization in air is directly measured by means of an extrapolation 

 chamber (Failla, 1944). 



2 See Am. J. Roentgenol. Radium Therapy, 39: 295-298 (1938). 



3 Sometimes the absorption coefficients of air and of certain atoms in tissue vary 

 with wave length to considerably different degrees. For example, in a tissue of fairly 

 representative atomic composition the energy absorbed in tissue per roentgen may 

 vary as much as 15 per cent when the effective wave length is varied from 0.014 A 

 (Ra 7 rays) to 0.363 A (soft X rays) (Lasnitzki and Lea, 1940). If the tissue of 

 interest contains an exceptionally large amount of atoms heavier than oxygen (e.g., 

 skin, which is high in sulfur), the variation of energy absorption with wave length may 

 be much greater (Gray, 1940). 



