42 BIOPHYSICALLY ACTIVE X-RAYS 



the beam of x-radiation so that all high-speed electrons may have suffi- 

 cient range in which to dissipate their energy. 



3. A saturation current value must be utilized to measure the ioniza- 

 tion. 



International Unit of X-Ray Quantity. The Roentgen 



In the report of the Committee on the Standardization of X-ray 

 Measurements as published in Radiology, Vol. 22, p. 289, 1934, a defini- 

 tion for a unit effective intensity for biological purpose is set up called 

 the roentgen (" r " unit). It has been accepted in the United States 

 and defined as follows: " The roentgen is the quantity of x-radiation 

 which, when the secondary electrons are fully utilized and the effects of all 

 scattered radiation avoided, produces in 1 cc of atmospheric air at 0° C and 

 76 cm mercury pressure such a degree of conductivity that 1 esu of charge is 

 measured wider saturation conditions."* 



What is wanted in roentgen therapy work or in biological reactions 

 to x-radiation is not the energy content of an x-ray beam but rather the 

 amount that will be utilized in the tissue. For example, suppose that 

 one considers the biological effectiveness of two beams of different wave- 

 lengths. The beam having the greater biological effect is not the one 

 with the greater energy content but the one that will lose the greater 

 amount of energy in 1 cc of tissue. If the total energy content were 

 available, to decide which beam was biologically more effective, it would 

 be necessary to know the distributions of energy in each beam for each 

 wavelength and the coefficients of absorption and scattering for each 

 wavelength. Ionization measurements, however, include all these 

 factors, since an electrometer, introduced into the electrical circuit, 

 indicates a current proportional to the energy absorbed. 



Parallel-Plate Standard Ionization Chamber 



The Bureau of Standards has constructed a standard ionization cham- 

 ber under the direction of L. S. Taylor [1930]. Figure 1-16 shows the 

 assembled chamber diagrammatically. In the standard chamber the 

 plate spacing P1P2 must be sufficiently great so that if any photoelec- 

 trons strike them their contribution to the ionization is negligibly small. 

 It was found that, for a parallel-plate ionization chamber with a 200-kv 



* The Fifth International Congress of Radiology held in Chicago [1937] provi- 

 sionally adopted the following definition: " The roentgen shall be the quantity of X- 

 or gamma-radiation such that the associated corpuscular emission per 0.001293 gram 

 of air produces, in air, ions carrying 1 esu of quantity of electricity of either sign." 

 This definition rules out any possible ionization by scattered x-rays, a matter left 

 uncertain in the original definition. 



