44 BIOPHYSICALLY ACTIVE X-RAYS 



bination. For this purpose a field strength of about 150 volts/cm is 

 sufficient. 



Since the degree of ionization is determined by the mass absorption 

 coefficient of x-radiation by the gas, corrections must be made for the 

 temperature and pressure of the air in the chamber, for by definition the 

 air must represent conditions at 0° C and 1012.9 millibars (760 mm) 

 pressure. 



Under these circumstances, if I is the current, measured by the elec- 

 trometer, in electrostatic units, L the effective length of the collector 

 electrode P2, A the area of the limiting diaphragm in square centimeters, 

 T the absolute temperature, and p the pressure in millimeters of mer- 

 cury, then the intensity of the x-ray beam as measured in roentgens per 

 second is 



_r_ _7_ T 760 



sec ~ L X A ' 273 ' p 



Thus, if the area A and the length L are each unity, so that we are 

 considering 1 cc of air at T = 273° K, and if p = 760 mm, then r/sec 

 = I. If now 7 is 1 esu of current (1/30,000 microampere), then, since 

 Q = It, r has the dimensions of quantity. The unit of quantity of 

 radiation defined this way is a unit of dosage. In order to get a clearer 

 conception of the magnitude of 1 roentgen, we may choose a technical 

 x-ray tube with glass walls, driven at 100-kv constant potential and 

 5-ma current. At a distance of 2 meters, using no filter, this tube emits 

 in 1 second about 0.1 r, and at 180 kv its intensity is about 0.2 r/sec. 



1 r is equal to 1 esu X 1 second 



Small Ionization Chambers 



For therapeutic or biological purposes it is, of course, impracticable to 

 handle such a large instrument as the one discussed above, because it 

 lacks mobility. A portable instrument has been developed with a small 

 ionization chamber, an electrostatic current-measuring device with 

 calibrated scale, and an electrostatic charger for supplying the potential 

 for the saturation current measurements, all housed as a self-contained 

 unit. 



The ionization chamber may be cylindrical, with the ion collector in 

 the form of a coaxial rod of graphite. The outer shell of the ionization 

 chamber is usually some very thin material comparable to an " air wall," 

 as for instance Bakelite, whose effective atomic number closely approxi- 

 mates that of air.* This outer shell, with its internally conducting layer 



* Effective atomic number of air is 7.69. 



