78 



BIOLOGICAL EFFECTS OF RADIATION 



Thus every time a certain definite quantity of radiation has fallen on the 

 ionization chamber, the glow tube functions, and the counting mechanism 

 indicates directly the number of roentgens. The constancy of the elec- 

 trical calibration is tested by connecting into the system a constant cur- 

 rent which is produced by a small uranium oxide ionization chamber. 

 The circuit is then adjusted by means of a variable condenser to produce 

 a redetermined time rate of indication. 



Measurement of X-ray Output. — In the measurement of the output of 

 an X-ray tube at a given point with a thimble chamber there are few 

 precautions which are not self-evident from the preceding discussion. 

 Scattering from nearby objects should be avoided, for example, by keeping 

 the thimble chamber always at least 15 cm. from any filters. There has 

 been some question in biological practice whether or not to include in 

 the measurement the back scattering from the irradiated object. 

 Although a knowledge of back scattering is of interest, its exact measure- 

 ment is very difficult in that it depends, among other factors, upon: (a) 

 size of field, (6) size and shape of irradiated object, (c) size and shape of 

 thimble chamber, (d) material of thimble chamber, and (e) depth to 

 which the chamber is immersed in the surface of the irradiated material. 

 To avoid the complication of these factors, the preferred method of 

 X-ray dosage measurement is to use the thimble chamber placed at the 

 position of the surface of the irradiated object but surrounded by at least 

 15 cm. of air. This gives the intensity of the X-ray beam in roentgens 

 per minute without modification by scattering. 



In view of the foregoing discussions it may be convenient to summarize 

 the factors which it is necessary to state in describing a biological applica- 

 tion of X-radiation. Such factors include at least the following: a. 



Table 7. — Comparative X-ray Outputs for Different Types of Generator 

 Factors: (a) Pyrex tube about 5 mm. thick; (6) 50-cm. target to ionization chamber; 



(c) filter 0.557 mm. Cu + 1.0 mm. Al 



* About the same for half-wave and mechanical rectifiers, 

 t Voltage doubling (Fig. 1, C). 

 t Voltage tripling (Fig. 1, D). 



Note: Outputs are about 9 per cent greater for thin glass tubes. Ratios between outputs are nearly 

 the same as given, for filtrations up to 2.0 mm. Cu. 



