XIV. X RAYS AND X IRRADIA'JION 



457 



other existed. This gap has been filled through the work on Droso- 

 phila eggs (4-8,44) followed by that of genetics workers on gene muta- 

 tion and on survival of Drosophila sperm (26). The measurements 

 are in agreement and show that the total energy absorbed in the tis- 

 sues as the X rays pass thi-ough them is the important element in 

 producing the effects. The physical unit of X radiation, r. unit or 

 roentgen, is the quantity of X rays "such that the associated corpus- 

 cular emission per 0.001293 gram of air produces, in air, ions carrying 

 1 electrostatic unit of quantity of electricity of either sign." The 

 dose in r. units, therefore, is not a measure of the total energy in an 



L M N 



Fig. 3. Sectional view of an X-ray dosimeter oi' r. meter (courtesy 

 Victoreen Instrument Co.): A, chamber tube containing condenser 

 and chamber; B, chamt)er tube cap (for end opposite chamber); C, 

 ionization chamber; D, insulating button to cover insulation at E; 

 F, discharge switch; G, static charger wheel; H, scale; I, objec- 

 tive; J, ocular; L, lamp; M, battery; N, light switch. 



X-ray beam, but only that fraction which can be dissipated by the 

 ionization process in a thin layer of air. This can be related to the 

 energy dissipated in tissue by ionization. For a further discussion, 

 the reader should consult the following chapter. 



A diagram of a small dosimeter based on this ionization principle 

 is shown in Figure 3. The ionization chamber is a small thimble- 

 shaped structure at the end of the tube. Its walls are easily pene- 

 trated by X rays, the charge being taken by the wire to the electro- 

 scope. The current causes the electroscope to discharge ; the amount 

 of the discharge is measured on an r. unit scale, which is read by look- 

 ing through the microscope. This type of dosimeter must be cali- 

 brated for the quality of X rays absorbed, however, by comparison 



