XIV. X RAYS AND X IRRADIATION 455 



well as for diagnostic and therapeutic uses. Their advantages are 

 better control, lower maintenance, and constant readiness for use. 

 The thermionic emission of electrons from the cathode, which consists 

 of a filament heated to incandescence by an electric current, pei-mits 

 convenient control of the X-ray output. The electrons are focused 

 by the cathode to strike the anode at a small spot. The X rays pass 

 out through the thin wall of the tube or through a special window . 

 The Coolidge type tubes are now available in a wide range of designs 

 and power ratings. Water-cooled models are preferred for heavA- 

 duty continuous operation. They permit higher dosage rates in thai 

 the samples can l)e placed closer to the anode, thus avoiding much loss 

 in intensity from the inverse square law factor. The form of the 

 radiation intensity versus wavelength curve depends on several fac- 

 tors besides the applied voltage to the tube. These will be discussed 

 later. Ordinarily, voltages are confined within the range of 30 to 250 

 kilovolts. Lower voltage tubes with especially thin windows are 

 also available for special purposes. 



For higher voltages special type equipment is available in some 

 laboratories and hospitals, as, for instance, the Sloan tube and genera- 

 tor. These tubes are capable of delivering 850 to 1000 kilovolt X 

 rays. The radiation is general, covering all wavelengths up to the 

 limit of the exciting voltage. The Sloan apparatus (5, p. 85) in- 

 volves an oscillator circuit with auxiliary tubes similar to those of a 

 short wave broadcasting set. For still other tvpes of tu})es, see Clark 



Betatrons and synchrotrons are two of the newer types of equip- 

 ment from which it is possible to obtain X-ray radiation of 10-100 

 million volts. Their use in biological experiments is just beginning. 

 Current physics periodicals must be consulted for details. Theii* 

 widespread use is expected, at least for industrial piuposes. 



When X radiation is obtained from a qualified physical labora- 

 tory, there probably will be no difficulty in the operation of the ap- 

 paratus or in obtaining the energy output. For the investigator 

 who wishes to do experimental work using hospital equipment the 

 difficulties may be greater. In general, he must acquaint himself 

 with the type of X-ray machine and its suitability for the purpose he 

 has in mind. One point to be carefully taken into consideration is 

 that for the treatment of biological materials the machine should be 

 capable of continuous operation over the necessary time without 

 damage to the tube or its power supply. This requirement precludes 



