4 BIOPHYSICALLY ACTIVE X-RAYS 



radiator. It can be designed to operate over a wide range of energy 

 ratings; filament current range 3.5 to 5.0 amp, tube current from 25 to 

 150 ma, and potentials from 30 to 100 kv. 



Fig. 1-2. A fluoroscopic and radiographic x-ray tube. It will operate at 30 ma 

 for long exposures and will take 100 ma at 85 kv for i 1 ^ second. Air-cooled radiator, 

 Pyrex glass bulb. 



In an x-ray tube of the stationary-anode type, the electron stream 

 is directed against a fixed area on the target which is known as the focal 

 spot. The rating of a tube is limited by the ability of the target to 

 dissipate the heat generated by the colliding electrons. In practice, 

 the magnitude of the voltage, electron current, and time must be such 

 that the bombarding area is not brought too close to the melting point. 



Fig. 1-3. A rotating-anode Coolidge (RT 1-2) tube in which a large disk of 

 tungsten replaces the conventional massive copper anode with tungsten button 

 insert. The disk rotates during exposure. Anode speed 3000 rpm. Effective focus 

 1 mm 200 ma, 73 kv peak, exposure i^ sec. A stationary anode with effective focus 

 3.8 mm uses 200 ma, 82 kv peak, exposure 2*s second. (By courtesy of the General 

 Electric X-Ray Corporation.) 



If it were possible to move the hot metal target out of, and a cool metal 

 target into, the electron path, the time of operation of the tube could 

 be extended. This is what is done in the General Electric Company's 

 rotating-anode tube, Fig. 1-3. The target in this tube is a disk of tung- 

 sten fastened to the end of a short shaft of the rotor of an induction 

 motor, mounted in ball bearings. Outside the glass wall of the tube 

 surrounding the circumference of the rotor is the stator of the motor. 

 A 60-cycle current energizes the motor and rotates the target at about 

 3000 rpm. During excitation the target rotates so that relatively cool 



