248 Applied Biophysics 



when used at 100 centimeters. The advantages, however, out- 

 weigh this imperfection, and in the future a Hght secondary 

 diaphragm may be added. 



Inside the outer sheath (a) on which is mounted the dia- 

 phragm, is a protective lead cyHnder (d), which itself surrounds 

 the steel vacuum envelope of the tube (e). This lead cylinder, 

 which weighs 8 tons, gives an effective protection of 6 inches 

 of lead in any direction relative to the focal spot on the target 

 (f). The protection is so effective that with the tube operating 

 at full excitation — one million volts 4.5 milliamperes — the X-ray 

 leakage into the treatment room is only one half of tolerance 

 dose (10^ rontgens per second), a degree of protection rarely 

 encountered in 200 kilovolt tubes. The lead cylinder is also 

 used as the X-ray shutter of the tube. There is one aperture 

 in the lead cylinder opposite the target head, which aperture (g) 

 in the safe position points upwards into a six-inch-thick lead 

 block suspended from the treatment room roof. This block pre- 

 vents the emergence of the X-rays upwards into the treatment 

 room. Providing the treatment room doors are shut, the whole 

 of the lead cylinder can be made to rotate by pushing a control 

 button in the control room, and by automatic interlocks it stops 

 rotating when its aperture is aligned to that of the diaphragm 

 on the outer sheath, so permitting the emergence of the X-ray 

 beam in* the required direction through the diaphragm stops. 

 Just behind the diaphragm is mounted a three-plate ionization 

 chamber (i), which indicates on an instrument on the control 

 desk either the X-ray intensity or the dose given during an 

 exposure. Mounted on tlie control desk are also direct-reading 

 kilovoltmeters, indicating the actual kilovoltage applied to either 

 end of the tube and the sum of these, irrespective of load current. 

 These are electrostatic voltmeters which operate from a definite 

 proportion of the kilovoltage applied to each end of the tube, 

 obtained from oil-immersed resistance potentiometers connected 

 from each end of the tube to earth. 



The high voltage for the tube is supplied by two 500 kilovolt 

 Cockcroft zb d.-c. generators, comprising transformer, con- 

 densers, and four continuously-evacuated thermionic rectifiers 



