212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
Charge Removing Points . F 
High Voltage Terminal 
Generating Voltmeter Steel Pressure Tonk 
Tungsten Filament 
X-Ray Tube 
Accelerating Diaphragms 
Insulating Rings 
Equipotential Planes Resistance Potential Divider 
Electron Beam 
Charge Conveyer Belt 
Driving Motor Ground Plane 
Charge Sprayer Points 
Transformer Rectifier 
for Charge Spraying 
Water-Cooled Gold Anode 
Lead Diaphragm System 
lonization Chamber for 
Measuring X-Ray Dosage 
ea X-Ray Beam 
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1 1 
Ficure 1.—Diagram of an electrostatic X-ray generator. Following the original suggestion 
of R. J. Van de Graaff, electric charge is transferred to the high-voltage terminal at a 
continuous rate by a belt of insulating material. This charge maintains the terminal at a 
steady negative potential with respect to ground, and supplies the electrons required by 
the acceleration tube. Both tube and belt are mounted within the insulating column 
which is especially designed to insure a uniform voltage distribution between terminal and 
the ground plane. Electrons emitted from the hot tungsten filament at the high-potential 
end of the tube are progressively accelerated and focused as they pass along the acceleration 
tube and thus acquire the full potential of the terminal. On impact with the water-cooled 
ed anode, the kinetic energy of the electrons is transferred into penetrating radiant energy. 
ince this radiation is emitted in all directions, a lead diaphragm system is used to absorb 
the unwanted radiation and to define the beam of ionizing radiation used in therapy. 
Except for the interior of the highly evacuated X-ray tube, the entire apparatus is insulated 
by compressed gas at a pressure of 27 atmospheres. It is this feature which permits these 
instruments to be built with unusual compactness. 
has suffered from the limitation that 
the radiation must enter through the 
relatively radiosensitive skin and 
undergo very considerable attenuation 
program the physical properties of 
these radiations and their action on 
nonliving matter are actively followed. 
An important therapeutic property 
of these supervoltage roentgen rays is 
the unusually high skin tolerance 
which almost completely eliminates 
skin damage in the treatment of deep 
tumors. In the past, X-ray therapy 
and misdirection by the absorption 
and scattering processes before it 
reaches the site of a deep tumor. 
Many of these difficulties are reduced 
by the skill of the radiologist in the 
