T.O. 33B-1-16-106.2.4 X-RayGenerators.6.2.4.1.1 TankTypeX-RayGenerators.Tank-type units are usually small and light in weight for ease of portability. The entire high voltage circuit is housedin a single housing, which is commonly known as the tubehead in portable X-ray units. This arrangement avoidshaving to transmit high voltage from the high voltage transformer to the X-ray tube by means of insulated conductors.The housing contains the X-ray tube, the high voltage transformer, and the filament transformer. Electrical insulationis usually by transformer oil or compressed insulating gas. The control box is a separate unit that can be positioned atsome remote distance to protect the operator from radiation. Different circuit designs are used in various tank-typegenerators.6.2.4.1.2 SeparatecomponentUnits.Separate component units are those units where the transformers are separated from the X-ray tube. The high voltageand filament connections are made between the transformers and the X-ray tube through insulated cables. These unitsoffer the advantage of ease of positioning the X-ray tube. The tube is contained in a protective housing with adequateinsulation for the high voltages to be applied to the tube. These separate component units are usually fixed installationsand parts to be inspected are transported to the X-ray equipment. Size or weight of this equipment is not of importancebecause they are usually intended for radiography in a shielded facility.6.2.4.2 ComponentsandPropertiesofanX-RayTube.The X-ray tube houses the cathode (negative terminal) and the anode (positive or ground terminal) under high vacuum.Traditionally the tube has been a glass envelope with a reduced thickness at the window, the point where the X-raysexit, to reduce X-ray absorption. The high vacuum reduces the problem of the electrons colliding with, and beingabsorbed by, molecules of air and provides electrical insulation between the cathode and anode. In some designs aberyllium window is incorporated to further reduce absorption of the X-ray beam, particularly the lower energies. Inmany applications glass envelopes are being replaced by metal-ceramic envelopes. These tubes usually involve a metalcylinder with a ceramic disk at each end to hold and insulate the cathode and anode assemblies. The metal-ceramictube is more durable than the glass tube and is less susceptible to thermal and mechanical shock.6.2.4.2.1 Cathode.A structure known as the cathode serves as the electron source. (See Figure 6-8). Actually, it is a filament or coil ofthoriated tungsten wire that emits electrons when heated to a high temperature. But because the filament gives offelectrons in all directions, some means must be used to focus them on a target. A reflector or focusing cup within thecathode structure, into which the filament is centered, serves to focus the electron beam much as light is focused by aflashlight reflector.Figure 6-8. Fundamentals of X-Ray Tube.
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