Tank Type X-Ray Generators.
Tank-type units are usually small and light in weight for ease of portability. The entire high voltage circuit is housed
in a single housing, which is commonly known as the tubehead in portable X-ray units. This arrangement avoids
having 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 insulation
is usually by transformer oil or compressed insulating gas. The control box is a separate unit that can be positioned at
some remote distance to protect the operator from radiation. Different circuit designs are used in various tank-type
Separate component Units.
Separate component units are those units where the transformers are separated from the X-ray tube. The high voltage
and filament connections are made between the transformers and the X-ray tube through insulated cables. These units
offer the advantage of ease of positioning the X-ray tube. The tube is contained in a protective housing with adequate
insulation for the high voltages to be applied to the tube. These separate component units are usually fixed installations
and parts to be inspected are transported to the X-ray equipment. Size or weight of this equipment is not of importance
because they are usually intended for radiography in a shielded facility.
Components and Properties of an X-Ray Tube.
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-rays
exit, to reduce X-ray absorption. The high vacuum reduces the problem of the electrons colliding with, and being
absorbed by, molecules of air and provides electrical insulation between the cathode and anode. In some designs a
beryllium window is incorporated to further reduce absorption of the X-ray beam, particularly the lower energies. In
many applications glass envelopes are being replaced by metal-ceramic envelopes. These tubes usually involve a metal
cylinder with a ceramic disk at each end to hold and insulate the cathode and anode assemblies. The metal-ceramic
tube is more durable than the glass tube and is less susceptible to thermal and mechanical shock.
A structure known as the cathode serves as the electron source. (See Figure 6-8). Actually, it is a filament or coil of
thoriated tungsten wire that emits electrons when heated to a high temperature. But because the filament gives off
electrons in all directions, some means must be used to focus them on a target. A reflector or focusing cup within the
cathode structure, into which the filament is centered, serves to focus the electron beam much as light is focused by a
Figure 6-8. Fundamentals of X-Ray Tube.