T.O. 33B-1-1
6-12
beryllium which is a light metal of low atomic number and low X-ray absorption. Because of tremendous pressures
exerted by the atmosphere on large evacuated containers, X-ray ports must be designed with sufficient thickness to
withstand these pressures without implosion. In center-grounded X-ray equipment, it is also necessary to provide gas
(e.g., sulfur hexafluoride, SF6) and solid insulation for electrical isolation of the X-ray tube. Excessive inherent
filtration reduces the X-ray output as well as the radiographic contrast on equipment of a given rating. In normal
practice it is acceptable to tolerate inherent filtration equivalent to 1 mm of aluminum up to 100 kVp (kilovolts peak); 3
mm of aluminum up to 175 kVp; 5 mm of aluminum equivalent up to 250 kVp; and higher filtration in 1,000 to 2,000
kVp units. Inherent filtration above these tolerances reduces contrast, and hence, sensitivity of radiographic inspection,
and as a result, limits the sensitivity of inspection, especially on thin sections and light alloys. For this reason, during
radiographic inspections using kilovoltage of 150 or less, the tubehead shall be configured so that generated radiation
will travel from the target through a beryllium window without passing through any media other than air or insulating
gas.
6.2.4.2.5
Cooling Requirements.
The product of mA and kV equals watts of electrical power in the electron beam striking the X-ray target. One watt of
electrical power is equal to one volt-ampere. Therefore, in an X-ray tube operating at 10 mA (or 0.01 amperes) and
140 kV (140,000 volts), 1400 watts of electrical power are in the electron beam. Only a very small amount of the
energy in the electron beam is converted into X radiation. This ranges from about 0.05 percent at 30 kV to
approximately 10 percent in the MeV energy range. Most of the electron beam energy is converted into heat. This
generation of heat in the X-ray tube target material is one of the limiting factors in the capabilities of the X-ray tube. It
is necessary to remove this heat from the target as rapidly as possible. Various techniques are used for removal of heat.
In some instances, the target is comparatively thin, and a suitable oil is circulated on the back surface to remove heat.
Others (where the anode is being operated at ground potential) use water-antifreeze mixture to conduct heat away from
the target. Most X-ray targets are mounted in copper, using the copper as a heat sink. Some units have no external
method of heat removal, but depend upon heat dissipation into the atmosphere by fins of a thermal radiator. Some
totally enclosed tubes depend upon the heat storage capacity of the anode structure to absorb the heat generated during
X-ray exposure. This heat is then dissipated after the unit is turned off. These units usually have a duty cycle as a
limiting factor of operation that is dependent upon the heat storage capacity of the anode structure and the rate of heat
dissipation by thermal radiation. The rate of heat removal from the X-ray target is the primary limiting factor in X-ray
tube operation.
6.2.4.3
Types Of X-Ray Tubes.
6.2.4.3.1
Directional Tubes.
In directional X-ray tubes, the anode is set at an angle to the electron beam. When the high-speed electrons strike the
target, X-radiation is generated in a solid spherical pattern. The massive anode functions as an absorber for the
radiation traveling into the anode. In most X-ray tubes, lead-absorbing materials are used to restrict the exiting
radiation to a cone-shaped field passing through a window. The shielding reduces the leakage radiation hazard to
personnel, and prevents additional scattered radiation from surrounding materials and areas. In some portable
equipment, shielding of the X-ray tube has been omitted for the advantage of lightweight. In some very high-energy
units such as betatrons and linear accelerators, the target is comparatively thin and offers little absorption to the very
high-energy radiation being generated. The radiation beam from the front of the target is shielded to provide a
directional pattern, conical in shape.
6.2.4.3.2
Rod Anode X-Ray tubes.
These tubes are designed to produce a radiation beam in a circular pattern. These tubes are used for circumferential
radiography, particularly weldments in pipe. By use of an absorbing sleeve the circular radiation pattern can be
reduced to a directional beam.
