T.O. 33B-1-1
6-17
6.2.5.2.3
Choice Of Focal Spot Size.
X-ray tubes are available with different focal spot sizes. The focal spot in an X-ray tube is the area of the target that
produces the primary X-ray energy. (See Figure 6-9). The actual size of the focal spot is determined by the electron
bombardment pattern on the target. The minimum size of this area is limited by the melting point of the target
material and the concentration of the bombarding electrons per unit area. Tungsten is most often used as target
material because of its high melting point, 6098 F, and high efficiency of x-ray production. An effort is made in X-
ray tube design to achieve the smallest possible focal spot consistent with voltage and current required, melting
temperature of the target material, and field coverage needed. The smaller the focal spot size, the sharper the
radiographic image. It is normal to expect a focal spot size of the order of 2 to 10 mm (millimeters), in the voltage
range of 100 to 2,000 kVp. For special application, equipment with focal spots less than 1 mm in diameter are
available. X-ray tubes with dual focal spots are often used so the operator can choose the focal spot size and
operational conditions compatible with the demands of inspection quality. New X-ray machines are also available with
focal spots called mini-focus (spot size in the range of 0.2 to 1 mm) and micro-focus (spot size in the range of 0.002 to
0.025 mm). These new small focal spot X-ray units provide excellent image sharpness and can also be used to enlarge
the X-ray image geometrically.
6.2.5.3
Equipment Protective Devices.
X-ray apparatus must be not only safe to use, but it must also be protected against damage through inadvertent misuse.
To accomplish this objective, X-ray equipment should have protective devices as discussed in the following paragraphs.
6.2.5.3.1
The overload thermal circuit breaker usually incorporated in the main line switch, provides protection to the equipment
should a component failure be encountered. This protection assures that the thermal circuit breaker will disconnect the
unit from the power supply before extensive damage is done to the control or X-Ray head.
6.2.5.3.2
The over voltage protection circuit can be accomplished either by spark gaps set to arc at the over voltage point, or by
means of a voltage sensitive relay in the control circuit of the high voltage section. Sometimes both methods are used
since it is possible that under extreme conditions of surges the over voltage relay circuit may not react. This eliminates
the possibilities of voltage damage due to operator carelessness or component failure.
6.2.5.3.3
There is also the possibility of inverse voltage damage in a high voltage X-ray circuit. This becomes a problem when
the line conditions vary widely, as is possible when using X-ray equipment in shop, field or factories. A circuit called
the inverse voltage suppressor, consisting of a resistor and rectifier network in the primary winding of the transformer,
is used to protect X-ray equipment under these conditions.
6.2.5.3.4
An over-current fuse is used in the control circuit of the filament supply to prevent damage to the tube due to incorrect
usage of the equipment or component failure. The alternative is to design components in which the combination of
variables will not result in damage to the unit. This is not desirable when attempting to achieve maximum utility in a
design.
6.2.5.3.5
Using the maximum safe working temperature of materials results in maximum efficiency from those materials.
Therefore, it is necessary to prevent over-temperature to materials such as oil and solid insulation used in high voltage
X-ray circuits. To accomplish this, an over-temperature thermostat installed in the X-ray head prevents damage to
those materials.
6.2.5.3.6
When using gas as insulation material, it is also necessary to provide pressurestats in the X-ray head to prevent
operation and consequent damage to the equipment should the gas pressure be below the safe level for insulation of the
high voltage parts. Flow switches and pressurestats in the oil and water circulators are also used to prevent operation of
