Choice of Radiation Energy. The relation of X-ray voltage to the penetration for steel or other common materials depends upon the density of the material and the absorption characteristics of the material in the X-ray beam. In general, Table 6-2 can be used as a guide for applying X-rays to inspection problems, assuming average radiographic results are expected. It is necessary to establish lower limits as well as upper limits on material thickness because using voltages higher than what is required to penetrate a given thickness will reduce the radiographic contrast.
1 Choice of Tube Type. The choice of a directional or a rod anode tube type should depend upon the type of radiographic inspection conducted. Circumferential specimens, such as pipe weldments, are compatible with the rod anode radiation. It should be noted that scattered radiation is greater with the rod anode and additional personnel protection is often necessary. The directional X-ray tubes restrict the radiation to a smaller area and have a comparatively smaller focal spot resulting in
better quality radiographic images.
2 Choice of Window. When the X-ray absorption of a test object is low, lower energy radiation is required. To take advantage of the higher contrast provided at lower energies, an X-ray tube with a beryllium window should be used since beryllium transmits the low energy radiation. Up to 150 kVp the beryllium window offers advantages. Above 150 kVp the typical glass window should prove satisfactory. Therefore, radiographic inspections using 150 kVp or less SHALL use a beryllium window X-ray tube. The beryllium window and the resultant soft (low energy) spectrum SHALL also be used for the inspection of composite laminates. For example, a graphite-epoxy composite laminate 0.100-inch thick might require the use of an X-ray energy in the order of 10-20 kV for optimum sensitivity. (Reasonable exposures with standard portable X-ray equipment are often difficult below 25-30 kV.) It should be noted that, at X-ray energies of 15 kV or less, the air between the source and object would scatter the X-rays. If the X-ray equipment will operate that low, one way to displace the air is to stuff a helium-filled plastic bag between source and object.
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.
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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 the X-ray unit when proper cooling is not available for the unit. The degree of dependability of the equipment will be determined by the type of protection provided in the unit.
Source: KARTA TECHNOLOGY, INC
