T.O. 33B-1-16-176.2.5.2.3 ChoiceOfFocalSpotSize.X-ray tubes are available with different focal spot sizes. The focal spot in an X-ray tube is the area of the target thatproduces the primary X-ray energy. (See Figure 6-9). The actual size of the focal spot is determined by the electronbombardment pattern on the target. The minimum size of this area is limited by the melting point of the targetmaterial and the concentration of the bombarding electrons per unit area. Tungsten is most often used as targetmaterial 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, meltingtemperature of the target material, and field coverage needed. The smaller the focal spot size, the sharper theradiographic image. It is normal to expect a focal spot size of the order of 2 to 10 mm (millimeters), in the voltagerange of 100 to 2,000 kVp. For special application, equipment with focal spots less than 1 mm in diameter areavailable. X-ray tubes with dual focal spots are often used so the operator can choose the focal spot size andoperational conditions compatible with the demands of inspection quality. New X-ray machines are also available withfocal 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 to0.025 mm). These new small focal spot X-ray units provide excellent image sharpness and can also be used to enlargethe X-ray image geometrically.6.2.5.3 EquipmentProtectiveDevices.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.6.2.5.3.1The overload thermal circuit breaker usually incorporated in the main line switch, provides protection to the equipmentshould a component failure be encountered. This protection assures that the thermal circuit breaker will disconnect theunit from the power supply before extensive damage is done to the control or X-Ray head.6.2.5.3.2The over voltage protection circuit can be accomplished either by spark gaps set to arc at the over voltage point, or bymeans of a voltage sensitive relay in the control circuit of the high voltage section. Sometimes both methods are usedsince it is possible that under extreme conditions of surges the over voltage relay circuit may not react. This eliminatesthe possibilities of voltage damage due to operator carelessness or component failure.6.2.5.3.3There is also the possibility of inverse voltage damage in a high voltage X-ray circuit. This becomes a problem whenthe line conditions vary widely, as is possible when using X-ray equipment in shop, field or factories. A circuit calledthe 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.6.2.5.3.4An over-current fuse is used in the control circuit of the filament supply to prevent damage to the tube due to incorrectusage of the equipment or component failure. The alternative is to design components in which the combination ofvariables will not result in damage to the unit. This is not desirable when attempting to achieve maximum utility in adesign.6.2.5.3.5Using 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 voltageX-ray circuits. To accomplish this, an over-temperature thermostat installed in the X-ray head prevents damage tothose materials.6.2.5.3.6When using gas as insulation material, it is also necessary to provide pressurestats in the X-ray head to preventoperation and consequent damage to the equipment should the gas pressure be below the safe level for insulation of thehigh voltage parts. Flow switches and pressurestats in the oil and water circulators are also used to prevent operation of
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