Would be required to remove the electron from the atom. Asĭescribed in a previous chapter the binding energy is the energy that Z) of the atom and the shell in which the electron is located. Specific amount of binding energy that depends on the size (atomic number, An interaction with electron shells producesĬharacteristic x-ray photons interactions with the atomic nucleus produceĮlectron-Atom Interactions That Produce X-Ray Photons Two types of interactions produce radiation. The electrons interact with individual atoms of the anode material, as shownīelow. When the electrons strike the surface of the anode, they are slowed very quickly and lose their kinetic energy the kinetic energy is converted into either x-radiation or heat. A 100-keV electron reaches the anode surface traveling at more than one half the velocity of light. At this point the electron is traveling with a relatively high velocity determined by its actual energy content. Just as the electron arrives at the surface of the anode its potential energy is lost, and all its energy is kinetic.
As the electron moves from cathode to anode, however, its electrical potential energy decreases as it is converted into kinetic energy all along the way. This increase in kinetic energy continues as the electrons travel from the cathode to the anode.
This force accelerates them, causing an increase in velocity and kinetic energy. Most x-ray tubes have two focal spot sizes (small and large), which can be selected by the operator according to the imaging procedure.Īfter the electrons are emitted from the cathode, they come under the influence of an electrical force pulling them toward the anode.
Tubes with smallįocal spots are used when high image visibility of detail is essential and the amount of radiation needed is relatively lowīecause of small and thin body regions as in mammography. When selecting an x-ray tube for a specific application. Less blurring and better visibility of detail, and large focal spots have a greater heat-dissipating capacity.įocal spot size is one factor that must be considered X-ray tubes are designed to have specific focal spot sizes small focal spots The dimensions of focal spots usually range from 0.1 mm to 2 mm. In most x-ray tubes, the focal spot isĪpproximately rectangular. The dimensions of the focal spot are determined by the dimensions of the electron beam arriving from the cathode. The radiation is produced in a very small area on the surface of the anode known as theįocal spot. Not all of the anode is involved in x-ray production. With x-ray tubes with pure tungsten anodes, radiation output is reduced with usage because of thermal damage to the surface. The use of a rhenium-tungsten alloy improves the long-term radiation output of tubes. This produces a higher energy and more penetrating radiation, which can be used to image dense breast. Some mammography tubes also have a second anode made of rhodium, which has an atomic number of 45. This material has an intermediate atomic number (Z = 42), which produces characteristic x-ray photons with energies well suited to this particular application. Most x-ray tubes used for mammography have molybdenum-surface anodes. The use of molybdenum as an anode base material should not be confused with its use as an anode surface material. Two such materials are molybdenum and graphite. The anode body under the tungsten-rhenium surface on many tubes is manufactured from a material that is relatively light and has good heat storage capability. In recent years an alloy of tungsten and rhenium has been used as the target material but only for the surface of some anodes. For many years, pure tungsten was used as the anode material. Tungsten is almost unique in its ability to maintain its strength at high temperatures, and it has a high melting point and a relatively low rate of evaporation. In addition to a high atomic number, tungsten has several other characteristics that make it suited for this purpose. Most x-ray tubes use tungsten, which has an atomic number of 74, as the anode material. The fraction of the total electronic energy that is converted into x-radiation (efficiency) depends on two factors: the atomic number (Z) of the anode material and the energy of the electrons. The ideal situation would be if most of the electrons created x-ray photons rather than heat. The material for the anode is selected to enhance these functions. The anode has two primary functions: (1) to convert electronic energy into x-radiation, and (2) to dissipate the heat created in the process. It is a relatively large piece of metal that connects to the positive side of the electrical circuit. The anode is the component in which the x-radiation is produced.
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