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Anode The anode in an X-ray tube consists of a tungsten target embedded in a copper stem. The purpose of the target in an X-ray tube is to convert the kinetic energy of the colliding electrons into X-ray photons. The target is made of tungsten, an element that has several characteristics of an ideal target material, including the following: High atomic number (74) High melting point (3422oC) High thermal conductivity (173 W. m-1. K-1) Low vapor pressure at the working temperature of an X-ray tube. The focal spot is the area on the target to which the focusing cup directs the electrons and from which X-rays are produced. The sharpness of a radiographic image increases as the size of the focal spot decreases. However, the heat generated per unit target area becomes greater as the focal spot decreases in size. To take advantage of a small focal spot while distributing the electrons over a larger area of the target, the target is placed at an angle to the electron beam. The apparent size of the focal spot seen from a position perpendicular to the electron beam (the effective focal spot) is smaller than the actual focal spot size. Typically, the target is inclined about 20 degrees to the central ray of the X-ray beam; this causes the effective focal spot to be approximately 1 mm x 1 mm, as opposed to the actual focal spot, which is about 1 mm x 3 mm. This smaller effective focal spot results in a small apparent source of X-rays and an increase in the sharpness of the image, with a larger actual focal spot size to improve heat dissipation. Key Concept