Heel effect

In X-ray tubes, the heel effect or more accurately, the anode heel effect is a variation of the intensity of X-rays emitted by the anode depending on the direction of emission. Due to the geometry of the anode, X-rays emitted towards the cathode are in general more intense than those emitted perpendicular to the cathode–anode axis. The effects stems from the absorption of X-ray photons before they leave the anode in which they are produced. The probability for absorption depends on the distance the photons travel within the anode material, which in turn depends on the direction of emission.