In: Physics
The lower field intensity towards the anode in comparison to the cathode due to lower x-ray emissions from the target material at angles perpendicular to the electron beam is known as Anode heel effect.
More target material needs to be traversed at emission angles that are perpendicular to the electron beam (closer to the anode) than at those more parallel to it (closer to the cathode). It will lead to more resorption of the x-rays by the target material resulting in fewer x-rays reaching the field at angles perpendicular to the electron beam. It also means that the x-rays emitted to angles closer to the incident beam travel through less target material and fewer are resorbed.
The end result is that the field intensity towards the cathode is more than that towards the anode.
Anode focal spot placement: The area of the anode surface which receives the beam of electrons from the cathode is known as focal spot. It is the apparent source of x-rays.
Size and shape of the focal spot is depend upon the size and shape of the electron beam when it strikes the anode.
To get good spatial resolution a small focal spot is used while a large focal spot is used when exposure time is short.