The source of x-ray emission within an x-ray tube is called the:

The actual focal spot is where x-ray photons are generated within the x-ray tube. This is the point at which the high-energy electrons produced by the cathode filament collide with the anode material, typically made of tungsten. When the electrons strike the target (the anode), their kinetic energy is converted into x-rays and heat. The actual focal spot is crucial because it determines the quality and sharpness of the x-ray image; a smaller focal spot can lead to greater image resolution.

In this context, it's important to distinguish between the actual focal spot and the effective focal spot. While the actual focal spot refers to the physical area on the anode where the electrons hit, the effective focal spot is affected by the angle of the anode and is what contributes to the overall image quality. The other options list parts of the x-ray tube or related concepts but do not refer to the site of x-ray emission, which is specifically the actual focal spot.