What voltage is applied between the anode and cathode in an X-ray tube and is proportional to the energy of X-rays?

The voltage applied between the anode and cathode in an X-ray tube, which is referred to as kilovolt peak (kVp), controls the energy and penetrability of the produced X-rays. The kVp setting determines the maximum energy (and therefore the maximum wavelength) of the X-ray photons generated. Higher kVp settings produce X-rays with higher energy, which can penetrate thicker tissues, making it crucial for imaging different parts of the body.

In contrast, mAs refers to the product of tube current (mA) and exposure time (seconds), and it affects the quantity or number of X-ray photons produced rather than their energy. mA indicates the rate of electron flow, which influences the intensity of the X-ray beam, while RBE (Relative Biological Effectiveness) is related to the biological effects of radiation and is not directly involved with the functioning of the X-ray tube. Therefore, kVp is the correct answer as it directly correlates with the energy level of the X-rays produced.