Radiology Half value layer refers to A The rate at which an X-ray photon transfer its energy to irradiated matter B The thickness of a substance required to reduce the number of X-ray photons by half C The time taken for X-ray photons to travel half the distance from the source to the object D The heel-effect seen when the anode is placed at an angle to the electron stream in the X-ray cathode tube 💡 Explanation Half value layer refers to ✓ Correct Answer: B. The thickness of a substance required to reduce the number of X-ray photons by half