When an x-ray photon interacts with an absorbing substance, in general, a portion of the photon's energy is radiated away from the absorber as scattered radiation, and the other portion is converted into the kinetic energy of a high-speed electron or positron. The mass attenuation coefficient u/p represents the total probability of interaction of incident X-rays with matter, which includes the sum of all possible interaction probabilities. The mass-energy transfer coefficient Utr/P expresses the sum of that share of the photon energy transferred to the charged particle during the interaction. The mass-energy attenuation coefficient Uen/P represents the share of that part of the energy given to the charged particle by the photon that is used to cause ionization, excitation, and thus actually absorbed by the matter, after subtracting the continuous radiation
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