kinetic energy release in medium ( KERMA )

Initial interaction of an X-Ray photn in a medium is  called KERMA. When X and 4-rays pass through a medium, they transfer their kinetic energy to the charged particle electrons and protons. KERMA (K)  refers to the energy transferred to the charged particle. This charge trajectory forward into the particle medium

During the process, some electrons interact with the atom to produce Bremsstrahlung Radiaiton and some electrons react with another electron to produce 𝛅-rays. In the human body, only Bremsstrahlung is significant,  the amount of rays-rays is very small, so it is ignored. Its unit is joule / kg (J / Kg). Its SI  Gray is | Its special unit is Rad. When the reference material is air, it is called air kerma. 

1 air kerma (Gy) = 114R