Kerma.
The Kerma stand for the kinetic energy released in the medium, which describe the initial interaction of photons with an atom in medium. When the x and gamma ray pass through a medium, the transfer kinetic energy to the charged particles. The unit of Kerma is jouls per kilogram (J/kg) . The SI units gray and the special unit is rad. When the reference material is air. The quantity is called air Kerma