Compton Scattering:

Compton scattering is an elastic collision among an electron and a photon, as display in above Figure. In that case, the photon has more energy than is needed to eject the electron from orbit, or it cannot provide up all of its energy in a collision along with a free electron.  Because all of the energy from the photon cannot be transferred, the photon has to be scattered; the scattered photon must have less energy, or a longer wavelength.  A result is ionization of the atom, the high energy beta and a gamma at a lower energy level than the real.

Figure: Compton Scattering

Compton scattering is most predominant along with gammas at an energy level within the 1.0 to 2.0 MeV range.