Gamma Ray

RADIATION TYPES Radiation Detectors Gamma Ray The gamma ray is a photon of electromagnetic radiation with a very short wavelength and high energy. It is emitted from an unstable atomic nucleus and has high penetrating power. There are three methods of attenuating (reducing Figure 2 Photoelectric Effect the energy level of) gamma-rays: photoelectric effect, compton scattering, and pair production. The photoelectric effect occurs when a low energy gamma strikes an orbital electron, as shown in Figure 2. The total energy of the gamma is expended in ejecting the electron from its orbit. The result is ionization of the atom and expulsion of a high energy electron. The photoelectric effect is most predominant with low energy gammas and rarely occurs with gammas having an energy above 1 MeV (million electron volts). Compton scattering is an elastic collision between Figure 3 Compton Scattering an electron and a photon, as shown in Figure 3. In this case, the photon has more energy than is required to eject the electron from orbit, or it cannot give up all of its energy in a collision with a free electron. Since all of the energy from the photon cannot be transferred, the photon must be scattered; the scattered photon must have less energy, or a longer wavelength. The result is ionization of the atom, a high energy beta, and a gamma at a lower energy level than the original. Compton scattering is most predominant with gammas at an energy level in the 1.0 to 2.0 MeV range. IC-06 Page 6 Rev. 0