When monoenergetic gamma-rays are collimated into a narrow beam and allowed to pass through an absorber of variable thickness to strike a detector, there are interactions between gamma-rays photon with absorber. Each of the interaction process, the gamma-rays photon is removed from the beam either by absorption or by scattering away from the detector direction. It can be characterized by a fix probability of occurrence per unit path length that the gamma rays removes from the beam according to Eq. (1) (Knoll, 2010):