Interactions Of Radiation PDF

Interactions Of Radiation ( Attenuation Of X Rays & Gamma Rays ) Prof.Magda Allam Teaching Assistant : Lorana Mustafa  When a beam of radiation of any kind penetrates matter some of the radiation may be absorbed completely, some may be scattered and some may pass...

Interactions Of Radiation ( Attenuation Of X Rays & Gamma Rays ) Prof.Magda Allam Teaching Assistant : Lorana Mustafa  When a beam of radiation of any kind penetrates matter some of the radiation may be absorbed completely, some may be scattered and some may pass straight through without any interaction at all.  1- Absorption  Definition :The energy deposited in the material per unit mass.  The transferred energy is converted into kinetic energy of the electrons within the medium, enabling them to move through the medium.  The electrons may interact with other atoms in the medium causing ionization and excitation.  This results in the chemical and biological changes  2- Scattering  It occurs following a collision interaction between the primary X -ray beam and the atoms of the medium  The incident photon is deflected out of the path of the primary beam and travels onward in a new direction. Absorption and Scattering  3- Transmission without any interaction  The size of the nucleus of an atom is extremely small compared with the overall size of the atom and so most of the atom is considered as space.  Therefore, there is a high probability that some X - rays will pass straight through a medium without undergoing absorption or scattering  The X -rays are said to be transmitted and it is these X -rays that play a part in the production of a radiographic image and contribute to the exit dose of a beam of X -rays used for radiation treatment.  Attenuation  When a beam of X -rays traverses matter, there is a reduction in the intensity of that beam.  Intensity is defined as the rate of flow of photon energy through a unit area  This reduction in intensity is referred to as attenuation and involves a process of absorption, scattering or a combination of both.  Exponential relationship  Experimentally it can be shown that for a narrow, homogeneous beam of X -rays, i.e. a beam of photons of similar energy, the intensity of radiation transmitted is reduced in an ‘ exponential ’ manner.  Equal thicknesses of uniform attenuating material placed in the path of the beam produce equal fractional reductions in the intensity of radiation transmitted, e.g. a thickness of attenuator ( t ) reduces the intensity of the beam initially by 50% (100% reduced to 50%), then again by 50% (50% reduced to 25%).  Half - value layer  The HVL is the thickness of a medium that will reduce the intensity of a narrow beam of X -rays to exactly half of its original value.  The first HVL is indicated in by the value of the thickness of attenuator that reduces the intensity of the beam from 100% to 50%;  The second HVL is the thickness that reduces the intensity of the beam from 50% to 25%.  For a homogeneous beam of radiation, these values will always be equal, the HVL is ‘ t ’.  However, an X -ray beam is heterogeneous and comprises photons of differing energies, so successive values of HVL cannot be the same due to filtration of the lower energy photons in the beam by the initial attenuation.  The beam increases in homogeneity as filtration occurs and later values of HVL will become equivalent.  Thank You Thank You