Interaction of Ionizing Radiation Presentation(4) Prepared by:Maha Esmeal ahmed (PhD degree candidate)
1. Introduction: • When an X-ray beam passes throug amedium ,interaction between photons and matter can take place with the result that energy is transferred to the medium.
2.Ionization: • The process by which aneutral atom acquires apostive or negative charge . • Removal of an orbital electron leaves the atom postively charged ,resluting in an ion pair. • Charged particles such as electrons ,protons are known as direct ionizing radiation provided they have sufficient kinetic energy to produce ioinization.
3.Photon beam description: • An x-ray beam emitted from target or gamma ray beam emitted from radioactive source consists of large number of photons ,usually with a variety of energies.
4.Half value layer: • Is define as the thickness of an absorber required to attenuate the intensity of the beam to half its original
5.Attenuation coefficient: • This coefficient is depend on the energy of the photons and the nature of the material . Since the attenuation produced by a thickness x depend on the number of electrons presented in that thickness.
6.Energy transfer coefficient: • When photon interacts with electrons in the material ,apart of all of its energy is converted into kinetic energy of electrons. • If only apart of the photon energy is given to electron ,the photon it self is scattered with reduced energy .the scattered photon may interact again with a partial or complete transfer of energy to the electrons.
7.Energy absorption coefficient: • Most of the electrons set in motion by the photons will lose theirenergy by ionization and excitation .afew ,depending on the atomic number of the material ,will lose energy by bremsstrahlung interactions with nuclei. • The bremsstrahlung energy is radiated out of the local volume as x-rays and is not included in the calculation of locally absorbed energy.
8.Interactions of photons with matter: • Attenuation of photon beam by an absorbing material is caused by five major types of interactions. • Coherent scattering ,the photoelectric effect , the Compton effect and the pair production each of these processes can be represented by its own attenuation coefficient which varies in its particular way with the energy of the photon and with atomic number of absorbing material.
9.Coherent scattering: • Also known as classical scattering . • The process can be visualized by considering the wave nature of electromagnetic radiation . • This interaction consists of an electromagnetic wave passing near the electron and setting it into oscillation.
10. Photo electric effect: • Is aphenomenon in which photon interacts with an atom and ejects one of the orbital electrons from the atom . • The entire energy of the photon is first absorbed by the atom and then transferred to the atomic electron . • The kinetic energy of the ejected electron called photoelectron .
11.Compton effect: • In the Compton process the photon interacts with an atomic electron as though it were free electron . • The term free here mean s that the binding energy of the electron is much less than the energy of the bombarding photon .
12.Dependence of compton effect on energy and atomic number: • Compton effect is an interaction between a photon and free electron . • This mean that the energy of the incident photon must be large compared with electron binding energy .This is in contrast to the photoelecteric effect which becomes most probabale when the energy of the incident photon is equal to or slightly greater than the binding energy of the electron .
13.Pair production: • If the energy of the photon is greater than 1.02MeV,The photon may interact with matter through the mechanism of pair production .In this process the photon interacts strongly with electromagnetic field of an atomic nucleus and give up all its energy in the process of creating pair consisting of a negative electron and positive electron because the rest mass energy is equal to 0.51MeV.Aminumum energy of1.02MeV is required to create the pair of electron
14.Aheavy charge particles: • The rate of energy loss or stopping power caused by ionization interactions for charged particles is proportional to the square of the particle charge and inversely proportional to the sequare of its velocity.
15.Interaction of nucleus: • Like x-ray and gamma ray neutrons are indirectly ionizing .However ,their mode of interaction with a matter is different . • Neutrons interact basically by two processes: a-recoiling proton from hydrogen and recoiling heavy nuclei from other elements. and b-nuclear disintegrations.