Alhanouf Alshedi Basic Interactions of Radiation with Matter 2 ed Lecture

Introduction Interaction of radiation with matter results in the transfer of energy to the surrounding material. This leads to the ionization and excitation of atoms and molecules, which are responsible for radiobiological effects. Medical radioisotopes used in diagnosis are  -ray emitters, so the discussion will be confined to the interaction of photons with matter.

As you see the diagram, you note the interaction of the photons with soft tissue: Remaining photons which are the total of the primary photons & attenuated photons. Primary photons that just enter the object. Attenuated photons may be absorbed or scattered. Attenuation means reduction in the intensity of the beam. Scattered photons that bend their path in small angle. Scatter photons can cause fog of the image which degrade the quality of the image. They can be reduced by minimize the amount of tissue irradiated ( collimation).

Process of Interactions Three basic types of interaction Photoelectric effect Compton scattering Pair Production

1. Photoelectric Effect The photoelectric effect is an atomic process in which an atom absorbs totally the energy of an incident photon. If the absorbed energy is sufficient, an orbital electron is ejected from the atom. This electron is called photoelectron. The photoelectric effect creates a vacancy in an orbital electron shell, which in turns leads to the emission of characteristic x-ray or AE. High-energy photoelectron may interact with other atoms causing the ionization or excitation of these atoms.

Compton scattering is a collision between a photon and a loosely bound outer shell orbital electron of an atom. Part of the photon energy is transferred to the recoil electron, and the photon does not disappear, but it is deflected through a scattering angle. The two byproducts of Compton scattering are the : (1).Compton scattered electron. (2).Scattered x-ray photon of lower energy. 2. Compton Scattering

3. Pair Production Pair production occurs when a photon interacts with the electric field of a nucleus. In pair production, the photon disappears and its energy is used to create a positron-electron pair. Therefore, the energy of the incident photon must be  MeV, because the rest mass of the positron- electron pair is 2  0.511=1.022 MeV. When the positron has lost its kinetic energy and stopped, it undergoes annihilation with a negative electron, and a pair of MeV annihilation photons are emitted in opposite directions.

4. Coherent Scattering (unmodified) It is interaction of x-rays and the matter in which the radiation doesnt loose its energy but change only. It occurs when the energy of the photons in the beam is small compared with electron binding energies in the attenuating medium. The x-ray photons does not have sufficient energy to remove the electron from the orbit, so no energy is transferred to the electron. This interaction contribute to film fog and radiation dose to the patient.

Type of interaction X-ray photon energy range Site of interaction Byproduct of interaction Photoelectric absorption 1 to 50 kvp Inner-shell electron. Usually k shell. Photoelectron. Characteristic radiation. Compton scattering 60 Kvp to 2 mev Outer-shell electron Recoil electron. Scattered x-ray photon of lower energy Coherent scattering 1 to 50 kvp An atom None. Pair production Mev Nucleus of atom Positron & negatron. Two Mev phtons

Organizations of Radiation Protection (A) International Commission on Radiological Protection (ICRP): It prepares recommendations to deal with the basic principles of radiation protection and leave to the various national protection committees the responsibility of introducing the detailed regulations. (B) International Atomic Energy Agency (IAEA): It promote the peaceful uses of nuclear energy & it concerned with the practical application of the ICRP recommendations.

(C) International Commission on Radiological Units and Measurements (ICRU): of quantities, units of radiation & their measurement procedure. (D) The World Health Organization (WHO): It has published several reports concerning radiological protection. (E) The International Labor Organization (ILO). (F) National Council on Radiation Protection and Measurements. (G) National Radiological Protection Board (NRPB)

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