INTERACTION OF IONISING RADIATION WITH MATTER

INTERACTION OF IONISING RADIATION WITH MATTER -Alpha, beta, gamma and X radiations are all ionizing radiation with the ability to produce ions in living tissue. Energy from radiation is transferred to matter in two ways: Ionization and Excitation. Ionization is the process of removal of an electron from an atom leaving the atom with a net positive charge. Yazeed al-ashban

INTERACTION OF IONISING RADIATION WITH MATTER In excitation, the energy of incoming radiation raises an outer electron to a higher energy state from which it returns very rapidly (10-8 s) to its original state emitting a photon of light in the process. Alpha and beta radiations are made up of charged particles and these interact with matter via electrostatic forces. -Gamma and X radiations are a type of electromagnetic wave and interact with matter via the Compton Effect and the Photoelectric Effect. Yazeed al-ashban

INTERACTION OF X-RAY WITH MATTER -X-rays entering a patient can be absorbed, scattered, or transmitted. -When an X-ray is absorbed in a patient, all of the X-rays energy is transferred into the patient's tissue. -Scattering changes the X-ray's direction and reduces its energy. - Transmitted x-rays pass through the patient without interaction. -Most diagnostic x-rays are absorbed or scattered. Yazeed al-ashban

Types of x-ray interactions: 1-Photoelectric Effect -The photoelectric effect is an absorption process and usually occurs for low energy photons (<0.1 MeV) such as X-rays. -The energy of the X-ray is transferred (absorbed) to an inner electron, normally a K-shell electron and this gives the electron sufficient energy to escape from the atom. -The atom is left positively charged (i.e., an ion) and in an excited state due to the vacancy left in the inner shell. -This vacancy is quickly filled by another electron dropping down from a higher shell with the subsequent release of a photon of frequency determined by the two shells involved. Yazeed al-ashban

Types of x-ray interactions: 2-Compton Effect -occurs for high-energy photons (>0.1 MeV) such as gamma rays. -An incoming photon of high energy ( - ray) collides with an electron, ejecting the electron from the atom, because energy of photon is greater than binding energy. -As in the Photoelectric Effect, the ejected electron will then travel through the surrounding medium creating ion pairs. Yazeed al-ashban

Types of x-ray interactions: 3-ion pair production The incident x-ray must have energy of least 1.02 MeV for pair production to occur. -In pair production, the incident high energy x-ray is interact with the atom in the matter, it enters into the nucleus, as the result, the x-ray photon energy converted into 2 particles; one is called electron(-ve) and other is called positron(+ve),positron cannot remain in the nature, it will combining with a normal electron and converted back into radiation energy. Yazeed al-ashban

Types of x-ray interactions -Pair production does not occur at diagnostic radiology energies. -Only the photoelectric effect and Compton scattering are important in the diagnostic energy range -The photoelectric effect is more important at lower energies. Compton scattering is more important at higher energies Yazeed al-ashban