© Jimoid.com 2005 Ionising Radiation There are two types of radiation; ionising and non-ionising. Radiation Ionising Non-ionising Indirectly ionising (neutral particles) Photons, neutrons… Directly ionising (charged particles) Electrons, protons, α-particles… Non-ionising Radiation As its name implies this does not have the ability to give or remove charge from a neutral particle and thus cannot ionise matter. Ionising Radiation This radiation can ionise matter in two ways: Directly ionising radiation (charged particles) electrons, protons, α-particles and heavy ions, orDirectly ionising radiation (charged particles) electrons, protons, α-particles and heavy ions, or Indirectly ionising radiation (neutral particles) photons (x-rays and γ- rays) and neutrons.Indirectly ionising radiation (neutral particles) photons (x-rays and γ- rays) and neutrons.

© Jimoid.com 2005 Directly Ionising Radiation Directly ionising radiation deposits energy in the medium with which it is interacting by Coulomb interaction of the charged particle (radiation) with electrons of atoms in the matter which is being ionised.

© Jimoid.com 2005 Indirectly Ionising Radiation Indirectly ionising radiation is not in the form of a charged particle and so cannot interact directly to ionise the medium through Coulomb interactions. It must first react with the matter to release a charged particle which can then go on to interact with the medium and ionise it through Coulomb interactions. E = hν

© Jimoid.com 2005 Ionising Photons There are four classifications of ionising photon radiation: Characteristic x-rays which result from electron transitions from atomic shellsCharacteristic x-rays which result from electron transitions from atomic shells Bremsstrahlung which results from electron-nucleus Coulomb interactionsBremsstrahlung which results from electron-nucleus Coulomb interactions γ -rays which result from nuclear transitionsγ -rays which result from nuclear transitions Annihilation quanta which result from positron-electron annihilations (511 keV)Annihilation quanta which result from positron-electron annihilations (511 keV) E = hν γ e-e- e+e+ E = 511 keV 180°

© Jimoid.com 2005 Electron Interactions As energetic electrons traverse matter they interact with it through Coulomb interactions and lose energy. There are two possible results of these interactions: The electron loses energy through collisions or radiative lossesThe electron loses energy through collisions or radiative losses The electron can be deflected from its original pathThe electron can be deflected from its original path Energy losses are described by the stopping power Scattering is described by scattering power

© Jimoid.com 2005 Electron Interactions The type of interaction of the incident electron with a particular atom depends on the impact parameter b. b>>a Soft collision between electron and electron. Only a small amount of the incident electron’s kinetic energy will be transferred to the orbital electrons. e-e- Electron Trajectory Nucleus Electron Cloud b a b≈a This will result in a hard collision and an appreciable amount of the electrons kinetic energy will be given to the orbital electrons. This can result in ionisation of the atom or excitation. b<<a Coulomb interaction of the electron with the nucleus. This results in x-ray production through Bremsstrahlung and electron scattering

© Jimoid.com 2005 Stopping Power Energy losses by an electron moving through a medium of density ρ are described by the total mass-energy stopping power (S/ρ) tot This is a measure of the loss in kinetic energy E k of the electron per unit path length x. The total stopping power consists of two components – the collision stopping powers (S/ρ) coll (atomic excitations and ionisations) and the radiative stopping powers (S/ρ) rad (Bremsstrahlung production) (S/ρ) tot = (S/ρ) coll + (S/ρ) rad

© Jimoid.com 2005 Linear Energy Transfer (LET) The stopping power focuses on the amount of energy lost by an electron traversing a medium. If we focus on how much energy the medium is gaining from the electron we can describe a linear rate of energy absorption. The rate of energy absorption by the material, called the Linear Energy Transfer (LET), is defined as the average energy locally imparted to the absorbing medium by an electron of specified energy traversing a given distance in the medium.

© Jimoid.com 2005 Photon Beam Attenuation The intensity of a beam of monoenergetic photons attenuated by an attenuator of thickness x is given by I(x) = I(0) e -μ(h ν, Z)x where I(0) is the intensity of the unattenuated beam, and μ(h ν, Z) is the linear attenuation coefficient which depends on the energy of the photon h ν and the atomic number Z of the attenuator.

© Jimoid.com 2005 Half Value Layer (HVL) The Half Value Layer (HVL or x ½ ) is defined as the thickness of the attenuator that will attenuate the photon beam to 50% of it’s original intensity From I(x) = I(0) e -μ(h ν, Z)x we have ½ = 1 e -μx ½ -ln 2 = -μx ½ x ½ = (ln 2)/μ

© Jimoid.com 2005 Linear Attenuation Coefficient μ The linear attenuation coefficient μ is related to the mass attenuation coefficient μ m, atomic attenuation coefficient a μ and electronic attenuation coefficient e μ as follows: μ = ρ μ m = (ρ N A a μ)/A = (ρ N A e μ Z)/A The units of the linear, mass, atomic and electronic attenuation coefficients are: cm -1, cm 2 /g, cm 2 /atom and cm 2 /electron. This implies that the thickness given in (–μx) must be quoted in units of: cm, g/cm 2, atoms/cm 2 and electrons/cm 2 respectively

© Jimoid.com 2005 The Photoelectric Effect In the photoelectric effect the photon interacts with an orbital electron and disappears, while the electron is ejected from the atom thus ionising it. The energy of the photoelectron is given by E k = h ν – E B Where E k is the kinetic energy of the ejected electron, h ν the energy of the photon and E B the binding energy of the electron.

© Jimoid.com 2005 The Photoelectric Effect The mass attenuation coefficient for the photoelectric effect is proportional to (Z/h ν ) 3 The plot of h ν versus mass attenuation shows some sharp discontinuities where h ν equals the binding energy of particular electronic shells. These discontinuities, called absorption edges, are caused because for a particular shell, the electrons cannot undergo the photoelectric effect without energy h ν greater than or equal to the binding energy of that shell. Mass attenuation coefficient (cm 2 /g) Photon energy (MeV) L edges K edge

© Jimoid.com 2005 Compton Effect The Compton effect represents a photon scattering off an atom and ejecting an orbital electron from that atom. As h ν >>E B the electron can be treated as free and stationary when compared to the photon. φ θ Incident photon Recoil electron Scattered photon The energy of the photon dictates the average angle of deflection. For θ = 0, φ = π/2 (no change in photon direction) and for θ = π, φ = 0 (back scattering of the photon). The following is a table of average scattering and recoil values Incident Photon Energy (MeV) Scattered Photon Energy (MeV) Recoil Electron Energy (MeV)

© Jimoid.com 2005 Pair Production In pair production, a photon in the nuclear Coulomb field of an atom converts to an electron-positron pair. e-e- e+e+ 180° E = hν There is a minimum activation energy for this conversion of hν ≥ 2m e c 2 = 1.02 MeV Any photonic energy above this minimum threshold is shared equally by the electron-positron pair as kinetic energy If the pair production occurs in the field of an orbital electron then three particles are created and this process is called triplet production. An electron-positron pair are created and an orbital electron. The minimum energy for this activation is 4m e c 2 and all particles share this energy.

© Jimoid.com 2005 Photonic Attenuation The above graph shows the individual and combined mass attenuation effects upon photons at varying photon energies Mass attenuation coefficient (cm 2 /g) Photon energy (MeV) L edges K edge 100 Photoelectric Effect mass attenuation coefficient Compton Effect mass attenuation coefficient Pair Production mass attenuation coefficient Combined mass attenuation coefficient

© Jimoid.com 2005 Photonic Attenuation Photon Energy (MeV) Atomic Number Z Photoelectric Effect Dominant Compton Effect Dominant Pair Production Dominant