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Ionising Radiation


Ionising Radiation There are two types of radiation; ionising and non-ionising. Non-ionising Radiation Directly ionising (charged particles) Electrons, protons, ... – PowerPoint PPT presentation

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Title: Ionising Radiation

Ionising Radiation
  • There are two types of radiation ionising and

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,
a-particles and heavy ions, or Indirectly
ionising radiation (neutral particles) photons
(x-rays and ?-rays) and neutrons.
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.

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 h?
Ionising Photons
  • There are four classifications of ionising photon
  • Characteristic x-rays which result from electron
    transitions from atomic shells
  • Bremsstrahlung which results from
    electron-nucleus Coulomb interactions
  • ?-rays which result from nuclear transitions
  • Annihilation quanta which result from
    positron-electron annihilations (511 keV)

E h?
E h?
E 511 keV
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 losses
  • The electron can be deflected from its original
  • Energy losses are described by the stopping power
  • Scattering is described by scattering power

Electron Interactions
  • The type of interaction of the incident electron
    with a particular atom depends on the impact
    parameter b.
  • bgtgta
  • Soft collision between electron and electron.
    Only a small amount of the incident electrons
    kinetic energy will be transferred to the orbital

baThis 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. bltltaCoulomb interaction of the
electron with the nucleus. This results in x-ray
production through Bremsstrahlung and electron
Stopping Power
  • Energy losses by an electron moving through a
    medium of density ? are described by the total
    mass-energy stopping power (S/?)tot This is a
    measure of the loss in kinetic energy Ek of the
    electron per unit path length x.
  • The total stopping power consists of two
    components the collision stopping powers
    (S/?)coll (atomic excitations and ionisations)
    and the radiative stopping powers (S/?)rad
    (Bremsstrahlung production)
  • (S/?)tot (S/?)coll (S/?)rad

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.

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?, Z)x
  • where
  • I(0) is the intensity of the unattenuated beam,
  • µ(h?, Z) is the linear attenuation coefficient
    which depends on the energy of the photon h? and
    the atomic number Z of the attenuator.

Half Value Layer (HVL)
  • The Half Value Layer (HVL or x½) is defined as
    the thickness of the attenuator that will
    attenuate the photon beam to 50 of its original
  • From
  • I(x) I(0) e-µ(h?, Z)x
  • we have
  • ½ 1 e-µx½
  • -ln 2 -µx½
  • x½ (ln 2)/µ

Linear Attenuation Coefficient µ
  • The linear attenuation coefficient µ is related
    to the mass attenuation coefficient µm, atomic
    attenuation coefficient aµ and electronic
    attenuation coefficient eµ as follows
  • µ ? µm (? NA aµ)/A (? NA eµ Z)/A
  • The units of the linear, mass, atomic and
    electronic attenuation coefficients are cm-1,
    cm2/g, cm2/atom and cm2/electron.
  • This implies that the thickness given in (µx)
    must be quoted in units of cm, g/cm2, atoms/cm2
    and electrons/cm2 respectively

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
  • Ek h? EB
  • Where Ek is the kinetic energy of the ejected
    electron, h? the energy of the photon and EB the
    binding energy of the electron.

The Photoelectric Effect
  • The mass attenuation coefficient for the
    photoelectric effect is proportional to (Z/h?)3
  • The plot of h? versus mass attenuation shows some
    sharp discontinuities where h? 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? greater than or equal to the
    binding energy of that shell.

L edges
K edge
Mass attenuation coefficient (cm2/g)
Photon energy (MeV)
Compton Effect
  • The Compton effect represents a photon scattering
    off an atom and ejecting an orbital electron from
    that atom. As h?gtgtEB the electron can be treated
    as free and stationary when compared to the

The energy of the photon dictates the average
angle of deflection. For ? 0, f p/2 (no
change in photon direction) and for ? p, f 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)
0.1 0.085 0.015
1 0.560 0.440
10 3.1 6.9
100 20 80
Pair Production
  • In pair production, a photon in the nuclear
    Coulomb field of an atom converts to an
    electron-positron pair.

E h?
There is a minimum activation energy for this
conversion of h? 2mec2 1.02 MeV Any photonic
energy above this minimum threshold is shared
equally by the electron-positron pair as kinetic
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 4mec2 and all particles share this energy.
Photonic Attenuation
  • The above graph shows the individual and combined
    mass attenuation effects upon photons at varying
    photon energies.

Photonic Attenuation