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Ionising Radiation

- There are two types of radiation ionising and

non-ionising.

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

radiation - 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-

e

180

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

path - 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

electrons.

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

scattering

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,

and - 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

intensity - 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

photon.

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

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

energy

180

e-

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