# For spRs sitting FRCR Part I Examinations Interaction of Radiation with Matter Amanda Barry, Ph.D.

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For spRs sitting FRCR Part I Examinations Interaction of Radiation with Matter Amanda Barry, Ph.D

Table of Contents 1.Course Overview 2.Introduction 3.Electromagnetic Radiation and its interaction with Matter 1.Elastic scattering 2.Compton effect 3.Photo-electric effect 4.Pair production 5.Photonuclear interactions 6.Auger effect 7.Scattered radiation 8.Secondary electrons 1.Linear energy transfer 2.Range versus energy

Table of Contents 4.Interaction of sub atomic particles with matter. 1.Ionisation and excitation due to charged particles 2.Electrons 1.collision loss 2.radiative loss 3.stopping power due to each and total stopping power, 4.Particle range 5.Bragg peak 3.Bremsstrahlung 4.Neutrons - elastic and inelastic collisions. 5.Protons, ionisation profile 6.Elementary knowledge of pions and heavy ions.

Course Overview Intended Audience spRs preparing for their FRCR part I examination. Nature of the course I will present 3 x 1 hour long lectures. Goals of the course Understand the fundamentals of the interactions of radiation with matter Understand the processes which underpin the absorption, scattering and creation of radiation Understand their dependence on the properties of irradiated material (e.g. density, atomic number), its variation with energy and the relative importance in therapy and imaging.

What is Physics? ? ? ? ? ? ? ?

E = mc 2

What is Physics? I.the science that deals with matter, energy, motion, and force. II.a science that deals with matter and energy and their interactions. III.How? Why? What use?

What is Physics?

The Interaction of Radiation with Matter- General Photon energy:h= hc/ where:h = Planck’s constant (6.63e -34 J.s) = frequency = wavelength

Interaction of Radiation with Matter - General Ionising RadiationNon-Ionising Radiation X-rays (indirectly) -rays (indirectly) Electron beams (directly) Protons (directly) -particles (directly) -particles (directly) Neutrons (indirectly) Lasers Ultra-violet Infra-red Ultrasound MRI Ionisation = process whereby a neutral atom acquires a +ve or a –ve charge.

Interaction of Radiation with Matter- General X-ray and -rays are photons. Photons are individual packets of electromagnetic energy have no mass have no charge have wave-like + particle-like properties travel at the speed of light, c have Energy, E = h+ momentum, ph/c X-rays and -rays are indirectly ionising radiation. That is: 1.They transfer energy to the medium (Kerma) 2.They produce high-speed electrons Electrons deposit energy in the medium through excitation and ionisation along their tracks The deposition of energy in this way is what gives rise to radiation dose (Absorbed dose) (Exposure: ability of X- and -rays to ionise air).

Interactions of Radiation with Matter Photons interact with matter through five major processes: 1. Elastic scattering () 2. Compton effect () 3. Photoelectric effect () 4. Pair production () 5. Photonuclear interactions () The probability of a particular process taking place is represented by its own attenuation coefficient (given in brackets above) The total attenuation coefficient represents the probability that photons will interact with the medium X X

Elastic Scattering h in = h out No loss of photon energy Incoming photon Outgoing photon

Elastic Scattering Elastic scattering is also known as called “Coherent” or “Rayleigh” scattering Photon scattering angle depends on Z and h * h0.1 MeV1 MeV10 MeV Al 15 o 2 o 0.5 o Pb 30 o 4 o 1.0 o Occurs mainly at low energies Large Z materials Contributes nothing to KERMA or dose, no energy transferred, no ionisation, no excitation No real importance in radiotherapy   Z 2 /(h) 2 * F. H. Attix, Introduction to Radiological Physics and Radiation Dosimetry

The Photoelectric Effect E e : maximum kinetic energy of the outgoing electron W: energy needed to remove electron E e = h - W Incoming photon Outgoing electron

The Photoelectric Effect Photoelectron emitted leaving atom in unstable, excited state Atom relaxes by X-ray emission Auger electron emission (The Auger Effect) Auger electron Characteristic X-ray

The Photoelectric Effect Process = attenuation and absorption Interaction of a photon with bound atomic electrons Total absorption of photon energy Photoelectron emitted, max. kinetic energy: E e = h - W Produces characteristic X-rays and/or Auger electrons Predominates at low energies Is highly Z dependent Example:  Pb / Pb is 300 times greater than  bone/  bone   Z 3 /( h) 3

The Photoelectric Effect If the photon energy is slightly higher than the energy required to remove an electron form a particular shell (e.g. K, L, M) around the nucleus, there is a sharp increase in . This increase is called an absorption edge.

The Photoelectric effect Absorption edges important: 1. In radiology because it influences the choice of material used in intensifying screens, photographic film, contrast agents 2. In radiation protection because it influences the choice of shielding materials 3.In radiotherapy because it influences the choice of filtering material 2040 60 80 Photon energykeV Mass absortpion coefficient K-edge for La Z=57 K-edge for W Z=74 2040 60 80 Photon energykeV Mass absortpion coefficient CaWO 4 Intensifying Screen K-edge for La Z=57 K-edge for W Z=74 LaOBr Intensifying Screen Example

The Photoelectric Effect Number of X-rays produced/no. of vacancies = Fluorescent Yield () Fluorescent Yield is high for high Z, low for low Z Low Z materials give low energy X-rays => X-rays absorbed locally For low Z materials, Auger electrons more probable Z  K 10 0 15 0.05 20 0.19 25 0.30 30 0.50 35 0.63 40 0.74 45 0.80 50 0.84 55 0.88 60 0.89 65 0.90 70 0.92 75 0.93 80 0.95 85 0.95 90 0.97 Fluorescent yield (K-shell vacancy) * * H. Johns & J. Cunningham, The Physics of Radiology, 4 th Edition

The Compton Effect Incoming photon Outgoing electron Outgoing photon  

The Compton Effect Interaction of photon with unbound atomic electrons Scatter + partial absorption of photon energy Scattered electron + scattered photon Change in photon wavelength depends on angle of scattered photon out - in = constant x (1- Cos ) in : wavelength of the outgoing electron, out : energy of incoming photon If photon makes a direct hit: 1.Electron will be scattered straight on with maximum energy 2.Photon will be scattered backwards i.e.  = 180 o with minimum energy 3.Scattered photon energy

The Compton Effect Legends = Incident Photon Energy

The Compton Effect Dominates over a wide range of X-ray energies Depends on electron density ( e ) Independent of Z /   e / h

Pair Production – Type 1 h  1.022 MeV h – 1.022 = E - + E + E -, E + are the kinetic energies of the electron and positron resp. Incoming Photon, h Outgoing Positron, E + Outgoing Electron, E -

Pair Production – Type 1 Photon interacts with Coulomb field of atomic nucleus and is absorbed Electron/Positron pair produced if h  1.022 MeV Example of conversion of energy into mass: E = mc 2 –Energy equivalent of one electronic mass is 0.511 MeV –As e + & e - produced, incoming photon must have energy: 2 x 0.511 MeV –e + and e - can receive any fraction of photon energy Dominates at high photon energies Dependent on Z   Z 2 / ln( h 

Pair Production – Type 1 e + produced in Pair Production dissipates energy locally Energy lost through excitation and ionisation of atoms along its track until it comes to rest It is annihilated by combining with a free electron producing two photons of energy 0.511 MeV slow e + free electron 0.511 MeV photon

Pair production –Type 2 Otherwise called Triplet Production Incident photon interacts with Coulomb field of atomic electrons & is absorbed Incident photon transfers energy to Host e - and e - /e + pair produced Conservation of momentum => threshold energy for this process is 4mc 2 Incoming Photon h 2.04 MeV Outgoing Positron, E + Outgoing Electron, E2 - Original Electron, E1 - h=1.022 MeV+E1 - +E2 - +E +

Summary Pb, W H Ca Zr Al Sn

Summary Photon Energy (MeV) Relative number of interactions (%)  0.01 0.026 0.060 0.150 4.00 10.00 24.00 100.00 95 50 7 0 5 50 93 100 94 77 50 16 0 6 23 50 84 Table illustrating importance of various interactions with energy in H 2 O * * F. Khan, The Physics of Radiotherapy

Summary ProcessSymbolType of Interaction Variation with h Variation with Z Elastic Photoelectric Compton Pair Production  Bound electrons  1/ h  Z 2

Summary ProcessSymbolType of Interaction Variation with h Variation with Z Elastic Photoelectric Compton Pair Production     Bound electrons Nearly free electrons Heavy nuclei  1/ h  1/ ( h    ~ const. 10-100 keV  1/ h  above 100 keV rapid increase above 1.02 MeV  Z 2  Z 3 ~ Indep. of Z  Z Table from: P. Dendy & B. Heaton, Physics for Diagnostic Radiology, 2 nd Edition

Photonuclear Interactions High energy photon interacts with atomic nucleus resulting in emission of a proton (p) or a neutron (n) Occurs for incident photons with energy > few MeV If p emitted, effect can contribute to dose. But relative importance is low If n emitted, there can be consequences w.r.t. radiation protection –must take account in shielding designs –n can escape maze more readily than photons –n may activate accelerator hardware e.g. in target –Biological effect in radiotherapy patient negligible compared with effects of photons 25 MV X-ray beam has order-of-magnitude greater neutron contamination than 10MV

The Auger Effect (Revisited) Mono-energetic Auger electrons will carry away any surplus energy of excited atom Multiple Auger electrons can be emitted resulting in an Auger shower. Vacancies continue to move to less tightly bound shells until they are eventually filled by conduction band (free) electrons K L M h = E K -E L X-ray K L M hole K L M Auger e - E = h - E M E = E K – E L - E M K L M hole holes in L- and M-shell Initial state: hole in K-shell

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