Amanda Barry, Ph.D

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Interaction of Radiation with Matter - Lecture 3
For spRs sitting FRCR Part I Examinations
Amanda Barry, Ph.D
2
Interaction of Charge Particles with Matter

• TO RECAP
• Scattered Radiation Secondary electrons -
  sources of scatter and effects
• Charged particles are surrounded by an
  electrostatic field
• Charged particle undergoes many interactions
• Energy loss due to interaction of Coulomb fields
  of incoming charged particle and that of atomic
  electron/nuclei
• Collisional Losses Ionisation/Excitation via
  Hard Soft Xns
• Radiative Losses Bremsstrahlung via interaction
  with
• nuclear field
• Stopping Power and Restricted Stopping Power
• Absorbed Dose
• Particle Range

3
Interaction of Sub-atomic Particles with Matter

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

4
Introduction to Hadrons

• What are Hadrons?
• Hadrons are subatomic particles which experience
  the strong nuclear force e.g. neutrons and
  protons
• They are composed of fundamental particles called
  quarks, anti-quarks and gluons
• Generally, cannot see free (anti-)quarks or
  gluons
• Hadrons are either Baryons (spin-1/2) or Mesons
  (spin-0)
• Examples of Baryons are Neutrons and Protons
• Examples of Mesons are Pions
• Where are Hadrons useful?

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Introduction to Hadrons

• 1. High Energy Nuclear Physics

• Particles are accelerated
• to energies of 1500 TeV
• before colliding
• 12,500 Tonnes
• Diameter15 m
• Length 21.5 m
• Magnetic Field 4T
• (largest solenoid ever built)
• Data Recorded/s
• 10,000 Britannica
• Encyclopaedias

Large Hadron Collider, CERN
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Introduction to Hadrons

• Home to the WWW
• Particle Physics
• Recreating the BIG BANG
• 27 km accelerator
• Crosses French/Swiss border 4 times
• 20 European nations
• 3000 Enployees

CERN
http//public.web.cern.ch/Public/Welcome.html
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Introduction to Hadrons

• 2. Cancer Therapy

Image from http//www.lns.infn.it/CATANA/CATANA/
documents/pabloICATPP2003.pdf
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Introduction to Hadrons

• Why are Hadrons useful in Cancer Therapy?
• In many cases
• penetration depth can be well-defined and
  adjustable
• most energy deposited at end-of-range
• no dose beyond target
• dose to normal tissue minimised
• good tumour kill

• If most HADRON energy deposited at a depth that
  depends precisely on the energy of the particles
• tumours can be targeted more accurately,
  allowing a larger
• radiation dose to be delivered
• speeding up the treatment programme.

HADRONS ENABLE DELIVERY OF HIGH DOSE TO THE
TUMOUR SPARING THE SURRONDING TISSUES
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Introduction to Hadrons
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Interaction of Neutrons with Matter

• Properties of Neutrons
• Mass 1.67 e-27 kg
• No Charge
• Indirectly Ionising Radiation
• Neutron half-life 10.3 minutes
• Types of Neutron
• Thermal neutrons, E lt 0.5 eV
• Intermediate-energy neutrons, 0.5 eV lt EN lt 10
  keV
• Fast neutrons, E gt 10 keV
• All neutrons are initially Fast Neutrons which
  lose kinetic energy through interactions with
  their environment until they become thermal
  neutrons which are captured by nuclei in matter 

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Interaction of Neutrons with Matter

• Some sources of neutrons
• Spontaneous fission of isotopes
• Photonuclear interactions
• Neutron generator
• Interactions of neutrons
• Collisions with atomic nuclei often in a
  billiard-ball type interaction.
• Rare events, because neutron and nucleus are tiny
  compared to atom.
• So, neutrons can travel long distances through
  matter before interacting.
• Types of neutron interaction
• Elastic scattering
• Inelastic scattering
• Neutron capture

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Interaction of Neutrons with Matter Elastic
Scattering

• Elastic Scattering
• Neutron collides with atomic nucleus
• Neutron deflected with loss of energy E
• E given to recoiling nucleus
• Energy of recoiling nucleus absorbed by medium.
• The recoil nuclei quickly become ion pairs and
  loose energy through excitation and ionisation as
  they pass through the biological material. This
  is the most important mechanism by which neutrons
  produce damage in tissue. 
• Struck atoms can also lose orbital electron

Total energy unchanged
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Interaction of Neutrons with Matter Elastic
Scattering

• Conservation of Energy and Momentum
• E energy of scattered neutron 
• Eo initial energy of neutron 
• M mass of the scattered nucleus 
• m mass of neutron
• Energy transferred to nucleus ? as target mass ?
  neutron mass. 
• Hydrogen good for stopping neutrons e.g. fat
  better than muscle.
• Elastic scattering important at low neutron
  energies (few MeV) and
• not effective above 150 MeV

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Interaction of Neutrons with Matter Inelastic
Scattering

• Inelastic Scattering
• Neutron momentarily captured by nucleus
• Neutron re-emitted with less energy
• Nucleus left in excited state
• Nucleus relaxes by emitting g-rays or charged
  particles
• (adds to dose)

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Interaction of Neutrons with Matter Inelastic
Scattering

• Interaction probability ? as neutron energy ?
• target size ?
• Important at high neutron energies in heavy
  materials
• Energy transferred to the target nucleus and
  emitted energy 
• E Eo - Eg
• E Energy of the neutron after collision 
• Eo Initial energy of the neutron 

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Interaction of Neutrons with Matter- Neutron
Capture

• 3. Neutron Capture
• Neutron captured by nucleus of absorbing
  material
• Only g-ray emitted.
• Probability of capture is inversely
  proportional to the energy of the
• neutron. 
• Low energy (thermal neutrons) have the highest
  probability for
• capture.

Na23
Na24
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Interaction of Neutrons with Matter

• Where are neutrons useful?
• Cancer Therapy
• To produce radioactive isotopes for radiotherapy
  or imaging
• To analyse composition and structure of unknown
  elements
• Bomb detectors in airports
• Construction of electronic devices
• Nuclear energy

Image from A. L. Galperin, Nuclear
Energy/Nuclear Waste. Chelsea House
Publications New York, 1992
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Interaction of Neutrons with Matter
p(66) Be(49) Neutron Therapy Beam (same as 8 MV
photon beam)
Depth Dose
Image from http//www-bd.fnal.gov/ntf/reference/
hadrontreat.pdf
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Interaction of Neutrons with Matter

• Neutrons for Radiotherapy
• Neutrons have good tumour killing capabilities
• Tissue damage is primarily by nuclear
  interactions
• Neutrons are high LET radiation have high B.E.
• Lower chance of tumour repair
• Often lower dose required
• Good for radioresistant tumours

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Protons

• Properties of Protons
• Mass 1.67 e-27 kg
• Positive Charge
• Directly Ionising Radiation
• Proton half-life 1035 years
• Types of Proton Interaction
• Electronic - Ionisation and Excitation of
  atomic electrons
• Nuclear Coulomb Scattering
• Elastic Collision
• Non-elastic nuclear collision (20)

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Protons

Proton vs Photon Depth Dose in Water
w.massgeneral.org/.../proton/principles.asp
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Protons

• Protons for radiotherapy
• Protons have good dose distribution
• Low entry dose
• Most of energy deposited at a specific depth
• No dose beyond specific range

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Protons
World-wide Proton Treatments
From Particles, Newsletter, (Ed Sisterton) No. 28
July 2001
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Heavy Ions

• What are Heavy Ions?
• Heavy ions are ionised atoms which are usually
  heavier than C.
• Heavy ions are composed of Hadrons.
• Heavy ions refers to atoms that are generally
  completely ionised, i.e. they are bare atomic
  nuclei.
• The nuclei can be directed to a fixed target, or
  can be split into two beams moving in opposite
  directions that are brought into collision at a
  well-defined spot.
• Heavy ion nuclei most often used in nuclear
  physics experiments include C, Si, W, Au, Pb, U

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Pions

• What are Pions?
• Pions ( Pi Mesons)
• Symbols P-,P0, P
• Pions are the lightest of the Mesons (0.15 x
  Mp,N)
• Mesons exist inside the nucleus i.e. they are
  sub-atomic particles which experience the strong
  nuclear forces.
• Pions hold the nucleus together .
• Pions are produced as a result of high energy
  collisions in a particle accelerator e.g. protons
  colliding with a C or Be target.
• Pions live for 26 billionths of a second.

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Pions

• Pions (P-) in radiotherapy
• When the P- reaches the tumour it has slowed
  down so much that a
• nucleus captures it.
• The nucleus is now unstable and breaks up
  violently into smaller
• fragments.
• These fragments damage surrounding cells within
  a small radius

Image from http//www.triumf.ca/welcome/pion_tr
tmt.html
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Hadron Comparison

• Hadron Comparison
• Low LET Protons Photons
• Similar RBE but protons have sharp dose fall off
  at a specific depth determined by proton energy
• High LET Neutrons, Heavy Ions Pions
• Have high RBE, good tumour kill, poor cell repair

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• End of Lecture 3

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• QUIZ

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• Heavy ions are ions that are heavier than which
  element?
• A Carbon

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• What type of interaction is most common for
  photons in the radiotherapy energy range?
• A Compton Effect

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• What do you call a sub-atomic particle that
  experiences the strong nuclear force?
• A Hadron

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• How does the photoelectric effect depend on
  energy?
• A 1/E3

34

• Which Hadron is used for detecting bombs in
  airports?
• A Neutron

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• What is another name for an energetic secondary
  electron?
• A Delta ray

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• What is produced as a result of Pair Production?
• A positron/electron pair

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• What is the mass of a proton?
• A 1.67 e-27 kg

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• When many electrons are produced as a result of
  the Auger Effect, we have an ?
• A Auger Shower

39

• Approximately, what is the LET of a 5 MeV
  neutron?
• A 50 keV/mm

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• How many interactions does a 1 MeV electron
  typically undergo before coming to a stop?
• A 100,000

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• What type of particle follows a tortuous path
  when passing through matter?
• A Electron

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• Neutrons belong to which group of Hadrons?
• A Baryons

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• How does the Compton effect depend on Z?
• A It is independent of Z

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• What type of radiation is produced when electrons
  come close to the atomic nucleus ?
• A Bremsstrahlung

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• Of these two sub-atomic particles, which has the
  largest LET?
• Photon? Neutron?
• A Neutron

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• What type of collision results in no net loss of
  energy?
• A Elastic

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• Hadrons are made from what type of fundamental
  particles?
• A Quarks

48

• What is the rest mass energy of an electron in
  MeV?
• A 0.511 MeV

49

• Which of these is a form of DIRECTLY ionising
  radiation?
• Electron? Neutron?
• A Electron

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• What type of particle collision is short-handed
  by b gtgt a?
• A Soft Collision

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• What is produced when an electron and a positron
  annihilate?
• A Two g-rays

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• What is the probability of photon interaction
  called?
• A Linear Attenuation Coefficient

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• In which material do electrons of the same energy
  have the longest range?
• Bone? Fat?
• A Fat

54

• Radiation that is easily stopped in matter, has a
  HIGH or LOW LET?
• A High

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What is the probability that a charged particle
will pass through a medium without
interaction? A Zero
56

• How much energy is required to form an ion pair
  in dry air?
• A 34 eV