Amanda Barry, Ph.D

Interaction of Radiation with Matter - Lecture 3For spRs sitting FRCR Part I Examinations

Interaction of Charge Particles with Matter

TO RECAP:1. Scattered Radiation & Secondary electrons - sources of scatter

and effects2. Charged particles are surrounded by an electrostatic field3. Charged particle undergoes many interactions4. Energy loss due to interaction of Coulomb fields of incoming

charged particle and that of atomic electron/nuclei1. Collisional Losses – Ionisation/Excitation via Hard & Soft Xns2. Radiative Losses – Bremsstrahlung via interaction with

nuclear field5. Stopping Power and Restricted Stopping Power

1. Absorbed Dose2. Particle Range

Interaction of Sub-atomic Particles with Matter

3. 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 range5. Bragg peak

3. Bremsstrahlung4. Neutrons - elastic and inelastic collisions.5. Protons, ionisation profile6. Elementary knowledge of pions and heavy ions.

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?

Introduction to Hadrons

1. High Energy Nuclear Physics

Large Hadron Collider, CERN

•Particles are accelerated to energies of ~1500 TeV before colliding •12,500 Tonnes•Diameter:15 m•Length: 21.5 m•Magnetic Field: 4T(largest solenoid ever built)•Data Recorded/s = 10,000 Britannica Encyclopaedias

Introduction to Hadrons

• Home to the WWW

• Particle Physics: “ Recreating the BIG BANG”

• 27 km acceleratorCrosses French/Swiss border 4 times

• 20 European nations3000 Enployees

CERN

http://public.web.cern.ch/Public/Welcome.html

Introduction to Hadrons

2. Cancer Therapy

Image from: http://www.lns.infn.it/CATANA/CATANA/documents/pabloICATPP2003.pdf

Introduction to HadronsWhy 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

HADRONS ENABLE DELIVERY OF HIGH DOSE TO

THE TUMOUR SPARING THE SURRONDING TISSUES

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.

Introduction to Hadrons

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 < 0.5 eV

• Intermediate-energy neutrons, 0.5 eV < EN < 10 keV

• Fast neutrons, E > 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 

Interaction of Neutrons with Matter

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:1. Elastic scattering 2. Inelastic scattering 3. Neutron capture

1. 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

Interaction of Neutrons with Matter – Elastic Scattering

Neutron, E’

Recoiling Nucleus

IncomingNeutron, Eo

NucleusTotal energy unchanged

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

2

mMmM

EE o

Interaction of Neutrons with Matter – Inelastic Scattering

2. Inelastic Scattering• Neutron momentarily captured by nucleus• Neutron re-emitted with less energy• Nucleus left in excited state• Nucleus relaxes by emitting -rays or charged particles

(adds to dose)

EmittedNeutron

-ray

IncomingNeutron

Nucleus

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 - E

E = Energy of the neutron after collision Eo = Initial energy of the neutron 

Interaction of Neutrons with Matter- Neutron Capture

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

capture.

SlowNeutron

-ray

Nucleus

Na23 Na24

Interaction of Neutrons with Matter

Where are neutrons useful?1. Cancer Therapy2. To produce radioactive isotopes for radiotherapy or imaging3. To analyse composition and structure of unknown elements4. Bomb detectors in airports5. Construction of electronic devices6. Nuclear energy

Image from: A. L. Galperin, Nuclear Energy/Nuclear Waste. Chelsea House Publications: New York, 1992

Interaction of Neutrons with Matter

% D

epth

Do

se

Image from: http://www-bd.fnal.gov/ntf/reference/hadrontreat.pdf

p(66) Be(49) Neutron Therapy Beam

(same as 8 MV photon beam)

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

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%)

Protons

Proton vs Photon Depth Dose in Water*•

*w.massgeneral.org/.../proton/principles.asp

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

Protons

From Particles, Newsletter, (Ed Sisterton) No. 28 July 2001

World-wide Proton Treatments

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

PionsWhat are Pions?

• Pions (= Pi Mesons)

• Symbols:-,0, +

• 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.

Pions

Pions (-) in radiotherapy:• When the - 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_trtmt.html

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

End of Lecture 3

QUIZ

Heavy ions are ions that are heavier than which element?

A: Carbon

What type of interaction is most common for photons in the radiotherapy energy range?

A: Compton Effect

What do you call a sub-atomic particle that experiences the

strong nuclear force?

A: Hadron

How does the photoelectric effect depend on energy?

A: 1/E3

Which Hadron is used for detecting bombs in airports?

A: Neutron

What is another name for an energetic secondary electron?

A: Delta ray

What is produced as a result of Pair Production?

A: positron/electron pair

What is the mass of a proton?

A: 1.67 e-27 kg

When many electrons are produced as a result of the Auger Effect, we

have an …?

A: Auger Shower

Approximately, what is the LET of a 5 MeV neutron?

A: ~ 50 keV/m

How many interactions does a 1 MeV

electron typically undergo before

coming to a stop?

A: 100,000

What type of particle follows a tortuous path when passing

through matter?

A: Electron

Neutrons belong to which group of Hadrons?

A: Baryons

How does the Compton effect depend on Z?

A: It is independent of Z

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

A: Bremsstrahlung

Of these two sub-atomic particles, which has the largest

LET?

Photon? Neutron?

A: Neutron

What type of collision results in no net loss of energy?

A: Elastic

Hadrons are made from what type of fundamental particles?

A: Quarks

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

A: 0.511 MeV

Which of these is a form of DIRECTLY ionising radiation?

Electron? Neutron?

A: Electron

What type of particle collision is short-handed by b >> a?

A: Soft Collision

What is produced when an electron and a positron annihilate?

A: Two -rays

What is the probability of photon interaction called?

A: Linear Attenuation Coefficient

In which material do electrons of the same energy have the

longest range?

Bone? Fat?

A: Fat

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

A: High

What is the probability that a charged particle will pass through a medium without

interaction?

A: Zero

How much energy is required to form an ion pair in dry air?

A: ~ 34 eV