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

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

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.

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 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 Large Hadron Collider, CERN

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

Introduction to Hadrons 2. Cancer Therapy Image from: http://www.lns.infn.it/CATANA/CATANA/documents/pabloICATPP2003.pdf

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

Introduction to Hadrons

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 < 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 only if neutron comes close to nucleus, then interaction is via neutron and short range nuclear force

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

Interaction of Neutrons with Matter – 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 Neutron, E’ Recoiling Nucleus Incoming Neutron, Eo Nucleus Total 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

Interaction of Neutrons with Matter – 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) Emitted Neutron g-ray Incoming 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 - Eg E = Energy of the neutron after collision  Eo = Initial energy of the neutron 

Interaction of Neutrons with Matter- 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. g-ray Nucleus When neutrons lose sufficient energy through scattering, they can interact directly with the nucleus of the absorbing material If the energy of the neutron is known, the probability of capture by a specific nucleus can be defined by a term called "Capture Cross Section" which is expressed in Barns (1 s =10-24 cm2 ).  The capture cross section is different for each target nucleus, each isotope of the target nucleus and for each energy level of neutron. The cross section of nuclei vary greatly from almost 0 for helium to 2.5 X 10 6 Barns for a Xenon nucleus. When a neutron is captured by a nucleus, the progeny has an increased mass number of 1 and will emit a particle, electromagnetic radiation or the fission of the nucleus. The progeny itself may also be unstable and decay emitting various type of ionizing radiation. Slow Neutron Na23 Na24

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

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

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

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

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.

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_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/mm

How many interactions does a 1 MeV electron typically undergo before coming to a stop?

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 g-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?

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