1. Amanda Barry, Ph.D 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
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?

5. 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: m
Magnetic Field: 4T
(largest solenoid ever built)
Data Recorded/s =
10,000 Britannica
Encyclopaedias
Large Hadron Collider, CERN

6. 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

7. Introduction to Hadrons
2. Cancer Therapy
Image from:

8. 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

9. Introduction to Hadrons

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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 

16. 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

17. 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

18. Interaction of Neutrons with Matter
p(66) Be(49) Neutron Therapy Beam
(same as 8 MV photon beam)
% Depth Dose
Image from:

19. 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

20. 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%)

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

22. 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

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

24. 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

25. 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.

26. 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:

27. 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

28. End of Lecture 3

29. QUIZ

30. Heavy ions are ions that are heavier than which element?
A: Carbon

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

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

33. How does the photoelectric effect depend on energy?
A: 1/E3

34. Which Hadron is used for detecting bombs in airports?
A: Neutron

35. What is another name for an energetic secondary electron?
A: Delta ray

36. What is produced as a result of Pair Production?
A: positron/electron pair

37. What is the mass of a proton?
A: 1.67 e-27 kg

38. 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

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

41. What type of particle follows a tortuous path when passing through matter?
A: Electron

42. Neutrons belong to which group of Hadrons?
A: Baryons

43. How does the Compton effect depend on Z? A: It is independent of Z

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

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

46. What type of collision results in no net loss of energy?
A: Elastic

47. Hadrons are made from what type of fundamental particles?
A: Quarks

48. What is the rest mass energy of an electron in MeV?
A: MeV

49. Which of these is a form of DIRECTLY ionising radiation?
Electron? Neutron?
A: Electron

50. What type of particle collision is short-handed by b >> a?
A: Soft Collision

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

52. What is the probability of photon interaction called?
A: Linear Attenuation Coefficient

53. 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?

55. 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