1. Interactions of Charged Particles with Matter
Nick Harding
Clinical Scientist
Radiotherapy Department
Castle Hill Hospital
Hull & East Yorkshire Hospitals NHS Trust

2. TYPES OF RADIATION α 4 +2 β 1/1800 ±1 Radiation Mass Electric Charge
Speed
15,000 km/s α 4 +2
270,000 km/s β
1/1800 ±1
300,000 km/s
γ / X
α – radiation: Helium nuclei
β – radiation: Electrons and positrons
γ/X – radiation: Electromagnetic radiation

3. CHARGED PARTICLES Charged particles (e.g. α2+, β±, p+) interact
Electromagnetically (i.e. via Coulombic forces)
Elastic and inelastic collisions
involve collisions with e- and nuclei in absorbing material
interactions with e- most common
Radiative collisions – Bremsstrahlung
charged particles accelerated by electric field of nucleus

4. INDIVIDUAL INTERACTIONS
Charged particles
suffer many interactions along their path
energy loss considered a continuous process
At each interaction
charged particles are deflected/scattered
Charged particles may pass near a nucleus
suffers large deflection
most pronounced for light particles

5. PATH VERSUS RANGE Particle path length Particle range
the actual distance the particle travels
is dependent on the mass of the particle
Particle range
the actual depth of penetration of the particle in matter
is dependent
on the mass and kinetic energy of the particle
the traversing material (i.e. atomic number Z)

6. PATH LENGTH (α) a-particles Path = Range large mass particles
dense linear ionisation track
Path = Range

7. PATH LENGTH (β) β-particles Path > Range small mass particles
multiple scattering events
follow tortuous path
Path > Range

8. α RANGE (in water) Range of α-particles of energy of 4 MeV
dependent on traversing material
Material
Range (mm)
Air 25
Tissue
0.014

9. α RANGE (in water) Range of α-particles of energy of 8 MeV
dependent on traversing material
Material
Range (mm)
Air 70
Tissue
0.042

10. β- RANGE Electron range is dependent on kinetic energy of electrons
traversing material (e.g. water)
Electron Energy (keV)
Range (mm) 20
0.01 40
0.03
100
0.14
400
1.30

11. β+ RANGE F-18 positron energy Emax = 0.6 MeV dependent on material
Maximum Range (mm)
Lead
0.05
Glass
0.9
Water
2.4
Air
2000!

12. SPECIFIC IONISATION Specific ionisation
number of primary/secondary ion pairs produced per mm
expressed in ion pairs (IP/mm)
increased with the electrical charge of the particle
decreased with incident velocity of the particle

13. SPECIFIC IONISATION

14. RADIOTHERAPY

15. LINEAR ENERGY TRANSFER
The linear energy transfer (LET) is defined as:
the amount of energy deposited per unit length (eV/mm)
The LET of a particular type of radiation determines
the biologic consequence of radiation exposure
high LET radiations (α-particles, protons, electrons)
low LET radiations (γ-rays and X-rays)

16. INTERACTION MECHANISMS
Electromagnetic interactions
extend over some distance
not necessary for particle to make direct collision
can transfer energy simply passing close by
atomic internal energy quantised
only certain energy values can be transferred
may or may not excite and/or ionise atoms

17. INTERACTION MECHANISMS (1)
Elastic collisions
with orbital electrons
with atomic nucleus
Inelastic collisions

18. ELASTIC COLLISIONS No energy transfer Low-angle diffusion
Coulomb interaction with electron cloud
High-angle diffusion
Coulomb interaction with atomic nucleus
Atom is not ionised/excited

19. INELASTIC COLLISIONS Energy transfer Incident electron
loses energy
Ionisation of the atom
Excitation of the atom

20. INELASTIC COLLISIONS Ionisation Excitation
ejection of a bound electron
characteristic radiation or Auger electrons
Excitation
bound electron “jumps” to higher energy state

21. AURORA

22. AURORA

23. INTERACTION MECHANISMS (2)
Radiative collisions
with atomic nucleus
Involves the emission of radiation
when a charged particle is accelerated

24. RADIATIVE COLLISIONS Inelastic collision Charged particle is deflected
with electric field of nucleus
Charged particle is deflected
by the positive charge of nucleus
with a loss of kinetic energy
Bremsstrahlung (braking radiation)
X-rays
Energy of X-ray is equal to
the energy lost by the electron

25. RADIATIVE COLLISIONS

26. X-RAY TUBE

27. PRODUCTION OF X-RAYS

28. X-RAY SPECTRUM

29. Radionuclide Therapy P-32 Sm-153 Ra-223 I-131 Polycythaemia rubra vera
Thrombocythaemia
Sm-153
Bone metastases
Ra-223
I-131
Thyrotoxicosis
Thyroid cancer

30. Xofigo® THERAPY Used in medical therapy Ra-223 dichloride (Xofigo®)
Used to treat prostate cancer once it has spreaded to the bones
Radium acts like calcium

31. Radionuclide Therapy P-32 Sm-153 Ra-223 I-131 Polycythaemia rubra vera
Thrombocythaemia
Sm-153
Bone metastases
Ra-223
I-131
Thyrotoxicosis
Thyroid cancer

32. How does it work?

33. How does it work?
Some of the 131I is accumulated in the thyroid gland
The remainder is excreted in
urine
faeces
perspiration
Saliva
131I is based on the radiation-induced cell damage caused by the high-energy radiation emitted }
possible risk of contamination

34. How does it work?
Irradiated thyroid cells lose the ability to multiply themselves
Total mass of the gland is steadily reduced
The thyroid gland is totally ablated (i.e. 131I ablation)
Patient has no thyroid after the therapy
The patient is then prescribed with T3 or T4

35. POSITRON ANNIHILATION

36. PET ISOTOPES positron annihilation positron emission (~0.6-1.7 MeV)
photon (511 keV)
neutrino
positron emission (~ MeV)
up to “a few mm”

37. Properties of PET Isotopes
Wide range of half-lives,
generally shorter than conventional NM
Mainly cyclotron produced but some generators

38. Variation of Resolution with Positron Energy
Wide range of positron energies
higher energy → worse spatial resolution

39. Physical Limits on Resolution in PET

40. PET WHOLE BODY SCANS
2004
2005

41. INTERACTION MECHANISMS (3)
Cherenkov radiation
a particle moves faster than the speed of light in a material
Shock wave of EM radiation

42. INTERACTION MECHANISMS
Inelastic interactions produce:
heating
visible light fluorescence
Bremsstrahlung
characteristic x-ray radiation
secondary electrons
Auger electron production

43. Thanks for listening