Transcript Interactions of Charged Particles with Matter (N Harding)x

TYPES OF RADIATION
Radiation
Mass
Electric Charge
Speed
α
4
+2
15,000 km/s
β
1/1800
±1
270,000 km/s
γ/X
0
0
300,000 km/s
• α – radiation: Helium nuclei
• β – radiation: Electrons and positrons
• γ/X – radiation: Electromagnetic radiation
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
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
PATH VERSUS RANGE
 Particle path length
 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)
PATH LENGTH (α)
 a-particles
 large mass particles
 dense linear ionisation track
 Path = Range
PATH LENGTH (β)
 β-particles
 small mass particles
 multiple scattering events
 follow tortuous path
 Path > Range
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
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
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
β+ RANGE
 F-18
 positron energy Emax = 0.6 MeV
 dependent on material
Material
Maximum Range (mm)
Lead
0.05
Glass
0.9
Water
2.4
Air
2000!
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
SPECIFIC IONISATION
RADIOTHERAPY
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)
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
INTERACTION MECHANISMS (1)
 Elastic collisions
 with orbital electrons
 with atomic nucleus
 Inelastic collisions
 with orbital electrons
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
INELASTIC COLLISIONS
 Energy transfer
 Incident electron
 loses energy
 Ionisation of the atom
 Excitation of the atom
INELASTIC COLLISIONS
 Ionisation
 ejection of a bound electron
 characteristic radiation or
Auger electrons
 Excitation
 bound electron “jumps” to
higher energy state
 characteristic radiation or
Auger electrons
AURORA
AURORA
INTERACTION MECHANISMS (2)
 Radiative collisions
 with atomic nucleus
 Involves the emission of radiation
 when a charged particle is accelerated
RADIATIVE COLLISIONS
 Inelastic collision
 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
RADIATIVE COLLISIONS
X-RAY TUBE
PRODUCTION OF X-RAYS
X-RAY SPECTRUM
Radionuclide Therapy
 P-32
 Polycythaemia rubra vera
 Thrombocythaemia
 Sm-153
 Bone metastases
 Ra-223
 Bone metastases
 I-131
 Thyrotoxicosis
 Thyroid cancer
How does it work?
How does it work?
 Some of the
131I
is accumulated in the thyroid gland
}
 The remainder is excreted in
 urine
 faeces
possible risk of contamination
 perspiration
 Saliva

131I
is based on the radiation-induced cell damage
caused by the high-energy radiation emitted
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
 Patient has no thyroid after the therapy
 The patient is then prescribed with T3 or T4
ablation)
POSITRON ANNIHILATION
PET ISOTOPES
photon
(511 keV)
positron emission
(~0.6-1.7 MeV)
positron
annihilation
photon
(511 keV)
neutrino
up to “a few mm”
Properties of PET Isotopes
Wide range of half-lives,
generally shorter than
conventional NM
Mainly cyclotron
produced but some
generators
Variation of Resolution with Positron Energy
Wide range of positron energies
higher energy → worse spatial resolution
Physical Limits on Resolution in PET
PET WHOLE BODY SCANS
2004
2005
INTERACTION MECHANISMS
 Inelastic interactions produce:
 heating
 visible light fluorescence
 Bremsstrahlung
 characteristic x-ray radiation
 secondary electrons
 Auger electron production