Types of radiation
and
Interaction of radiation with matter
How different types of matter interacts with different media. Includes interaction of photon, neutron, proton, electron and alpha particles.
3. X-ray Gamma ray
Production Extra-nuclear Nuclear
Source Artificial Natural
Electron Beta ray
Production Extra-nuclear Nuclear
Source Artificial Natural
Dr BIKRAMJIT CHAKRABARTI
4. NATURE VELOCITY PENETRATION
POWER UP TO
IONISATION
ALPHA Heavy,
positive
charged
particle
1/10 of
light
Paper STRONG
BETA Light,
negative
charged
particle
9/10 of
light
Plastic WEAK
GAMMA Electro-
magnetic
radiation,
neutral.
100% of
light
Lead MODERATE
Dr BIKRAMJIT CHAKRABARTI
9. • High Z material
• Photon energy is low enough that the quantum effects of the interaction are unimportant
and the bound electron(s) can be regarded as essentially “free,”
• EM wave passes near electron
Oscillating
electron re-
irradiates
energy of same
frequency and
wavelength.
Coherent / classical scattering
1. Thomson scattering (single orbital electron)
2. Rayleigh scattering (group of electrons)Dr BIKRAMJIT CHAKRABARTI
10. Photon with specific energy
Photo-electric effect
Z3
specific differential attenuation causes contrast in X-ray and CT images
High Z material (lead) used for protection
1.Photo-electron:
• E= Ep-Eb
• Direction of emission depends on Ep
2. Characteristic (fluorescent) X-ray:
• Energy depends on Z & shell
specific Eb.
3. Auger-
electron
Probability = attenuation
τ/ρ = Z3
/E3
Probability peaks when Ep is
just greater than Eb
↑ Increasing
energy
Dr BIKRAMJIT CHAKRABARTI
12. Photon with high energy
The binding energy of the electron is insignificant
(considered ‘free’) compared with the incident photon’s
energy
Maximum energy for
photon during
• scatter at right
angle = 0.511 MeV
• back-scatter =
0.255 MeV
θ
Remember, angle
φ for photon!
Probability = attenuation
σc/ρ =
•Independent of Z
•Decreases with
increasing E
•Proportional to
electron/gm which is
essentially same for all
atoms (except H)
•Denser material (high
gm/cc) will have smaller
volume for same
attenuation. Compton effect
Therapeutic energy range
MV images are blurred
m0c2
= rest
energy of
electron =
0.511 MeV
Dr BIKRAMJIT CHAKRABARTI
13. Pair production along with annihilation
Energy of photon > 1.02 MeV
Photon 0.51 MeV
Photon 0.51 MeV
e+
e-
The probability of pair production (π/ρ)
• increases rapidly with incident photon
energy above the 1.02-MeV threshold
• proportional to Z2
per atom, Z per electron,
and approximately Z per gram.
Dr BIKRAMJIT CHAKRABARTI
Energy converted to mass
(positron)
Mass (positron-electron)
converted to energy
(annihilation)
14. Pair production along with annihilation
Energy of photon > 1.02 MeV
Photon 0.51 MeV
Photon 0.51 MeV
e+
e-
The probability of pair production (π/ρ)
• increases rapidly with incident photon
energy above the 1.02-MeV threshold
• proportional to Z2
per atom, Z per electron,
and approximately Z per gram.
Dr BIKRAMJIT CHAKRABARTI
Energy converted to mass
(positron)
Mass (positron-electron)
converted to energy
(annihilation)
16. Attenuation coefficients
• Linear attenuation coefficient (μ) (unit = cm-1
),
• Mass attenuation coefficient (μ/ρ) (unit = g-1
cm2
),
• Mass energy-transfer coefficient (μt/ρ),
• Mass energy-absorption coefficient (μen/ρ).
– Division by ρ, the physical density of the medium, makes
the coefficient medium independent.
N = N0e-µx
Dr BIKRAMJIT CHAKRABARTI
17. 30 KeV – 24
MeV
10-150 KeV 1.02 MeV
and higher
Dr BIKRAMJIT CHAKRABARTI
18. LET Stopping power
Explanation Energy deposition
per unit length
Ability of medium to stop fluence of
radiation
Unit KeV/µm J/m or Mev/cm (linear)
J/(kg/m2
) or MeV/g/cm2
) (mass)
Dr BIKRAMJIT CHAKRABARTI
Exposure = output Dose Kerma
Explanation Ionization/unit
mass
Energy absorbed/
unit mass
Energy released
SI unit C/kg Gy (J/kg) Gy (J/kg)
Other units R (esu/cm3
at STP) rad (100 ergs/g) -
Relation 1 R = 2.58 X 10-4
C/kg
1 Gy = 100 rad
= 0.876 R (air)
-
Equivalent dose Effective dose
Unit is Sv (J/kg) Energy absorbed to volume of
tissue
Energy absorbed to whole
body
Radiation WF (WR) Tissue WF (WT)
19. Interaction of electrons
1. Elastic collision (excitation): With atomic electron OR nuclei
→ No loss of kinetic energy, only change in direction of incident electron.
2. In-elastic collision:
– Ionisation of atom
(with orbital
electron) → Ejected
electron (if produces
further ionisations,
are known as δ ray.
– Bremsstraughlung X-
ray = radiative loss
(with nucleus)
Dr BIKRAMJIT CHAKRABARTI
20. Interaction of heavy, charged particles
1. Ionization and excitation
2. Interaction of
coulomb forces →
radiative loss
3. Nuclear
reactions
producing radio-
active nuclei
Proton: Hydrogen ion
Alpha particle: Helium ion
Carbon ion
Meson Dr BIKRAMJIT CHAKRABARTI
21. Why Bragg peak?
• Stopping power (rate of energy loss / unit
length)
• Also depends on electron density of media.
• The range of a charged particle is the distance
it travels before coming to rest. Range
proportional to (charge)2
X rest mass.
• The mass stopping power of a material is obtained by dividing the
stopping power by the density ρ.
Dr BIKRAMJIT CHAKRABARTI
23. Interaction of NEUTRONS
(High LET)
Main energy loss occurs when
interacts with hydrogen atom
= Recoil proton
Therefore, excess damage to
hydrogen containing tissues
(fat), nerve cells.
Hydrogenous material is good
for shielding
Nuclear disintegration .
Dr BIKRAMJIT CHAKRABARTI
Proton
Neutrons
Deuterium
γ
24. HIGH LET
(High RBE, low OER)
[Useful for hypoxic
tissue / low α:β
tumors]
BRAGG PEAK
(No exit / lateral dose)
[Useful for tumors at
close proximity to OAR]
NEUTRON PROTON & other heavy,
charged particles
CARBON IONS
Dr BIKRAMJIT CHAKRABARTI
25. Physico-chemical event
• Excitation followed by
ionization of water
molecule:
H2O → H2O+
+ e-
• Production of free
radicals
H2O+
→ H+
+ OH*
Dr BIKRAMJIT CHAKRABARTI
The typical energy loss in tissue for a therapeutic electron beam, averaged over its entire range, is about 2 MeV/cm in water.
The complete description of the energy and depth of penetration of the moving electrons at any point in the medium is complicated by the fact that the electrons are very much lighter than the atomic nuclei. As a result, the electron can lose a very large fraction of its energy in a single process and thus can be deflected by very large angles. This means that even if the electron beam is monoenergetic when first impinging on a medium, there will be a large variation among all the moving electrons as to where in the medium each will stop. This is referred to as range straggling.
In the physical processes of proton interaction in bio-materials, most of the proton energy is transferred to electrons. Ionization and excitation occur most frequently around the Bragg peak region, where nuclear reactions also exist. Protons generate numerous neutrons via nuclear reactions. Particularly, neutrons with relatively low energies produce recoil protons by elastic collisions with the hydrogen atoms. Around the Bragg peak, low-energy primary protons (slowed-down protons) are prevalent, whereas recoil (secondary) protons gradually become dominant behind the distal falloff region of the Bragg peak. Therefore, around the Bragg peak, the main contribution to the absorbed dose is that of the primary protons (from 80 to 90%), whereas secondary protons created by primary proton-induced reactions contribute to the dose from 20 to 5%. Behind the distal endpoint of the Bragg peak, the absorbed dose is mainly due to the protons produced by (1)H(n, p), and the contribution of these is about 70%.
Depth–dose distributions for a spread-out Bragg peak (SOBP, red), its constituent pristine Bragg peaks (blue), and a 10 MV photon beam (black). The SOBP dose distribution is created by adding the contributions of individually modulated pristine Bragg peaks. The penetration depth, or range, measured as the depth of the distal 90% of plateau dose, of the SOBP dose distribution is determined by the range of the most distal pristine peak (labeled 'Pristine peak'). The modulation width, measured as the distance between the proximal and distal 90% of plateau dose values, of the SOBP dose distribution is controlled by varying the number and intensity of pristine Bragg peaks that are added, relative to the most distal pristine peak, to form the SOBP. The dashed lines (black) indicate the clinical acceptable variation in the plateau dose of 2%. The dot–dashed lines (green) indicate the 90% dose and spatial, range and modulation width, intervals. The SOBP dose distribution of even a single field can provide complete target volume coverage in depth and lateral dimensions, in sharp contrast to a single photon dose distribution; only a composite set of photon fields can deliver a clinical target dose distribution. Note the absence of dose beyond the distal fall-off edge of the SOBP.