Interaction of radiation with matter
Dr. BIKRAMJIT CHAKRABARTI,
MD, DNB
Physics Lecture 2
Dr BIKRAMJIT CHAKRABARTI
Dr BIKRAMJIT CHAKRABARTI
X-ray Gamma ray
Production Extra-nuclear Nuclear
Source Artificial Natural
Electron Beta ray
Production Extra-nuclear Nuclear
Source Artificial NaturalDr BIKRAMJIT CHAKRABARTI
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
A. Electro-magnetic radiation
B. Particle radiationPARTICLE SYMBOL CHARGE MASS
PHOTON hv, γ 0 0
ELECTRON e, e-, β- -1 5.49 X 10-4 amu
POSITRON e+, β+ +1 5.49 X 10-4 amu
PROTON p, 1H1 +1 1.007277 amu
NEUTRON n, 0n1 0 1.008665 amu
ALPHA α, 2He4 +2 4.002604 amu
NEUTRINO v 0 <1/2,000 mo
PI MESONS π+, π-, π0 +1, 0, -1 273 mo, 264 mo,
MU MESON µ+, µ- +1, -1 207 mo
K MESON K+, K-, K0 +1, 0, -1 967 mo,,973 mo
1 amu = 1.66043 X 10-27 kg, m0 (rest mass of electron: 9.1091X 10-31 kgDr BIKRAMJIT CHAKRABARTI
Interaction depends on Radiation: Energy, charge, rest massMedia: Atomic configuration, density
Dr BIKRAMJIT CHAKRABARTI
Dr BIKRAMJIT CHAKRABARTI
• 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 scattering1. Thomson scattering (single orbital electron)
2. Rayleigh scattering (group of electrons)Dr BIKRAMJIT CHAKRABARTI
Photon with specific energy
Photo-electric effectZ3 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 = attenuationτ/ρ = Z3/E3
Probability peaks when Ep is just greater than Eb
↑ Increasing energy
Dr BIKRAMJIT CHAKRABARTI
Dr BIKRAMJIT CHAKRABARTI
Photon with high energyThe 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 rangeMV images are blurred
m0c2 = rest energy of electron = 0.511 MeV
Dr BIKRAMJIT CHAKRABARTI
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)
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)
Photo-disintegrationLow energy neutrons emitted
Neutron contamination
Photon energy > 10 MV
Dr BIKRAMJIT CHAKRABARTI
Attenuation coefficients
• Linear attenuation coefficient (μ) (unit = cm-1), • Mass attenuation coefficient (μ/ρ) (unit = g-1cm2),
• 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
30 KeV – 24 MeV
10-150 KeV 1.02 MeV and higher
Dr BIKRAMJIT CHAKRABARTI
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)
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
Interaction of heavy, charged particles
1. Ionization and excitation2. Interaction of coulomb forces → radiative loss
3. Nuclear reactions producing radio-active nuclei
Proton: Hydrogen ionAlpha particle: Helium ion
Carbon ionMeson Dr BIKRAMJIT CHAKRABARTI
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
Dr BIKRAMJIT CHAKRABARTI
Interaction of NEUTRONS(High LET)
Main energy loss occurs when interacts with hydrogen atom
= Recoil protonTherefore, excess damage to hydrogen containing tissues
(fat), nerve cells.Hydrogenous material is good
for shielding
Nuclear disintegration .
Dr BIKRAMJIT CHAKRABARTI
Proton
Neutrons
Deuterium
γ
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
Physico-chemical event
• Excitation followed by ionization of water molecule:
H2O → H2O+ + e-
• Production of free radicals
H2O+ → H+ + OH*
Dr BIKRAMJIT CHAKRABARTI
Cellular effects of radiation - DNA
Dr BIKRAMJIT CHAKRABARTI
Cellular effects of radiation – cell structure
Damage to• Membranes• Lysosome
Bystander effectDr BIKRAMJIT CHAKRABARTI
Lecture 3
• Clinical radiation generators
Dr BIKRAMJIT CHAKRABARTI