bikramjit radiation physics (lecture2)

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