bikramjit radiation physics (lecture2)

Interaction of radiation with matter

Dr. BIKRAMJIT CHAKRABARTI,

MD, DNB

Physics Lecture 2

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


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


• 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


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


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.


Energy converted to mass (positron)Mass (positron-electron) converted to energy (annihilation)

Photo-disintegrationLow energy neutrons emitted

Neutron contamination

Photon energy > 10 MV


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


30 KeV – 24 MeV

10-150 KeV 1.02 MeV and higher


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)


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)


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.


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 .


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


Physico-chemical event

• Excitation followed by ionization of water molecule:

H2O → H2O+ + e-

• Production of free radicals

H2O+ → H+ + OH*


Cellular effects of radiation - DNA


Cellular effects of radiation – cell structure

Damage to• Membranes• Lysosome

Bystander effectDr BIKRAMJIT CHAKRABARTI

Lecture 3

• Clinical radiation generators


bikramjit radiation physics (lecture2)

Health & Medicine