Week 3 Radiation Physics

Attenuation is?

The reduction of energy (e.g x-ray) as it passes through a medium or tissue.

Linear attenuation coefficient (μ) is?

The measure of a material’s ability to attenuate an x-ray beam per unit thickness.

Thickness (cm) is a linear quantity

Large μ = high % of attenuation per cm

Low μ = vice versa

The total linear attenuation of coefficient is ?

The fraction of x-ray removed from the beam per unit thickness of the irradiated beam.

Mass attenuation coefficient (μ/p) is?

Linear attenuation coefficient (μ) divided by density of a material (p)

μ measured in cm² a

p measured in g.

Each x-ray interaction has a linear attenuation coefficient and mass attenuation coefficient

Mass attenuation coefficient for PE, Compton scatter, and pair production

(μ/p)_PE, (μ/p)_C, (μ/p)_PP

What is the total mass attenuation equal to?

(μ/p)_{TT =} (μ/p)_{PE +} (μ/p)_{C +} (μ/p)_PP

Photoelectric absorption

x-ray hits inner shell electron (usually k-shell)

X-ray is completely absorbed

When x-ray energy is more than binding energy of electron the electron is ejected from the atom (photo-electron)

Usually followed by characteristic x-ray emission

What increases PE occuring?

High atomic number of absorbing material

Decreases with increased x-ray photon energy.

Compton Scattering interactions explained

x-ray hits outer shell electron

x-ray is scatter and changes direction with less energy, some energy is transferred to electron which recoils

Electron energy = energy loss from x-ray

Scattered photon escapes target material or interacts again (PE or CS)

Compton scatter

X-ray is scattered angled 0-180 degrees

Small energy transfer from x-ray to electron = small scattering angle

Small incident x-ray energy = large scattering angle

Larger incident x-ray energy = smaller scattering angle

What increases probability of CS to occur?

1. High atomic number

2. Decreasing photon energy

Pair production

Incident x-ray passes close to the nucleus and interacts with nuclear field

X-ray absorption occurs and energy is converted to electron positron pair (E=mc²)

For PP to occur, x-ray energy must be 2× 0.511 MeV (1.02 MeV) this is the threshold energy

Energy greater than 1.02 MeV given to electron + positron as kinetic energy

Positron will annihilate with an electron and two gamma rays are produced at 180^o

Attenuation of x- and y-rays

depends on thickness and type of absorber, I = I ₀e^-μx I

I = intensity of transmitted beam (W/m²)

I₀ = initial intensity (W/m²)

μ = linear attenuation coefficient of absorber material (

x= thickness of absorber (cm)

can be rearranged for -ln (I/I₀) = μx

Half value thickness or Layer (HVT/HVL)

Thickness of absorber required to attenuate 50% of incident beam. = HVT = ln2/μ

Tenth value thickness

Thickness of absorber required to attenuate 10% of incident beam.

TVT = ln10/μ

Electron interactions

electrons are charged particles with mass and interact differently to photons (x-rays and gamma rays)

Interact with the electric field (coloumb field) of other charged particles (protons and orbital electrons)

Through these collisions the electrons may lose some kinetic energy or scatter.

Electron interactions with orbital electrons

Inelastic collisions between the incident electron and orbital electron are coulomb interactions that result in

1. Atomic ionisation - ejection of orbital elctron from the absorber atom

2. Atomic excitation - the transfer of an atomic orbital electron from one shell) to a higher shell

Atomic ionisation and excitations cause energy loss for incident electron

Electrons lose energy in 1000s of collisions losing small amounts of energy until it is stopped.

Electron interactions with nucleus

THey may pass close to nucleus and experience loss of energy due to electrostatic force

This results in x-ray photon emission (aka bremsstrahlung)

The energy of the -ray photons is distributed over a range (max energy = energy of the incident e-_ and is dependent on how close the e- passes to the atomic nucleus

x-rays electrons and dose

Electrons cause energy to be deposited in a material e.g tissue

X-ray interact through CS, PE, and PP

Results in electron

The electron then travels soem small distance losing energy along its track in multiple energy loss collisions (ionisation or excitation with orbital electrons)

Also radiative losses (bremsstrahlung from the nucleus)