Radiation Interactions

Ionizaiton

removing bound electrons

excitation

exciting bound electrons

nuclear

interactions involving a nuclear change

Photon Interactions with matter

Rayleigh Scattering, photoelectric effect, compton scattering, pair production, photonuclear interactions

Rayleigh Scattering (Coherent Scattering)

little concern in radiation therapy
For low-energy photons
Electric field of photon causes an electron to oscillate which is scattered in another direction with no change in energy

coherent scattering interaction probability

Z/ E²

Coherent Scattering

Photoelectric effect

photoelectric effect

a photon interacts with electron on inner shell where it is asborded and electron is ejected
either an a characteristic xray or auger electro emittedc

characteristic xray

photon emitted when electron drops from higher to lower energy state
binding energy equals difference between energy levels

Auger electron

characteristic xray interacts another bound electron and is absorbed, ejecting electron

Photoelectric Effect probability

Z³ / E³

Compton Scattering

Compton Scattering

occurs when a photon interacts with an outer shell electron and is scattered thru some angle ejecting electron with energy related to scattering angle

Compton Scattered Photon energy formula

Photons scattered at 90

side scatter
independent of energy with a max of 0.511 MeV

Photons scattered at 180

backscattered
independent of energy with a max of 0.255 MeV

Photons scattered between 90 and 180

Max energy between two max energies

Compton Scattering Probability

1/E for everything but hydrogen
As Energy increases, probability of compton decreases

Pair Production

Pair Production

occurs when a photon near a nucleus of an atom and an electron and positron pair are created
Min thershold is 1.022 MeV

Pair Production probability

Z ln(E)
More probable than Compton interactions about 25 MeV in tissue
slowly increases with increasing energy

Photonuclear interactions

high energy photons (above 8 MeV) interact with nucles and ejects neutrons (photodisintegrations)

• more common around 10 MeV

• why neutron shielding is important for linacs

Charged particle interactions

for a given medium is proportional to C² / v²
C = particle charge
v = particle speed

Bragg peak

occurs at end of charged particle’s path
as particle slows down, its rate of energy deposition increases rapidly

CSDA

continuous slowing down approximation (CSDA)
shortcut for approx range of charge particles
average energy loss per unit path length in a given medium

Proton Interactions

• usually interact by ionizing atom by knocking off electrons

• if too high energy, they reach nucleus and create a new atom (ex. F-18 in cyclotron)

• relatively straight path due to large size, bragg peak

Beta interactions

• scatter off bound electrons and eject them similar to compton interactions in photons

  • produces characteristic xrays

  • Bragg peak but cant observe it due to electrons scattering electrons that have random paths

Range Straggling

range of ranges bein exhibted for electrons due to energy straggling

energy straggling

each particle presents with different interactions, which varies energy of particles at a given depth
Beta particles

Bremsstrahlung radiation

electrons are decelerated in the electric field of nucleus so that change direction and radiate a photon
photons are emitted with a huge spectrum p to max energy of incident electrons (average = 1/3 Emax)
very inefficient (1%)

alpha interactions

can ionize electrons or interact with nucleus causing spontaneous or delayed radioactive decay
outer layer of skin is enough to stop them (but harmful to ingest)

Neutron Interactions

two categories:
Recoil reactions and nuclear activations

Recoil reactions

where a neutron hits a nucleus

Nuclear Activations

since neutrons carry no charge, they can easily reach a nucleus as electrons do not screen them with any repulsive forces due to charge
can be absorbed into nucleus and results in spontaneous decay

Photon interaction in Tissue

photoelectric effect - dominant from 10-30 keV
compton scattering - dominant from 30 keV to 25 MeV
pair production - above 25 MeV