Module 8: Interactions with Matter

what is a photon

bundle of energy which has no mass and moves through space as a waveform at the speed of light

• also called electromagnetic radiaiton

what is am important characteristic of light

• the amount of photons moving through space (intensity) decreases the farther away you are from the source of this radiation

what does radiation follow the principle of

1/r²

• I(o)/I(n) = d²(n)/d²(o)

what happens when photons hit matter

they can

• be transmitted (light passing through clear glass)

• be reflected (like light hitting a mirror)

• refracted like bending an image under water

• diffracted like light out of a prism

• be attenuated/absorbed/adsorbed

• be scattered (light passing through opaque glass)

what happens as the energy of the photon increases to the probability of interacting with matter

the probability of interacting with matter decreases

what happen when an atom becomes ionized

it becomes much more chemically reactive

where does the photoelectric effect occur relative to Z

• lower end of the energy spectrum and probability increases with Z

• the relative probability of this interaction increase at a rate of Z³ a material - 2 times the Z will have a 8 times higher probability of having photoelectric interactions

what happens during the photoelectric effect

• photon gets completely absorbed by an inner orbit electron and as long as the energy absorbed is greater than the binding energy it is ionized otherwise it is just converted into kinetic energy

when is photoelectric effect more likely to occur

• when the photon’s energy is close to but slightly higher that the binding energy of the electron its ionizing

how does the probability of the photoelectric effect decrease

• at a rate of (photon energy - binding enegy)³

where does the photoelectric effect occur with the highest probability

• with tightly bound K and L shell electrons

what is produced when an atom has been ionized

• an ion pair consisting of a positively charged atom and a negatively charged energetic recoil electron

what happens to the ionized atom

it will quickly absorb a nearby electron

what happens to the recoil electron

it will interact with close by atom causing additional ionizations

what is compton scattering

• an imcoming photon is absorbed and results in an electron being removed, BUT the incoming photo is only partially absorbed

• the photon is diverted from its original path of travel called “Scattering”

• sinilar to photoelectric effect

what does the scattering range from in the compton scattering effect

• a range from a few degrees to a max of 180 degrees (called backscatter)

• the scattered photon always has less energy than the original photon

the larger the scattering angle, the __ energy is lost by the photon

more

Emax in compoton scattering

• scatter occurs at the small angles of deflection

• called compton edge

• measured in MeV

Emin in compton scattering

• scatter occurs at 180 degree of deflection

• is called backscatter

• measured in MeV

what happens as the energy of photons increase to the probability of compton scattering occuring instead of photoelectric effect

• the probabiltiy of compton scattering effect happening increases

as photon energies increase further, the probability of compton scattering starts to

decrease

when does coherent scatter (rayleigh or thompson scatter) occur

• occurs with low energy photons velow 10 keV

what happens in coherent scatter (rayleigh or thompson scatter)

• the absorbed photon just excites the atom but doesn’t ionize it

• the energy is then released as another photon of the same energy as the one aborbed but in a different direction

why is compton scatter important in radiation safety

• compton scatter profoundly impacts radiation safety as well as image quality

• this is why is there a debate about x ray workers

what happens when a photon with energy greater than 1.02 MeV is absorbed by a nucleus

• the energy can get converted directly into mass

what do the electrons of the energies have

two electrons get created with excess energy going to kinetic energy

• two electrons with 0.511 MeV

• each have an opposite charge

• the pair have no net or overall charge

what happens to a photon with energy greater than 10 MeV is absorbed by a nucleus

• the energy causes the nucleus to break apart

what is the Cerenkov effect

• unique interaction where a charged particle travels in a medium at a speed greater than the speed of light in that medium

• created an electromagnetic shockwave in teh form of visible light

as radiation passes through matter it can either…?

• interact

• or not interact

what happens when radiation passing through matter interacts

• it deposits energy in the matter it comes in contact with

• this process can leave a wake of destruction around the emission as it travels through space

the more massive, more energetic, or larger the charge of the emission the ___ the probability of interactiion as well as the __ the path of destruction

• larger

• larger

which interacts with the most matter

• alpha

the probabilitiy of a x ray or gamma ray interacting is much __

• lower than particle radiation

Linear energy transfer (LET)

• the energy that gets deposited locally along an emissions track

what is LET expressed in

KeV/um

Linerar stopping power

• also includes energy loss to distant effects like bremsstrahlung

Specific Ionization (SI)

refers to the total number of ion pairs produced by primary and secondary ionization

SI increases as a particle __

• slows down

• important in planning radiotherapy treatments since we want the maximum damage to occur in