Interactions of Radiation With Matter

RADL 70Kyle Thornton

Basic Concepts Of Interaction

Three possible occurrences when x or gamma photons in the primary beam pass through matter: No interaction at all

Known as transmission Absorption Scatter

The latter two are methods of attenuation

Attenuation

The reduction of x-ray photons as they pass through matterPrimary radiation – attenuation = remnant or exit radiation

Attenuation Of An X-Ray Photon

The Five Interactions Of X and Gamma Rays With Matter

Photoelectric effect Very important in diagnostic radiology

Compton scatter Very important in diagnostic radiology

Coherent scatter Not important in diagnostic or therapeutic

radiology

Pair production Very important in therapeutic radiology

Photodisintegration Very important in therapeutic radiology

Photoelectric Effect

All of the energy of the incoming photon is totally transferred to the atom Following interaction, the photon ceases to exist

The incoming photon interacts with an orbital electron in an inner shell – usually KThe orbital electron is dislodgedTo dislodge the electron, the energy of the incoming photon must be equal to, or greater than the electron’s energy

Photoelectric Effect

The incoming photon gives up all its energy, and ceases to existThe ejected electron is now a photoelectronThis photoelectron now contains the energy of the incoming photon minus the binding energy of the electron shellThis photoelectron can interact with other atoms until all its energy is spentThese interactions result in increased patient dose, contributing to biological damage

Photoelectric Effect

Photoelectric Effect

A vacancy now exists in the inner shellTo fill this gap, an electron from an outer shell drops down to fill the gapOnce the gap is filled, the electron releases its energy in the form of a characteristic photonThis process continues, with each electron emitting characteristic photons, until the atom is stableThe characteristic photon produces relatively low energies and is generally absorbed in tissue

Characteristic Radiation Cascade

The Byproducts of the Photoelectric Effect

PhotoelectronsCharacteristic photons

The Probability of Occurrence

Depends on the following: The energy of the incident photon The atomic number of the irradiated object It increases as the photon energy decreases, and

the atomic number of the irradiated object increases

When the electron is more tightly bound in its orbit When the incident photon’s energy is more or

close to the binding energy of the orbital electron This type of interaction is prevalent in the

diagnostic kVp range – 30 - 150

What Does This All Mean?

Bones are more likely to absorb radiation This is why they appear white on the film

Soft tissue allows more radiation to pass through than bone These structures will appear gray on the film

Air-containing structures allow more radiation to pass through These structures will appear black on the film

Compton Scattering

An incoming photon is partially absorbed in an outer shell electronThe electron absorbs enough energy to break the binding energy, and is ejectedThe ejected electron is now a Compton electronNot much energy is needed to eject an electron from an outer shellThe incoming photon, continues on a different path with less energy as scattered radiation

Compton Scatter

Byproducts Of Compton Scatter

Compton scattered electron Possesses kinetic energy and is capable of ionizing

atoms Finally recombines with an atom that has an electron

deficiency

Scattered x-ray photon with lower energy Continues on its way, but in a different direction It can interact with other atoms, either by

photoelectric or Compton scattering It may emerge from the patient as scatter

Contributes to radiographer dose or

Contributes to film fog

Probability Of Compton Scatter Occurring

Increases as the incoming photon energy increasesMore probable at kVp ranges of 100 or greaterResults: Most of the scattered radiation produced

during a radiographic procedure The scatter is isotropic

Sidescatter, backscatter, or small-angle (forward)

Coherent Scatter

Occurs at low energies – below 30 kVpAn incoming photon interacts with an atomThe atom vibrates momentarilyEnergy is released in the form of an electromagnetic waveA combination of these waves form a scatter waveThe photon changes its direction, but no energy is transferredMay result in radiographic film fog

Pair Production

Does not occur in the diagnostic energy rangeIncoming photon must have an energy of at least 1.02 MeVThis process is a conversion of energy into matter and then matter back into energyTwo electrons are produced in this interaction

Pair Production

An incoming photon of 1.02 MeV or greater interacts with the nucleus of an atomThe incoming photon disappearsThe transformation of energy results in the formation of two particlesNegatron Possesses negative charge

Positron Possesses a positive charge

Pair Production

PositronsConsidered antimatterDo not exist freely in natureCannot exist near matterWill interact with the first electron they encounterAn electron and the positron destroy each other during interaction Known as the annihilation reaction

This converts matter back into energyBoth the positron and electron disappearTwo gamma photons are released with an energy of .51 MeV

Pair Production

The produced gamma photons may interact with matter through pair production or Compton scatterPair production is used for positron emission tomography, a nuclear medicine imaging procedureIt is also used in radiation therapy

Photodisintegration

Occurs at above 10 MeVA high energy photon is absorbed by the nucleusThe nucleus becomes excited and becomes radioactiveTo become stable, the nucleus emits negatrons, protons, alpha particles, clusters of fragments, or gamma raysThese high energy photons are found in radiation therapy

Photodisintegration

Interactions Of Particulate Radiation With Matter

Alpha radiation is monoenergeticBeta particles and positrons are also monoenergeticThese particles lose energy in the form of ion pairsAs they pass near or through a neutral atom, they remove energy through the force of attraction or repulsion

Interactions Of Particulate Radiation With Matter

Alpha particles ionize by attracting an electron from an atomBeta particles ionize by repelling an electron from an atom

Two Mains Types Of Particulate Interaction

Elastic interaction No change in kinetic energy, it is transferred

from one particle to another Alpa particles colliding with outer shell orbital

electrons

Inelastic interaction The total kinetic energy is changed after the

interaction Beta particles interacting with inner shell orbital

electrons and slow down This produces low penetrating secondary radiation

Summary Of Interactions