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Medical Physics: Radiation Safety > Interactions of Radiation and Matter > Flashcards

Flashcards in Interactions of Radiation and Matter Deck (19)
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What three (general) processes occur when an x-ray beam interacts with a patient, and what do they cause?

Transmission - Forms image
Scattering - Staff Dose
Absorption - Patient Dose


State and explain the equation for Exposure (X).

X = dQ/dm
X - The amount of radiation that produces, in air, ion of either sign equal to 1C/kg
dQ - The sum of all the charges (of all ions of one sign) produced when all electrons produced in a volume (dV) are stopped in air.
dm - The mass of air in volume dV


State and explain the equation for Absorbed Dose (D).

D = dE/dm
the energy absorbed per unit mass, measured in Gy (J/kg), need to specify the material in which the energy is absorbed.


Explain how to go from Exposure to Dose in Air.

X = Q/m
X/e = No. Ion pairs produced
W - average energy to produce 1 ion pair.
E = W(X/e)


What two assumptions are needed to equate KERMA to absorbed dose-in-air?

- Negligible Bremsstrahlung
- Short electron ranges


How is dose in air converted to dose in another medium?

D(m)/D(a) = [u/p](m)/[u/p](a)
u - mass attenuation coefficient for the medium
p - density of the medium


Explain the photoelectric effect.

X-Ray photon transfers all energy to inner (k) shell electron
Electron liberated with energy of photon - binding energy
Vacancy in k-shell filled by outer electron
Characteristic X-ray emitted (may be absorbed locally in low z material)
Dominant at lower energies.


How does the photoelectric effect change with increasing atomic number?

Proportional to Z^3


How does the photoelectric effect change with increasing energy?

Proportional to 1/E^3


What is the name of an outer shell electron emitted instead of the photon through the photoelectric effect?

Auger Electron


What is the equation for fluorescent yield (probability of no Auger Electrons)?

w = no of k x-ray photons/no of k shell vacancies


Explain compton scattering.

Photon incident on "free" electron
Photon scatters at reduced energy
Electron recoils from collision
Change in photon energy depends on initial energy and angle of scatter.


What is the change in wavelength from compton scatter?

Δλ = (h/mc)*(1-cosθ)


What is the change in energy from compton scatter?

ΔE = E(0)*[α(1-cosθ)/(1 + α(1-cosθ))]
α = E(0) (in MeV)/0.511


How does compton scatter change with increasing energy?

Constant (below 100keV)
Proportional to 1/E (above 100keV)


How does compton scatter change with increasing Z

Independent of Z.


Explain elastic scattering.

Whole atom absorbs recoil
Bound electrons vibrate at frequency of photons
Electrons re-radiate energy at same frequency as photon
Scattering in forward radiation
Less than 10% of interaction - of little practical importance
Proportional to (Z^2)/E


Explain pair procuction.

High energy (>1.02MeV) photon produces an electron-positron pair.
Positron annihilates with a local free electron.
Produces two photons (0.511MeV) in opposite directions.
Proportional to E-1.02 and Z


Explain how k edge matching is used in X-Ray imaging

Match detector/contrast material with k-edge at peak of x-ray spectrum.
Maximises absorption, (k-shell electrons can now be emitted) for fewer photons.
Lower dose to patient for same image quality.