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MED 107- MEDICAL PHYSICS II > 2. IONISING RADIATION: INTERACTIONS OF PHOTONS WITH MATTER > Flashcards

2. IONISING RADIATION: INTERACTIONS OF PHOTONS WITH MATTER Flashcards

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1
Q
  1. What are the three types of interactions of photons with matter?
A
  1. Photoelectric Absorption
  2. Compton Scattering
  3. Pair Production
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2
Q
  1. How can a photon be absorbed?
A
  • it can be absorbed by transferring all of its energy to
    an inner orbital electron
  • this happens in an atom absorber
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3
Q
  1. What happens after the photon has transferred all of its energy to an inner orbital electron?
A
  • the electron is ejected from the atom
  • the photon disappears
  • this is because the photon has lost all of its energy
  • the photon also did not have any mass
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4
Q
  1. What happens with the atom now that the inner orbital electron has been ejected?
A
  • the atom is now left with a vacant inner electron orbit
  • it fills this inner electron vacant orbit with one of the
    outer electrons
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5
Q
  1. What happens when the atom fills the vacant inner electron orbit with an outer electron?
A
  • the electron releases a small amount of energy
  • this energy is released in the form of a characteristic
    X- Ray Photon
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6
Q
  1. Why is the X-Ray called a Characteristic Photon?
A
  • its energy is characteristic of the absorbing material
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7
Q
  1. Explain, step by step, what is happening in this diagram.
A
  1. There is an incoming X-Ray Photon
  2. The photon collides with the electron of the inner
    orbital
  3. The energy is absorbed by the electron
  4. The electron energy is now greater than the
    photon energy
  5. The photon is annihilated
  6. The electron is ejected from the inner orbital of the
    atom
  7. An outer electron now fills the empty void in the inner
    orbital
  8. This electron now has excess energy that is expels in
    the form of an X-Ray Photon
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8
Q
  1. How can we describe all the interactions that happen between photons and matter?
A
  • these interactions are Probabilistic
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9
Q
  1. What does the odd that a photon is absorbed by Photoelectric absorption dependent on?
A
  • it depends on the chemical elements in the absorbing
    material
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10
Q
  1. What is an important quantity when it comes to Photoelectric Absorption?
A

THE ATMOIC NUMBER:
- the number of protons in the nucleus of an atom of
that element

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11
Q
  1. What is the mathematical relationships between Photoelectric Absorption and Atomic number?
A
  • the likelihood of electric absorption increases as a cube of the atomic number
  • ⬆ in likelihood of electric absorption = ⬆ Z³
    ∴ Photoelectric absorption is more likely to occur in
    materials that have many atoms with larger atomic
    values (Z)
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12
Q
  1. Give me an example of a good absorber of X-Ray photons?
A
  • Heavy Metals
  • EG: Lead
    : it has an atomic number of 82
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13
Q
  1. What other quality makes a material a good absorber for X-Ray photons?
A
  • a material with a higher density will make a good
    absorber
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14
Q
  1. What effect does an X-Ray beam have on Air?
A
  • negligible
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15
Q
  1. What effect does an X-Ray beam have on Bone?
A
  • significant
  • this is due to the relatively high density of the bone
  • the photoelectric effect is high in the bone
  • the bone is a good absorber of X-Ray photons
  • the X-Rays beams are very defined in the bone
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16
Q
  1. What effect does an X-Ray beam have on Soft Tissue?
A
  • it has a similar effect to that of water
  • this means that it will appear as the light grey and dark
    grey areas of an X-Ray
  • X-rays can pass easily though soft tissue and fluid
    without being absorbed
17
Q
  1. What effect does an X-Ray beam have on Fat Tissue?
A
  • it has a less important effect than it does on water and
    soft tissue
18
Q
  1. What effect does an X-Ray beam, have on the lungs?
A
  • a very weak effect
  • this is due to the low density of the lungs
19
Q
  1. What is the Atomic Mass Number of bone?
A
  • the same as Carbon
  • 40.078 u
20
Q
  1. What effect does a higher kVp have on the photoelectric effect?
A
  • it reduces the photoelectric effect
  • this allows bone and lung structures to be
    simultaneously visualised
  • this is because there is a lower absorption of X-Ray
    beams
  • this produces a better image quality
21
Q
  1. What is kvP?
A
  • it is the Kilovoltage peak (kVp)
  • this is the peak potential applied to the x-ray tube
  • this potential accelerates electrons from the cathode to
    the anode in radiography or tomography
22
Q
  1. What is the Compton Effect?
A
  • this is the interactions with the outer electrons
  • these electrons are not tightly bound to an atom
23
Q
  1. What happens in the Compton Effect?
A
  • the photon collides with an electron
  • the photon gives some of its energy to the electron
24
Q
  1. What happens if the collision with the photon and the outer orbital electron is head on?
A
  • the photon’s original direction of travel reversed
  • the photon loses the maximum amount of energy
25
Q
  1. What happens if the collision with the photon and the electron in the outer orbital is a glancing one?
A
  • the energy gives from the photon to the electron will
    be much less
26
Q
  1. How many electrons can a single photon undergo?
A
  • several
  • it will lose some of its energy on each occasion
  • the photon will eventually be absorbed by the
    photoelectric effect
27
Q
  1. What is the actual result of photon energy loss due to Compton Scattering?
A
  • the angle through which the photon is scattered
28
Q
  1. For which energies is the Compton effect the dominant one?
A
  • it is dominant for photon energies above 200 keV
  • it is dominant for photon energies up to 2 MeV
29
Q
  1. When do both the Photoelectric Effect and the Compton effect occur?
A
  • they occur for photon energies between 60 keV and
    90 keV
30
Q
  1. How do photons interact with electrons during the Compton effect?
A
  • they interact as though the electrons are not bound to
    an atom
  • this means that only the total number of electrons in
    the material matters
31
Q
  1. When it comes to the Compton Effect, what is important for the photon absorption at higher energies?
A
  • the thickness of the absorber
  • the density of the absorber
    (this is usually relatively constant)
    (it has nothing to do with the atomic number)
32
Q
  1. How does the Compton Effect distinguish between materials with different chemical compositions?
A
  • it does this based on their densities only
33
Q
  1. What can be said about the sensitivity of the Compton Effect?
A
  • it is relatively insensitive to variations in anatomy
    compared to the photoelectric effect
  • this is because most soft tissues have very similar
    densities

MED 107- MEDICAL PHYSICS II (108 decks)

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