Nuclear medicine Flashcards

1
Q

What is a scintillation counter?

What are its components?

A

measures the activity of incident radiation

components:

  • scintillator crystal
  • photomultiplier tube
  • electronics
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2
Q

How are light flashes (= scintillations) in the scintillator produced?

A

ionized either via:

  • incident γ-/X-ray-photon produces ionizing e- via secondary processes (photoeffect, Compton scattering, pair production)
  • incident ionizing α-, β-particle

process:

  1. ionization excites scintillation crystal
  2. returns to ground state → fluorescence photons emitted (= scintillation)
  3. reflected into PMT
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3
Q

What is the photoeffect/photoelectric effect?

Formula.

A

an incident γ-photon removes a photoelectron from the bound e- of an atom, while being absorbed

Eγ = A + Ee

  • Eγ = energy of incident γ-photon
  • A = work needed to remove e-
  • Ee = energy of removed photoelectron
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4
Q

What is Compton scattering?

Formula.

A

incident γ-photon removes a Compton e- from the outer shell e- of the atom, while a Compton photon of lower E (longer λ) is emitted
⇒ E is split btw Compton e- and Compton photon

Eγ = A + Ee + Eγ

  • Eγ = energy of incident γ-photon
  • A = work needed to remove e-
  • Ee = energy of Compton e-
  • Eγ’ = energy of Compton photon
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5
Q

What is pair production?

Formula.

A

incident γ-photon of sufficiently high E (h*f > 1022 keV) is absorbed near the nucleus and and electron-positron pair is created

Eγ = 2 mec2 + Ee + Ep

  • Eγ = energy of incident γ-photon
  • 2 mec2 = combined rest mass of electron-positron
  • Ee = energy of e-
  • Ep = energy of e+
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6
Q

What is a photomultiplier tube or PMT?

What are its components?

A

amplifier device that converts the weak incident light via secondary emissions into a measurable electric current

components:

  • photocathode
  • dynode array
  • anode
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7
Q

What are secondary emissions?

A

when a primary e- of high speed hits a metal surface (e.g. dynodes in a PMT) several secondary e- are emitted

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8
Q

What is signal-to-noise ratio?

A
  • signal = useful information
  • noise = useless information

measure of the successfulness of detection → the higher the better

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9
Q

What are sources of internal and external noise in a scintillation counter?

A
  • external: background noise (e.g. wall, furniture, cosmic rays, etc.)
  • internal: from electronics, esp. PMT (electric field)
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10
Q

What is an integral discriminator?

A

device that

  • lets through pulses of voltage amplitude higher than a certain level, the discriminator level
  • blocks those of smaller amplitude
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