Nuclear medicine

Question 1

What is a scintillation counter?

What are its components?

Answer 1

measures the activity of incident radiation

components:

• scintillator crystal
• photomultiplier tube
• electronics

Question 2

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

Answer 2

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

Question 3

What is the photoeffect/photoelectric effect?

Formula.

Answer 3

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

E_γ = A + E_e

• E_γ = energy of incident γ-photon
• A = work needed to remove e^-
• E_e = energy of removed photoelectron

Question 4

What is Compton scattering?

Formula.

Answer 4

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

E_γ = A + E_e + E_γ‘

• E_γ = energy of incident γ-photon
• A = work needed to remove e-
• E_e = energy of Compton e-
• E_γ’ = energy of Compton photon

Question 5

What is pair production?

Formula.

Answer 5

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

E_γ = 2 m_ec² + E_e + E_p

• E_γ = energy of incident γ-photon
• 2 m_ec² = combined rest mass of electron-positron
• E_e = energy of e^-
• E_p = energy of e⁺

Question 6

What is a photomultiplier tube or PMT?

What are its components?

Answer 6

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

components:

• photocathode
• dynode array
• anode

Question 7

What are secondary emissions?

Answer 7

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

Question 8

What is signal-to-noise ratio?

Answer 8

• signal = useful information
• noise = useless information

measure of the successfulness of detection → the higher the better

Question 9

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

Answer 9

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

Question 10

What is an integral discriminator?

Answer 10

device that

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