NMTCB Radiation Physics & Detection 2 — Questions and Answers
Question 1: Which interaction mechanism is primarily responsible for photoelectric absorption in diagnostic nuclear medicine energy ranges?
- Ejection of an inner-shell electron by a photon that is completely absorbed (Correct answer)
- Elastic scattering of a photon without energy loss
- Pair production creating an electron-positron pair
- Compton scattering with partial photon energy transfer
Correct answer: Ejection of an inner-shell electron by a photon that is completely absorbed
In photoelectric absorption, an incident photon transfers all its energy to an inner-shell electron, ejecting it and leaving no scattered photon.
Question 2: The Q-value of a nuclear reaction represents:
- The quality factor for radiation weighting
- The net energy released or absorbed in a nuclear reaction (Correct answer)
- The charge-to-mass ratio of emitted particles
- The quenching efficiency in liquid scintillation counting
Correct answer: The net energy released or absorbed in a nuclear reaction
The Q-value is the difference in rest-mass energy between reactants and products, indicating whether energy is released (positive Q) or required (negative Q).
Question 3: In a sodium iodide (NaI) detector, thallium doping primarily serves to:
- Increase the density of the crystal for better stopping power
- Shift the emission spectrum to visible light wavelengths detectable by the PMT (Correct answer)
- Reduce the decay time of scintillation pulses
- Increase the atomic number of the scintillator
Correct answer: Shift the emission spectrum to visible light wavelengths detectable by the PMT
Thallium impurities create luminescence centers that emit visible light (~415 nm) compatible with photomultiplier tube sensitivity.
Question 4: The dead time of a radiation detector refers to:
- Time required for the crystal to cool after use
- Period after an event during which the detector cannot register another event (Correct answer)
- Interval between calibration cycles
- Duration of the photomultiplier high-voltage warm-up
Correct answer: Period after an event during which the detector cannot register another event
Dead time is the minimum time interval that must elapse after detecting one event before the detector can register a subsequent event.
Question 5: Which property makes technetium-99m particularly ideal for nuclear medicine imaging?
- Alpha emission that provides high LET for tumor therapy
- 140 keV gamma emission with 6-hour half-life and no particulate radiation (Correct answer)
- Beta emission suitable for dosimetry calculations
- Long half-life allowing extended imaging protocols
Correct answer: 140 keV gamma emission with 6-hour half-life and no particulate radiation
Tc-99m emits a 140 keV gamma ray ideal for gamma camera collimation, has a 6-hour half-life minimizing patient dose, and produces no particulate radiation.
Question 6: The Auger electron emission following electron capture occurs because:
- The nucleus emits an electron during beta-minus decay
- An outer-shell electron fills an inner-shell vacancy, releasing energy as a low-energy electron instead of X-ray (Correct answer)
- Pair production releases two electrons simultaneously
- Internal conversion transfers gamma energy to orbital electrons
Correct answer: An outer-shell electron fills an inner-shell vacancy, releasing energy as a low-energy electron instead of X-ray
When an inner-shell vacancy is filled, the transition energy can be transferred to another orbital electron (Auger electron) rather than emitted as a characteristic X-ray.
Question 7: A gas-filled detector operating in the Geiger-Müller region produces:
- A pulse size proportional to initial ionization energy
- A maximum-sized pulse independent of primary ionization amount (Correct answer)
- Only ion pairs without electron multiplication
- Pulses proportional to particle LET only
Correct answer: A maximum-sized pulse independent of primary ionization amount
In the GM region, avalanche multiplication is so extensive that all pulses reach the same maximum size regardless of the energy deposited by the initial radiation.
NMTCB Radiation Physics & Detection 2
Which interaction mechanism is primarily responsible for photoelectric absorption in diagnostic nuclear medicine energy ranges?