NMTCB Radiation Physics & Detection 3 — Questions and Answers
Question 1: The resolving time correction formula for a detector with observed count rate n and dead time τ gives the true count rate as:
- N = n / (1 - nτ) (Correct answer)
- N = n × (1 + nτ)
- N = n × e^(-nτ)
- N = n / (1 + nτ)
Correct answer: N = n / (1 - nτ)
For a paralyzable or non-paralyzable detector approximation at moderate count rates, true count rate N = n/(1 - nτ) corrects for events lost during dead time.
Question 2: Internal conversion coefficient (α) is defined as the ratio of:
- Alpha particle emission rate to total decay rate
- Internal conversion electron emission rate to gamma ray emission rate (Correct answer)
- Photoelectric interactions to Compton interactions
- Characteristic X-ray yield to Auger electron yield
Correct answer: Internal conversion electron emission rate to gamma ray emission rate
The internal conversion coefficient α = Ne/Nγ, where Ne is the number of conversion electrons and Nγ is the number of gamma rays emitted per unit time.
Question 3: Which detector type is used in PET scanners primarily because of its:
- High sensitivity to single gamma photons at all energies
- Ability to detect coincident 511 keV annihilation photons with high stopping power (Correct answer)
- Low cost and easy manufacturing scalability
- Compatibility with room-temperature operation without cooling
Correct answer: Ability to detect coincident 511 keV annihilation photons with high stopping power
PET detectors must efficiently stop 511 keV photons and detect coincident pairs; scintillators like LSO/LYSO with high density and Z provide this stopping power.
Question 4: The Bateman equations describe:
- Statistical fluctuations in radioactive decay measurement
- Buildup and decay kinetics of daughter nuclides in a radioactive decay chain (Correct answer)
- Energy transfer during Compton scattering events
- Absorbed dose calculations in tissue-equivalent phantoms
Correct answer: Buildup and decay kinetics of daughter nuclides in a radioactive decay chain
Bateman equations give the activity of each member of a radioactive decay series as a function of time, accounting for both production and decay of each nuclide.
Question 5: Energy resolution of a scintillation detector is expressed as:
- The maximum detectable photon energy divided by detector volume
- FWHM of a photopeak divided by the peak centroid energy, expressed as a percentage (Correct answer)
- Signal-to-noise ratio at the photomultiplier output
- Number of photoelectrons produced per keV of absorbed energy
Correct answer: FWHM of a photopeak divided by the peak centroid energy, expressed as a percentage
Energy resolution (%) = (FWHM/E₀) × 100, where FWHM is the full width at half maximum of the photopeak and E₀ is the photopeak centroid energy.
Question 6: Pair production requires a minimum photon energy of 1.022 MeV because:
- This is the K-shell binding energy of high-Z absorber atoms
- Two electron rest-mass energies (2 × 0.511 MeV) must be supplied to create the electron-positron pair (Correct answer)
- Compton scattering cross-section peaks at this energy
- Nuclear binding energy thresholds require this minimum
Correct answer: Two electron rest-mass energies (2 × 0.511 MeV) must be supplied to create the electron-positron pair
Pair production converts photon energy into an electron (0.511 MeV rest mass) and positron (0.511 MeV rest mass), requiring at least 1.022 MeV.
Question 7: The secular equilibrium condition in a parent-daughter decay system is established when:
- Parent and daughter have identical half-lives
- Parent half-life is much longer than daughter half-life and sufficient time has elapsed (Correct answer)
- Daughter activity exceeds parent activity by a factor of 10
- Both nuclides emit radiation of the same energy
Correct answer: Parent half-life is much longer than daughter half-life and sufficient time has elapsed
Secular equilibrium occurs when the parent's half-life vastly exceeds the daughter's, causing the daughter activity to equal the parent activity after ~7 daughter half-lives.
NMTCB Radiation Physics & Detection 3
The resolving time correction formula for a detector with observed count rate n and dead time τ gives the true count rate as: