NRRPT Radiation Fundamentals & Detection 3 — Questions and Answers
Question 1: The half-value layer (HVL) of a gamma-ray beam is 2 cm of lead. What thickness reduces the intensity to 12.5% of the original?
- 4 cm
- 6 cm (Correct answer)
- 8 cm
- 10 cm
Correct answer: 6 cm
12.5% = (1/2)³, so three HVLs are needed: 3 × 2 cm = 6 cm of lead.
Question 2: In neutron activation analysis, what process produces radioactive isotopes from stable elements?
- Neutron-proton reaction (n,p)
- Radiative neutron capture (n,γ) (Correct answer)
- Neutron fission (n,f)
- Inelastic neutron scattering (n,n')
Correct answer: Radiative neutron capture (n,γ)
Radiative neutron capture (n,γ) adds a neutron to a stable nucleus, forming a heavier isotope that is often radioactive.
Question 3: What does the energy resolution of a radiation detector describe?
- The minimum energy of radiation the detector can sense
- The ability to distinguish between two close energy peaks (Correct answer)
- The maximum dose rate the detector can measure
- The efficiency of converting radiation to electrical signal
Correct answer: The ability to distinguish between two close energy peaks
Energy resolution is the ability of a detector to separate two photon energies that are close together, expressed as FWHM/peak energy.
Question 4: A radionuclide has a physical half-life of 8 days and a biological half-life of 24 days. Its effective half-life is approximately:
- 32 days
- 16 days
- 6 days (Correct answer)
- 4 days
Correct answer: 6 days
Effective half-life = (T_p × T_b)/(T_p + T_b) = (8 × 24)/(8 + 24) = 192/32 = 6 days.
Question 5: Which radiation type has the highest linear energy transfer (LET) in tissue?
- Cobalt-60 gamma rays
- 1 MeV beta particles
- 5 MeV alpha particles (Correct answer)
- Diagnostic X-rays at 80 kVp
Correct answer: 5 MeV alpha particles
Alpha particles have very high LET (~80 keV/μm in tissue) due to their large mass and charge compared to beta particles and photons.
Question 6: What is the primary purpose of using a discriminator circuit in a radiation counting system?
- To amplify the detector signal before counting
- To reject pulses below a set threshold, reducing noise counts (Correct answer)
- To convert analog pulses to digital signals
- To measure the energy of individual photons
Correct answer: To reject pulses below a set threshold, reducing noise counts
A discriminator rejects low-amplitude noise pulses by only passing signals above a preset voltage threshold.
Question 7: Pair production interactions require incident photon energies above:
- 0.511 MeV
- 1.022 MeV (Correct answer)
- 1.25 MeV
- 2.044 MeV
Correct answer: 1.022 MeV
Pair production requires at least 1.022 MeV to create an electron-positron pair (2 × 0.511 MeV rest mass energy).
NRRPT Radiation Fundamentals & Detection 3
The half-value layer (HVL) of a gamma-ray beam is 2 cm of lead.
What thickness reduces the intensity to 12.5% of the original?