NRRPT Radiation Fundamentals & Detection 5 — Questions and Answers
Question 1: A rem is related to a rad by which formula?
- rem = rad × Q (quality factor)
- rem = rad / Q (quality factor)
- rem = rad × WR (radiation weighting factor)
- Both A and C are correct expressions (Correct answer)
Correct answer: Both A and C are correct expressions
Dose equivalent (rem) = absorbed dose (rad) × quality factor Q; in SI, effective dose (Sv) = absorbed dose (Gy) × radiation weighting factor WR — both express the same biological weighting concept.
Question 2: What phenomenon causes the 'wall effect' artifact in ionization chamber measurements of photon beams?
- Secondary electrons generated in chamber walls contributing to ionization (Correct answer)
- Backscattering from the collecting electrode
- Electric field distortion near the chamber stem
- Recombination of ions before collection
Correct answer: Secondary electrons generated in chamber walls contributing to ionization
Energetic secondary electrons ejected from the chamber wall can travel into the gas volume and produce additional ionization, affecting measurement accuracy.
Question 3: In nuclear stability, which statement about the neutron-to-proton (N/Z) ratio is correct?
- Stable nuclei always have N/Z = 1.0
- The stable N/Z ratio increases above 1.0 for heavier nuclei (Correct answer)
- Nuclei with N/Z > 1 always undergo alpha decay
- The N/Z ratio has no bearing on decay mode
Correct answer: The stable N/Z ratio increases above 1.0 for heavier nuclei
Heavy stable nuclei require more neutrons than protons to overcome increasing Coulomb repulsion; the stable N/Z ratio rises above 1 for Z > 20.
Question 4: Coherent (Rayleigh) scattering differs from Compton scattering in that it:
- Transfers significant energy to a recoil electron
- Scatters the photon with essentially no energy loss (Correct answer)
- Only occurs at photon energies above 1 MeV
- Produces characteristic X-rays of the absorber
Correct answer: Scatters the photon with essentially no energy loss
In Rayleigh scattering, the photon interacts with the atom as a whole and is deflected with negligible energy loss, unlike Compton which involves energy transfer to a free electron.
Question 5: Which statement correctly describes the operation of a silicon semiconductor detector?
- It requires a high-pressure fill gas to increase sensitivity
- Ionizing radiation creates electron-hole pairs that are swept to electrodes by an applied bias (Correct answer)
- It operates in gas multiplication mode similar to a proportional counter
- It must be operated at elevated temperatures to reduce leakage current
Correct answer: Ionizing radiation creates electron-hole pairs that are swept to electrodes by an applied bias
In silicon detectors, radiation creates electron-hole pairs; reverse bias voltage sweeps carriers to respective electrodes, producing a charge pulse proportional to deposited energy.
Question 6: The nuclide Am-241 is commonly used in smoke detectors because it emits:
- Beta particles that ionize a smoke-sensing gas
- Alpha particles that ionize air in an ionization chamber (Correct answer)
- Gamma rays that activate a photovoltaic sensor
- Neutrons that activate a BF₃ detector tube
Correct answer: Alpha particles that ionize air in an ionization chamber
Am-241 emits 5.49 MeV alpha particles that ionize air between two electrodes; smoke particles reduce this ionization current, triggering the alarm.
Question 7: What is the purpose of the 'guard ring' electrode in a parallel-plate ionization chamber?
- To increase the electric field strength near the collecting electrode
- To eliminate field distortions and collect only ions from the defined volume (Correct answer)
- To shield the chamber from external electromagnetic interference
- To accelerate positive ions toward the cathode more efficiently
Correct answer: To eliminate field distortions and collect only ions from the defined volume
The guard ring maintains a uniform electric field in the collection region and intercepts edge-effect ions so only ionization from the defined sensitive volume is measured.
NRRPT Radiation Fundamentals & Detection 5
A rem is related to a rad by which formula?