NRRPT Physics and Interactions 4 — Questions and Answers
Question 1: The concept of 'buildup factor' in gamma-ray shielding accounts for:
- Secondary scattered photons that add to the transmitted dose (Correct answer)
- Energy buildup due to electron cascade in the shield
- Attenuation of the primary beam only
- Activation of shielding material by gamma rays
Correct answer: Secondary scattered photons that add to the transmitted dose
The buildup factor (B) accounts for scattered and secondary photons that contribute to dose beyond what the uncollided (primary) beam attenuation predicts.
Question 2: The energy of a photon emitted during isomeric transition (IT) equals:
- The difference in energy levels between the metastable and ground state (Correct answer)
- The sum of binding energies of all nucleons
- The kinetic energy of ejected internal conversion electrons plus 1.022 MeV
- The total rest mass energy of the nucleus
Correct answer: The difference in energy levels between the metastable and ground state
In isomeric transition, the nucleus de-excites from a metastable state to the ground state, emitting a gamma photon equal to the energy difference between those nuclear energy levels.
Question 3: What is the primary biological significance of high-LET radiation compared to low-LET radiation?
- High-LET radiation produces more widely spaced ionization events
- High-LET radiation is more densely ionizing and causes complex, clustered DNA damage (Correct answer)
- High-LET radiation has longer range in tissue
- High-LET radiation is less effective per unit of absorbed dose
Correct answer: High-LET radiation is more densely ionizing and causes complex, clustered DNA damage
High-LET radiation produces dense clusters of ionization along its track, causing complex double-strand breaks and clustered DNA damage that is harder for cells to repair.
Question 4: An atom undergoes electron capture. What is emitted as a direct result of the orbital electron vacancy created?
- A beta particle
- A neutrino only
- Characteristic X-rays or Auger electrons (Correct answer)
- A positron and a neutrino
Correct answer: Characteristic X-rays or Auger electrons
Electron capture creates a vacancy in an inner electron shell; this vacancy is filled by outer-shell electrons, releasing characteristic X-rays or Auger electrons.
Question 5: The stopping power of a material for charged particles is described by the Bethe-Bloch equation and depends primarily on:
- The charge and velocity of the particle, and the electron density of the medium (Correct answer)
- Only the mass of the particle
- The nuclear binding energy of the medium
- The photon fluence rate in the medium
Correct answer: The charge and velocity of the particle, and the electron density of the medium
The Bethe-Bloch equation shows that stopping power depends on the particle's charge (z²), its velocity (β), and the electron density (Z/A) of the absorbing medium.
Question 6: In nuclear fission of U-235, the average number of prompt neutrons released per fission event is approximately:
- 0.5
- 1.0
- 2.5 (Correct answer)
- 5.0
Correct answer: 2.5
Each thermal fission of U-235 releases an average of approximately 2.43 prompt neutrons (often cited as ~2.5), which is essential for sustaining a chain reaction.
Question 7: The half-value layer (HVL) of a photon beam is related to the linear attenuation coefficient (μ) by:
- HVL = ln(2)/μ = 0.693/μ (Correct answer)
- HVL = μ/ln(2)
- HVL = 1/μ²
- HVL = 2μ
Correct answer: HVL = ln(2)/μ = 0.693/μ
From the exponential attenuation equation I = I₀e^(−μx), setting I/I₀ = 0.5 and solving gives HVL = ln(2)/μ ≈ 0.693/μ.
NRRPT Physics and Interactions 4
The concept of 'buildup factor' in gamma-ray shielding accounts for: