NRRPT Physics and Interactions 3 — Questions and Answers
Question 1: Bremsstrahlung radiation efficiency increases with:
- Lower atomic number (Z) of the target and lower electron energy
- Higher atomic number (Z) of the target and higher electron energy (Correct answer)
- Lower electron energy and higher target density only
- Higher electron charge and lower photon frequency
Correct answer: Higher atomic number (Z) of the target and higher electron energy
Bremsstrahlung production is proportional to Z² and to the electron kinetic energy, so high-Z targets (like tungsten) and high-energy electrons maximize bremsstrahlung yield.
Question 2: The mass attenuation coefficient (μ/ρ) of a compound material can be calculated using:
- The mixture rule: sum of weight-fraction-weighted elemental coefficients (Correct answer)
- The simple arithmetic mean of elemental coefficients
- Only the coefficient of the highest-Z element present
- The geometric mean of elemental mass attenuation coefficients
Correct answer: The mixture rule: sum of weight-fraction-weighted elemental coefficients
The mixture rule states that μ/ρ for a compound equals the sum of each element's weight fraction multiplied by its elemental mass attenuation coefficient.
Question 3: Which quantity is conserved in all nuclear reactions?
- Kinetic energy
- Number of neutrons
- Total baryon number and charge (Correct answer)
- Number of protons only
Correct answer: Total baryon number and charge
Conservation laws in nuclear reactions require that total baryon number (A) and electric charge (Z) are conserved, along with energy and momentum.
Question 4: Positron annihilation produces two photons, each with an energy of:
- 0.511 MeV traveling in opposite directions (Correct answer)
- 1.022 MeV traveling in the same direction
- 0.511 MeV traveling in the same direction
- 1.022 MeV at 90° to each other
Correct answer: 0.511 MeV traveling in opposite directions
When a positron annihilates with an electron, two 0.511 MeV photons are emitted in opposite directions (180° apart) to conserve momentum.
Question 5: The range of alpha particles in tissue is approximately:
- Several meters
- Several centimeters
- A few tens of micrometers to ~100 μm (Correct answer)
- Less than 1 nanometer
Correct answer: A few tens of micrometers to ~100 μm
Alpha particles (4–8 MeV) have very short ranges in tissue, typically a few tens to about 100 micrometers, due to their large charge and mass causing dense ionization.
Question 6: In the Compton effect, the energy of the scattered photon is LOWEST when the scattering angle is:
- 0°
- 45°
- 90°
- 180° (Correct answer)
Correct answer: 180°
At 180° (backscatter), the photon transfers maximum energy to the recoil electron, leaving the scattered photon with the lowest possible energy.
Question 7: Which type of radiation has the greatest penetrating power in matter?
- Alpha particles
- Beta particles
- Thermal neutrons
- High-energy gamma rays (Correct answer)
Correct answer: High-energy gamma rays
High-energy gamma rays are highly penetrating because they are uncharged photons that interact only probabilistically; thick dense shielding is required to attenuate them significantly.
NRRPT Physics and Interactions 3
Bremsstrahlung radiation efficiency increases with: