RSO Radiation Physics and Fundamentals 2 — Questions and Answers
Question 1: The traditional unit of radioactivity is the Curie (Ci). One Curie is defined as:
- 3.7 × 10^10 disintegrations per second (Correct answer)
- 1 disintegration per second
- 3.7 × 10^6 disintegrations per minute
- 1 joule of energy absorbed per kilogram
Correct answer: 3.7 × 10^10 disintegrations per second
One Curie equals 3.7 × 10^10 disintegrations per second, based on the activity of one gram of radium-226.
Question 2: In electron capture decay, the nucleus captures an orbital electron and emits:
- A gamma ray only
- A neutrino and characteristic X-rays (Correct answer)
- A positron and a neutrino
- An alpha particle
Correct answer: A neutrino and characteristic X-rays
During electron capture, a proton combines with the captured electron to form a neutron, emitting a neutrino; the resulting electron vacancy produces characteristic X-rays.
Question 3: The specific activity of a radioactive material is defined as:
- Activity per unit volume
- Activity per unit mass (Correct answer)
- Activity per unit time
- Activity per unit area
Correct answer: Activity per unit mass
Specific activity is the radioactivity per unit mass of a radionuclide or radioactive material, typically expressed as Ci/g or Bq/g.
Question 4: Which of the following radionuclides is a pure beta emitter with no associated gamma radiation?
- Co-60
- Cs-137
- H-3 (Tritium) (Correct answer)
- I-131
Correct answer: H-3 (Tritium)
Tritium (H-3) decays by beta-minus emission with a very low-energy beta and produces no gamma radiation, making external dose a minor concern but internal uptake significant.
Question 5: The mean free path of a photon in a material is best described as:
- The distance at which intensity drops to 50% of the original
- The average distance a photon travels before interacting (Correct answer)
- The thickness of shielding required to stop all photons
- The distance at which dose rate equals the background level
Correct answer: The average distance a photon travels before interacting
Mean free path is the average distance a photon travels in a medium before undergoing an interaction such as photoelectric absorption, Compton scattering, or pair production.
Question 6: If a radionuclide has a physical half-life of 6 hours and a biological half-life of 12 hours, what is its effective half-life in the body?
- 18 hours
- 9 hours
- 4 hours (Correct answer)
- 6 hours
Correct answer: 4 hours
Effective half-life = (T_physical × T_biological) / (T_physical + T_biological) = (6 × 12) / (6 + 12) = 72/18 = 4 hours.
Question 7: Secular equilibrium between a parent and daughter radionuclide occurs when:
- Parent and daughter have equal half-lives
- The daughter's half-life is much longer than the parent's
- The parent's half-life is much longer than the daughter's (Correct answer)
- The activity of the parent equals zero
Correct answer: The parent's half-life is much longer than the daughter's
Secular equilibrium occurs when the parent half-life is so much longer than the daughter's that the daughter activity equals the parent activity and remains constant.
RSO Radiation Physics and Fundamentals 2
The traditional unit of radioactivity is the Curie (Ci).
One Curie is defined as: