ARRT Radiation Physics and Radiobiology 2 — Questions and Answers
Question 1: Which type of ionizing radiation has the greatest linear energy transfer (LET)?
- X-rays
- Gamma rays
- Alpha particles (Correct answer)
- Beta particles
Correct answer: Alpha particles
Alpha particles have the highest LET because they are heavy, doubly charged particles that deposit energy very densely over a short range.
Question 2: The oxygen enhancement ratio (OER) is MOST relevant in which radiobiological context?
- Determining the dose rate effect
- Comparing tumor hypoxia sensitivity to radiation (Correct answer)
- Calculating the four R's of repair
- Measuring bremsstrahlung production
Correct answer: Comparing tumor hypoxia sensitivity to radiation
OER quantifies how much more radiation dose is needed to achieve the same cell kill in hypoxic versus well-oxygenated conditions, directly relevant to tumor radiobiology.
Question 3: Which interaction predominates when a 100 keV photon passes through soft tissue?
- Pair production
- Coherent scattering
- Compton scattering (Correct answer)
- Photoelectric effect
Correct answer: Compton scattering
At 100 keV in soft tissue, Compton scattering predominates because this energy range is above the threshold for photoelectric dominance in low-Z materials.
Question 4: The unit of radiation absorbed dose in the SI system is:
- Roentgen
- Rem
- Gray (Correct answer)
- Becquerel
Correct answer: Gray
The Gray (Gy) is the SI unit of absorbed dose, equal to 1 joule of energy deposited per kilogram of matter.
Question 5: Which radioactive decay mode results in the emission of an electron from the nucleus?
- Alpha decay
- Beta-minus decay (Correct answer)
- Electron capture
- Isomeric transition
Correct answer: Beta-minus decay
Beta-minus decay occurs when a neutron converts to a proton, releasing an electron (beta particle) and an antineutrino from the nucleus.
Question 6: What is the primary mechanism by which ionizing radiation damages DNA indirectly?
- Direct strand rupture by photons
- Free radical formation from water radiolysis (Correct answer)
- Activation of repair enzymes
- Cross-linking of histone proteins
Correct answer: Free radical formation from water radiolysis
Indirect DNA damage occurs when radiation ionizes water molecules, producing hydroxyl radicals (·OH) that then attack and break DNA strands.
Question 7: Which of the following best describes the inverse square law?
- Intensity decreases linearly with distance from the source
- Intensity is proportional to the square of the distance
- Intensity decreases by a factor of four when distance doubles (Correct answer)
- Intensity doubles when distance is halved
Correct answer: Intensity decreases by a factor of four when distance doubles
The inverse square law states that radiation intensity is inversely proportional to the square of the distance, so doubling distance reduces intensity to one-quarter.
ARRT Radiation Physics and Radiobiology 2
Which type of ionizing radiation has the greatest linear energy transfer (LET)?