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ABR Medical Physics Part 1 — 100 Original Practice Questions
Basic Physics & Atomic Structure (1–15)
1. The half-life of a radioactive isotope is defined as the time required for:
A. Its activity to double
B. Half of its nuclei to decay
C. All of its nuclei to decay
D. Its energy to reduce by half
Answer: B. Half-life is the time in which 50% of radioactive nuclei undergo decay.
2. The activity of a radioactive source decreases from 1600 MBq to 200 MBq in 12 hours.
The half-life of the radionuclide is:
A. 2 h
B. 3 h
C. 4 h
D. 6 h
Answer: C. 1600 → 800 → 400 → 200 requires 3 half-lives. 12 h / 3 = 4 h per half-life.
3. The SI unit of absorbed dose is:
A. Rad
B. Gray (Gy)
C. Rem
D. Sievert
Answer: B. Gray (Gy) is the SI unit for absorbed dose, defined as 1 J/kg.
4. Which interaction predominates for diagnostic X-rays in soft tissue (20-150 keV)?
A. Pair production
B. Photoelectric effect
C. Compton scattering
D. Coherent scattering
Answer: C. Compton scattering dominates in soft tissue at diagnostic energies, affecting
image contrast.
5. The half-value layer (HVL) of an X-ray beam is defined as:
A. The thickness of material that absorbs all photons
B. The distance at which beam intensity doubles
C. The thickness of material that reduces beam intensity by 50%
D. The time required for half of a radioactive source to decay
Answer: C. HVL is the thickness of a specified material that attenuates the beam to half its
original intensity, characterizing beam quality.
6. In linear accelerators, the target used to generate X-rays is typically made of:
A. Copper
, 2
B. Aluminum
C. Tungsten
D. Lead
Answer: C. Tungsten has a high atomic number and melting point, making it efficient for
bremsstrahlung production.
7. The buildup region in megavoltage photon beams refers to:
A. The depth at maximum electron density
B. The depth at which dose rises to its maximum
C. The surface dose only
D. The dose beyond the maximum
Answer: B. High-energy photons have a dose that increases with depth until dmax due to
electron scatter buildup.
8. Compton scattering probability:
A. Increases with atomic number
B. Decreases with photon energy
C. Is independent of electron density
D. Only occurs in metals
Answer: B. Compton scattering is primarily dependent on electron density and decreases as
photon energy increases.
9. The quantity that best describes energy deposited per unit mass in a medium is:
A. Exposure
B. Kerma
C. Absorbed dose
D. Activity
Answer: C. Absorbed dose is energy deposited per unit mass.
10. Which of the following is NOT a source of radiation?
A. Radioactive material
B. X-ray generating equipment
C. Ultrasound transducer
D. Linear accelerator
Answer: C. Ultrasound uses mechanical sound waves, not ionizing radiation.
11. The primary purpose of a parallel-plate ionization chamber is to:
A. Measure beam energy
B. Measure dose in a buildup region
C. Determine beam symmetry
D. Calibrate a thermoluminescent dosimeter
Answer: B. Parallel-plate chambers have a shallow depth of maximum response, ideal for
surface and near-surface dose measurement.
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12. In radiation protection, effective dose accounts for:
A. Only absorbed dose
B. Only equivalent dose
C. Both tissue weighting and radiation type
D. Only high-energy photons
Answer: C. Effective dose considers radiation type and tissue sensitivity to provide a risk-
based measure.
13. Which of the following is a gas-filled detector?
A. Thermoluminescent dosimeter
B. Geiger-Müller counter
C. Film badge
D. Optically stimulated luminescent dosimeter
Answer: B. Geiger-Müller counters are gas-filled detectors.
14. The primary advantage of flattening-filter-free (FFF) photon beams in radiotherapy is:
A. Increased dose homogeneity
B. Reduced penetration depth
C. Higher dose rate and reduced treatment time
D. Lower skin dose
Answer: C. FFF beams remove the flattening filter, increasing dose rate and reducing
treatment time.
15. Which type of radioactive decay emits a helium nucleus?
A. Beta minus decay
B. Beta plus decay
C. Alpha decay
D. Gamma decay
Answer: C. Alpha decay emits a helium nucleus (2 protons + 2 neutrons).
Radiation Measurement & Instrumentation (16–25)
16. The principal limitation of film dosimetry is:
A. Low spatial resolution
B. Energy dependence and non-linear response
C. Poor tissue equivalence
D. Inability to measure large doses
Answer: B. Film dosimetry can have energy-dependent response and nonlinear optical
density-to-dose conversion.
17. A thermoluminescent dosimeter (TLD) works by:
A. Ionizing gas
B. Emitting light when heated after radiation exposure
C. Changing color
ABR Medical Physics Part 1 — 100 Original Practice Questions
Basic Physics & Atomic Structure (1–15)
1. The half-life of a radioactive isotope is defined as the time required for:
A. Its activity to double
B. Half of its nuclei to decay
C. All of its nuclei to decay
D. Its energy to reduce by half
Answer: B. Half-life is the time in which 50% of radioactive nuclei undergo decay.
2. The activity of a radioactive source decreases from 1600 MBq to 200 MBq in 12 hours.
The half-life of the radionuclide is:
A. 2 h
B. 3 h
C. 4 h
D. 6 h
Answer: C. 1600 → 800 → 400 → 200 requires 3 half-lives. 12 h / 3 = 4 h per half-life.
3. The SI unit of absorbed dose is:
A. Rad
B. Gray (Gy)
C. Rem
D. Sievert
Answer: B. Gray (Gy) is the SI unit for absorbed dose, defined as 1 J/kg.
4. Which interaction predominates for diagnostic X-rays in soft tissue (20-150 keV)?
A. Pair production
B. Photoelectric effect
C. Compton scattering
D. Coherent scattering
Answer: C. Compton scattering dominates in soft tissue at diagnostic energies, affecting
image contrast.
5. The half-value layer (HVL) of an X-ray beam is defined as:
A. The thickness of material that absorbs all photons
B. The distance at which beam intensity doubles
C. The thickness of material that reduces beam intensity by 50%
D. The time required for half of a radioactive source to decay
Answer: C. HVL is the thickness of a specified material that attenuates the beam to half its
original intensity, characterizing beam quality.
6. In linear accelerators, the target used to generate X-rays is typically made of:
A. Copper
, 2
B. Aluminum
C. Tungsten
D. Lead
Answer: C. Tungsten has a high atomic number and melting point, making it efficient for
bremsstrahlung production.
7. The buildup region in megavoltage photon beams refers to:
A. The depth at maximum electron density
B. The depth at which dose rises to its maximum
C. The surface dose only
D. The dose beyond the maximum
Answer: B. High-energy photons have a dose that increases with depth until dmax due to
electron scatter buildup.
8. Compton scattering probability:
A. Increases with atomic number
B. Decreases with photon energy
C. Is independent of electron density
D. Only occurs in metals
Answer: B. Compton scattering is primarily dependent on electron density and decreases as
photon energy increases.
9. The quantity that best describes energy deposited per unit mass in a medium is:
A. Exposure
B. Kerma
C. Absorbed dose
D. Activity
Answer: C. Absorbed dose is energy deposited per unit mass.
10. Which of the following is NOT a source of radiation?
A. Radioactive material
B. X-ray generating equipment
C. Ultrasound transducer
D. Linear accelerator
Answer: C. Ultrasound uses mechanical sound waves, not ionizing radiation.
11. The primary purpose of a parallel-plate ionization chamber is to:
A. Measure beam energy
B. Measure dose in a buildup region
C. Determine beam symmetry
D. Calibrate a thermoluminescent dosimeter
Answer: B. Parallel-plate chambers have a shallow depth of maximum response, ideal for
surface and near-surface dose measurement.
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12. In radiation protection, effective dose accounts for:
A. Only absorbed dose
B. Only equivalent dose
C. Both tissue weighting and radiation type
D. Only high-energy photons
Answer: C. Effective dose considers radiation type and tissue sensitivity to provide a risk-
based measure.
13. Which of the following is a gas-filled detector?
A. Thermoluminescent dosimeter
B. Geiger-Müller counter
C. Film badge
D. Optically stimulated luminescent dosimeter
Answer: B. Geiger-Müller counters are gas-filled detectors.
14. The primary advantage of flattening-filter-free (FFF) photon beams in radiotherapy is:
A. Increased dose homogeneity
B. Reduced penetration depth
C. Higher dose rate and reduced treatment time
D. Lower skin dose
Answer: C. FFF beams remove the flattening filter, increasing dose rate and reducing
treatment time.
15. Which type of radioactive decay emits a helium nucleus?
A. Beta minus decay
B. Beta plus decay
C. Alpha decay
D. Gamma decay
Answer: C. Alpha decay emits a helium nucleus (2 protons + 2 neutrons).
Radiation Measurement & Instrumentation (16–25)
16. The principal limitation of film dosimetry is:
A. Low spatial resolution
B. Energy dependence and non-linear response
C. Poor tissue equivalence
D. Inability to measure large doses
Answer: B. Film dosimetry can have energy-dependent response and nonlinear optical
density-to-dose conversion.
17. A thermoluminescent dosimeter (TLD) works by:
A. Ionizing gas
B. Emitting light when heated after radiation exposure
C. Changing color
