NMTCB Certification Exam Study Guide 2026/2027 | 500+
Practice Questions, Answers & Detailed Rationales | CNMT
DOMAIN I — RADIATION PHYSICS & DETECTION
Questions 1–35
1. What is the SI unit of absorbed radiation dose?
A. Sievert
B. Gray
C. Roentgen
D. Curie
Answer: B. Gray
Rationale: The gray (Gy) measures absorbed radiation energy per unit
mass. The sievert measures equivalent/effective dose, while the curie
measures activity.
2. Which particle has a negative electrical charge?
A. Alpha particle
B. Proton
C. Electron
D. Neutron
Answer: C. Electron
Rationale: Electrons carry a negative charge. Protons are positive, while
neutrons have no electrical charge.
3. Which type of radioactive emission has the greatest penetrating
ability?
,A. Alpha
B. Beta
C. Gamma
D. Positron
Answer: C. Gamma
Rationale: Gamma photons have no mass or charge and can penetrate
matter much more effectively than alpha or beta particles.
4. What happens during beta-minus decay?
A. A proton becomes a neutron
B. A neutron becomes a proton
C. A proton becomes a positron
D. An electron combines with a proton
Answer: B. A neutron becomes a proton
Rationale: In beta-minus decay, a neutron converts into a proton while
emitting an electron and an antineutrino. The atomic number increases
by one.
5. What is the half-life of a radionuclide?
A. Time required for all activity to disappear
B. Time required for activity to double
C. Time required for half the radioactive atoms to decay
D. Time required for radiation to reach equilibrium
Answer: C. Time required for half the radioactive atoms to decay
Rationale: Physical half-life is the time required for one-half of the
radioactive nuclei in a sample to decay.
,6. A radionuclide has a physical half-life of 6 hours. What fraction
remains after 18 hours?
A. 1/2
B. 1/4
C. 1/8
D. 1/16
Answer: C. 1/8
Rationale: Eighteen hours represents three half-lives: 6, 12, and 18
hours. The remaining activity is (1/2)³ = 1/8.
7. Which interaction is primarily responsible for producing scattered
photons at diagnostic nuclear medicine energies?
A. Pair production
B. Compton scattering
C. Photodisintegration
D. Nuclear fission
Answer: B. Compton scattering
Rationale: Compton scattering is an important interaction for gamma
photons in the energy range encountered in nuclear medicine.
8. The photoelectric effect results in:
A. Complete absorption of the photon
B. Production of two photons
C. Emission of a neutron
D. Deflection without energy loss
Answer: A. Complete absorption of the photon
, Rationale: In the photoelectric effect, the incident photon transfers all
of its energy to an electron and disappears.
9. Pair production requires a photon energy of at least approximately:
A. 140 keV
B. 364 keV
C. 511 keV
D. 1.022 MeV
Answer: D. 1.022 MeV
Rationale: Pair production requires at least 1.022 MeV, equal to twice
the electron rest-mass energy of 511 keV.
10. What is produced when a positron annihilates with an electron?
A. One 140-keV photon
B. Two 511-keV photons traveling approximately 180° apart
C. Two 140-keV photons
D. One 1.022-MeV photon
Answer: B. Two 511-keV photons traveling approximately 180° apart
Rationale: Positron-electron annihilation normally produces two 511-
keV photons emitted in nearly opposite directions.
11. What principle explains why radiation intensity decreases with
increasing distance from a point source?
A. Half-value layer
B. Inverse square law
C. Beer-Lambert law
D. Reciprocity law
Practice Questions, Answers & Detailed Rationales | CNMT
DOMAIN I — RADIATION PHYSICS & DETECTION
Questions 1–35
1. What is the SI unit of absorbed radiation dose?
A. Sievert
B. Gray
C. Roentgen
D. Curie
Answer: B. Gray
Rationale: The gray (Gy) measures absorbed radiation energy per unit
mass. The sievert measures equivalent/effective dose, while the curie
measures activity.
2. Which particle has a negative electrical charge?
A. Alpha particle
B. Proton
C. Electron
D. Neutron
Answer: C. Electron
Rationale: Electrons carry a negative charge. Protons are positive, while
neutrons have no electrical charge.
3. Which type of radioactive emission has the greatest penetrating
ability?
,A. Alpha
B. Beta
C. Gamma
D. Positron
Answer: C. Gamma
Rationale: Gamma photons have no mass or charge and can penetrate
matter much more effectively than alpha or beta particles.
4. What happens during beta-minus decay?
A. A proton becomes a neutron
B. A neutron becomes a proton
C. A proton becomes a positron
D. An electron combines with a proton
Answer: B. A neutron becomes a proton
Rationale: In beta-minus decay, a neutron converts into a proton while
emitting an electron and an antineutrino. The atomic number increases
by one.
5. What is the half-life of a radionuclide?
A. Time required for all activity to disappear
B. Time required for activity to double
C. Time required for half the radioactive atoms to decay
D. Time required for radiation to reach equilibrium
Answer: C. Time required for half the radioactive atoms to decay
Rationale: Physical half-life is the time required for one-half of the
radioactive nuclei in a sample to decay.
,6. A radionuclide has a physical half-life of 6 hours. What fraction
remains after 18 hours?
A. 1/2
B. 1/4
C. 1/8
D. 1/16
Answer: C. 1/8
Rationale: Eighteen hours represents three half-lives: 6, 12, and 18
hours. The remaining activity is (1/2)³ = 1/8.
7. Which interaction is primarily responsible for producing scattered
photons at diagnostic nuclear medicine energies?
A. Pair production
B. Compton scattering
C. Photodisintegration
D. Nuclear fission
Answer: B. Compton scattering
Rationale: Compton scattering is an important interaction for gamma
photons in the energy range encountered in nuclear medicine.
8. The photoelectric effect results in:
A. Complete absorption of the photon
B. Production of two photons
C. Emission of a neutron
D. Deflection without energy loss
Answer: A. Complete absorption of the photon
, Rationale: In the photoelectric effect, the incident photon transfers all
of its energy to an electron and disappears.
9. Pair production requires a photon energy of at least approximately:
A. 140 keV
B. 364 keV
C. 511 keV
D. 1.022 MeV
Answer: D. 1.022 MeV
Rationale: Pair production requires at least 1.022 MeV, equal to twice
the electron rest-mass energy of 511 keV.
10. What is produced when a positron annihilates with an electron?
A. One 140-keV photon
B. Two 511-keV photons traveling approximately 180° apart
C. Two 140-keV photons
D. One 1.022-MeV photon
Answer: B. Two 511-keV photons traveling approximately 180° apart
Rationale: Positron-electron annihilation normally produces two 511-
keV photons emitted in nearly opposite directions.
11. What principle explains why radiation intensity decreases with
increasing distance from a point source?
A. Half-value layer
B. Inverse square law
C. Beer-Lambert law
D. Reciprocity law
