Radiation Dosimetry Certification Exam
Questions with Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
Question 1
Which type of radioactive decay emits a helium nucleus?
A. Beta-minus decay
B. Alpha decay
C. Gamma decay
D. Positron emission
Rationale: Alpha decay releases an alpha particle, which consists of two protons
and two neutrons and is equivalent to a helium-4 nucleus. Alpha particles have
relatively high mass and charge compared with beta particles and photons.
Question 2
What is the primary source of bremsstrahlung x-rays in a medical linear
accelerator?
A. Nuclear fission
B. Electron capture
C. Positron annihilation
D. Electron interactions with a high-Z target
Rationale: High-energy electrons strike a dense target, commonly made from a
high-Z material such as tungsten, and undergo deceleration. The resulting loss of
kinetic energy produces bremsstrahlung x-rays.
,Question 3
Which interaction is primarily responsible for transferring photon energy to an
electron through complete absorption of the photon?
A. Compton scattering
B. Photoelectric effect
C. Pair production
D. Coherent scattering
Rationale: In the photoelectric effect, the incident photon is completely absorbed
and an electron is ejected from the atom. The probability of this interaction
depends strongly on photon energy and the atomic number of the absorbing
material.
Question 4
Which photon interaction is most closely associated with an incident photon
scattering from an electron while transferring only part of its energy?
A. Pair production
B. Photoelectric absorption
C. Compton scattering
D. Nuclear fission
Rationale: Compton scattering involves an interaction between the incident
photon and an electron in which the photon transfers part of its energy and
changes direction. A scattered photon and recoil electron result.
Question 5
What is the main physical distinction between photons and charged particles used
in radiation therapy?
A. Photons always have greater stopping power
B. Charged particles have no interactions with matter
C. Photons have rest mass while electrons do not
D. Charged particles directly ionize matter along their tracks
,Rationale: Charged particles interact directly through their electric charge and
produce ionization along their paths. Photons are indirectly ionizing and transfer
energy to charged particles through photon interactions.
Question 6
Which characteristic is most associated with the Bragg peak of a proton beam?
A. Maximum dose at the skin surface
B. Concentration of dose near the end of the proton range
C. Continuous dose beyond the proton range
D. Complete absence of entrance dose
Rationale: Protons deposit relatively low dose along much of their entrance path
and deposit a substantial amount of energy near the end of their range,
producing the Bragg peak. This property is important in proton treatment
planning.
Question 7
A radionuclide has a physical half-life of 10 days. Approximately what fraction of
the original activity remains after 30 days?
A. 50%
B. 25%
C. 12.5%
D. 6.25%
Rationale: Thirty days represents three half-lives. The activity decreases to 1/2
after 10 days, 1/4 after 20 days, and 1/8, or 12.5%, after 30 days.
Question 8
Which radionuclide is commonly used as the source in high-dose-rate
brachytherapy?
, A. Cobalt-60 only
B. Cesium-137
C. Iodine-125
D. Iridium-192
Rationale: Iridium-192 is widely used in HDR brachytherapy because it has
suitable photon emissions, a relatively short half-life, and can be incorporated
into a small source.
Question 9
What is the Hounsfield unit assigned to water on a conventional CT scale?
A. −1000 HU
B. 0 HU
C. +100 HU
D. +1000 HU
Rationale: Water is conventionally assigned a CT number of approximately 0 HU.
Air is approximately −1000 HU, while dense materials such as cortical bone have
substantially positive values.
Question 10
What is the approximate Hounsfield unit of air?
A. −1000 HU
B. −100 HU
C. 0 HU
D. +1000 HU
Rationale: Air is conventionally represented at approximately −1000 HU on the
CT scale. This large difference from water allows air-containing regions to be
readily distinguished.
Question 11
Questions with Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
Question 1
Which type of radioactive decay emits a helium nucleus?
A. Beta-minus decay
B. Alpha decay
C. Gamma decay
D. Positron emission
Rationale: Alpha decay releases an alpha particle, which consists of two protons
and two neutrons and is equivalent to a helium-4 nucleus. Alpha particles have
relatively high mass and charge compared with beta particles and photons.
Question 2
What is the primary source of bremsstrahlung x-rays in a medical linear
accelerator?
A. Nuclear fission
B. Electron capture
C. Positron annihilation
D. Electron interactions with a high-Z target
Rationale: High-energy electrons strike a dense target, commonly made from a
high-Z material such as tungsten, and undergo deceleration. The resulting loss of
kinetic energy produces bremsstrahlung x-rays.
,Question 3
Which interaction is primarily responsible for transferring photon energy to an
electron through complete absorption of the photon?
A. Compton scattering
B. Photoelectric effect
C. Pair production
D. Coherent scattering
Rationale: In the photoelectric effect, the incident photon is completely absorbed
and an electron is ejected from the atom. The probability of this interaction
depends strongly on photon energy and the atomic number of the absorbing
material.
Question 4
Which photon interaction is most closely associated with an incident photon
scattering from an electron while transferring only part of its energy?
A. Pair production
B. Photoelectric absorption
C. Compton scattering
D. Nuclear fission
Rationale: Compton scattering involves an interaction between the incident
photon and an electron in which the photon transfers part of its energy and
changes direction. A scattered photon and recoil electron result.
Question 5
What is the main physical distinction between photons and charged particles used
in radiation therapy?
A. Photons always have greater stopping power
B. Charged particles have no interactions with matter
C. Photons have rest mass while electrons do not
D. Charged particles directly ionize matter along their tracks
,Rationale: Charged particles interact directly through their electric charge and
produce ionization along their paths. Photons are indirectly ionizing and transfer
energy to charged particles through photon interactions.
Question 6
Which characteristic is most associated with the Bragg peak of a proton beam?
A. Maximum dose at the skin surface
B. Concentration of dose near the end of the proton range
C. Continuous dose beyond the proton range
D. Complete absence of entrance dose
Rationale: Protons deposit relatively low dose along much of their entrance path
and deposit a substantial amount of energy near the end of their range,
producing the Bragg peak. This property is important in proton treatment
planning.
Question 7
A radionuclide has a physical half-life of 10 days. Approximately what fraction of
the original activity remains after 30 days?
A. 50%
B. 25%
C. 12.5%
D. 6.25%
Rationale: Thirty days represents three half-lives. The activity decreases to 1/2
after 10 days, 1/4 after 20 days, and 1/8, or 12.5%, after 30 days.
Question 8
Which radionuclide is commonly used as the source in high-dose-rate
brachytherapy?
, A. Cobalt-60 only
B. Cesium-137
C. Iodine-125
D. Iridium-192
Rationale: Iridium-192 is widely used in HDR brachytherapy because it has
suitable photon emissions, a relatively short half-life, and can be incorporated
into a small source.
Question 9
What is the Hounsfield unit assigned to water on a conventional CT scale?
A. −1000 HU
B. 0 HU
C. +100 HU
D. +1000 HU
Rationale: Water is conventionally assigned a CT number of approximately 0 HU.
Air is approximately −1000 HU, while dense materials such as cortical bone have
substantially positive values.
Question 10
What is the approximate Hounsfield unit of air?
A. −1000 HU
B. −100 HU
C. 0 HU
D. +1000 HU
Rationale: Air is conventionally represented at approximately −1000 HU on the
CT scale. This large difference from water allows air-containing regions to be
readily distinguished.
Question 11
