RAPHEX EXAM QUESTIONS AND
ANSWERS GRADED A+ 2025/2026
In the equation for radioactive decay, lambda represents _________ - ANS the fraction of
atoms that decay per unit time
Purpose of rounded leaf ends for MLCs - ANS keep the geometric penumbra relatively
constant for different leaf positions
Neutron contamination is greatest in a _______ beam
a. 6 MV photon
b. 10 MV photon
c. 18 MV photon
d. 6 MeV electron
e. 20 MeV electron - ANS c. 18 MV photon
remember neutrons are produced through photon interactions, begin happening above 10 MV
photons
Loss of charged particle equilibrium is greatest in lung radiotherapy for field sizes _______ cm2
and photon energies _______ MV - ANS < 5x5; >= 15 MV
effect is greater for higher photon energies since the average energy of the secondary electrons
is greater. effect is greater for small field sizes, harder to maintain CPE in small space
1 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
,What is the relationship between HVL1 and HVL2 for a polyenergetic kV photon beam -
ANS HVL1 < HVL2 always
As a polyenergetic kV photon beam traverses a material, lower-energy photons will be
preferentially attenuated. Therefore, as the beam travels farther into a material, it will become
hardened and its average energy will increase. Therefore, the HVL will increase as well.
Compton scattering angles for minimum and maximum energy transference - ANS MAX
ENERGY TRANSFER --> electron scattered 0 degrees (forward) and the photon is scattered at 180
degrees (backward)
MIN ENERGY TRANSFER --> electron scattered at 90 (sideways) and photon is scattered at 0
degrees (forward), no energy is transferred to the electron
NOTE: all of the photon energy cannot be transferred to the electron, must always be some
energy available for the scattered photon.
what is the relationship between the mass energy transfer coefficient (u_tr/ρ) and the mass
energy absorption coefficient (u_en/ρ) - ANS u_tr/ρ ≤ u_en/ρ always
u_tr/ρ includes radiative losses, u_en/ρ does not
The quantity f_med (or f-factor) - ANS roentgen-to-rad conversion factor
0.876 * (u_en/ρ)air/(u_en/ρ)med
as such, function of both medium density and photon energy
KERMA is defined as _______ - ANS the energy per unit mass transferred from incident
photons to charged particles in the medium
(energy of charged particles liberated by indirectly ionizing radiation)
Energy dependence of dosimeters:
2 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
, a. radiographic film
b. radiochromic film
c. Al2O3 OSLD
d. LiF thermoluminescents - ANS Energy dependence will be based on Atomic Number
a. A = 42.8
b. A = 6.84
c. A = 11.2
d. A = 8.2
In the kV range, radiochromic film will have the least energy dependence (as in the
Photoelectric effect range, attenuation is proportional to Z^3)
_____________ is the most accurate for measuring surface dose
a. extrapolation chamber
b. farmer chamber
c. plane parallel chamber
d. OSLD chamber
e. film - ANS a. extrapolation chamber
according to AAPM TG 176, the extrapolation chamber is the gold standard for surface dose
measurement
The charge measured by an ionization chamber is ___________ proportional to the pressure
and __________ proportional to the temperature of the gas in the chamber - ANS directly;
inversely
as the pressure increases or the temperature decreases, the density of air in the chamber will
increase, resulting in an increase in charge measured.
_____________ is NOT suitable for small field dosimetry
3 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
ANSWERS GRADED A+ 2025/2026
In the equation for radioactive decay, lambda represents _________ - ANS the fraction of
atoms that decay per unit time
Purpose of rounded leaf ends for MLCs - ANS keep the geometric penumbra relatively
constant for different leaf positions
Neutron contamination is greatest in a _______ beam
a. 6 MV photon
b. 10 MV photon
c. 18 MV photon
d. 6 MeV electron
e. 20 MeV electron - ANS c. 18 MV photon
remember neutrons are produced through photon interactions, begin happening above 10 MV
photons
Loss of charged particle equilibrium is greatest in lung radiotherapy for field sizes _______ cm2
and photon energies _______ MV - ANS < 5x5; >= 15 MV
effect is greater for higher photon energies since the average energy of the secondary electrons
is greater. effect is greater for small field sizes, harder to maintain CPE in small space
1 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
,What is the relationship between HVL1 and HVL2 for a polyenergetic kV photon beam -
ANS HVL1 < HVL2 always
As a polyenergetic kV photon beam traverses a material, lower-energy photons will be
preferentially attenuated. Therefore, as the beam travels farther into a material, it will become
hardened and its average energy will increase. Therefore, the HVL will increase as well.
Compton scattering angles for minimum and maximum energy transference - ANS MAX
ENERGY TRANSFER --> electron scattered 0 degrees (forward) and the photon is scattered at 180
degrees (backward)
MIN ENERGY TRANSFER --> electron scattered at 90 (sideways) and photon is scattered at 0
degrees (forward), no energy is transferred to the electron
NOTE: all of the photon energy cannot be transferred to the electron, must always be some
energy available for the scattered photon.
what is the relationship between the mass energy transfer coefficient (u_tr/ρ) and the mass
energy absorption coefficient (u_en/ρ) - ANS u_tr/ρ ≤ u_en/ρ always
u_tr/ρ includes radiative losses, u_en/ρ does not
The quantity f_med (or f-factor) - ANS roentgen-to-rad conversion factor
0.876 * (u_en/ρ)air/(u_en/ρ)med
as such, function of both medium density and photon energy
KERMA is defined as _______ - ANS the energy per unit mass transferred from incident
photons to charged particles in the medium
(energy of charged particles liberated by indirectly ionizing radiation)
Energy dependence of dosimeters:
2 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
, a. radiographic film
b. radiochromic film
c. Al2O3 OSLD
d. LiF thermoluminescents - ANS Energy dependence will be based on Atomic Number
a. A = 42.8
b. A = 6.84
c. A = 11.2
d. A = 8.2
In the kV range, radiochromic film will have the least energy dependence (as in the
Photoelectric effect range, attenuation is proportional to Z^3)
_____________ is the most accurate for measuring surface dose
a. extrapolation chamber
b. farmer chamber
c. plane parallel chamber
d. OSLD chamber
e. film - ANS a. extrapolation chamber
according to AAPM TG 176, the extrapolation chamber is the gold standard for surface dose
measurement
The charge measured by an ionization chamber is ___________ proportional to the pressure
and __________ proportional to the temperature of the gas in the chamber - ANS directly;
inversely
as the pressure increases or the temperature decreases, the density of air in the chamber will
increase, resulting in an increase in charge measured.
_____________ is NOT suitable for small field dosimetry
3 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
