RAPHEX 2025 | Complete Exam Questions with All Correct Answers | Radiation Physics & Dosimetry Review

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RAPHEX 2025 EXAM QUESTIONS WITH ALL
CORRECT ANSWERS/NEWEST UPDATE Questions
and Correct Answers/ Latest Update / Already Graded
Photonuclear disintegration

Ans: Photon energy > binding energy of nucleus, particle removed from nucleus
Generates neutrons in therapy, activates high Z materials in linac head
At energies > 10 MV, effect increases


Speed (waves)

Ans: speed = wavelength x frequency


Speed of light

Ans: 3 x 10^8 m/s


Photon energy

Ans: hf = hc/lambda


Planck's constant

Ans: 6.63 x 10^-34 kgm2/sec


Charge on an electron

Ans: -1.6 x 10^-19 C


1 eV

Ans: 1.6 x 10^-19 J



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Proton mass

Ans: 938 MeV


Neutron mass

Ans: 940 MeV


Electron mass

Ans: 0.511 MeV


N(t)

Ans: N0e^-lambda(t)


Half-life

Ans: T1/2 = 0.693/decay constant


Mean lifetime

Ans: 1.44(T1/2)


1 Curie

Ans: 3.7 x 10^10 Bq


Naturally occurring radioactive materials

Ans: Thorium-232, Uranium-238, Uranium-235


effective half-life



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Ans: 1 / effective = 1/physical + 1/biological


Heel effect

Ans: Decrease in radiation intensity toward anode side of tube


Increased x-ray tube kVp

Ans: Increased mean output energy and output


Output of an x-ray tube

Ans: proportional to kV^2


Increased x-ray tube mAs

Ans: Increased tube output only


Photoelectric interactions

Ans: photon absorbed, ejects inner shell electron
observed at diagnostic energies (30-120 kV), especially high atomic number
materials


Probability of photoelectric interactions

Ans: Z^3/E^3


Compton (incoherent) scattering




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Ans: photon collides with outer shell electron, electron recoils and photon
scatters
dominant mechanism at therapy energies
only depends on electron density


Backscattered photons

Ans: 180 degrees, energy no greater than 256 keV


Side-scattered photons

Ans: energy no greater than 512 keV


Pair production

Ans: Photon interacts with Coulomb field from nucleus, creates electron-
positron pair
Photon energy must be > 1.022 MeV
Higher therapy energies, higher atomic number materials


Photon beam attenuation

Ans: I=I0(e^-ux)
x=material thickness
u=linear attenuation coefficient (dependent on photon energy and material
composition)
Estimate 3.5% per cm for a 6 MV photon beam
Estimate 2.5 % per cm for a 15 MV photon beam


Mass attenuation coefficient

Ans: linear attenuation coefficient / density




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