Energy and mass are equivalent according to Einstein, and both can be measured in electron volts. (1 eV is the energy acquired by an electron when accelerated through 1 volt.) If the rest mass of an electron is 511 keV, what is the threshold energy of a gamma-ray photon in order that it can undergo electron pair production?
A)1.533 MeV
B)1.022 MeV
C)0 eV, because it produces a particle-antiparticle pair
D)511 keV

Question

Quizplus · Accepted Answer

According to the law of conservation of energy, the energy of the gamma-ray photon must be converted into the mass of the electron-positron pair created (as well as the kinetic energy of the created particles). Therefore, the threshold energy of the gamma-ray photon must be greater than or equal to the combined rest mass energy of the electron and positron, which is 2 x 511 keV = 1.022 MeV. Therefore, choice B is the correct answer.

Energy and Mass Are Equivalent According to Einstein, and Both