An electron and a positron, each of mass 0.00055 u, meet and annihilate each other. Their kinetic energies are negligible. Taking \(1\ \mathrm{u}=931.5\ \mathrm{MeV}/c^{2}\), the energy given out as gamma-ray photons is about
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(1.02\ \mathrm{MeV}\)
An electron and a positron have the same mass. When they annihilate, the mass of both particles turns into the energy of gamma-ray photons, as given by \( E=mc^{2} \).
\( \begin{aligned} m &= 2\times0.00055 \\ &= 0.00110\ \mathrm{u} \end{aligned} \)
\( \begin{aligned} E &= m\times931.5\ \mathrm{MeV/u} \\ &= 0.00110\times931.5 \\ &= 1.02\ \mathrm{MeV} \end{aligned} \)
Therefore, about 1.02 MeV is given out, shared by the gamma-ray photons.
NCERT (2026-27):
Points to Ponder – “7. Electron and positron are a particle-antiparticle pair. They are identical in mass; their charges are equal in magnitude and opposite. (It is found that when an electron and a positron come together, they annihilate each other giving energy in the form of gamma-ray photons.)”
Nuclear fusion – energy generation in stars – “… The proton-proton (p, p) cycle by which this occurs is represented by the following sets of reactions:”
…
\( e^{+}+e^{-}\rightarrow\gamma+\gamma+ \)\( 1.02\ \mathrm{MeV} \)\( \qquad \text{(ii)} \)