In pair production near a nucleus, which statement is true?

In pair production near a nucleus, which statement is true?

• A. The photon is absorbed without creating particles.
• B. The energy of the photon remains unchanged.
• C. The energy is converted into the mass of two electrons, with any remaining energy as kinetic energy.
• D. The nucleus emits a gamma ray only.

Answer: (C)

Pair production happens when a sufficiently energetic photon converts into an electron-positron pair in the electromagnetic field of a nearby nucleus. The nucleus is needed to conserve momentum, since a lone photon cannot produce a massive pair and satisfy both energy and momentum conservation. The photon's energy goes first into creating the rest mass of the electron and the positron, which together require 2 m_e c^2 of energy. Any energy beyond that threshold shows up as kinetic energy of the produced particles (and a tiny share can go as recoil energy of the nucleus). So the true statement is that the photon's energy is converted into the mass of the electron-positron pair, with any remaining energy appearing as kinetic energy. The other ideas—no particles created, energy unchanged, or the nucleus merely emitting a gamma ray—do not fit what actually happens in pair production.

Pair production happens when a sufficiently energetic photon converts into an electron-positron pair in the electromagnetic field of a nearby nucleus. The nucleus is needed to conserve momentum, since a lone photon cannot produce a massive pair and satisfy both energy and momentum conservation. The photon's energy goes first into creating the rest mass of the electron and the positron, which together require 2 m_e c^2 of energy. Any energy beyond that threshold shows up as kinetic energy of the produced particles (and a tiny share can go as recoil energy of the nucleus). So the true statement is that the photon's energy is converted into the mass of the electron-positron pair, with any remaining energy appearing as kinetic energy. The other ideas—no particles created, energy unchanged, or the nucleus merely emitting a gamma ray—do not fit what actually happens in pair production.