Answer: Two gammas, each 0.511 MeV emitted in opposite directions.
- A One single gamma photon carrying 1.022 MeV in one direction
- B Two gammas, each 0.511 MeV emitted in opposite directions
- C A single alpha particle ejected from the annihilation point
- D A pair of neutrinos with no accompanying photon emission
Correct answer: B. Two gammas, each 0.511 MeV emitted in opposite directions
Explanation: Pair annihilation: e⁺ + e⁻ → 2 gamma. Two 511 keV photons emitted back-to-back (momentum conservation). Basis of PET scans.
Binding energy per nucleon rises sharply for light nuclei, peaks around iron-56 (the most stable nucleus), then slowly declines for heavier nuclei - which is exactly why fusing light nuclei or splitting heavy nuclei both release energy: each moves the products toward this stability peak.
Concept context
Radioactivity, nuclear reactions, fission, fusion, and binding energy.