Which process is commonly described as the inverse of fission?

• A. Beta decay
• B. Gamma decay
• C. Fission
• D. Fusion

Answer: (D)

The idea being tested is recognizing what acts as the inverse of fission. Fission splits a heavy nucleus into lighter pieces, releasing energy. The reverse process would take light nuclei and bind them together to form a heavier nucleus, which is fusion. Fusion is the process that combines nuclei, often releasing energy when light nuclei join to form a nucleus with higher binding energy per nucleon, as seen in stars when hydrogen isotopes fuse to helium. The other processes don’t reverse fission: beta decay changes a neutron into a proton (or vice versa) with particle emission, and gamma decay emits a photon without changing the number of nucleons. Thus, fusion best fits as the inverse of fission.