Final answer:
Alpha decay is a type of radioactive decay where a nucleus emits an alpha particle. Californium-251 undergoes alpha decay, resulting in the formation of the daughter nucleus Thorium-234.
Step-by-step explanation:
Alpha decay is a type of radioactive decay where a nucleus emits an alpha particle, which is composed of two protons and two neutrons. Californium-251, for example, undergoes alpha decay, resulting in the formation of a different nucleus. In this case, the parent nucleus Californium-251 decays into the daughter nucleus Thorium-234 by emitting an alpha particle.