Nuclear fusion is a process whereby the nuclei of atoms are joined, or fused,  and in which energy is released  One of the by-products of fusion is helium-4 gas  A recent fusion experiment was conducted using heavy water contained in a sealed flask  The flask was, in turn, contained in an air-filled chamber designed to eliminate extraneous vibration  After the experiment, a measurable amount of helium-4 gas was found in the air of the chamber
The experimenters cited this evidence in support of their conclusion that fusion had been achieved
Helium-4 gas rapidly breaks down, forming ordinary helium gas after a few hours.