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
When fusion is achieved, it normally produces several by-products, including tritium and gamma rays.