Lithium-6 is valuable as the source material for the production of tritium (hydrogen-3) and as an absorber of neutrons in nuclear fusion reactions. Natural lithium contains about 7.5 percent lithium-6, with the rest being lithium-7. Large amounts of lithium-6 have been separated out for placing into hydrogen bombs. The separation of lithium-6 has by now ceased in the large thermonuclear powers, but stockpiles of it remain in these countries. Lithium-6 acts as a fermion in interactions with other particles because it has three protons, three neutrons, and three electrons, and these give the atom a total atomic "spin" of plus or minus 1/2 - and not the integral spin of a boson.