Since then, the U.S. has scaled back the use of the substance in these monitors, but there is not enough of both regular helium and helium-3 to go around. Researchers are adapting by "using cryogen-free technology that (although prone to vibration) is compatible with dilution refrigerators and superconducting magnets," Nature reported. Helium-3 derives from the decay of tritium, a radioactive isotope of hydrogen produced in the production of nuclear weapons--as nuclear weapon production has fallen, so has helium-3 abundance. It can also be found on the moon, so if we ever needed a plentiful supply of helium-3, we could get it there. But that would obviously be quite an undertaking.