The heaviest stable krypton isotope, krypton 86, rose to prominence in the second half of the last century with a tad over one and a half million wavelengths of its orange-red spectral line being used ...

No, it’s krypton - although unfortunately it has nothing to do with Superman. That doesn’t make this gas any less heroic though: during the Cold War, its radioactive isotope Kr-85 was used to ...