The most significant radiation sources are the fission products from the primary fission stage, and in the case of fission-fusion-fission weapons, from the fission of the fusion stage uranium tamper. Many more neutrons per unit of energy are released in a thermonuclear explosion in comparison with a purely fission yield influencing the fission products composition. For example, the uranium-237 isotope is a unique thermonuclear explosion marker, as it is produced by a (n,2n) reaction from uranium-238, with the minimal neutron energy needed being about 5.9 MeV. Considerable amounts of neptunium-239 and uranium-237 are indicators of a fission-fusion-fission explosion. Minor amounts of uranium-240 are also formed, and capture of large numbers of neutrons by individual nuclei leads to formation of small but detectable amounts of higher transuranium elements, e.g. einsteinium-255 and fermium-255.