An observation of Cyg X-3 a year later with the same instrument yielded an optically thin thermal spectrum for this source and provided the first evidence for strong spectral iron K-line emission from an X-ray binary.
12.
The plutonium-239 ( or the fissile uranium-235 ) fission cross-section is much smaller in a fast spectrum than in a thermal spectrum, as is the ratio between the 239 Pu / 235 U fission cross-section and the 238 U absorption cross-section.
13.
Finally, the ratio of neutrons released per neutron absorbed ( ? ) in is greater than two over a wide range of energies, including the thermal spectrum; as a result, thorium-based fuels can be the basis for a thermal breeder reactor.
