Information on mass chains and nuclides available on this website:

WWW TUNL

⁶n has not been observed in the interaction of 700 MeV protons or of 400 GeV protons with uranium: the cross section is < 1.1 x 10^-3 μb (TU77; 700 MeV), < 9 μb (TU77B; 400 GeV). See also (DE77M) [and footnote 10 in (TU77B) and (GO76J, SA78G; theor.).