I believe neutrons can also come from “star formation” which occurs when a nucleus is hit by a gamma photon having more energy than the binding energy of said nucleus. The nucleus blows up (leaving a star pattern on cloud-chamber photographs, hence the name) and not shown on those photographs, some of the remnants are neutrons. This only happens at gamma energies well above 5 or 10 MeV. Even if the gammas didn’t penetrate far into the atmosphere, they wouldn’t need to as there are nitrogens up there, and anyway the neutrons can penetrate.
Then again, maybe the Vikings had nuclear weapons. (Which is why they were so feared.)
Also called spallation. I have seen spallation at the HERMESIII accelerator, but usually with a high-Z target. Not much from Al, plenty from W.
It’s a possibility to get n from high energy photons, but the lower the Z, the smaller the cross section.
I kind of like the suggestion that it was Viking nukes! Or maybe a “natural reactor” like at Gabon that has not been discovered yet.