Another one along these lines is antineutrinos. They're created by fission reactions, so a nuclear reactor puts out a lot of them. From what's available in the open literature, this seems like it might be getting close to being practical.
This publication[1] says neutrinos, but the article it references says antineutrinos[2].
That explains my misunderstanding.
1 https://www.energy.gov/science/np/articles/detecting-neutrin...
2. https://www.osti.gov/pages/biblio/1959810-evidence-antineutr...