While I share your skepticism regarding the numbers quoted by the original article, a few remarks about the cosmic muons: I think you are overestimating the co

While I share your skepticism regarding the numbers quoted by the original article, a few remarks about the cosmic muons: I think you are overestimating the cosmic muon flux quite a bit, see the particle data book for the discussion of cosmic muons. http://pdg.lbl.gov/2012/reviews/rpp2012-rev-cosmic-rays.pdf http://pdg.lbl.gov/2012/reviews/rpp2012-rev-cosmic-rays.pdf (page 5) Also ionization loss by fast muons is pretty low, it's in the "minimum ionizing particle" regime, and most cosmic muons we see "down here" are in the >1GeV/c area. http://pdg.lbl.gov/2013/reviews/rpp2012-rev-passage-particles-matter.pdf http://pdg.lbl.gov/2013/reviews/rpp2012-rev-passage-particle... (page 4 is muons on Copper) But again, I think your doubts about the numbers is certainly warranted. With a experimental setup needing such a long exposure time to yield statistically usable numbers, I think one cannot really prove a lot. Certainly a detector more suitable to the task of counting radioactive decay, such as a calibrated large-volume geiger counter, would be better. At least one should run both setups in parallel for an extended period of time to be able to do cross-corralations.