In 1966 Kenneth Greisen, Vadim Kuzmin and Georgiy Zatsepin showed that high-energy charged particles (cosmic rays are mostly atomic nuclei, and thus positively charged) should be slowed by collisions with the photons of the cosmic microwave background (radiation left over from the Big Bang that permeates all space).
Using the first completed section of the NOvA neutrino detector, scientists have begun collecting data from cosmic rays-particles produced by a constant rain of atomic nuclei falling on the Earth's atmosphere from space .
It will examine neutrons (the electrically neutral components of atomic nuclei) that are ejected from the lunar surface after being bombarded by cosmic rays (energetic particles that constantly streak through space).
Since every isotope that spews gamma or X rays or charged particles does so in a unique way--an atomic fingerprint, of sorts--the color and intensity of the flash tells the detector which isotope is present and in what quantity.