Some of their work involved X-rays produced by particle accelerators, devices that bring atomic particles into high energies.
VOA: special.2009.12.08
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 .