Mysterious radiation burst recorded in tree rings (774–775 AD)
Cosmic rays showered Earth 1230 years ago, but
where from? Los Angeles Times, June 4, 2012
Mysterious radiation burst recorded in tree rings
(Spike in carbon-14 levels indicates a massive
cosmic event — but supernovae and solar flares
ruled out.) by Richard A. Lovett, June 2, 2012
The paper is:
Miyake, F., K. Nagaya, K. Masuda, and T.
Nakamura, 2012, A signature of cosmic-ray
increase in ad 774–775 from tree rings in
Japan. Nature. Published online June 3, 2012.