1999. Found this searching the word "sun":
Chemical Action around Metal in the Soil
"... the detector reading is much weaker than the original one, now that the surrounding soil has been disturbed. I have observed this phenomenon on many occasions."
"To me this suggests that there is some type of “chemical actionâ€, which takes place around a long-time buried coin (or any metal) when placed in the soil."
"A few years back, Dr. Manuel Ortiz called this “field†F.E.R.F. I believe this was his term, which stood for Free Electron Radiation Field." 
"Is radiation from the Sun involved?"
MY BIG
YES TO THE LAST QUESTION.
http://www.geotech1.com/forums/showt...&highlight=sun