A very good summary on the search and study of geo-neutrinos in this month's issue of CERN Courier.
Geo-neutrinos are the (anti)neutrinos produced by the natural radioactivity inside the Earth. In particular, the decay chains of 238U and 232Th include six and four β− decays, respectively, and the nucleus of 40K decays by electron capture and β− decay with branching ratios of 11% and 89%, respectively. The decays produce heat and electron antineutrinos, with fixed ratios of heat to neutrinos (table 1). A measurement of the antineutrino flux, and possibly of the spectrum, would provide direct information on the amount and composition of radioactive material inside the Earth and so would determine the radiogenic contribution to the heat flow.
It's interesting that the article mentioned that the Earth emits mainly electron antineutrino, while the sun emits mainly electron neutrino. In light of the recent possible error in estimating the amount of electron antineutrino that is emitted from such nuclear reaction, I wonder if this makes the study of geo-neutrinos even more difficult. Or maybe the 3% difference doesn't matter.