Nuclear reactors are not the only interesting source of electron anti-neutrinos (νe) for SNO+.

Anti-neutrinos are also released during the beta decay of radioactive isotopes in the Earth's mantle and crust; in particular, 40K and isotopes in the decay chains of 238U and 232Th. These anti-neutrinos, called "geo-neutrinos", are interesting from a geophysics point of view because they can tell us the amount of radioactivity present deep inside the Earth. This radioactivity is important because it is thought to be responsible for a large fraction (roughly 50%) of the heat produced in the Earth [1], and as such, it plays a very important role in determining how the interior of the Earth has cooled over time and how it will behave in the future.

The KamLAND experiment was the first to detect geo-neutrinos in 2005, in Japan  [2], and since then they have updated their measurement [3], and an independent measurement has been made by the Borexino experiment in Italy [4]

The experimental location of SNO+ in Sudbury, Canada is a good site for a third geo-neutrino detector in the world because the local geology in this region has been extensively characterized. This will allow the SNO+ geo-neutrino measurement to be combined with data from KamLAND and Borexino in a global analysis to infer the abundance of uranium and thorium deep in Earth's mantle, otherwise inaccessible.

Geo-neutrino energy spectra

Figure 1. The expected geo-neutrino signal in SNO+. The geo-neutrinos stand out clearly above the reactor anti-neutrinos at low energy.



[1] Fiorentini, G. et al., hep-ph/0409152 v1 (2004)
[2] Araki, T. et al., Nature  436, 499-503 (2005)
[3] Gando, A. et al., Phys. Rev. D 88 no.3, 033001 (2013)
[4] Agostini, M. et al., Phys. Rev. D 92, 031101 (2015)