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G. Fiorentini: GeoNeutrinos

March 15, 2011

Not many news on Geoneutrinos in the Neutel11 Conference, but a really comprehensive talk by G. Fiorentini.

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So, first of all what geoneutrinos are and why we’re such interested in catching and knowing them.

They’re “just” antineutrinos coming from Earth, emitted in U, Th and K decay chains together with heat in very well fixed rates. Thus geoneutrinos represent a probe of Earth’s interior, giving the possibility to check the standard geochemical model (BSE) of natural radioactivity in Earth and to answer to some still open questions: what is the radiogenic contribution to terrestrial heat production? How much U and Th are there in the crust and in the mantle? What is hidden in the Earth’s core?

From an experimental point of view, geoneutrinos are distinguishable from solar neutrinos essentially for being antineutrinos (instead of neutrinos), and different production processes can be recognized thanks to different energy spectra; but it must be also reminded that only a fraction of geoneutrinos coming from U and Th  are above threshold for inverse beta-decay on protons, thus being detectable.

Frithermore, geoneutrinos detection is much sensitive to anti-neutrinos produced at reactors, not only for background reasons (reactors anti-nu must be subtracted to the signal spectrum, mainly in loe energy region) but also because reactors anti-nu in the high energy region are used to calibrate the detectors. Thus, in principle recalculations of the reactor anti-neutrino fluxes are interesting, but the recent 3% flux increase estimate is negligible with respect to  statistical errors and geological uncertainties.

So far only two running experiments are performing geoneutrinos search: Borexino, which results were presented in March 2010, and KamLAND, with results presented in June 2010.

Borexino published the detection of 9.9 +4.1 -3.4 geoneutrinos events, excluding the no geoneutrinos scenario at 4 sigma. The central value is close to the fully radiogenic model and some 1sigma from the BSE prediction

KamLAND published 111 +45 -43 geoneutrinos events (having reduced the background with respect to previous data release), with the best fit value close to the BSE prediction and 2.5 sigma away from fully radiogenic model. This last result can be used to estimate, for the very first time, the terrestrial heat power from U and Th: H(U+Th) = 16 ± 8 TW (exp) ± 5 TW (geology).

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