B. Ricci: Antineutrinos from the Earth: Status and Perspectives
Geo-neutrinos were born on board of the Santa Fe Chief train, in 1953. During an experiment Gamow wrote to Reines: “It just occurred to me that your background may just be coming from high energy beta-decaying members of U and Th families in the crust of the Earth.”
Reines answered: “Heat loss from Eath’s surface is 50 erg cm-2 s-1. If assume all due to beta decay then have only enough energy for about 10^8 one-Mev neutrinos per cm-2 per second.”
U, Th and 40K release heat together with anti-neutrinos, in a well-fixed ratio. A fraction of geo-neutrinos from U and Th are above the threshold (1.8 MeV) to produce inverse beta decay processes on protons. The antineutrinos with highest energy are from Uranium, with a maximum energy of 3.3 MeV.
98% of uranium geo-neutrinos come from two transitions, from 214Bi and 234Pa. The spectrum is inferred from theory only. Detectors are sensitive to background from reactor antineutrinos. Above 3.3 MeV only reactor antineutrinos contribute; below, these account for 30% of the detectable flux.
THe total uncertainties on the background reactor flux is of about 3%.
Borexino and KamLAND have results for geo-neutrinos. Borexino in 2007-2012 collected 14+-4 events. KamLAND from 2002 to 2012 obtained a signal of 116+-28 events. What can we learn from these data ?
The speaker showed a very nice graph of heat emission from the earth versus the signal from Th and U, which showed various theoretical models and the data points. Only one model fits the measurements (see figure, below).
Our knowledge of the Earth’s interior are sketchy. We dug holes only up to 12 km deep. Samples from the crust and teh upper portion of the mantle are available for geochemical analysis. Seismology can reconstruct the density profile, but not the composition. Recently, a refined geophysical structure of the continental crust and new compilations of geochemical data have been made available. A new approach for evaluating the composition of the middle crust is also available.
Above: various models (coloured circles) predict different heat versus signal fluxes. The experimental measurements (triangle and square) are only compatible with one of the predictions (Krauss et al.,1984).
KL and Borexino can give information about radiogenic heat. A usual way to represent this is again the 2D graph of heat versus neutrino yield. The slope of the correlation between the two quantities is universal; the intercept depends on the site, and the width of the band depends also on the site, through crust-related effects.
The debate on the terrestrial heat flow is still open. Various estimates ranging from 31 to 47 terawatts exist.
To extract global information from geo-neutrinos, the contribution from “regional” geology (of the order of 500×500 km^2) has to be controlled. For each element the expected signal of geo-neutrinos is the sum of contributions from local, farl field crust, and mantle sources. We can derive the flux from the mantle by subtracting the crust contribution. The total estimates of KL and Borexino are of 31.1+-7.3 and 38.8+-12 Terawatts, respectively. The best fit for the mantle sigmal common to both is 14.1+-8.1 TNU.
For borexino, 68% of the signal should come from the crust, the rest from the mantle. In KamLand the percentages are 71% and 29%, respectively. The same proportion of the latter is expected in Juno, which will start data taking in 5 years. In SNO+ the crust contribution is 74%. So all these experiments will extract mainly information from the crust. At the hawaii the signal will instead come for only 28% from the crust, so an experiment there would have more information about mantle neutrinos.
In conclusion, geoneutrinos represent a unique probe of the Earth interior. A big effort is going on to calculate the expected signal. Future experiments are needed to better determine the radioactive content of the deep Earth. A conference devoted to this will be “Neutrino Geoscience 2015, Paris June 15th-17th.