I2 : Geoparticles

Elementary particles can nowadays be used as probes of their sources or of the matter they have traversed.

We propose to investigate the use of particles as probes of the Earth’s interior in two ways:

1) neutrinos from the decay of radioactive elements in the crust and mantle, called geoneutrinos, to gather information on the composition and distribution of the radioactive elements whose decay provides a large fraction of the Earth’s thermal power.

2) geophysical tomography with muon telescopes, to map the internal density structure of volcanoes, unstable cliffs, and geological layers located over mines, nuclear waste storages, and CO2 reservoirs.

The main difficulty in geo-neutrino detection lies in their weak flux with respect to other antineutrinos, mainly those issued from power reactors (which are, on the other hand, the signal for reactor neutrino experiments such as Double Chooz in the Ardennes region, in which the APC group has a leading role). Conversely, muons represent one of the most dangerous backgrounds for neutrino experiments located underground.

The research program we propose is along the following lines:

1) The modelling of muon propagation through the rock is critical to extract the relevant information from the muon telescope data. Detailed simulations have been carried out by the APC group to predict the background to the Double Chooz experiment, and independently by the IPGP group to predict the signal in muon telescopes. A synergy of the two teams will lead to the development of the most accurate methods for the inverse tomography problem.

2) An accurate modelling of reactor anti-neutrino spectra can be obtained from the analysis of the data that will come from the Double Chooz experiment (2011-2016). This will improve the knowledge of geo-neutrino backgrounds and thus the precision on current and future measurements. In addition, the APC group is involved in the Borexino experiment, which has recently reinforced the evidence for geo-neutrinos and will provide a more precise measurement in the near future. Participating in this data analysis will be useful to gain experience on the topic.

3) The results of current geo-neutrino experiments are dominated by the thick continental crust. However, the importance of geo-neutrinos lies in the possibility to probe the geochemistry of the mantle. We propose two approaches, to be studied jointly by IPGP and APC: either detect geo-neutrinos at specific locations, or precisely subtract the contribution from the crust. The option of a movable detector could be interesting in this regard.

4) Future detectors for geo-neutrinos demand R&D on specific items. We propose to construct a test bench to address some key topics. In parallel with the R&D on future large detectors, a selection of the most promising sites for geosciences, i.e. where to deploy these detectors, has to be performed.