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Marco Pallavicini: Borex Source Experiment

March 13, 2013

Borexino is mainly a solare neutrino experiment, detecting neutrinos in organic liquid scintillator, with very low backgrounds obtained with selection, shielding and purification of the liquid. The detector can also detect anti-neutrinos, e.g. from Earth sources.

Borexino is a sphere in a tank of 3300 cubic meters of water. The detector is well-known and the speaker did not spend much time describing it. A talk was given yesterday and readers can check the report of that one for more details.

SoX is an acronym for Short distance neutrino oscillations with Borexino. This is a search for sterile neutrinos or other short-distance effects. It means to use a neutrino source from 51Cr, and an antineutrino source from 144Ce.

The idea is old – it was already in the original proposal of Borexino. The idea was succcessfully implemented by Gallex and SAGE. The science case does not need to be made again, but the origin was LSND in 2001, and this was later complemented by Gallium and reactor anomalies. One should also mention the astrophysics evidences from WMAP9. Planck results are coming soon!

What is planned is to test the existence of low L/E anomalies by placing well-known artificial sources close or inside Borexino. In a phase-A the source will be a Chromium source at 8.25 meters from the detector center. The size of the detector is large with respect to the expected neutrino oscillation length. In a phase B, one would use 144Ce, and in a phase C put it inside the detector. The useful antineutrinos from the 144Ce-Pr source come from the latter element above the detection threshold of 1.8 MeV.

The data analysis will consist of two techniques. One can do a standard disappearance experiment, since the total number oef events depends on theta_14. The sensitivity depends on how well one knows the activity of the source. The short lifetime yields useful time-events correlation: the background is constant but the signal is not.

One can also do spatial waves analysis, where the distribution of events as a function of the distance to the source can be observed, extracting well the parameters.

The two methods can of course be combined for maximum sensitivity. One can study the distance versus energy correlation pattern. The speaker showed a simulation of the observed spectrum, which would allow very precise determinations of the mixing parameters.

Finally the speaker discussed ancillary measurements: they can probe the weinberg angle at low energy, with a 2.6% error; and they can study the neutrino magnetic moment.

The 51Cr source needs a careful thermal design to handle the 10 MCi (2kW) power. The external temperature must be acceptable – expected 90 degrees. The internal one will be 260 degrees celsius.The source in phase A will be in a pit below the detector, in a 1-m-wide tunnel. THe speaker also showed diagrams for the proposed shielding designs.The source will end up weighing about 2 tons.With such a source inside the gamma background would prevent the neutrino detection, and only antineutrinos could be detected.

The 144Ce source instead would be very massive – 4 tons of shielding. The source would be made with spent nuclear fuel from Russia.

In summary, the plan is to perform an extensive search of sterile neutrinos with both nu and anti-nu sources. They have envisioned three phases of operation. The first two phases are expected for 2015-2016, while the third has no schedule yet.


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