Borexino

Borexino is a 300 ton liquid scintillator detector placed at the Laboratori Nazionali del Gran Sasso (LNGS), the largest underground laboratory in the world. Coverage by the 1400 m thick rock provides a cosmic-ray-flux reduction by one million times. The experiment is contained in a stainless steel dome of 18 m in diameter and consists of the Outer Detector (OD) and the Inner Detector (ID).

  • The OD serves as a shielding against external background and as a Cherenkov veto for cosmogenic muons. It is filled with 2400 tons of ultra pure water and is equipped with 208 PMTs.
  • The ID consists of a stainless steel sphere and two nested nylon vessels. It is filled with 1040 tons of shielding liquid outside and 280 tons of liquid scintillator inside the inner nylon vessel. There are 2200 PMTs installed inside.
  • The liquid scintillator is based on pseudocumene (PC, 1,2,4-trimethylbenzene) with addition of PPO (2,5-diphenyloxazole) as a fluorescent dye.
The Borexino collaboration's primary goal is detecting solar neutrinos, particularly those below 2 MeV. The experiment’s distinctive feature is the unprecedented ultra-low radioactive background, which is the basis of the outstanding achievements obtained by the experiment. Less than one year after the start of the data taking in May 2007, the first real-time measurement of the 7Be solar neutrinos was performed [1]. Since then, the following results concerning solar neutrinos have been obtained:
  • the measurement of 7Be solar neutrinos with 5% precision [2];
  • exclusion of any significant day-night asymmetry in the 7Be solar neutrino rate [3];
  • the first direct observation of the pep neutrinos [4];
  • the best available upper limit on the flux of solar neutrinos produced in the CNO (carbon, nitrogen, oxygen) cycle [4];
  • the first real-time measurement of neutrinos from the primary proton–proton fusion process in the Sun [5].

In 2010 the Borexino collaboration reported the first observation of geoneutrinos at more than 3$\sigma$ C.L. [6]. Geoneutrinos, antineutrinos emitted along the U and Th decay chains, hold clues about the Earth radiogenic heat. Its more precise determination would bring precious insights about the formation, composition, and dynamics of our planet. Borexino, together with KamLAND in Japan, are the only two experiments being currently able to measure geoneutrinos. The latest Borexino update on geoneutrinos was published in 2015 [7].

Thanks to an unprecedentedly high radiopurity and relatively large volume, Borexino has placed stringent limits on several rare processes. The most recent are:

  • the search for correlations between (anti)neutrinos and gamma-ray bursts [8];
  • a test of electric charge conservation [9].
The data gathered since 2011 (Phase II) are characterised by even lower radioactive background than in Phase I (2007-2010). These data are currently the main focus of the solar neutrino program, in which the IKP group is deeply involved. In contrast with the previous analysis, the spectral fit is being performed on the whole energy scale up to about 1.5-2 MeV, providing a simultaneous result for 7Be, pp, and pep neutrinos. The possibility of the first measurement of CNO neutrinos, expected to contribute to the solar power by less than 1%, is currently under investigation.

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