The BOREXINO experiment

Since 2007, the Borexino experiment has been performing a spectroscopic measurement of the solar neutrino flux. Due to the low energy threshold and the unprecedented level of radiopurity achieved in the liquid scintillator target, Borexino is able to cover the full energy range of the solar neutrino spectrum. By now, all spectral components originating from the solar proton-proton fusion chain have been measured, verifying the energy dependence of oscillation probabilities expected by the stanard MSW-LMA oscillation scenario. Moreover, Borexino has performed a measurement of the geoneutrino flux produced by beta-decays of radioactive isotopes embedded in the Earth’s crust and mantle.


The BOREXINO detector

The Borexino detector is located at the Gran Sasso underground laboratory in central Italy. The rock shielding provided by the Abruzzo mountains significantly reduces the background by cosmic radiation, especially cosmic muons. This is a prerequisite for the detection of particles at extremely low count rate - as in the case of neutrinos.

The detector consists of two subdetectors (cf. Figure 1):

Figure 1: Layout of the BOREXINO detector

The Inner Detector contains the organic liquid scintillator (pseudocumene+PPO) and is subdivided by two concentric nylon vessels that separate the central ultrapure target volume from the shielding buffer region. Only the innermost 100 tons of the target are used for neutrino detection. More than 2000 photomultiplier tubes (PMTs) are mounted to the surrounding stainless steel sphere of 13.7 m diameter and detect the scintillation light created by neutrino interactions in the target.

The Outer Detector provides both passive shielding against external radioactivity from the surrounding rock and an active veto for cosmic muons. The water Cherenkov detector consists of 2400 tons of ultrapure water and is equipped with 208 PMTs.


Solar neutrino spectroscopy

In the Sun, the thermonuclear burning of hydrogen to helium mostly happens via the fusion reactions of the proton-proton (pp) chain depicted in Figure 2. Depending on the fusion process, the kinetic energies of the emerging neutrinos will vary. Figure 3 shows the neutrino spectrum that can be calculated based on the reaction rates predicted by the Standard Solar Model (SSM).

Figure 2: Solar proton-proton chain
Figure 3: Solar neutrino spectrum

The high energy resolution, low detection threshold and ultralow background levels achieved in Borexino allow to resolve the spectral contributions of the individual pp fusion reactions. In this way, not only the solar spectrum but also neutrino oscillations can be studied. Figure 4 depicts the survival probability of electron neutrinos as a function of neutrino energy. Borexino has been able to confirm the MSW-LMA scenario that predicts a transition from vacuum oscillations at sub-MeV neutrino energies to matter-enhanced oscillations in the high-energy regime above 5 MeV.


Figure 4: Solar electron neutrino survival probability as a function of neutrino energy. Data points from Borexino, band follows MSW-LMA prediction.

In 2014, Borexino published the results of a first spectral measurement of the neutrinos originating from the most basic proton-proton fusion reaction. These so-called pp neutrinos constitute nearly the entirety of the solar neutrino flux, vastly outnumbering those emitted in the reactions that follow. The new Borexino result demonstrates that about 99 per cent of the power of the Sun, 3.84 x 10^{26} W, are generated by the proton–proton fusion process.


Contribution by the working group at JGU Mainz

The group is involved in the data analysis towards an improved measurement of the pep neutrino flux as well as a possible first measurement of CNO neutrinos. The CNO fusion cycle is subdominant in the Sun but prevails in heavier stars and constitutes an excellent probe for solar metallicity.


Interesting links

  • BOREXINO homepage at the LNGS.
  • Press release by EC PRISMA:
      Detection of pp-neutrinos provides first direct measurement of solar power at its production.
  • Scientific American: Strange Neutrinos from the Sun Detected for the First Time.


    Related Publications

    BOREXINO collaboration (2017)
    Seasonal Modulation of the Be-7 Solar Neutrino Rate in Borexino

    BOREXINO collaboration (2017)
    Borexino's search for low-energy neutrino and antineutrino signals correlated with gamma-ray bursts
    Astropart. Phys. 86 (2017) 11-17, arXiv:1607.05649

    BOREXINO collaboration (2015)
    A test of electric charge conservation in Borexino
    Phys. Rev. Lett. 115 (2015) 231802, arXiv:1509.01223

    BOREXINO collaboration (2015)
    Spectroscopy of geo-neutrinos from 2056 days of Borexino data

    BOREXINO collaboration (2014)
    Neutrinos from the primary proton–proton fusion process in the Sun
    Phys. Rev. D91 (2015) 3, 032005, 10.1038/nature13702

    BOREXINO collaboration (2014)
    Final results of Borexino Phase-I on low energy solar neutrino spectroscopy
    Phys. Rev. D89 (2014) 11, 112007, arXiv:1308.0443