Borexino

The Borexino experiment aims at the detection of low-energy neutrinos in a large (270 ton) liquid-scintillator target. Important results have been obtained especially in the detection of solar neutrinos. Both the standard solar model predictions of the solar neutrino flux and spectrum (by J. Bahcall et al.) and the energy-dependence of neutrino oscillation probability induced by matter effects have been successfully verified. Moreover, Borexino was able to detect antineutrinos produced by beta-decays of radioactive isotopes embedded in the Earth’s crust and by nuclear reactors.

The Borexino detector is located at the LNGS (Laboratori Nazionali del 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 Borexino experiment consist of an Inner an Outer Detector (ID/OD): The ID volume contains the liquid scintillator (based on pseudocumene) and is further subdivided by two concentric nylon vessels that avoid radioactive contamination by convection and diffusion in the liquid. The central target volume consists of 280 tons of ultrapure scintillator. Only the innermost 100 tons are used for neutrino detection. The inactive buffer liquid contained in the two outer volumes (1040 tons) serve as shielding from external radiation. The ID is confined within a stainless steel sphere of 13.7 m diameter. On its inside, 2200 photomultiplier tubes (PMTs) detect the scintillation light created by neutrino interactions in the target volume.

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

Fig. 4: Schematic view of the Borexino experiment

Solar neutrinos

Inside our Sun, the fusion of hydrogen to helium mostly takes place via the proton-proton chain shown in figure 5. The detection of the corresponding neutrinos is at the heart of the Borexino physics program. The high energy resolution, low detection threshold and ultralow radioactive background achieved in Borexino allow to resolve the contributions of single steps in the pp-chain to the total solar neutrino spectrums. Moreover, the spectroscopic measurement allows to investigate the energy-dependence of neutrino oscillation probabilities, especially in the transition from vacuum to matter-dominated oscillations (figure 7).

Fig. 5: Solar pp chain for the fusion of hydrogen to helium. Neutrinos from reactions measured by Borexino are indicated by color.

Borexino detects solar neutrinos by elastic scattering off electrons. The recoiling electron produces scintillation light. The number of photons collected by the photosensors allows to reconstruct the particle energy.

 

Neutrinos from the Berillium-7 reaction were the first to be detected by Borexino [http://arxiv.org/abs/1104]. As these neutrinos are mono-energetic (862 keV) their electron recoils produce a Compton-like shoulder in the detected spectrum (cf. figure 6). In accordance with the standard solar model and the MSW-LMA oscillation scenario, Borexino measured a rate of 46 ± 3 events per day and 100 tons.

   
Fig. 6: Fit to the signal by solar Be-7 neutrinos measured in Borexino (backgrounds are included)   Fig. 7: Transition from vacuum to matter-dominated oscillations as measured by Borexino and further neutrino experiments

Moreover, Borexino performed a first measurement of the pep neutrino flux as well as the low-energy region of the Boron-8 neutrino spectrum below 5 MeV. The resulting rates correspond to the predictions of the standard solar model including MSW-LMA oscillations (figure 7). [[http://arxiv.org/abs/1110.3230, http://journals.aps.org/prd/abxstract/10.1103/PhysRevD.82.033006].

Geoneutrinos

Geoneutrinos are electron antineutrinos emitted in the beta-decay of radioactive elements embedded in the Earth’s crust and mantle. They are mainly produced by the elements of the natural uranium and thorium decay chains and the decay of potassium-40. Detection in Borexino is by the inverse beta decay reaction on hydrogen,


As the reaction threshold is at 1.8 MeV, only antineutrinos from the U/Th chains can be detected. The coincidence signature of the inverse beta decay allows for a very effective reduction of single-event backgrounds: The prompt annihilation of the positron is followed by the delayed capture of the neutron on a free proton in the scintillator that releases at 2.2 MeV gamma quantum after ~250 µs.

 

Figure 8 shows the prompt event energy spectrum of electron antineutrinos in Borexino. The contribution of geoneutrinos can be clearly distinguished from the background introduced by reactor antineutrinos by their respective spectral shapes. In 1353 live days, Borexino measured 14.3 ± 4.4 geoneutrino events [http://arxiv.org/abs/1303.2571]. The resulting flux is in accordance with predictions from the Bulk Silicate Earth model.

Fig. 8: Antineutrino spectrum detected by Borexino. The total signal is composed of geoneutrinos (yellow) and reactor neutrinos (orange). 500 p.e. correspond to 1 MeV of prompt event energy.

Future program of Borexino

  • pp-neutrinos: The fusion of two protons to a deuteron forms the basic reaction of the pp-chain and is closely related to the overall solar luminosity. The corresponding pp-neutrino flux is very large but is found at the low-energy end of the solar neutrino spectrum. The primary background is the beta-decay of the carbon isotope C-14 that is contained in the hydrocarbons of the scintillator.
  • CNO neutrinos: The catalytic production of helium by the successive fusion of protons to carbon is known as the CNO-cycle. In the sun, this process contributes only about 1% of the total energy conversion by fusion, while it is the dominant process in heavier stars. The CNO fusion rate is also very sensitive to the abundance of heavy elements (metals) in the solar center.
  • Sterile Neutrinos - see SOX