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Poster excerpt 6: Borexino, from the Sun to the Earth

March 2, 2015

(The following text is from Romain Roncin)

The Borexino experiment dedicates its existence to the neutrino physics. Even if the neutrinos are elusive particles, physicists managed over the last decades to build large detectors in order to unlock the secrets of these fascinating particles. Borexino is one of these detectors which provides outstanding results since it started taking data, in May 2007.

We call solar neutrinos the electron neutrinos which are produced through nuclear reactions inside the Sun. Neutrinos are detected inside Borexino through elastic scattering reaction. This reaction allows a neutrino to give a part of its kinetic energy to an electron of the liquid scintillator. This scattered electron produces then scintillation light which can be measured. Thanks to the intrinsic radiopurity achieved in its scintillator, Borexino already measured the rate of neutrinos coming from the 8B, 7Be and pep processes which occur inside the Sun [1,2,3].

The latest achievement of Borexino stands in the measurement of the solar neutrinos flux from the pp process. This reaction consists of the fusion of two protons and provides low energy neutrinos. These neutrinos contribute almost to the entire solar neutrinos flux but are difficult to access due to their low energies. The suppression of the radioactive backgrounds in the energy region below 420 keV is the key of such a measurement. Whereas previous experiments such as GALLEX and SAGE were able to extract indirectly the pp solar neutrino flux, Borexino managed to measure it directly, in good agreement with the prediction of the Standard Solar Model (SSM).

This measurement can be considered as a new milestone achieved in solar spectroscopy, allowing the measurement of the energy of the Sun in real time. The figure below shows the fit of the energy spectrum between 165 and 590 keV together with the residuals. The pp solar neutrino spectrum is represented in the red line and corresponds to an interaction rate of 144 ± 13 (stat) ± 10 (syst) counts / (day × 100 ton) [4].

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In addition to the physics of the Sun, Borexino is also interested in the physics of the Earth via the study of the so-called geo-neutrinos. They consist of electron antineutrinos which are produced through the decay of the radioactive isotopes present in the crust and the mantle of our planet.

The knowledge of the Earth is limited by indirect measurements. While seismology con- strains the density profile, geochemistry allows to study the chemical composition of rocks which can be accessed. Chondritic meteorites as well as the photosphere of the Sun are also potential evidence of the Earth structure and composition. The interdisciplinar field of neutrino geoscience aims to take advantage of the neutrino experiments skills to study the Earth interior with direct messengers, the geo-neutrinos. Due to its low background level as well as its position in Italy, a nuclear free country, Borexino is leading this interdisciplinary field.

The detection of geo-neutrinos relies on the signature of the inverse β decay reaction where the positron scintillation and annihilation, the « prompt » signal, is followed by the neutron capture on hydrogen, the « delayed » signal. The prompt and delayed signals are correlated in space and time, allowing to apply specific cuts determined by neutron capture physics. The main background comes from the electron antineutrinos which are emitted by the nuclear reactors around the world.

Between December 2007 and August 2012, Borexino managed to observe 46 electron antineutrino candidates with a very low background. The figure below shows the prompt energy spectrum of these candidates. The red dashed line corresponds to the expected electron antineutrino spectrum produced by nuclear reactors whereas the blue dashed line represents the expected geo-neutrino spectrum. Since these two spectra overlap, we used an unbinned maximal likelihood method in order to extract the number of geo-neutrinos we found to be equal to 14.3 ± 4.4 [5].

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References 

[1] “Measurement of the solar 8B neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector”. Phys.Rev., D82:033006, 2010.

[2] “Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino”. Phys.Rev.Lett., 107:141302, 2011.

[3] “First Evidence of pep Solar Neutrinos by Direct Detection in Borexino”. Phys.Rev.Lett., 108:051302, 2012.

[4] “Neutrinos from the primary proton–proton fusion process in the Sun”. Nature, 512:383-386, 2014.

[5] “Measurement of geo-neutrinos from 1353 days of Borexino”. Phys.Lett., B722:295–300, 2013.

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