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Interactions News Wire #28-07
29 May 2007  http://www.interactions.org
Source: Laboratori Nazionali del Gran Sasso
Content: Press Release
Date Issued: 29 May 2007

The Borexino experiment at Gran Sasso begins the data taking

The Borexino detector for low-energy solar neutrino studies has been completely filled (May 15th) with highly purified scintillator and high purity shielding liquids (pseudocumene and water) and is now fully operational at the Laboratori Nazionali del Gran Sasso. This achievement became reality after several years of technical developments (leading to background radioactive levels then never achieved before), of construction and commissioning and through the solution of several problems involving the underground laboratory and the local authorities, which caused to the experiment three years of stop (mostly due to worries for environmental damages).

The main goal of Borexino is the measurement of the monoenergetic (862 keV) Be-7 solar neutrinos, a central feature in the on-going story of solar neutrinos and neutrino oscillations. Borexino will accomplish this goal by detecting neutrino-electron scattering events taking place in real time in its well shielded 100 tonnes mass fiducial volume.

The detector is an unsegmented liquid scintillator featuring 300 tonnes of sensitive mass, viewed by 2,200 photomultipliers, for the detection of the energy released by the neutrino-induced recoil electrons. The detector core is a transparent nylon spherical vessel (125 µm thick), 8.5 m of diameter, filled with liquid scintillator and surrounded by 1,000 tonnes of high-purity shielding buffer liquid. The scintillator mixture is PC (1-2-4,trimethylbenzene) and PPO (2,5 diphenyl oxazole, 1.5g/l) as a fluor, while the buffer liquid is PC with the addition of DMP as light quencher. The photomultipliers are supported by a stainless steel sphere, which also separates the inner part of the detector from an additional external shielding, provided by 2,400 tons of purified water.
This outermost water shield is instrumented with 200 outward-pointing photomultipliers serving as a veto for penetrating muons, the only significant residual cosmic background at the 3800 mwe (meters of water equivalent) depth of the underground laboratory.

Since the Homestake pioneering experiment, observations of solar neutrinos have offered a first hint of physics beyond the Standard Model of particle physics: neutrino mass and oscillations. A spectacular confirmation of the oscillation mechanism in the solar regime came during the last decade through a series of experiments culminating with the final SNO direct demonstration of solar neutrino oscillations.
These challenging experiments were able to observe in real time the high energy part of the solar neutrino spectrum, which amounts to only 0.01% of the total flux. Radioactivity as well as other backgrounds imposed constraints on the lowest achievable threshold which was at best around 5 MeV, leaving the experimental exploration of the low energy part only to radiochemical experiments.

Borexino, with its ultrapure scintillator mass will be able to lower this detection threshold well below the MeV limit, with the potential capability of exploring for the first time solar signals like Be-7, CNO, pep, and also part of the pp.

At present the phenomenology of solar neutrinos is explained in terms of the so-called LMA solution. This model, however, still needs to be confirmed with these low energy solar neutrinos. As a matter of fact the upturn of the electron neutrino survival probability between 0.7 and 4 MeV (transition between the oscillations in vacuum and in matter) has never been observed. Therefore, sub-MeV solar neutrinos may offer a unique opportunity to check the LMA oscillations scenario and search for other phenomena and sub-leading effects. Besides, sub-MeV solar neutrinos detection allows the possibility to test at the few percent level the astrophysical model of the Sun.
Beyond the solar neutrino program, a variety of other physics topics can be explored, such as antineutrinos from Supernovae (with the unique possibility of a very low threshold at 0.25 Mev, particularly useful for the detection of neutrino-proton scatterings)., terrestrial antineutrinos (geoneutrinos) and neutrino magnetic moment. The study of geoneutrinos is particularly favoured at Gran Sasso due to the low level of the background from nuclear reactors.
Further reading

Gianpaolo Bellini
the Borexino spokesperson

Roberta Antolini
LNGS Public Affairs