Borexino: a real-time detector for low-energy neutrinos

Borexino was a particle physics experiment which has been performed by an international collaboration in the “Hall C” of the Laboratori Nazionali del Gran Sasso (LNGS).  Its physics program was centered on solar neutrinos, but also included other relevant topics in low background neutrino detection (including geoneutrinos) and underground physics.

The name “Borexino” derives from that of a former proposal for a (rather different) solar neutrino experiment to be performed at LNGS, called “BOREX” (BORon solar neutrino EXperiment). Borex was never realized; it has been superseded by the (relatively) smaller but more advanced and ambitious Borexino detector.
Borexino detector

The Borexino detector was a large, low background, liquid scintillator detector. It was designed to reveal low energy (sub-MeV) neutrinos in real time. Its main goals were to perform solar neutrinos spectroscopy and to measure the 7Be neutrino flux from the Sun.

Extreme radiopurity

Achieving such goals required a very low energy threshold. In turn, that requires an extreme radiopurity. Radioactive background in the Borexino detector had to be at least 10 orders of magnitude lower than in typical natural conditions!

Nothing in Borexino was standard

To achieve the required radiopurity levels, all detector parts and components had to be developed ad hoc. All the involved materials had to be carefully selected for radiopurity. Measurements on materials has been performed using germanium detectors and mass spectrometers. Everything had to go through clean rooms. Five clean rooms had to be built around the detector. The inside of the detector itself was equipped as a clean room. All welds were TIG in nitrogen or argon atmosphere. All surfaces were pickled and passivated. The nitrogen for stripping was cryogenically purified. Etc. (please check the relevant papers for more details).

The Counting Test Facility

A Borexino prototype, called the Counting Test Facility (CTF), has been built and operated in the Hall C of LNGS between 1994 and ’96.  This detector demonstrated the achievement of ultra-low count rates (radiopurity of the order of 10-16g/g of 238U equivalent) on the several-ton scale.

The Borexino detector has been built on the CTF experience.

Timeline
Results

The extraordinary radiopurity achieved by Borexino is unprecedented for such a large detector. Further improved over the years, this has made it possible to accomplish not only its primary goals, but also to produce many other interesting results, both within and beyond the Standard Model of particle physics.

Open Data

A selection of the experimental data acquired and analysed by the Borexino collaboration for some of the most significant publications is freely available as “Open Data”.

Legacy

In its 32 years of operation, Borexino has developed techniques and methods to achieve unprecedented levels of radio-purity, which still represent the state of the art. These pioneering techniques and methods set a new standard for ultra-low background physics, a legacy for future experiments studying low-energy neutrinos and/or searching for rare events with detectors operating deep underground.

Borexino design, techniques and methods have already inspired other experiments. In particular, JUNO (Jiangmen Underground Neutrino Observatory, currently under construction in China) could be considered a direct successor. It builds on the legacy of Borexino also in terms of expertise, with the direct participation of many of our people.

Borexino has left an equally influential scientific legacy, with discoveries and precise measurements of the nuclear processes that make the Sun and stars shine. Thanks to its unparalleled low background in the energy window from ∼150 keV to ∼15 MeV, it has contributed significantly to the understanding of neutrino oscillations, with the observation of the energy-dependent matter to vacuum-dominated flavor conversion probability of solar neutrinos. In addition to textbook-quality results on solar neutrinos, Borexino has also contributed to the study of radioactivity in the Earth’s mantle with its background-free measurements of geoneutrinos.

For more information and details please refer to these articles:
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