Borexino Experiment Research Articles

Overview and accomplishments of the Borexino experiment

The Borexino experiment is running at the Laboratori del Gran Sasso in Italy since 2007. Its technical distinctive feature is the unprecedented ultralow background of the inner scintillating core, which is the basis of the outstanding achievements accumulated by the experiment. In this talk, after recalling the main features of the detector, the impressive solar data gathered so far by the experiment will be summarized, with special emphasis to the most recent and prominent result concerning the detection of the fundamental pp solar neutrino flux, which is the direct probe of the engine mechanism powering our star. Such a milestone measurement puts Borexino in the unique situation of being the only experiment able to do solar neutrino spectroscopy over the entire solar spectrum; the counterpart of this peculiar status in the oscillation interpretation of the data is the capability of Borexino alone to perform the full validation across the solar energy range of the MSW-LMA paradigm. The talk will be concluded highlighting the perspectives for the final stage of the solar program of the experiment, centered on the goal to fully complete the solar spectroscopy with the missing piece of the CNO neutrinos. If successful, such a measurement would represent the final crowning of the long quest of Borexino to unravel all the properties of the neutrinos from the Sun.

First real–time detection of solar pp neutrinos by Borexino

Solar neutrinos have been pivotal to the discovery of neutrino flavour oscillations and are a unique tool to probe the reactions that keep the Sun shine. Although most of solar neutrino components have been directly measured, the neutrinos emitted by the keystone pp reaction, in which two protons fuse to make a deuteron, have so far eluded direct detection. The Borexino experiment, an ultra-pure liquid scintillator detector running at the Laboratori Nazionali del Gran Sasso in Italy, has now filled the gap, providing the first direct real time measurement of pp neutrinos and of the solar neutrino luminosity.

Test of the electric charge conservation law with Borexino detector

The new limit on the electron lifetime is obtained from data of the Borexino experiment. The expected signal from the e → γν decay mode is a 256 keV photon detected in liquid scintillator. Because of the extremely low radioactive background level in the Borexino detector it was possible to improve the previous measurement by two orders of magnitude.

Recent Borexino results and prospects for the near future

The Borexino experiment located in the Gran Sasso National Laboratory, is an organic liquid scintillator detector conceived for the real time spectroscopy of low energy solar neutrinos. The phase-I of the data taking campaign (2007 – 2010) has allowed the first independent measurements of 7 Be and pep solar neutrino fluxes as well as the first measurement of anti-neutrinos from the Earth. After a purification of the scintillator, Borexino is now in phase-II since 2011. Thanks to the unprecedented background levels, we have performed the first flux measurement of neutrinos from the fundamental pp reaction which powers the Sun. We review this breakthrough result and other recent results, including the latest review of our terrestrial neutrino analysis. We also discuss the upcoming measurements on middle energy solar neutrino spectral components (pep, CNO) and the new project SOX devoted to the study of sterile neutrinos via the use of a neutrino source placed in close proximity of the detector’s active material.

The status of the study of solar CNO neutrinos in the Borexino experiment

Although less than 1% of solar energy is generated in the CNO cycle, it plays a critical role in astrophysics, since this cycle is the primary source of energy in certain more massive stars and at later stages of evolution of solar-type stars. Electron neutrinos are produced in the CNO cycle reactions. These neutrinos may be detected by terrestrial neutrino detectors. Various solar models with different abundances of elements heavier than helium predict different CNO neutrino fluxes. A direct measurement of the CNO neutrino flux could help distinguish between these models and solve several other astrophysical problems. No CNO neutrinos have been detected directly thus far, and the best upper limit on their flux was set in the Borexino experiment. The work on reducing the background in the region of energies of CNO neutrinos (up to 1.74 MeV) and developing novel data analysis methods is presently under way. These efforts may help detect the CNO neutrino flux in the Borexino experiment at the level predicted by solar models.

Neutrino velocity and local Lorentz invariance

We discuss the possible violation of local Lorentz invariance (LLI) arising from a faster-than-light neutrino speed. A toy calculation of the LLI violation parameter [Formula: see text], based on the (disclaimed) OPERA data, suggests that the values of [Formula: see text] are determined by the interaction involved, and not by the energy range. This hypothesis is further corroborated by the analysis of the more recent results of the BOREXINO, LVD and ICARUS experiments.

Solar Neutrinos and the Borexino experiment

Solar neutrinos are produced in the core of the Sun in different nuclear reactions all based on the conversion of hydrogen into helium, releasing energy and making the Sun shine. Until now, the observation of solar neutrinos has demonstrated: a) the nuclear origin of the Sun's energy; b) that the ve produced were undergoing lepton flavor transformation into vμ or vτ, the neutrino oscillation mechanism. In the recent years, the Borexino experiment, in the Gran Sasso underground laboratory, has made significant contributions to the solar neutrino spectroscopy: first observation and precision measurement of the 7Be neutrinos, first observation of the pep reaction, stringent limit on CNO neutrinos, observation of 8B neutrinos with a 3 MeV threshold. These measurements reinforce the so-called LMA solution of the neutrino oscillation explaining the solar ve survival probability as a function of energy.

Expected geoneutrino signal at JUNO

Constraints on the Earth’s composition and on its radiogenic energy budget come from the detection of geoneutrinos. The Kamioka Liquid scintillator Antineutrino Detector (KamLAND) and Borexino experiments recently reported the geoneutrino flux, which reflects the amount and distribution of U and Th inside the Earth. The Jiangmen Underground Neutrino Observatory (JUNO) neutrino experiment, designed as a 20 kton liquid scintillator detector, will be built in an underground laboratory in South China about 53 km from the Yangjiang and Taishan nuclear power plants, each one having a planned thermal power of approximately 18 GW. Given the large detector mass and the intense reactor antineutrino flux, JUNO aims not only to collect high statistics antineutrino signals from reactors but also to address the challenge of discriminating the geoneutrino signal from the reactor background. The predicted geoneutrino signal at JUNO is $$ {39.7}_{-5.2}^{+6.5} $$ terrestrial neutrino unit (TNU), based on the existing reference Earth model, with the dominant source of uncertainty coming from the modeling of the compositional variability in the local upper crust that surrounds (out to approximately 500 km) the detector. A special focus is dedicated to the 6° × 4° local crust surrounding the detector which is estimated to contribute for the 44% of the signal. On the basis of a worldwide reference model for reactor antineutrinos, the ratio between reactor antineutrino and geoneutrino signals in the geoneutrino energy window is estimated to be 0.7 considering reactors operating in year 2013 and reaches a value of 8.9 by adding the contribution of the future nuclear power plants. In order to extract useful information about the mantle’s composition, a refinement of the abundance and distribution of U and Th in the local crust is required, with particular attention to the geochemical characterization of the accessible upper crust where 47% of the expected geoneutrino signal originates and this region contributes the major source of uncertainty.

The calibration system for the Borexino experiment

Borexino is a large liquid scintillator detector designed to study solar neutrinos in real-time. Since the beginning of data-taking in 2007, Borexino has been able to perform a complete spectroscopy of neutrinos from the Sun, thanks to its unprecedented radiopurity. Calibrations have been crucial for the success of the experiment. In this paper, we describe the Borexino calibration system, emphasizing its most critical aspects, in particular to those related to radiopurity. We also discuss some of the results of the calibration campaigns performed in Borexino Phase-1.

Counting test facility for the Borexino experiment

A fundamental breakthrough which opened the way to the realization of the Borexino detector was the demonstration of exceptionally low, unprecedented radioactive contaminations in the liquid scintillator, obtained with its pilot prototype Counting Test Facility. Though of limited dimension, with its 4.8 m3 of active liquid core, CTF has however been a key milestone not only for Borexino, but also for the entire field of the ultra-low background searches. Here, we succinctly remind the motivations, which concurred to lay down the project, as well as the specific radiopurity challenge, which guided the design. After the description of the technical elements of the detector, the main outcomes are summarized, both regarding optical and purity scintillator properties, with special emphasis on the exceptional achievements in term of ultra-low traces of radioactive contaminants. The discussion is completed with the description of how CTF was employed for the pre-qualification of the entire inventory of the Borexino scintillator, confirming also in the final phase of its life its essential role for the success of the overall Borexino solar neutrino program.

Water purification in Borexino

Water Purification is often an important methodological tool in low radioactivity experiments. A variety of techniques have been exploited in the frame of the Borexino experiment to the goal of using water as a high purity shielding and as a reagent for cleaning and purification processes. This paper describes the water purification strategies and the purification results obtained in Borexino.

The nylon scintillator containment vessels for the Borexino solar neutrino experiment

The neutrino event rate in the Borexino scintillator is very low (~ 0.5 events per day per ton) and concentrated in an energy region well below the 2.6 MeV threshold of natural radioactivity. The intrinsic radioactive contaminants in the photomultipliers (PMTs), in the Stainless Steel Sphere, and in other detector components, play special requirements on the system required to contain the scintillator. The liquid scintillator must be shielded from the Stainless Steel Sphere and from the PMTs by a thick barrier of buffer fluid. The fluid barrier, in addition, needs to be segmented in order to contain migration of radon and daughters emanated by the Stainless Steel Sphere and by the PMTs. These requirements were met by designing and building two spherical vessel made of thin nylon film. The inner vessel contains the scintillator, separating it from the surrounding buffer. The buffer region itself is divided into two concentric shells by the second, outer nylon vessel. In addition, the two nylon vessels must satisfy stringent requirements for radioactivity and for mechanical, optical and chemical properties. This paper describes the requirements of the the nylon vessels for the Borexino experiment and offers a brief overview of the construction methods adopted to meet those requirements.

The filling strategy of the Borexino experiment

The detection of solar neutrinos by means of a liquid scintillator requires very low levels of radioactivity throughout the whole active volume. The selection of clean detector materials and the purification of the scintillator are necessary, but not sufficient conditions. It is also crucial that during the filling operations the scintillator is not contaminated, either by air or by getting in contact with dirty surfaces. This paper describes the procedures which we adopted to minimize these risks and achieve very good levels of radio-purity which we finally got in Borexino.

The Borexino scintillator and solvent procurement

The Borexino experiment for solar neutrino physics and other rare phenomena requires an extremely low radioactive background to disentangle the very few events due to neutrino interactions. Therefore, the Borexino scintillator has to satisfy the most stringent radiopurity requirements, being about eight orders of magnitude less radioactive than an ordinary material. This was achieved by means of scintillator purification techniques and of a special care during all the production, handling and procurement of the scintillator solvent. This paper describes the methodology and the quality control procedures that were employed during the production, handling and shipping of the solvent.

Characterization of positronium properties in doped liquid scintillators

Ortho-positronium (o-Ps) formation and decay can replace the annihilation process, when positron interacts in liquid scintillator media. The delay induced by the positronium decay represents either a potential signature for anti-neutrino detection, via inverse beta decay, or to identify and suppress positron background, as recently demonstrated by the Borexino experiment. The formation probability and decay time of o-Ps depend strongly on the surrounding material. In this paper, we characterize the o-Ps properties in liquid scintillators as function of concentrations of gadolinium, lithium, neodymium, and tellurium, dopers used by present and future neutrino experiments. In particular, gadolinium and lithium are high neutron cross section isotopes, widely used in reactor anti-neutrino experiments, while neodymium and tellurium are double beta decay emitters, employed to investigates the Majorana neutrino nature. Future neutrino experiments may profit from the performed measurements to tune the preparation of the scintillator in order to maximize the o-Ps signature, and therefore the discrimination power.

Searches for short-baseline electron-neutrino oscillations in the SAGE and BOREXINO experiments involving artificial neutrino sources

The potential of the SAGE and BOREXINO experiments with 51Cr artificial neutrino sources for searches for and determination of parameters of short-baseline oscillations.

Probing the Sun’s inner core using solar neutrinos: A new diagnostic method

The electronic density in the Sun's inner core is inferred from the 8B, 7Be and pep neutrino flux measurements of the Super-Kamiokande, SNO and Borexino experiments. We have developed a new method in which we use the KamLAND detector determinations of the neutrino fundamental oscillation parameters: the mass difference and the vacuum oscillation angle. Our results suggest that the solar electronic density in the Sun's inner core (for a radius smaller than 10% of the solar radius) is well above the current prediction of the standard solar model, and by as much as 25%. A potential confirmation of these preliminary findings can be achieved when neutrino detectors are able to reduce the error of the electron-neutrino survival probability by a factor of 15.

Cosmogenic Backgrounds in Borexino at 3800 m water-equivalent depth

The solar neutrino experiment Borexino, which is located in the Gran Sasso underground laboratories, is in a unique position to study muon-induced backgrounds in an organic liquid scintillator. In this study, a large sample of cosmic muons is identified and tracked by a muon veto detector external to the liquid scintillator, and by the specific light patterns observed when muons cross the scintillator volume. The yield of muon-induced neutrons is found to be Yn = (3.10±0.11)·10−4 n/(μ·(g/cm2)). The distance profile between the parent muon track and the neutron capture point has the average value λ = (81.5±2.7) cm. Additionally the yields of a number of cosmogenic radioisotopes are measured for 12N, 12B, 8He, 9C, 9Li, 8B, 6He, 8Li, 11Be, 10C and 11C. All results are compared with Monte Carlo simulation predictions using the FLUKA and GEANT4 packages. General agreement between data and simulation is observed for the cosmogenic production yields with a few exceptions, the most prominent case being 11C yield for which both codes return about 50% lower values. The predicted μ-n distance profile and the neutron multiplicity distribution are found to be overall consistent with data.

The Earthʼs mantle and geoneutrinos

The KamLAND and Borexino experiments have observed, each at ∼4σ level, signals of electron antineutrinos produced in the decay chains of thorium and uranium in the Earthʼs crust and mantle (Th and U geoneutrinos). Various pieces of geochemical and geophysical information allow an estimation of the crustal geoneutrino flux components with relatively small uncertainties. The mantle component may then be inferred by subtracting the estimated crustal flux from the measured total flux. We find that crust-subtracted signals show hints of a residual mantle component, emerging at ∼2.4σ level by combining the KamLAND and Borexino data. The inferred mantle flux, slightly favoring scenarios with relatively high Th and U abundances, within ∼1σ uncertainties is comparable to the predictions from recent mantle models.

Neutrino speed: a report on the [formula omitted] speed measurements of the BOREXINO, ICARUS and LVD experiments with the CNGS beam

We report the measurement of the speed of muon neutrinos performed by the Borexino, ICARUS and LVD experiments, using narrow bunch CNGS neutrino beams with an average energy E = 17 GeV and a baseline of ∼ 730 km between Cern and Laboratori Nazionali del Gran Sasso (LNGS). The final result for the difference in time-of-flight between such muon neutrinos and a particle moving at the speed of light in vacuum is δt=0.8±0.7stat±2.9sys ns for Borexino, δt=−0.1±0.7stat±2.7sys ns for ICARUS and δt=−0.3±0.6stat±3.2−sys ns for LVD, well consistent with zero.