International Journal of Modern Physics A

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.

Purification of 278 tons of liquid scintillator and 889 tons of buffer shielding for the Borexino solar neutrino detector is performed with a system of combined distillation, water extraction, gas stripping and filtration. The purification system removed K, U and Th by distillation of the pseudocumene solvent and the PPO fluor. Noble gases, Rn, Kr and Ar were removed by gas stripping. Distillation was also employed to remove optical impurities and reduce the attenuation of scintillation light. The success of the purification system has facilitated the first time real time detection of low energy solar neutrinos.

Borexino detector is composed of 2214 photomultipliers (PMTs) collecting the scintillation light produced by the liquid scintillator. Phototubes are partially immersed in the scintillator and partially in high purity water. A proper electrical and mechanical selection of the devices as well as a custom-made encapsulation was mandatory for a long-term immersion in these two aggressive liquid. Underwater cables and connectors and optical fibers are also described in detail.

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 performance of the Borexino Outer Detector is described. It is a large water Cherenkov detector for identifying cosmic muons, penetrating the whole detector system. The Outer Detector is important for tagging cosmic muon generated background events in Borexino. Here, we present the muon identification efficiency and show the capability of muon tracking reconstruction.

Borexino achieved radiopurity world records. During each phase of the Experiment, starting from the fabrication up to the commissioning, various techniques to reach this important result have been adopted. A special care has been taken in the material screening and selection, in the component cleaning and installation and in the scintillator purification and handling. In this paper, the installation part will be described.

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.

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.

BOREXINO has set new standards in the field of ultra-low background experiments. Such a success was only possible by a strict quality control program during the construction phase of the experiment. In this paper, we describe how construction materials and auxiliary systems of the BOREXINO detector were screened for their residual radioactivity with unprecedented high sensitivity. The highly sensitive assay techniques developed for this purpose were also used to validate the purity of water and nitrogen used in the experiment. Moreover, we report on the production of ²²²Rn-free nitrogen and synthetic air with very low ²²²Rn concentration as well as on the successful search for commercial nitrogen with a particularly low concentration of argon and krypton.

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.

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 signal produced in Borexino by solar neutrinos interacting in the liquid scintillator has no particular signatures that allow to distinguish it from the background on a event-by-event basis. I recall the main features of the analysis method and, in particular, I describe three procedures that have been particularly implemented in Borexino and that are essential to extract the signal from the background: the reconstruction of the event position, the pulse shape discrimination and the three fold coincidence.