The Borexino detector at the Laboratori Nazionali del Gran Sasso (0806.2400v1)

Abstract: Borexino, a large volume detector for low energy neutrino spectroscopy, is currently running underground at the Laboratori Nazionali del Gran Sasso, Italy. The main goal of the experiment is the real-time measurement of sub MeV solar neutrinos, and particularly of the mono energetic (862 keV) Be7 electron capture neutrinos, via neutrino-electron scattering in an ultra-pure liquid scintillator. This paper is mostly devoted to the description of the detector structure, the photomultipliers, the electronics, and the trigger and calibration systems. The real performance of the detector, which always meets, and sometimes exceeds, design expectations, is also shown. Some important aspects of the Borexino project, i.e. the fluid handling plants, the purification techniques and the filling procedures, are not covered in this paper and are, or will be, published elsewhere (see Introduction and Bibliography).

• The paper presents a comprehensive overview of Borexino’s design and performance in detecting sub-MeV solar neutrinos in real time.
• It details the innovative use of photomultipliers, gateless electronics, and a robust calibration system to ensure high precision and radiopurity.
• The study highlights improved energy resolution and background rejection, paving the way for expanded applications in neutrino and astrophysics research.

Overview of the Borexino Detector at the Laboratori Nazionali del Gran Sasso

The paper presents an extensive examination of the Borexino detector, an advanced low-energy neutrino spectroscopy apparatus located at the Laboratori Nazionali del Gran Sasso, Italy. The primary aim of Borexino is the real-time detection of sub-MeV solar neutrinos, with a particular focus on the mono-energetic (862 keV) electron capture neutrinos from $^7\text{Be}$. This paper provides comprehensive details on Borexino’s structural and operational attributes, including the photomultiplier tubes, electronics, and trigger and calibration systems. The apparatus's performance not only meets but often surpasses its design expectations.

Detector Design and Functionality

Borexino is housed deep underground to minimize interference from cosmic rays, utilizing roughly 3800 meters of water equivalent for muon flux reduction. The design primarily involves a large volume liquid scintillator for neutrino-electron scattering detection. Key components include the stainless steel sphere (SSS) supporting the photomultipliers and containing the scintillator, and the water tank which acts both as shielding against radiation and a Cherenkov detector for muons.

The scintillator mixture, primarily pseudocumene (PC) with a fluor PPO addition, allows for high photon yield, excellent transparency, and fast decay times suitable for precise temporal and spatial resolution. The detector’s radiopurity is critical, necessitating contamination levels orders of magnitude lower than typical environmental radioactivity.

Photomultipliers and Electronics

The paper details the selection of 8" E.T.L. 9351 phototubes as central to Borexino's light detection capability. These were chosen based on criteria encompassing quantum efficiency, dark count rates, and after-pulsing characteristics to ensure reliable detection of low-energy events.

A unique feature of Borexino’s electronics is the integration of a gateless charge integrator, designed to operate without external gating, thereby minimizing dead time and ensuring high accuracy in charge measurement. This is supplemented with a secondary reading system for higher energy events using fast waveform digitizers to extend dynamic range and cope with potential saturation at high photonic outputs.

Calibration and Data Collection

The calibration system utilizing a multiplexed optical fiber network ensures accurate photomultiplier timing and gain alignment crucial for maintaining positional and energy resolution within the detector. The system's design allows simultaneous light distribution to each PMT, thus facilitating frequent and efficient calibrations.

Borexino employs a sophisticated data acquisition system capable of handling the high data throughput generated by natural $^{14}$C decay, with considerations for long-term data management and network infrastructure robustness.

Current Performance and Future Prospects

Preliminary results indicate the Borexino detector's success in achieving low radiation background levels and effective muon detection, with improvements observed over initial testing phases such as reduced PMT failure rates and enhanced energy resolutions. The current capability to efficiently detect $^{214}$Bi-$^{214}$Po coincidences offers promising means for background rejection and energy calibration.

Beyond its primary focus on solar neutrinos, Borexino’s setup positions it for contributions in diverse physics areas, including geophysical anti-neutrino detection, potential supernova event monitoring, and tests of neutrino magnetic moments and rare decay processes. Continued operational data from Borexino will not only test the robustness of current neutrino models but also pave the way for future experimental innovations in astrophysics and particle physics.