A. Perotti

A large-scale low-background liquid scintillation detector: the counting test facility at Gran Sasso

A 4.8 m3 unsegmented liquid scintillation detector at the underground Laboratori Nazionali del Gran Sasso has shown the feasibility of multi-ton low-background detectors operating to energies as low as 250 keV. Detector construction and the handling of large volumes of liquid scintillator to minimize the background are described. The scintillator, 1.5 g PPO/L-pseudocumene, is held in a flexible nylon vessel shielded by 1000 t of purified water. The active detector volume is viewed by 100 photomultipliers, which measure time and charge for each event, from which energy, position and pulse shape are deduced. On-line purification of the scintillator by water extraction, vacuum distillation and nitrogen stripping removed radioactive impurities. Upper limits were established of < 10−7 Bq/kg-scintillator for events with energies 250 keV < E < 800 keV, and < 10−9 Bq/kg-scintillator due to the decay products of uranium and thorium. The isotopic abundance of 14C12C in the scintillator was shown to be approximately 10−18 by extending the energy window of the detector to 25–250 keV. The 14C abundance and uranium and thorium levels in the CTF are compatible with the Borexino Solar Neutrino Experiment.

Ultra-low background measurements in a large volume underground detector

A large volume (4.8 m3) liquid scintillator detector has been running in Hall C of the Gran Sasso Underground Laboratory since February 1995. This detector is called the “Counting Test Facility” (CTF). The main goal of the detector facility is the measurement of ultralow background levels in scintillators and the development of processes able to purify them at this level. The detector has been designed to have exceptional sensitivity using a variety of methods to identify backgrounds. With the CTF, records were achieved in the domain of low background large volume detectors. Limits of 3.5 ± 1.3 × 10−16 g/g and 4.4−1.2+1.5 × 10−16 g/g for the 238U and 232Th daughters, respectively, and 1.85 ± 0.13 ± 0.01 × 10−18 for the isotopic abundance of 14C relative to 12C were obtained. These results are very encouraging and point towards the feasibility of low energy, real time scintillation detectors for solar neutrinos, such as Borexino.

Measurement of the 14C abundance in a low-background liquid scintillator

Abstract The 14 C/ 12 C ratio in 4.8 m 3 of high-purity liquid scintillator was measured at (1.94±0.09)×10 −18 , the lowest 14 C abundance ever measured. At this level the spectroscopy of low-energy solar neutrinos, in particular a measurement of the 7 Be neutrino flux, will not be obstructed by the 14 C β decay intrinsic to a liquid scintillator detector. A comprehensive study of the deviation of the shape of the 14 C β spectrum from the allowed statistical shape reveals consistent results with recent observations and calculations. Possible origins of the 14 C in the liquid scintillator are discussed.

Light propagation in a large volume liquid scintillator

The fluorescence light propagation in a large volume detector based on organic liquid scintillators is discussed. In particular, the effects of the fluor radiative transport and solvent Rayleigh scattering are emphasized. These processes have been modelled by a ray-tracing Monte Carlo method and have been experimentally investigated in the Borexino prototype which was a 4.3 ton, 4π sensitive detector. The comparison between the model prediction and the experimental data shows a satisfactory agreement indicating that the main aspects of these processes are well understood. Some features of the experimental time response of the detector are still under study.

The water purification system for the low background counting test facility of the Borexino experiment at Gran Sasso

The Borexino experiment, for the study of solar neutrino physics, requires radiopurity at the level of 5 × 10−16 g/g 238U equivalent (or 6 × 10−9 Bq/kg) on a detector mass of many tons of scintillator. Feasibility studies are performed in a counting test facility now operating at LNGS, which consists of 4 t of liquid scintillator viewed by 100 photomultipliers and shielded by 100 t of water. The accomplishment of this goal requires the shielding liquid, water, to be at the 10−13 g/g contamination level (1.2 × 10−6 Bq/kg) or better. This paper describes the water purification system; it consists of a combination of several purification processes to remove particulate, radioactive ions, dissolved gases and other impurities. Residual contaminations are measured by analytical or direct-counting techniques. For radon measurement, particularly challenging at this low activity levels, a low background counting method has been developed.

Borex: Solar neutrino experiment via weak neutral and charged currents in boron-11

Borex, an experiment to observe solar neutrinos using boron-loaded liquid scintillation techniques, is being developed for operation at the Gran Sasso underground laboratory. It aims to observe the spectrum of electron type 8B solar neutrinos via charged current inverse β-decay of 11B and the total flux of solar neutrinos regardless of flavor by excitation of 11B via the weak neutral current.

A new solar neutrino detector

Abstract This paper describes the main features of the proposed low energy solar neutrino detector Borexino, planned to be installed at the Gran Sasso Laboratory. This real time detector is based on a massive, calorimetric, liquid scintillation spectroscopy technique, whose high luminosity is the base for the attempt to achieve a low signal detection threshold. After a description of the main structural components of the detector, of its performances in term of spatial and energy resolution, and of the neutrino reactions occurring in the liquid scintillator, a description of the crucial background topic is given. Finally the main implications of the physics program of the experiment are briefly illustrated.

A new detector for solar neutrino

Abstract The design of a new neutrino detector, Borex, is illustrated in this paper. The main experimental features of this detector along with its principal physical goals are thoroughly discussed. The key features of the liquid scintillation spectroscopy detection technique adopted in the design of the detector are deeply explained. The main advantages of this technique, i.s. high energy resolution, low detection threshold, ability to distinguish between different neutrino detection modes, background discrimination, make Borex a powerful and flexible experimental tool able to test and measure the flux, the flavor content, the energy spectrum, the antineutrino content of the solar neutrino flux arriving at Earth. Borex is thus equipped to probe completely the particle physics of solar neutrinos. Due to its characteristics, this experiment, planned to be carried out at the Gran Sasso Laboratory, is expected to be a fundamental step towardsthe solution of the solar neutrino problem.