Low Energy Solar Neutrino Spectroscopy Research Articles

Final results of Borexino Phase-I on low-energy solar neutrino spectroscopy

Borexino has been running since May 2007 at the LNGS with the primary goal of detecting solar neutrinos. The detector, a large, unsegmented liquid scintillator calorimeter characterized by unprecedented low levels of intrinsic radioactivity, is optimized for the study of the lower energy part of the spectrum. During the Phase-I (2007-2010) Borexino first detected and then precisely measured the flux of the 7Be solar neutrinos, ruled out any significant day-night asymmetry of their interaction rate, made the first direct observation of the pep neutrinos, and set the tightest upper limit on the flux of CNO neutrinos. In this paper we discuss the signal signature and provide a comprehensive description of the backgrounds, quantify their event rates, describe the methods for their identification, selection or subtraction, and describe data analysis. Key features are an extensive in situ calibration program using radioactive sources, the detailed modeling of the detector response, the ability to define an innermost fiducial volume with extremely low background via software cuts, and the excellent pulse-shape discrimination capability of the scintillator that allows particle identification. We report a measurement of the annual modulation of the 7 Be neutrino interaction rate. The period, the amplitude, and the phase of the observed modulation are consistent with the solar origin of these events, and the absence of their annual modulation is rejected with higher than 99% C.L. The physics implications of phase-I results in the context of the neutrino oscillation physics and solar models are presented.

The Borexino detector: Photomultipliers system

Borexino, a real-time device for low energy solar neutrino spectroscopy has completed the construction phase at the middle of 2006 in the underground laboratories at Gran Sasso, Italy. The detector has been filled with 1300 tons of highly purified scintillator and 2400 tons of ultra-pure water as external shield and since May 16th is under data taking. The experimental goal is the direct measurement of the flux of Be7 solar neutrinos of all flavors via neutrino-electron scattering in an ultra-pure scintillation liquid. The poster describes the design of the Borexino detector and the features of the construction phase. It also presents in deep details the photomultiplier detection apparatus: the device electrical performances, the encapsulation design, the mechanical support, the R&D ageing test and finally the ‘in situ‘ installation procedure.

Performances and calibrations of the Borexino detector

Borexino is a real-time detector for low energy Solar neutrino spectroscopy installed in the underground laboratories (3800 mwe depth) at Gran Sasso, Italy (LNGS). Borexino is specifically designed to measure the mono-energetic (866 keV) 7Be flux via neutrino-electron elastic scattering in ultra-pure organic scintillation liquid. The separation of signal and background relies on the identification of the Compton-like edge in the recoil electron energy spectrum at 667 keV. The physics potential of Borexino strongly depends on the unprecedented requirements of low level background. The exceptional radio-purity reached by the Borexino, in order to guarantee the success of the experiment, must be complemented with the identification and discrimination of the residual intrinsic contaminants, as well as of the external and cosmogenic backgrounds. Crucial in this respect are the energy and spatial reconstruction capabilities of the detector, which thus require a careful and precise assessment. We report on the Borexino performances after 1 year of data taking in reconstructing event energy and position, in discriminating on the nature of the contaminant with the pulse shape analysis and in the identification of fast coincidences looking at time and space correlations.

Low energy tracking and particles identification in the MUNU Time Projection Chamber at 1 bar: possible application in low energy solar neutrino spectroscopy

In this paper we present the results from the measurements made with the MUNU Time Projection Chamber (TPC) at 1 bar pressure of CF4 in the energy region below 1 MeV. Electron events down to 80 keV are successfully measured. The electron energy and direction are reconstructed for every contained single electron above 200 keV. As a test the 137Cs photopeak is reconstructed by measuring both the energy and direction of the Compton electrons in the TPC.

Prototype scintillator cell for an In-based solar neutrino detector

We describe the work carried out at MPIK to design, model, build and characterize a prototype cell filled with a novel indium-loaded scintillator of interest for real-time low energy solar neutrino spectroscopy. First, light propagation in optical modules was studied with experiments and Monte Carlo simulations. Subsequently a 5 cm × 5 cm × 100 cm prototype detector was set-up and the optical performances of several samples were measured. We first tested a benchmark PXE-based scintillator, which performed an attenuation length of ∼ 4.2 m and a photo-electron yield of ∼ 730 pe / MeV . Then we measured three In-loaded samples. At an In-loading of 44 g / l , an energy resolution of ∼ 11.6 % and a spatial resolution of ∼ 7 cm were attained for 477 keV recoil electrons. The long-range attenuation length in the cell was ∼ 1.3 m and the estimated photo-electron yield ∼ 200 pe / MeV . Light attenuation and relative light output of all tested samples could be reproduced reasonably well by MC. All optical properties of this system have remained stable over a period of > 1 year .

Ytterbium-based scintillators, a new class of inorganic scintillators for solar neutrino spectroscopy

The observed deficit of the solar neutrino flux is now well established. This puzzling problem of today's particle physics could be resolved soon. The most likely explanation would be the vacuum neutrino oscillation phenomenon, indirectly proving the non-zero mass of these fleeting particles. Following the proposition of Raghavan of using 176Yb as a target for low-energy solar neutrino spectroscopy, an intense R&D work has started a few years ago to define a suitable scintillator incorporating a large amount of ytterbium. Recently, the observation of UV scintillation in mixed yttrium/ytterbium aluminium garnets opened the field of investigation to a new class of scintillating crystals with interesting luminescence properties, very attractive not only for neutrino physics but also for radiation detection, in general. Their luminescence properties present some peculiarities that make them interesting by themselves.

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

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