Solar Neutrino Experiment Borexino Research Articles

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.

226Ra, 210Pb, 210Bi and 210Po deposition and removal from surfaces and liquids.

Deposition of 226Ra from water on nylon was investigated. Measurements performed for different pH and different radium concentrations in the water gave similar absolute activities deposited on the foil surface. Obtained results were used to estimate the amount of 226Ra plated-out on the nylon scintillator vessel in the solar neutrino experiment BOREXINO during filling of the detector. Another problem studied in the frame of BOREXINO was the removal of 210Pb from its organic liquid scintillator by applying distillation and water extraction. After several tests had been performed for both methods it was found that after the water extraction the initial lead content in the scintillator sample was reduced only accordingly to the ratio of the volumes of the applied liquids (simple dilution). In contrast to this, distillation was very effective providing in the best case a 210Pb reduction factor higher than 100. Removal efficiencies of the long-lived 222Rn daughters during etching from surfaces of standard and high purity germanium were investigated in the frame of the GERDA experiment, which aims to search for neutrino-less double beta decay of 76Ge. The standard etching procedure of Canberra used during production of high purity n-type germanium diodes was applied to germanium discs, which had been exposed earlier to a strong 222Rn source for its progenies deposition. In contrast to copper and stainless steel, 210Pb, 210Bi and 210Po was removed from germanium very efficiently. An evidence of a reverse process was also observed—the isotopes were transferred from the etchant to the clean germanium surface.

Spectroscopy of solar neutrinos

AbstractIn the last years, liquid‐scintillator detectors have opened a new window for the observation of low‐energetic astrophysical neutrino sources. In 2007, the solar neutrino experiment Borexino began its data‐taking in the Gran Sasso underground laboratory. High energy resolution and excellent radioactive background conditions in the detector allow the first‐time spectroscopic measurement of solar neutrinos in the sub‐MeV energy regime. The experimental results of the Beryllium 7 neutrino flux measurements (Arpesella et al. 2008b) as well as the prospects for the detection of solar Boron 8, pep and CNO neutrinos are presented in the context of the currently discussed ambiguities in solar metallicity. In addition, the potential of the future SNO+ and LENA experiments for high‐precision solar neutrino spectroscopy will be outlined (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Results from the solar neutrino experiment BOREXINO

We report about recent results from the solar neutrino experiment BOREXINO. With BOREXINO solar neutrino spectroscopy in the sub-MeV range became possible. In particular we demonstrate the measured counting rate of solar 7Be neutrinos. Additionally we show data on solar 8B neutrinos with an energy threshold of 2.8 MeV and discuss the implications for solar and neutrino physics. Finally further prospects of BOREXINO are discussed.

Pulse-shape discrimination with the Counting Test Facility

Pulse shape discrimination (PSD) is one of the most distinctive features of liquid scintillators. Since the introduction of the scintillation technique in the field of particle detection, many studies have been carried out to characterize intrinsic properties of the most common liquid scintillator mixtures in this respect. Several application methods and algorithms capable of achieving optimum discrimination performances have been developed. However, the vast majority of these studies have been performed on samples of small dimensions. The Counting Test Facility, prototype of the solar neutrino experiment Borexino, as a 4 ton spherical scintillation detector immersed in 1000 tons of shielding water, represents a unique opportunity to extend the small-sample PSD studies to a large-volume set-up. Specifically, in this work we consider two different liquid scintillation mixtures employed in CTF, illustrating for both the PSD characterization results obtained either with the processing of the scintillation waveform through the optimum Gatti's method, or via a more conventional discrimination approach based on the charge content of the scintillation tail. The outcomes of this study, while interesting per se, are also of paramount importance in view of the expected Borexino detector performances, where PSD will be an essential tool in the framework of the background rejection strategy needed to achieve the required sensitivity to the solar neutrino signals.

Light concentrators for Borexino and CTF

Light concentrators for the solar neutrino experiment Borexino and the Counting Test Facility (CTF) have been developed and constructed. They increase the light yield of these detectors by factors of 2.5 and 8.8, respectively. Technical challenges like long-term stability in various media, high reflectivity and radiopurity have been addressed and the concepts to overcome these difficulties will be described. Gamma spectroscopy measurements of the concentrators show an upper limit of 12μBq/g for uranium and a value of 120μBq/g for thorium. Upper limits on other possible contaminations like 26Al are presented. The impact of these results on the performance of Borexino and the CTF are discussed and it is shown that the design goals of both experiments are fulfilled.

Ar and Kr concentrations in nitrogen as a measure of the 39Ar and 85Kr activities in connection with the solar neutrino experiment Borexino

Among other radionuclides, 39Ar and 85Kr are potential background sources in the solar neutrino detector Borexino. The expected low event rate requires that the nitrogen used in the experiment needs to have lower concentrations than 0.5 μBq/m 3 for 39Ar and 0.2 μBq/m 3 for 85Kr, corresponding to volume concentrations in N 2 of 0.4×10 −6 m 3/m 3 for Ar and 0.2×10 −12 m 3/m 3 for Kr. Applying gas mass spectrometry and special care in the sampling technique we succeeded in finding the required purity. In addition, we studied the possibility to purify nitrogen from Kr by the adsorption method.

Measurements of extremely low radioactivity levels in BOREXINO

The techniques researched, developed and applied towards the measurement of radioisotope concentrations at ultra-low levels in the real-time solar neutrino experiment BOREXINO at Gran Sasso are presented and illustrated with specific results of widespread interest. We report the use of low-level germanium gamma spectrometry, low-level miniaturized gas proportional counters and low background scintillation detectors developed in solar neutrino research. Each now sets records in its field. We additionally describe our techniques of radiochemical ultra-pure, few atom manipulations and extractions. Forefront measurements also result from the powerful combination of neutron activation and low-level counting. Finally, with our techniques and commercially available mass spectrometry and atomic absorption spectroscopy, new low-level detection limits for isotopes of interest are obtained.

Low level γ-ray germanium-spectrometer to measure very low primordial radionuclide concentrations

A new germanium spectrometer especially suited for large sample measurements is described in detail. It is operated in the Gran Sasso underground laboratory under shielding rock of 3300 m water equivalent, which reduces the muon flux by six orders of magnitude. The integral background counting rate in the energy range from 50 to 2750 keV is about 0.15 min−1. The low peak count rates of mostly less than 1 count per day together with a relative efficiency of 102% and the high sample capacity makes this spectrometer one of the most sensitive worldwide. Some sample measurements for the solar neutrino experiment BOREXINO and the detector efficiency calibration by the Monte Carlo method are discussed as well.

222Rn emanation measurements at extremely low activities

For the solar neutrino experiment Borexino, a system has been set up to measure emanation of 222Rn in samples of up to 80 litres volume. The apparatus has been constructed to high vacuum standard and consists mainly of parts made from electropolished stainless steel. The low blank activity of the system together with a highly sensitive detection method for the extracted Rn leads to a sensitivity level in the range 70–100 μBq 222Rn. Due to the large sample capacity of the emanation chambers, it is possible to measure specific emanation rates as low as several μBq/m2. Some measurements characterizing the performance of the apparatus are discussed.

Records in ultra low radioactivity measurements with neutron activation analysis

Neutron Activation Analysis has emerged to be an analytical method sensitive enough to detect fg/g traces of unstable primordial nuclides in complex matrices. Especially low count rate experiments in the field of solar neutrino physics and dark matter search can profit from the detection capabilities of this method which had been unattained so far. This gain in sensitivity has been achieved by combining neutron activation, radiochemical separation methods, and efficient low level counting systems at the new underground laboratory of the accelerator laboratory in Garching. Recent improvements which have been made in the purification and analysis of the liquid scintillator, as foreseen for the solar neutrino experiment BOREXINO, are being presented as an example in this paper

New records on low activity measurements with neutron activation analysis

A very sensitive NAA technique of liquid organic scintillators has been developed at the Technische Universitat Munchen focusing on quasistable radionuclides. Combining important input factors for sensitive NAA very low detection limits are obtained, e.g. 2 · 10−16 g/g for 238U. This article describes these input factors and how they have been realized for the NAA of liquid organic scintillators. The results are discussed in the context with the solar neutrino experiment BOREXINO, for which this work has been started. Possibilities for a further improvement of sensitivity are presented as an outlook.

Neutron activation analysis of detector components for the solar neutrino experiment BOREXINO

The proposed solar neutrino experiment BOREXINO requires detector components of high radiopurity. The acceptable levels for U and Th contaminations reach down to the sub-ppq region. To select suitable materials a very sensitive analysis method have to be applied. In this article a highly developed neutron activation analysis techniques are presented, which have been used to determine the K, Th and U content of detector components, namely nylon, water and liquid scintillator.

A low-energy HPGE detector dedicated to radioactivity measurements far below environmental levels

Successive background reduction factors of a low-background high-purity semi-planar Germanium detection system, N type, were achieved with various shielding conditions, at ground level and underground locations. The optimal working conditions, achieved at a depth of 500 m water equivalent (m w.e.) lead to a background reduction factor of about 100 in the energy region from 5 to 1000 keV in comparison with a measurement at sea level. The integral count rate as low as (0.00361 ± 0.00004) s −1 over this energy interval is mainly attributable to the influence of the cosmic rays but also to the residuals of the natural decay chains inside the various constituents of the detection system, the radon and progenies from the environment, the weak level of man-made radionuclides and finally the cosmogenic activation of the shielding materials formerly stored at sea level. About one event per day, or less, corresponding to the the γ-rays of interest, are recorded in energy intervals of four times the FWHM resolution of the detector. Detection limits were deduced for 210Pb and uranium metal activities and for a few radionuclides of interest. Materials studied are; clay standard, uranium metal, biological matrix exposed to radon flux; gold and silver foils, various lead and copper batches, as a contribution to material selection with respect to their low level of radiocontaminants, in particular for the solar neutrino and dark matter experiment BOREXINO.

An analytical approach to the evaluation of the pulse shape discrimination properties of scintillators

In this paper a general methodology to compute the performances of different pulse shape discrimination techniques, applied to exploit the intrinsic PSD features of organic and of some inorganic scintillators, is presented. The most popular methods, i.e. the rise time technique and the charge integration approach, are evaluated and compared. As a specific example, detailed calculations are carried out for a binary organic liquid scintillator under consideration for the low energy solar neutrino experiment Borexino. It is demonstrated, in particular, that the charge integration method offers superior performances, the difference being remarkable for pulses comprising a low number of photoelectrons.

Characterization and magnetic shielding of the large cathode area PMTs used for the light detection system of the prototype of the solar neutrino experiment Borexino

In this article we discuss the results of the testing of ∼ 50 Thorn Emi 9351 phototubes used for the light detection system of the Counting Test Facility, prototype of the solar neutrino experiment Borexino, presently under installation at the Gran Sasso Laboratory. The tests were aimed to a systematic evaluation of the spread of the main characteristics of the device and to a thorough evaluation of the shielding effectiveness of μ-metal shields of different lengths.

Performances of the photomultiplier Hamamatsu R4558 for underground physics applications

Abstract In this article we describe the main results of the measurements on the large cathode-area phototube Hamamatsu R4558. This tube, whose forerunner R1408 has been extensively adopted in large-scale Cherenkov and liquid-scintillator detectors, has been considered as a potential candidate for the use in the proposed low-energy solar-neutrino experiment Borexino. Hence the tests carried out have been particularly focused to evaluate the values of the parameters of this device more crucially impacting on the overall physics performances of the detector.

Tests on the photomultiplier Burle C83061E for underground physics applications

In this article we describe the main results of the test activity performed on the large cathode area phototube Burle C83061E. This tube, originally developed for a large-scale Cherenkov detector, has been considered as a potential candidate for the use in the proposed low energy solar neutrino experiment Borexino. Thus the measurements carried out have been particularly focused to evaluate the parameters of this device more critically affecting the overall physics performances of the detector.