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Topic Name: Measurement of the monoenergetic (862 keV) Be-7 solar neutrinos & the phenomenology of solar neutrinos .
Category: Nuclear Magnetic Resonance
Research persons: Borexino Researchers
Location: S.S. 17bis Km. 18,910,67010 Assergi (Aq) - Italy, Italy
Details
The Borexino detector for low-energy solar neutrino studies has been completely filled (May 15th) with highly purified scintillator and high purity shielding liquids (pseudocumene and water) and is now fully operational at the
Laboratory Nazionali del Gran Sasso. This achievement became reality after several years of technical developments (leading to background radioactive levels then never achieved before), of construction and commissioning and through the solution of several problems involving the underground laboratory and the local authorities, which caused to the experiment three years of stop (mostly due to worries for environmental damages).
The main goal of Borexino is the measurement of the mono energetic (862 keV) Be-7 solar neutrinos, a central feature in the on-going story of solar neutrinos and neutrino oscillations. Borexino will accomplish this goal by detecting neutrino-electron scattering events taking place in real time in its well shielded 100
tones mass fiducially volume.
The detector is an unregimented liquid scintillator featuring 300 tones of sensitive mass, viewed by 2,200
photo multipliers, for the detection of the energy released by the neutrino-induced recoil electrons. The detector core is a transparent nylon spherical vessel (125 µm thick), 8.5 m of diameter, filled with liquid scintillator and surrounded by 1,000 tonnes of high-purity shielding buffer liquid. The scintillator mixture is PC (1-2-4,trimethylbenzene) and PPO (2,5 diphenyl oxazole, 1.5g/l) as a fluor, while the buffer liquid is PC with the addition of DMP as light quencher. The
photo multipliers are supported by a stainless steel sphere, which also separates the inner part of the detector from an additional external shielding, provided by 2,400 tons of purified water.
This outermost water shield is instrumented with 200 outward-pointing photo multipliers
serving as a veto for penetrating muons, the only significant residual cosmic background at the 3800
mew (meters of water equivalent) depth of the underground laboratory.
Since the Homestake pioneering experiment, observations of solar neutrinos have offered a first hint of physics beyond the Standard Model of particle physics: neutrino mass and oscillations. A spectacular confirmation of the oscillation mechanism in the solar regime came during the last decade through a series of experiments culminating with the final SNO direct demonstration of solar neutrino oscillations.
These challenging experiments were able to observe in real time the high energy part of the solar neutrino spectrum, which amounts to only 0.01% of the total flux. Radioactivity as well as other backgrounds imposed constraints on the lowest achievable threshold which was at best around 5 MeV, leaving the experimental exploration of the low energy part only to radiochemical experiments.
Borexino, with its ultra pure scintillator mass will be able to lower this detection threshold well below the MeV limit, with the potential capability of exploring for the first time solar signals like Be-7, CNO, pep, and also part of the pp.
At present the phenomenology of solar neutrinos is explained in terms of the so-called LMA solution. This model, however, still needs to be confirmed with these low energy solar neutrinos. As a matter of fact the upturn of the electron neutrino survival probability between 0.7 and 4 MeV (transition between the oscillations in vacuum and in matter) has never been observed. Therefore, sub-MeV solar neutrinos may offer a unique opportunity to check the LMA oscillations scenario and search for other phenomena and sub-leading effects. Besides, sub-MeV solar neutrinos detection allows the possibility to test at the few percent level the astrophysical model of the Sun.
Beyond the solar neutrino program, a variety of other physics topics can be explored, such as antineutrinos from Supernovae (with the unique possibility of a very low threshold at 0.25 Mev, particularly useful for the detection of neutrino-proton scatterings)., terrestrial antineutrinos (geoneutrinos) and neutrino magnetic moment. The study of geoneutrinos is particularly
favored at Gran Sasso due to the low level of the background from nuclear reactors.
About Borexino -
Borexino is an experiment performed by an international collaboration and located in the "Hall C" of the Laboratori Nazionali del Gran Sasso (LNGS).
The Borexino physics program is centered on Solar Neutrino Physics, but also includes other relevant topics in low background neutrino detection and underground physics.
The Borexino detector, currently under construction in the "Hall C" of LNGS, is a real time detector for low energy (sub-MeV) Solar Neutrinos, with the specific goal of measuring the Be-7 neutrino flux from the Sun. The very low energy experimental threshold (250 keV) requires extreme radiopurity of the detector.
A Borexino prototype, called Counting Test Facility (CTF) was built and operated in the Hall C of
LNGS. This detector demonstrated the achievement of
ultralow count rates (radio purities of the order of E-16 gr/gr of U-238 equivalent) on the several-ton scale.
The Borexino detector is being built on the CTF experience and is scheduled for completion on the year 2007.
Researchers of Borexino-
Alessandra
Di Credico
Davide
D'angelo
Aldo
Ianni
István
Manno
Paolo
Saggese
Oleg
Ju. Smirnov
Alexandre
Derbin
In The Images-
1.Low Energy Solar Neutrinos cross section
2. Borexino detector-The
detector core is a transparent spherical vessel (Nylon Sphere, 100 micron
thick), 8.5 m of diameter, filled with 300 tonnes of liquid scintillator and
surrounded by 1000 tonnes of a high-purity buffer liquid.The scintillator
mixture is made out of pseudocumene (PC) and 1.5 g/l of PPO (a fluor), while the
buffer liquid is pure PC alone (with the possible addition of a light quencher,
DMP).
The photomultipliers are supported by a
Stainless Steel Sphere (SSS) which also separates the inner part of the detector
from the external shielding, provided by 2400 tons of pure water (water
buffer).An additional containment vessel (Nylon film Radon barrier) is
interposed between the scintillator Nylon Sphere and the photomultipliers, to
the goal of reducing Radon diffusion toward the internal part of the detector.
The outer water shield is instrumented with
200 outward-pointing photomultipliers serving as a veto for penetrating muons,
the only significant remaining cosmic ray background at the Gran Sasso depth
(about 3500 meters of water equivalent).
Moreover, the 2200 photomultipliers viewing the internal part of the detector
are divided in two sets: 1800 photomultipliers are equipped with light cones so
that they see light only from the Nylon Sphere region, while the remaining 400
PMT's are sensitive to the total volume of the stainless steel sphere.
This design greatly increases the capability of the system to identify muons
crossing the PC buffer (and not the scintillator).
The Borexino design is based on the concept of
a GRADED SHIELD of progressively lower intrinsic radioactivity as one approaches
the sensitive volume of the detector; this culminates in the use of 200 tonns of
the low background scintillator to shield the 100 tonnes innermost Fiducial
Volume.In these conditions the ultimate background will be dominated by the
intrinsic contamination of the scintillator, while all backgrounds from the
construction materials and external shieldings are negligible.In addition to the
purity of the construction material and the predicted rejection efficienty of
the muon system, the detection of the 7Be neutrino signal in the 100
tonnes of the Borexino Fiducial Volume requires the intrinsic radiopurity of the
scintillator to be below 5x10-15 g/g of U,Th equivalent.
This radiopurity have been reached in the
development of the Counting Test
Facility, which also served as a test of the whole Borexino concept.
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