We report on the study of a new liquid scintillator target for neutrino interactions in the framework of the research and development program of the Borexino solar neutrino experiment. The ...scintillator consists of 1,2-dimethyl-4-(1-phenylethyl)-benzene (phenyl-o-xylylethane, PXE) as solvent and 1,4-diphenylbenzene (para-Terphenyl, p-Tp) as primary and 1,4-bis(2-methylstyryl)benzene (bis-MSB) as secondary solute. The density close to that of water and the high flash point makes it an attractive option for large scintillation detectors in general. The study focused on optical properties, radioactive trace impurities and novel purification techniques of the scintillator. Attenuation lengths of the scintillator mixture of 12
m at 430
nm were achieved after purification with an alumina column. A radiocarbon isotopic ratio of
C
14
/
C
12
=
9.1
×
10
-
18
has been measured in the scintillator. Initial trace impurities, e.g.
238U
at
3.2
×
10
-
14
g
/
g
could be purified to levels below
1
×
10
-
17
g
/
g
by silica gel solid column purification.
We observed, for the first time, solar neutrinos in the 1.0-1.5 MeV energy range. We determined the rate of pep solar neutrino interactions in Borexino to be 3.1+ or -0.6 sub(stat)+ or -0.3 sub(syst) ...counts/(day times 100 ton). Assuming the pep neutrino flux predicted by the Standard Solar Model, we obtained a constraint on the CNO solar neutrino interaction rate of <7.9 counts/(day times 100 ton) (95% C.L.). The absence of the solar neutrino signal is disfavored at 99.97% C.L., while the absence of the pep signal is disfavored at 98% C.L. The necessary sensitivity was achieved by adopting data analysis techniques for the rejection of cosmogenic super(11)C, the dominant background in the 1-2 MeV region. Assuming the MSW-LMA solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of (1.6+ or -0.3)x10 super(8) cm super(-2)s super(-1) and <7.7x10 super(8) cm super(-2)s super(-1) (95% C.L.), respectively, in agreement with both the High and Low Metallicity Standard Solar Models. These results represent the first direct evidence of the pep neutrino signal and the strongest constraint of the CNO solar neutrino flux to date.
An experiment with an artificial neutrino source in Borexino is presented. The neutrino source can be located outside the detector or eventually, at the end of the solar neutrino phase, could be ...deployed inside. The physics case for the source experiment includes the search for short-baseline neutrino oscillations, neutrino-electron scattering at sub-MeV range, neutrino magnetic moment. Preliminary predictions of the sensitivity are reported for the sterile neutrino search.
A very high radiopurity level of its scintillator and detector location far away from the European nuclear reactors makes Borexino a very sensitive tool for antineutrino studies. Spectral ...contributions corresponding to two known antineutrino sources were reported in a recently published paper 1: v sub(e)s produced in European nuclear reactors and geo-neutrinos, produced in beta decays of isotopes along the decay chains of long-lived super(238)U and super(232)Th distributed within the Earth's interior. A sensitive search for other antineutrino sources has been performed.
We report on a search for the day-night asymmetry of the Be-7 solar neutrino rate measured by Borexino at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. The measured value, Adn=0.001 +- 0.012 ...(stat) +- 0.007 (syst), shows the absence of a significant asymmetry. This result alone rejects the so-called LOW solution at more than 8.5 sigma. Combined with the other solar neutrino data, it isolates the Large Mixing Angle (LMA) -- MSW solution at DeltaChi2 > 190 without relying on the assumption of CPT symmetry in the neutrino sector. We also show that including the day-night asymmetry, data from Borexino alone restricts the MSW neutrino oscillations to the LMA solution at 90% confidence level.
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