Development of a Liquid Scintillator and of Data ... - Borexino - Infn

2.2 The Detection Principle
intrinsic background present in the organic scintillator (mainly C), the upper limit is chosen
to accommodate the finite energy resolution of the detector (about 7 % at 665 keV).
BOREXINO is sensitive to all neutrino flavours, as and can interact via neutral current
interactions while only can additionally interact via charged current interactions. The ratio
of the cross-sections for and is about 5:1 in the Be- signal window. In principle, the
shape of the recoil spectrum is slightly different for and , but the difference is probably
too small to be distinguishable in practice.
As the scintillation light is emitted isotropically, the directional information of the scattered
neutrino is lost and no directional cuts on the neutrino source can be applied. Since the experimental
signature is simply a few hundred keV energy deposition in the scintillator, the
requirements on the radiopurity of the detector components, especially the scintillator, are
very high. The different sources of background and their suppression will be discussed in
detail in section 2.4.
Solar neutrinos from other branches of the pp-cycle also contribute to the count rate in the Be
neutrino window (250 - 800 keV). These are summarized in table 2.1. With a threshold energy
of 250 keV for the recoil electron, the lowest energy for neutrinos that can be be detected is
410 keV.
neutrino branch SSM LMA SMA LOW
pp 1.3 1.0 0.6 0.6
Be 45.7 26.0 9.7 25.1
pep 2.0 1.0 0.4 1.0
Æ 4.2 2.3 0.9 2.3
Ç 5.5 2.8 1.1 2.8
F 0.07 0.03 0.01 0.03
B 0.01 0.04 0.05 0.05
Sum 58.78 33.17 12.76 31.78
Table 2.1: Expected signal rates (counts per day in 100 tons of scintillator in the energy range
from 250 - 800 keV) in BOREXINO from neutrinos of the individual branches for different oscillation
scenarios.
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