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

2 The Borexino Experiment
volume would be of the same order or even higher than the expected neutrino signal. It is
therefore important to exclude systematic errors of the position reconstruction well below the
percent level.
Material U (g/g) Th (g/g) K (g/g)
scintillator
nylon vessel
buffer liquid
PMTs
stainless steel tank
Table 2.3: Background requirements for the BOREXINO detector components.
2.4.3 Background from Cosmic Ray Events
The flux of cosmic rays is dramatically suppressed by locating the detector underground. For
this reason the detector is installed in the Gran Sasso underground laboratory, with an overburden
of about 1400 m of rocks which provides a shielding of about 3600 mwe and suppresses
the cosmic ray flux by a factor of . The remaining flux of about 1.1 Ñ is composed
essentially of ultrarelativistic muons with energies of several hundred GeV.
A muon passing through the scintillator will produce a large amount of scintillation photons
and can hence easily be identified. A fraction of muons, though, cross only the buffer without
entering the scintillator. They could produce few enough photons (by Cerenkov light or scintillation
of the PC in the buffer) to be misidentified as a low energy electron. These muons will
be identified over the Cerenkov photons they produce in the outer water tank, which will be
detected by 200 outward looking PMTs mounted on the outside of the Stainless Steel Sphere.
Another source of background are short-lived radioactive nuclides produced on-site by muon
inelastic interactions in the scintillator (see table 2.4). Most of the isotopes have half-lives in
the sub-seconds scale. With the identification of the muon by the outer Cerenkov detector, it
is possible to discard these activities with a moderate loss of measuring time by imposing a
veto cut subsequent to a muon signal in the off-line analysis. However, this is not the case
for Be (¬ ,EÑÜ MeV, t s) and Be (t d, EC, 10 % ­ with
478 keV). From the measurement of the cross sections for the production of these isotopes
one can expect in BOREXINO a background of less than 1 event per day in the Be neutrino
window (250 - 800 keV) in 100 t of scintillator [Hag00b]. The large background contribution
of C in the energy region from 0.8 - 1.4 MeV however jeopardizes the detection of the solar
pep-neutrinos, with an expected event rate of only 1.5 counts/(day 100 t) according to the
SSM.
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