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

Summary and Outlook
possible to distinguish clearly between the different currently allowed oscillation scenarios.
The LENS experiment aims at a real-time charged current detection of the Be and, eventually
the pp neutrinos. By measuring the CC and the neutrino-electron scattering (CC + NC)
rate of the Be neutrinos, their total flux can be determined and compared with solar model
predictions. The comparison of the results from BOREXINO and LENS can then give an unambiguous
proof for oscillations of the Be neutrinos.
The identification of the neutrino signal in BOREXINO relies on the distinction of a Comptonlike
edge in the electron energy spectrum, due to the monoenergetic (862 keV) Be neutrinos
scattered off electrons. Since the neutrino signal does not have any event-by-event signature,
reduction and rejection of the background signals from radioactive decays are essential. As
the expected signal rate in BOREXINO is less than ÓÙÒØ× ØÓÒ ¡ Ý in the energy range
from 0.25 - 0.8 MeV, the requirements on the radiopurity of the detector components are extremely
challenging: for the liquid scintillator, a maximum concentration of U and Th
of g/g and of Kof g/g is allowed in order to achieve a signal/background ratio
even in the case of total conversion of into . The necessary background suppression
can be achieved only if a large fraction of the background events can be rejected by their pulse
shape characteristics (alpha decays) and by spatial cuts (discrimination of external background
by fiducial volume cut). It is therefore crucial to provide several independent analysis methods
which can be used for cross checks of the efficiency and systematic uncertainty of the applied
cuts.
In this work, a new method for the spatial reconstruction of scintillation events for BOREXINO
and the CTF was developed and successfully tested with Monte Carlo simulations and CTF2
data. From laboratory measurements and source tests in the CTF1, the parameters for an optical
model of the light propagation in the liquid scintillator have been evaluated and applied in
a tracking program developed for BOREXINO and the CTF. The reconstruction compares the
ADC and TDC data from a real event with values from simulations on a spatial grid. The resolution
of this reconstruction method therefore depends strongly on the understanding of the
light propagation mechanisms. Calibration measurements with localized sources in the CTF2
were performed in order to study the systematic behaviour of the reconstruction code.
A new liquid scintillator based on phenyl-o-xylylethane (PXE) was tested as an alternative
choice for BOREXINO. Its physical and chemical properties make it superior to pseudocumene
(PC) in some respects, e.g. the higher density (target mass) and the higher flash point (safety),
but there are also some limitations which need further investigation. In particular, the C
concentration was about 6 times higher than in the PC measured in the CTF1 ( C/ C
¡ ). The C level may vary, though, depending on the petroleum source, and also for
the PC a new batch of scintillator from another source could have a higher C/ C ratio.
The very low U concentration that was measured in PXE by NAA ( ¡ g/g) could
not be confirmed by CTF measurements, as in the CTF1 the sensitivity of the detector was
substantially reduced due to the failure of a large fraction of the PMTs, and in the CTF2 a
Rn background of the order of several g/g from an unidentified source was present.
The Th concentration measured in the CTF2 ( ¦ ¡ g/g) is compatible with the
limit of ¡ g/g that was measured with NAA, confirming the high intrinsic radiopurity
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