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

2 The Borexino Experiment
2.2 The Detection Principle
The neutrino detection in BOREXINO is based on neutrino-electron scattering using the wellknown
technique of liquid scintillation spectroscopy. The Feynman diagram of the process of
neutrino-electron scattering was shown in fig. 1.3. Solar neutrinos scattering off the electrons
in the scintillator will result in a recoil electron which can be detected via scintillation light.
The cross section for this pure leptonic reaction can be calculated using the standard theory of
electroweak interactions [t’H71]:
Ì
Ä Ê
Ì
ÄÊ
Ì
where: Ì = kinetic energy of the scattered electron
= energy of the incoming neutrino
= Ñ ¡ cm
Ê = ×Ò Ï ¦
Ä = ×Ò Ï ¦ , + for , – for .
For a neutrino with a given energy the maximum kinetic energy of the electron is
ÌÑÜ
Ñ
A monoenergetic neutrino source produces an electron recoil spectrum with a sharp edge and
thus a good signature. For the Be-neutrinos at 862 keV this edge lies at 665 keV (see fig. 2.5).
Even though the recoil spectrum goes from zero to the edge, in practice the optimum Be-
signal window in BOREXINO is from 250 - 800 keV. The lower threshold has to be set due to
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Figure 2.5: The cross section for scattering for monoenergetic neutrinos with
keV.