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

5 Particle Identification with a Neural
Network
In the first section of this chapter I will describe the scintillation process in organic scintillators,
and explain how the ionization density of the exciting radiation is related to the timing
behaviour of the emitted scintillation photons. This difference can be exploited for «/¬ discrimination.
In the next section I will introduce the method of neural networks, that I have
applied to the problem of «/¬ identification. In the last section results obtained with Monte
Carlo simulations of the CTF and BOREXINO and with data from a CTF2 source run are
reported and confronted with the standard ’charge integration’ method.
5.1 Pulse Shape Discrimination in Liquid Scintillator
5.1.1 The Scintillation Process
Organic scintillators are aromatic compounds with planar molecules built up mainly from
condensed or linked benzenoid rings (see e.g. fig. 6.1). One fundamental characteristics of
the molecular structure is the presence of delocalized electrons occupying so called -orbitals
which extend above and below the molecular plane. The -electronic states are of particular
interest because transitions between these states lead to the luminiscence observed in the
scintillation process.
The electronic levels of an organic molecule with a -electron structure can be represented
schematically as shown in fig. 5.1. The ground state is a singlet in which the -electron spins
are fully paired. Excitation can lead from the ground state S to an excited singlet state S
(electron spins paired) or triplet state T (electron spins unpaired). Associated with each electronic
level is a vibrational band of substates SÜ, TÜ. The absorption transition from the
ground state S to the triplet states T is spin-forbidden and unlikely to occur due to photon
excitation. Excitement into triplet states is possible with ionizing radiation. The higher singlet
electronic states are quickly (on the order of pico seconds) de-excited to the S state through
radiationless internal conversion. Fluorescence light is emitted by radiative transitions between
the first excited -electron state S and the vibrational substates of the ground state S Ü,
hence the fluorescence spectrum lies predominantly at wavelengths longer than the singlet
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