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

5 Particle Identification with a Neural Network
5.1.3 Pulse-Shape-Discrimination
The intensity of photons from a scintillation event as a function of time can be divided into
at least two components, fast and slow. The fast component has a decay time equal to the
fluorescence lifetime of the fluorescing species (the fluorescence solute). The slow component
has a decay time which is diffusion controlled. It has been shown to be due to the annihilation
of two triplet excited molecules Ì Ì Ë Ë . The production of triplet excited molecules
is related in part to the specific ionization of the excitation particle: The higher the specific
ionization of the particle, the more triplet excited molecules are created. Two different types of
particle which produce the same intensity of the fast component will have different intensities
of the slow component. Thus it is possible with pulse shape discrimination to distinguish two
types of particles with different specific ionization, especially electrons from heavy particles
like neutrons or alpha particles. The ability to discriminate between « and ¬ or ­ radiation is
based on the difference in the tail of the time distributions of the scintillator decay, as shown
in fig. 5.2 (see also table 4.1).
5.1.4 The Flash-ADC
In collaboration with the company CAEN, a 400 MHz waveform digitizer was developed for
BOREXINO, which was first tested in the CTF2. Its basic function is to digitize the analog
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Figure 5.2: Time decay distribution of È ÈÈÇ Ð for emission excited by « or ¬
radiation.