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

4.2 The Tracking of the Scintillation Photons
Figure 4.4: In the left picture the shape of the BOREXINO concentrators is shown. In the right
picture the angular acceptance of this cone shape is shown.
Ñ Ñ ¢ reflectivity 92 % ) [Obe01]. The shape and the angular acceptance of
the light concentrators are shown in fig. 4.4.
PMT Quantum Efficiency, Time Jitter and Counting Statistics
The PMTs used in the CTF as well as in BOREXINO are the 8” Thorn EMI 9351 photomultipliers.
Their characteristics are: high quantum efficiency (26 % at 420 nm), limited transit time
spread ( ns), good pulse height resolution for single photoelectron pulses (peak/valley
2.5), low dark noise rate (1 kHz), low after pulse probability (2.5 %), gain . The quantum
efficiency of the photo cathode of the PMT is shown in fig. 4.5. A typical pulse height distribution
of single photoelectron pulses and the transit time distribution is shown in fig. 4.6 for one
of the PMTs used in BOREXINO. In the simulation, if a photon hits a PMT, the probability for
areflection at the water-glass interface is calculated. If the photon is not reflected, it creates a
photoelectron with a probability according to the PMT quantum efficiency. The pulse height
signal is calculated according to a Poisson distribution. For the time signal, the transit time of
the PMT has to be taken into account. The mean transit time is 4.5 ns, with a Gaussian time
jitter of 1 ns.
The Tracking Program
The data flow of the tracking program is outlined here for BOREXINO (the tracking in the CTF
is analogous):
1. The tracking algorithm starts with the generation of a scintillation photon at the position
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