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

4.3 The Reconstruction Program
detector, the time delay introduced by scattering and/or absorption-reemission processes, and
the time jitter of the PMT. The probability density function for a single photoelectron Ø is
derived from a Monte Carlo simulation of the detector and archived in a look-up table. For the
CTF, it is shown in fig. 4.7.
Multiple Hits
In the CTF the data acquisition registers only the arrival time of the first photoelectron for
each electronic channel while the total number of photoelectrons per channel arriving within
a time window of a few hundred nano seconds is recorded with an ADC. In BOREXINO the
arrival time of each photoelectron will be recorded, if they arrive with at least 140 ns distance.
In BOREXINO the probability for multiple hits is greatly reduced compared to the CTF, as
each PMT covers a smaller solid angle. In the CTF, for a 1 MeV event the average value is 3
hits per PMT, while in BOREXINO it will be only 0.2 hits per PMT.
The probability for PMT , that the first one out of Ò photoelectrons has been registered at the
time Ø can be calculated to be
Ô ØÒ Ò ¡ Ø ¡
Ø
Ò
where Ø is derived from the measured time Ø in the following way:
Ø Ìevent Ø emission Ø flight Øscatter ¦ Ø jitter
Ìevent Ø min Ü Ý Þ Ø
Ø Ìevent Ø min Ü Ý Þ Ø
Ø Ø Ø Ø min Ü Ý Þ Ø Ø min Ü Ý Þ
The Ø min Ü Ý Þ are archived in a large multidimensional look-up table for each grid point
and PMT (they are derived from the Monte Carlo simulation, which was described in the last
section).
Noise Hits
The problem of (white) noise hits, which occur uncorrelated with the scintillation event, is
equivalent to a probability density constant in time. This is already solved by substituting
Ø with the maximum probability Å Ø . More correctly one has to add to Ø
and renormalize afterwards. In practice this is equivalent to an additional constant bias to
ÐÓ ÄÆØ× ¡ ÐÓ , which thus only slightly affects the minimization process. For a noise
rate of 1 kHz one gets ¡ Ò× .
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