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

4. If the photon reaches a surface:
4.2 The Tracking of the Scintillation Photons
- Inner Vessel Radius, 425 cm (only for water buffer): Reflection? Refraction? Calculate
new direction (Ù Ù ), index of refraction in the new medium, and continue
with 2.
- Radius of PMTs with cones, 635 cm: Cone aperture hit? Is it reflected by the cone?
If it is accepted, proceed to the PMT. Is it reflected on the surface? If not, go to 5,
otherwise continue with 2.
- Radius of PMTs without cones, 655 cm: PMT hit? If yes, is it reflected? If not,
goto 5, otherwise calculate reflection angle and continue with 2.
- Stainless Steel Sphere, 685 cm: absorption or reflection on stainless steel?
5. PMT hit: the photon is converted into a photoelectron according to the photocathode
efficiency. Calculate the time jitter and the pulse height signal. If the pulse height is
above the threshold, store the time of the signal.
When the grid for the reconstruction is computed, the program loops over a spatial grid, where
a fixed number of photons starts from each grid point. In the end, only the time of the first
photoelectron of each PMT is stored for each grid point. This simulation has to be done with
a very high statistics, in order to get the earliest possible arrival time of a photon at each PMT
Ø min Ü Ý Þ .
The probability density function for a single photoelectron Ø is derived from a Monte Carlo
simulation, where a number of photons starts from the center of the detector (so that the distance
to all PMTs is the same) and is stored in a separate file. This probability density function
reflects the decay time of the primary scintillation light emission, additional delays introduced
through absorption and reemission, scattering, reflection, etc. of the photon on its way to the
PMT, and the time jitter of the PMT. In principle, this distribution also depends on the distance
between the primary emission point and each individual PMT, but in the first order it is the
same for all positions inside the scintillator. Fig. 4.7 shows the averaged probability density
function for the CTF for two different positions, in the center and in a distance of 94 cm from
the center (corrected for the time of flight to the PMTs). There are notable differences only
for late arrival times, which do not provide good information for the position reconstruction,
anyway.
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