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

4 Position Reconstruction of Scintillation Events
4.4 Results Obtained with Simulated Data
Monte Carlo for Alpha, Beta and Gamma events
To study the behaviour of the reconstruction code depending on the position, energy and decay
type, I have simulated energy deposits from «, ¬ and ­ decays at different positions in the
BOREXINO and CTF detector.
The simulation uses the scintillation decay times measured for different types of particle excitation
(see table 4.1). The photon yield of the scintillator is assumed to be photons/MeV.
This number reproduces approximately the photoelectron yield measured in the CTF (
photoelectrons/MeV). The energy deposits of « and ¬ particles are assumed to be pointlike.
The quenching of the alpha energies is included as measured in laboratory (see table 5.1). The
energy of the « or ¬ decay is ‘translated’ into the corresponding number of photons which
start at the point of the energy deposit with a time delay according to the scintillation decay
time, and are tracked throughout the detector as described in section 4.2.
The interaction of ­-rays in the scintillator is simulated including Compton scattering and
photo effect. Pair production plays no significant role in the energy range of interest (
MeV). The ­ travels through the detector with the speed of light (index of refraction ).
At each interaction point, the number of photons corresponding to the energy of the scattered
electron, start with a time delay according to the excitation by ¬ particles, and are tracked
throughout the detector.
Energy Dependence of the Spatial Resolution for Different Particles
The approximation of the resolution of the spatial reconstruction for a point-like event is given
by
¡Ö Ø
Ô ÒÔ
where is the speed of light, Ø is the error of one time measurement and ÒÔ is the number of
measurements (= number of detected photons).
The Monte Carlo simulation for the CTF leads to the following results, which are plotted in
fig. 4.9:
For beta events the Monte Carlo gives an energy dependence of the spatial resolution as
Ü
Ñ
Ô ÒÔ
Ñ
Ô ÅÎ
assuming a photoelectron yield of 300 photoelectrons per MeV in the CTF.
For alpha events the spatial resolution is slightly worse, as more scintillation light is emitted
at late times. The energy dependence is still approximately Ô ÒÔ.
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