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

4.2 The Tracking of the Scintillation Photons
4.2 The Tracking of the Scintillation Photons
In the Monte Carlo simulation each photon is tracked from the place where it was produced
to the place where it is absorbed or detected. The propagation of the photon in the transparent
media of the detector (scintillator, water) is described by optical laws. In the tracking procedure
the scintillation decay, the absorption and reemission of photons, the light scattering,
the reflection or refraction on the border of different media, the reflection on the light guide
surface and the photo multiplier time jitter are taken into account.
The Scintillation Process
Ionizing radiation passing through the organic scintillator transfers energy to the scintillator
molecules. A fraction of this energy goes into the excitation of scintillator molecules which
relax back to their ground state and sometimes emit light in this process, the remainder is
dissipated nonradiatively. Due to the decay into lower energy states of the excited scintillator
molecules the generation of scintillation photons is delayed a time Ø. The distribution of this
delay is described by an exponential probability density function Ø where the parameter
is the scintillation decay time. Generally the scintillator has more than one component,
where each component is characterized by its decay time and its intensity Õ. The different
components are due to different excited states of the molecules, which will be explained in
more detail in chapter 5. The probability density function is
Ø Õ Õ
Ø
Õ
Figure 4.1: Normalized emission spectra of PC and È ÈÈÇ Ð .
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