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

4.2 The Tracking of the Scintillation Photons
probability of 0.8 (the PPO quantum efficiency). The band-gap of the lowest electronic transitions
in the PC molecule is at 320 nm, so that the absorption of PC drops rapidly at higher
wavelengths. At longer wavelengths the measured extinction coefficient obeys roughly a
dependence typical for Rayleigh scattering.
In the simulation the interaction probabilities with PC and PPO were extracted from the extinction
coefficient measurements. The interactions with PC were simulated as absorption for
wavelengths shorter than 320 nm and as Rayleigh scattering for longer wavelengths. The interactions
with PPO were modeled as absorptions for wavelengths shorter that 400 nm, and
as Rayleigh scattering for longer wavelengths. In the case of an absorption in the scintillator,
the reemission occurs with the quantum efficiency of PPO (0.8), as in the case of absorption
by PC the excitation energy is shifted non-radiatively to the PPO, just as in the primary scintillation
process. The reemission probability of the pure PC in the buffer region is only 0.34.
The reemitted fluorescence photons were modeled with an isotropic distribution and with an
emission time delay following the exponential fluorescence decay with ns for PPO
and ns for pure PC. The spectrum of the reemitted photons was assumed to be the
same as the primary emission spectrum.
The elastic Rayleigh scattering was simulated with an angular distribution of È
Ó× , with no effective time delay and no shift in the wavelength of the scattered photon.
Reflection and Refraction at Borders and Surfaces
In the CTF as well as in BOREXINO the scintillator will be È ÈÈÇ Ð , whereas
the buffer liquid is different for the two detectors: water in the CTF and È ÅÈ Ð
in BOREXINO. If the refractive index is not the same for the two media, the processes of
refraction and reflection at the interface can occur. The refraction angle at the interface can be
calculated according to Snellius’ law
×Ò
×Ò
Ò
Ò
The probability that a photon is reflected at the interface scintillator-water is given by the
Fresnel’s law, depending on the polarization of the photon,
×Ò
Ê
Ê
×Ò
ØÒ
ØÒ
As the refractive index of the scintillator ( ) is larger than that of water ( ), for
incident angles Ö ×Ò Ò Ò Æ total reflection occurs. If the scintillator/water
interface is an ideal sphere, then this totally reflected light is ‘trapped’ by a number of successive
internal reflections (as the incidence angle relative to the sphere stays the same). This
‘trapped light’ may be detected at the PMTs only after a long delay, depending on whether it
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