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

4 Position Reconstruction of Scintillation Events
eventually escapes due to imperfections of the Inner Vessel (IV) shape and material. This creates
unpredictable distortions in the energy spectrum and the time distributions, and therefore
has negative effects on position reconstruction (fiducial volume) and pulse shape discrimination.
The exact behaviour is hard to predict by simulations, since the shape of the vessel would
have to be known very accurately.
While in the CTF the inside tank walls are coated with a black epoxy with a very low reflectivity,
in BOREXINO reflections from the surface of the stainless steel sphere have to be taken
into account. The diffuse reflectivity of passivated stainless steel has been measured with a
photo spectrometer using an integrating sphere. The wavelength dependence of the diffuse
reflectivity for two different surface treatments of the stainless steel is shown in fig. 4.3. For
the BOREXINO steel sphere, the passivated surface has been chosen.
Reflections at the Stainless Steel Sphere are simulated according to Lambert’s law (reflection
of rays on rough surfaces)
Ï Ó× ª Ó× ×Ò
For incident angles greater than 80 Æ mirror reflection is assumed.
reflectivity (%)
45
40
35
30
25
20
15
10
5
passivated stainless steel
darkened stainless steel (Polygrat)
250 300 350 400 450 500
wavelength (nm)
Figure 4.3: The diffuse reflectivity of passivated stainless steel and darkened stainless steel. The
BOREXINO Stainless Steel Sphere has a passivated surface.
Light Concentrators
The shape of the concentrators is designed to produce a uniform response to events inside the
IV with a misalignment tolerance of 5 degrees. The angular acceptance of the concentrators
drops rapidly for angles greater than 36 degrees, i.e. for events at a radius bigger than 5 m.
The concentrators for BOREXINO are made of high purity aluminum. The reflectivity of aluminum
is 92 % at 400 nm. The amplification factor of the concentrators has been measured
to be ¦ , which is very close to the optimal amplification factor (geometrical factor
50