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

6 Test of a PXE based scintillator for Borexino
flash ADC board with the analog sum of all PMT signals was tested during some runs of the
CTF2 (see chapter 5).
In order to determine the efficiency of the «¬ discrimination a clean sample of « and ¬ events
in the same energy range are needed. The only events that can be unambiguously identified as
« or ¬ decays are the delayed coincidences from the Bi(¬)- Po(«) decay. I selected these
events from all the 4 ton runs with the same selection criteria as for the determination of the
U contamination. Further I restricted the sample to Bi events having energies in the same
range as the Po alpha events: 200 - 400 photoelectrons for the Po alpha events and 100 -
500 photoelectrons for the Bi beta events.
The efficiency of the «¬ discrimination was determined by fitting a gaussian to the distribution
of rØÓØ for « and ¬ events (see fig. 6.11). The ¬ identification efficiency was then
fixed at 98 % and the corresponding « identification efficiency was calculated to be 84.6 %.
Applying a radial cut on the Bi and Po events of Ö cm, yields for an « identification
efficiency of 92.4 %. This improvement is due to the fact, that for events in the outermost
region of the Inner Vessel, where total internal reflection plays an important role, the fraction
of light registered at late times, is higher than in the inner regions of the Inner Vessel.
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Figure 6.11: «¬ discrimination capability of PXE. Plotted is the tail-to-total ratio r ØÓØ of
Bi beta events (solid line) and Po alpha events (dashed line). For a ¬ identification efficiency
of 98.0 %, the fit yields an « identification efficiency of 84.6 %.