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

7 The Source Runs in CTF2
7.4 Analysis of the Source Runs
7.4.1 Event Selection
To select the Po alpha decay events, which originated from the Rn source, the following
cuts were applied:
- they are detected by the second electronic group;
- they have a coincidence time delay of 10 s-1ms;
- they have a pulse height in the range Ô Õ Ô ( Ô Õ Ô for
source no. 2);
- the preceding group 1 event ( Bi) has a pulse height between 100 and 1200 pe (the
lower energy threshold is set to reject accidental coincidences involving C events);
- there is no muon flag on this event or the preceding group 1 event.
With these cuts, the Po alphas and the preceding Bi betas can be tagged very efficiently
with negligible background. Their energy spectra are shown in fig. 7.6. Fitting an exponential
to the coincidence time distribution reproduces very well the decay time of Po,
Ø × ÐÒ
Ø
¡ ×
while the fit gives ¡ × (see fig. 7.6, lower plot).
Background
The two sources of potential background events are Bi- Po events in the bulk scintillator
volume and accidental coincidences:
- The background due to Rn present in the scintillator containment vessel can be estimated
to be at about the same level as in the previous runs before the source insertion,
counts per hour. Due to the fact that the scintillator inside the source vial was
quenched and the energy of the Po alpha peak shifted towards lower energies, the
actual background contribution is even smaller.
- The accidental background can be estimated by applying the energy cuts mentioned
above for the group 1 and group 2 events, but taking a coincidence time window shifted
to much greater delay times (7010 - 8000 s). For run 797 this gives an accidental background
rate of counts per hour.
Compared to the source activity of 200 - 800 counts per hour (for source 1 and 2 respectively)
both background contributions are negligible.
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