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

7 The Source Runs in CTF2
7.1 Motivation
At the end of the PXE campaign in CTF2, in September 2000, a series of calibration measurements
was performed using a Rn-source. The source consisted of a small quartz vial filled
with Rn spiked scintillator. Positioning the Rn-source at various locations in the Inner
Vessel, the homogeneity of the energy response of the detector and systematic shifts of the
position reconstruction could be tested as well as the dependence of the energy and position
resolution on position, energy and kind of decay.
Rn offers several advantages as a radioactive source material, including the ease with which
it can be detected. The decay of the short lived Rn-daughters Bi (¬) and Po («) occurs
with a half life of 164 s, so that these events can be tagged very efficiently using a cut on the
energy of the Po decay and the coincidence time. The relatively short 3.8 day half life of
Rn is also advantageous. This would allow its decay in the unfortunate case that a source
contaminated the detector.
As introducing a radioactive source bears the risk of contaminating the scintillator these measurements
could be done only at the end of the PXE campaign though for a better understanding
of the detector performance it would be wishful for BOREXINO to do some calibration
measurements with a localized source before the solar neutrino data taking starts. For the
planned calibration strategy for BOREXINO see e.g. [Man00].
7.2 Production of the Sources
For the calibration measurements a few ml of liquid scintillator loaded with a Rn-activity of
0.1 – 1 Bq are needed. A lower activity would lead to a worse signal/background ratio (background
comes from Rn homogeneously distributed in the scintillator originating from a
U/ Ra-contamination of the scintillator itself or introduced with air together with the
sources), whereas a higher activity would result in a high deadtime of the detector (the data
acquisition of the CTF2 needs 110 ms for the read-out of one event, so that for rates higher
than a few Hz there is a significant dead time) and an increase of the background by accidental
coincidences. The Rn-loaded scintillator is contained in small quartz vials. Quartz is UV
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