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

3.3 Results from the CTF1
All the structure is immersed in 1000 tons of deionized water contained within a carbon steel
tank (11 m diameter, 10 m high), whose inside walls are coated with a black epoxy resin in
order to minimize light reflections from the tank walls. The water provides shielding from the
external ­-ray background coming from the PMTs, the steel, and the surrounding rocks.
Fig. 3.2 shows the data acquisition logic. The 100 photomultiplier outputs are processed in
64 electronic channels, 72 PMTs are fanned together in pairs into 36 channels, and 28 PMTs
are read as single channels. Each electronic channel has a time-to-digital-converter (TDC) and
an analog-to-digital-converter (ADC). The trigger is provided by a Majority Logic Unit. The
trigger condition is set so that a certain number of PMTs (usually 6) must receive a photon
within a time window of 30 ns. This trigger condition is necessary because each PMT has a
dark noise of approximately 1 kHz. For each event the charge and timing of the hit phototubes
are recorded. To avoid missing fast time correlated events, each processing channel is doubled
by a second channel to record events coming within a time window of 8 ms after the first event.
The delay time is measured with a precision of 1 ns. For longer correlation times, the internal
clock of the computer is used (with a resolution of s). The PMT analog signals are also
summed up to provide the total charge and the charge integrated over the pulse tail (with a
delay of 32 ns and 48 ns), to discriminate between alpha and beta events, and to record the
pulse shape of each event by a waveform digitizer. The time calibration of the PMT signals is
done with a pulsed laser (pulse width 50 ps) with a wavelength of 410 nm, which is coupled to
a bunch of 100 optical fibers leading to each PMT. The intensity of the laser signal is attenuated
so that each PMTs receives photoelectron from each laser pulse.
3.3 Results from the CTF1
From March 1995 until August 1996 the scintillator È ÈÈÇ Ð was tested in the
CTF. With the CTF the scintillator radiopurity was measured at previously unachieved levels.
It is the largest nuclear detector ever built with sensitivity down to the sub 100 keV spectral
regime; and with a background of 0.03 events kg keV yr in the 0.25 - 2.5 MeV energy
window [Ali98c] it is the detector with the lowest background activity in this regime, even
better than detectors searching for neutrino-less double beta decay.
The C content in the scintillator was determined by normalizing the CTF low energy spectrum
(50 - 150 keV) to the spectral shape expected from the decay of C (see fig. 3.3), giving
a value of
¦ Ali98b℄
The concentration of Rn and Th in the scintillator was determined by tagging the correlated
decays of their short-lived daughters Po (Ø ×) and Po
(Ø Ò×). Assuming secular equilibrium in the decay chains, this activity can be ex-
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