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

2.3 Detector Design
the CTF for radioactive impurities and for optical properties and then stored in three 120 m
stainless steel storage vessels until filling of the BOREXINO detector. The raw material for the
PC may come from different petroleum sources, which can cause variability in the C level.
The CTF will be used to test the C level to assure that the scintillator is appropriate for BO-
REXINO. The fluor PPO will be purified off-line as concentrated solution in pseudocumene.
The existing CTF purification plant will be modified to process the fluor solution, by a combination
of water extraction and distillation. The concentrated fluor solution will be stored in
two 4 m vessels until it is added to the solvent during the filling operation to a concentration
of 1.5 g/l. The PC used for shielding will be delivered and purified on-line for direct filling of
the buffer. The DMP is added on-line to a concentration of 5 g/l .
Water Purification System
Water is a major shielding component for both BOREXINO and the CTF, attenuating
the gamma ray flux from the rock surrounding hall C. The design purity goal is about
Í in the CTF and about Í in BOREXINO (where additional shielding
is provided by the PC buffer). The water purification system consists of a purification
system for raw water from the rock, and a recirculation loop to maintain the purity of the water.
The water is processed by reverse osmosis, a continuous deionization unit, ultra filtration,
an ion exchanger and a final nitrogen stripping to remove Rn. For details see [Bal96].
Nitrogen System
Nitrogen gas is used in BOREXINO as a purge gas to remove gaseous contaminations ( Rn,
Kr) from the scintillator and buffer fluids as well as a buffer gas against recontamination with
atmospheric air. Two qualities of gaseous nitrogen with the following Rn-purity are provided
by the nitrogen plant: regular nitrogen (RN , 1 mBq/m ) and high purity nitrogen (HPN ,
10 ÕÑ ). The primary source of the nitrogen will be liquid nitrogen commercially delivered
to the LNGS and stored in three 6 m tanks. The regular nitrogen is produced by direct
evaporation of the liquid. The high purity nitrogen is obtained from two high purity charcoal
traps operated at Æ in line with the liquid flow from the storage tanks to a special evaporator
and heater. The high purity line is fabricated of electropolished stainless steel to maintain
the Rn-purity level. For a detailed description of the system see [Rau99, Heu00].
Electronics and Data Acquisition
In order to get the information on time, position and energy of an event, the time and pulse
height information of each PMT are needed. The basic block diagram of the electronics is
shown in fig. 2.8. The PMTs will work mainly in the single photoelectron mode (events in
the neutrino window have pulse heights between 50 and 300 photoelectrons), but can also
give very large signals due to cosmic rays. The PMT signals arrive at the Front End Board,
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