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

2 The Borexino Experiment
2.3 Detector Design
2.3.1 The Main Detector
The BOREXINO detector is being constructed in hall C of the Gran Sasso underground laboratory
(LNGS) in Italy. The laboratory is shielded by an overburden of 3600 mwe which leads
to a suppression of the cosmic muon flux by a factor of , down to a value of Ñ .
The detector is designed in such a way as to reduce the external gamma ray background in
the central fiducial volume to a level well below the expected solar neutrino signal. A detailed
description of the BOREXINO detector can be found in [Ali01]. A sketch of the detector is
shown in fig. 2.6.
The active part of the detector is an unsegmented volume of 300 tons of liquid scintillator,
contained in a thin (0.1 mm) spherical nylon vessel of diameter 8.5 m (Inner Vessel). The
scintillator consists of the solvent pseudocumene (PC, 1,2,4 trimethylbenzene) with 1.5 g/l
PPO (3,5 diphenyloxazole) added as fluor.
The scintillation light is observed by 2200 8” diameter photomultiplier tubes (PMTs) mounted
on the inside surface of a stainless steel sphere with a diameter of 13.7 m. 1800 PMTs are
equipped with light concentrators for an efficient collection of the scintillation light from the
Inner Vessel. Signals from the 400 remaining PMTs without concentrators will be used for
distinguishing muon tracks producing Cerenkov light in the buffer region from events inside
the scintillator. The three dimensional arrangement of the photomultipliers allows the position
of the neutrino interactions within the detector volume to be reconstructed using the timing
information of the triggered PMTs. Thus, a fiducial volume of 100 tons of scintillator (radius
3 m) can be defined by software cuts to reduce the external background.
The volume between the Inner Vessel and the stainless steel sphere is filled with 1000 m of a
buffer liquid which serves as shielding against radioactive background from the outer detector
parts. This buffer liquid has to fulfill several requirements: it should be optically transparent to
the fluorescence emission of PPO (i.e. it should not absorb the scintillation light from the active
scintillator volume), and it should not emit fluorescence light itself, as this would lead to very
high counting rates mainly due to the background from the PMTs. Ideally, the refractive index
of the buffer should match that of the scintillator. The density of the buffer liquid should be
very close to the density of the scintillator to minimize the buoyancy-induced stress on the thin
nylon vessel (thickness only 0.1 mm). The buffer liquid was chosen to be pseudocumene with
the addition of 5 g/l DMP (dimethylphthalate) as a quencher to suppress the fluorescence in the
buffer PC by a factor of 20 [Che99]. A second nylon barrier with a diameter of 10.5 m should
prevent the Rn atoms emanating from the steel, PMTs and concentrators from getting close
to the active scintillator volume.
The external steel tank with a height of 17 m and 18 m diameter filled with deionized water
provides a final shield against gamma rays and neutrons from the surrounding rocks. The
thickness of the water buffer is 2 m, except at the bottom where it is only 1 m. Steel plates
under the floor of the tank provide the equivalent of 1 m of water shielding. 200 phototubes
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