The Borexino Detector

Borexino is a solar neutrino experiment at the Laboratori Nazionali del Gran Sasso, in Italy, designed to detect low-energy solar neutrinos, in real time, using 300 tons of liquid scintillator in an unsegmented detector. The detector is located in Hall C of the Gran Sasso underground laboratory. Neutrino-electron scattering in the scintillator produces flashes of scintillation light which are observed by 2000 photomultiplier tubes (20 cm diameter). With a fiducial volume of about 100 tons, the expected neutrino count rate is about 30 events per day above 250 keV (Standard Solar Model and Large Mixing Angle neutrino oscillation model, assuming vacuum oscillations), due mostly to ⁷Be solar neutrinos.

The emphasis in Borexino is on the ultimate in radiopurity, for the energies where the ⁷Be neutrino signals occur are filled with false positives from many different species of radioactive decay. In particular, the uranium and thorium chains are great problems, as well as the radioactive noble gas isotopes ³⁹Ar, ⁸⁵Kr, and ²²²Rn, all present in the Earth's atmosphere. Numerous methods of getting rid of these isotopes have been used, ranging from careful materials selection, through construction of the most delicate parts of the detector in a radon-free clean room, up to a several month sequence of purging the entire detector with special nitrogen free of argon, krypton and radon.

Structure of Borexino

A schematic of the detector is shown at right. The concentric volumes of the detector are based on the principle of graded shielding: the cleanest parts are at the very center. First, of course, the underground location of the detector, under 3500 meters of water equivalent, reduces the cosmic ray flux to a fraction of its value at the surface. Next, a shield of ultra-pure water protects internal parts of the detector from neutrons and gamma rays, emitted by radioactive decays in the rock walls of the laboratory.

Within the ultra-pure water is a stainless steel sphere (SSS) that acts as a support structure for > 2000 PMTs. About 200 of these are located in the volume of water; they act as a muon detector by observing the tracks of Cherenkov light left by muons passing through. The remainder are on the inner surface of the SSS, pointed inward.

The scintillator and its containment vessels

Two of the main contributions of the Princeton group to the experiment are the majority of the scintillator purification plants, and the scintillator containment vessels. Because the scintillator and its containment vessels are the most sensitive parts of the detector in terms of radiopurity requirements, great care was taken in the design and manufacture of both components.

Inside the SSS are three more concentric spherical volumes, separated by the two thin nylon membranes built in the Princeton University Physics Department clean room, under a radon-purged atmosphere. The outer two volumes are filled with buffer fluid. This acts as a passive buffer to increase the distance between the PMTs (which themselves are quite radioactive by our standards) and the active part of the detector. The outer nylon vessel in addition acts as a barrier to prevent radon atoms emanating from the PMTs and SSS from migrating inward. Both vessels have quite an elaborate architecture to provide for structural support and intrumentation to monitor the parameters of the detector, including vessel shapes and positions, differential pressures, and fluid temperatures. A detailed description of the vessels has been published by Nuclear Instrumentation and Methods A, and may also be read at the arXiv.

The scintillator itself is pseudocumene (also called 1,2,4-trimethylbenzene), an organic compound similar to benzene, with the addition of 1.5 grams/liter of 2,5-diphenyloxazole, a fluor. It is purified through techniques of nitrogen stripping and distillation. It is this material in which neutrino interactions with its electrons yield scintillation light. The maximum possible kinetic energy transferred to an electron by an 0.86 MeV ⁷Be solar neutrino is 0.66 MeV. At this energy, roughly 7000 scintillation photons will be produced, of which only about 330 will be detected.

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