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Abstract
Borexino is an experiment for low energy (<1 MeV) solar neutrino spectroscopy approaching completion at the underground Gran Sasso laboratories in Italy. It is specifically designed to measure in real time the flux of mono-energetic {\berillium} neutrinos produced by fusion reactions in the Sun. Its 300-ton liquid scintillator target is contained in an 8.5 meter diameter nylon inner vessel (IV) and is surrounded by 1000 tons of buffer fluid. A second, 11.5 meter diameter concentric nylon outer vessel (OV) around the IV serves as a barrier for radon emanated at the periphery of the detector.
Borexino requires unprecedented low levels of radioactive impurities to be a success (∼1 background event/day in the central 100-tons of scintillator). The IV, which is in direct contact with the scintillator, also has to meet extremely stringent radioactive and cleanliness requirements. Intrinsic levels ∼10 −12 g/g for U and Th and ∼10−8 g/g for K are needed.
The vessels, assembled in a clean room in Princeton, made of a 125 micron-thick membrane, need to be leak tight at the 10−2 cc/s and 1 cc/s level for the IV and OV respectively, and have to withstand mechanical stresses due to density differences and temperature gradients between the fluids they contain. Their requirements and assembly process are presented in detail. An upper limit on the inner vessel leak rate of 10−3 cc/s was measured. The performance of a matrix of light sources, placed on both vessels for monitoring its shape with digital cameras, is demonstrated.
The problem of surface contamination by radon in the air is extensively addressed, strategies for minimizing it are analysed and the effectiveness of their application evaluated. In particular, an original radon filter based on vacuum swing adsorption on activated charcoal has been developed for use in connection with the clean room. Such a technique yielded radon abatement factors in excess of 104 in a small-scale prototype, and ∼100 in the final system.
Finally, trace scintillator radioactivity data are reported from CTF3, a counting test facility for Borexino now in its third data-taking phase. An intrinsic 14C isotopic contamination of the scintillator ∼5 × 10−18 has been measured. Upper limits of ∼3.5 × 10−16 g/g on {\uranium} contamination and of ∼10 −4 Bq/ton for the 85Kr residual activity have also been set. (Abstract shortened by UMI.)
