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Free keywords:
solar neutrinos; BOREXINO experiment; radon emanation; radium; proportional counters
Abstract:
BOREXINO is a large-scale liquid scintillator detector for solar neutrinos with a low expected event rate, therefore requiring an extremely low radioactive background. In this paper we present a method, which makes it possible to measure the surface and bulk activities of Ra-226 in thin nylon foils for use in BOREXINO at unprecedented concentration levels, as low as 0.5 muBq/m(2) and 10 muBq/kg, respectively. It is based on high-sensitivity Rn-222 emanation detection, taking advantage of the fact that the permeability of nylon for radon is strongly enhanced in the presence of water. A mathematical diffusion model is outlined to distinguish the surface and bulk activity of Ra-226 from the Rn-222 emanation rates of dry and wet film. The experimental method is described in connection with screening measurements of nylon foils of different origin. One foil could be identified with a low Ra-226 contamination (corresponding to similar to10(-12) g/g U-238-equivalent) for application in BOREXINO. This film will be used for the construction of a liquid scintillator containment vessel as the central part of the BOREXINO solar neutrino detector. (C) 2002 Elsevier Science B.V. All rights reserved.
