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Summary
An important measure to achieve in Borexino the required radio-purity was careful material screening with very sensitive devices. For this purpose the world’s most sensitive gamma ray spectrometer was developed (detection limit of <10 micro_Bq/kg for U/Th). Even more important is the elimination of Rn-222 sources, because Rn-222 is a relatively long-lived radioactive noble gas isotope which may diffuse into the fiducial volume of the liquid scintillator. A screening technique dedicated to the measurement of Rn-222 with few atom sensitivity will be discussed along with results obtained from various subsystems of Borexino. Studies of very weak radon diffusion/permeation through the Borexino scintillator vessel will also be outlined. One of the crucial points in the experiment was the radio-purity of gases used in Borexino. For continuous monitoring of their radon level we developed a ∼ 400 liter radon detector based on electrostatic collection of charged radon daughters. We also built a plant which produces permanently almost Rn-222-free nitrogen (less than 1 atom in 4 m3 at STP) at high flow-rate and we successfully searched for nitrogen with a particularly low argon (<ppb) and krypton (<0.1 ppt) concentrations. We also designed and realized cryogenic system maintaining the purity an making possible to deliver it to the experimental site. This is the radio-purest gas used by any experiment ever. Production of synthetic air with a very low Rn-222 level by mixing oxygen with radon-free nitrogen will also be briefly described.
