Setup of a Drift Tube Muon Tracker and Calibration of Muon ...

decays which usually show a different time profile of the emitted light in organicscintillators. A detailed description of the scintillator can be found in [68].High transparency and similar optical properties to those of the scintillator aredesired for the Buffer. However, it should not produce a lot of scintillation light byitself. The buffer is also based on PC. This has the advantage that the density ofthe fluids are equal and no buoyancy forces occur inside the nylon vessels. About5g/l of dimethylphtalate (DMP) is added to the PC as a light quencher to preventgammas emanating from the PMTs from reaching the target and to further suppressscintillation processes in the buffer thus acting as a passive shielding. This way, theouter vessel (OV) allows scintillation light originating inside the target area to travelthrough without generating a lot of signals itself. The buffer is divided into two partsby the second nylon vessel. This acts as a barrier mainly for radon emmanating fromthe PMTs.4.2.3 Radio PurityA high radio purity is essential for detecting low energetic neutrinos in Borexino.Borexino expects a rate of the order of a few ten counts per day for 100 tons ofliquid scintillator from the interaction of sub-MeV solar neutrinos above 200 keV.This corresponds to an equivalent activity of the order of 10 −9 Bq/kg. Radioactivebackground within the scintillator mainly occurs from the decays of 238 U and 232 Thdaughters as well as 40 K. Also, nitrogen used to remove oxygen from the scintillatoris contaminated, mainly with noble gases like 222 Rn, 39 Ar and 85 Kr. The goal wasto reduce the abundances of radioactive contaminants to a level where the rate oftheir decays is well below the expected neutrino rate. Namely, this corresponds torequired abundances of < 10 −16 g/g for 238 U and 232 Th and < 10 −14 g/g of 40 Kwithin the scintillator. Nitrogen used to flush the detector has to provide a radiopurity of 0.36 ppm for Ar and 0.16 ppt for Kr, corresponding to approximately 1count per day in the detector. This limit is also set for events caused by external γradiation [68].The actual cleanliness of the scintillator even surpasses these requirements. Theabundances for 238 U have been found to be (1.6 ± 0.1) × 10 −17 g/g, those of 232 Thhave been measured as (6.8 ±1.3) ×10 −18 g/g [69]. However, even small abundancesof radioactive elements have to be considered. Their contribution and ways to treatthem are presented in the next section.4.3 BackgroundIn a low rate experiment like Borexino, a special care has to be given to all sourcesof background. Background for neutrino events mainly occurs from two sources:natural radioactivity and cosmic radiation. The background signals can disturb theneutrino measurements in two ways: On the one hand, decays with Q values ofthe order of the neutrinos energy mimic neutrino events and therefore distort themeasurement, and on the other hand, if the background events exceed a certainrate, the detector is blinded. The effect of radioactive decays can be minimized byobtaining a high radio purity of all materials involved and by having an effectiveshielding against all external radiation. The flux from cosmic radiation, especially57