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

muons, is reduced by going deep underground. However, a complete shielding againstbackground is not possible and it has thus to be studied thoroughly. This sectiongives a brief overview of background sources in Borexino and describes how they canbe discriminated. Special care is given to the uncorrelated cosmogenic backgroundinduced by muons.4.3.1 RadioactivityNatural radioactive elements are intrinsic to both the scintillator and the surroundingmaterials. The most prominent background source in Borexino is the naturalabundance of 14 C in organic materials and hence in the scintillator. At the Earth’ssurface, a 14 C has a relative abundance to 12 C of approximately 1.2 × 10 −12 g/g.However, this concentration is already significantly reduced due to the fact that thescintillator is based on a mineral oil that has been formed underneath the Earth’ssurface during a long time compared to the 14 C half-life of 5730 years. In Borexinoa 14 C cleanliness of the order of 10 −18 compared to 12 C has been reached, resultingin a rate of 3.5×10 4 counts per day and ton. While decays of 14 C limit the detectionof neutrinos to energies above the 14 C end point energy of ∼156 keV, their numberis reduced sufficiently to not constantly blind the detector.Further intrinsic radioactivity arises mainly from the decay chains of 238 U and232 Th, as well as 40 K. Special care also has to given to the presence of 222 Rn. It is anoble gas formed in the 238 U decay chain. In Hall C of the LNGS laboratory, radonactivities around 80Bq/m 3 are measured. It is hence utterly important to preventany contact between the air and the scintillator. The same is true to prevent acontamination of the scintillator with 85 Kr, which has a Q value of 690 keV, a valueclose to the expected Compton edge from the 7 Be neutrinos. Its content in thescintillator can be probed through the rare decay sequence 85 Kr → 85 Rb ∗ + e + + ν ewith the delayed coincidence of 85 Rb ∗ → 85 Rb + γ. The activity of 85 Kr has thusbeen estimated to be 29 ± 14 counts per day and 100 tons.4.3.2 NeutronsNeutrons knocked out of the surrounding materials by cosmic muons are a seriousthreat to the Borexino measurements. Neutrons created by muons passing next tothe detector are particularly critical, because their parent cannot be identified andvetoed. High energy neutrons may travel a distance of a few meters through differentmaterials. Through elastic scattering, the neutron loses its energy to protons andthus leaves a signal in the detector. If it is thermalized within the scintillator, adelayed 2.2 MeV γ will be released from the neutron capture on Hydrogen. Thissignature can mimic the signal from the inverse β decay.4.3.3 Cosmogenic BackgroundThe Borexino experiment is well shielded from cosmic radiation by ∼ 3800 m.w.e.of rock. The cosmic muon flux in Hall C of the LNGS is reduced by almost 6orders of magnitude to ∼ 1.2µ/m 2 /h compared to the flux on the Earth’s surface(∼ 6.5 ×10 5 µ/m 2 /h). However, this still corresponds to approximately 4300 muonscrossing the detector each day. The mean energy of these muons is about 320 GeV,58