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

Figure 4.4: Principle of the three fold coincidence for the detection of cosmogenicbackground (Figure from [70]). If both the muon track and the position of the neutroncapture are reconstructed, events within the intersection of a cylinder around themuon track and a sphere around the neutron can be vetoed for a few half-lives of thecosmogenic induced radioisotope.since the low-energetic part of the cosmic muon spectrum is absorbed on its waythrough the rock.Muons crossing the IV of the Borexino detector typically deposit hundreds ofMeV of ionization energy in the detector and are thus easily discriminated fromthe solar neutrino signals. Special care has to be given to muons crossing onlythe buffer in the ID without passing through the IV. The light produced throughboth the Čerenkov effect in the buffer and from the remaining scintillation givesa significant signal despite the quenching effect of the DMP. Unfortunately, thesesignals can mimic the desired neutrino events. It is therefore necessary to effectivelytag and veto these muons.An even larger challenge are muon-induced cosmogenic events caused by spallation.The most prominent process is the knock-out of a neutron from the 12 C beingone of the main constituents of the scintillator. The remaining 11 C decays via a β +with a half-life of 20.39 minutes and a Q value of 1982.2 keV. The emitted positronthen almost instantly annihilates thus depositing a visible energy between ≈ 1 and2MeV in the detector which cannot be correlated to the incident muon anymore.Unfortunately, this signal overlays the energy range of CNO neutrinos and thereforehinders their detection. The expected rate for Borexino is 25 counts per day anddominates the CNO signal by approximately a factor of five [71].To suppress the cosmogenic background, the so-called three fold coincidence isused [72]. The neutron knocked out by the muons will be captured by a hydrogenatom after a typical time of ∼ 250ñs, which will then de-exite and release a 2.2 MeVphoton. This can be correlated to the crossing muon. Also, the position of theneutron capture can be reconstructed. Since its mean free path is rather short59