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

4.1.3 Čerenkov LightČerenkov detectors are well suited for the detection of high energetic charged particles.Relativistic particles traveling through a medium with refractive index n at aspeed faster than the speed of light within this medium (β ≥ c n) generate light in aconical shape. It is emitted at an angle αcos α = 1βn .Since only particles with high velocities produce Čerenkov radiation, such a detectorcan only be used for high energetic particles. Applying photo multipliers around thedetection volume allows to observe the Čerenkov light. A particle whose track iscontained in the target can be identified by a light ring whereas a passing particleproduces a disk like shape. Shape and dimensions of these structures can be usedto obtain information on the direction and energy of the particle. The Borexinoexperiment is surrounded by a large water tank acting both as a passive shieldingagainst external background sources as well as a Čerenkov veto for cosmic muons.The signals recorded in this tank are also used for muon tracking. Čerenkov light isnot only emitted in the water tank, it also occurs when high energetic particles likecosmic muons cross the scintillator.4.1.4 Background in Liquid Scintillator DetectorsElastic scattering and inverse β decay offer excellent detection methods of low energeticneutrinos in liquid scintillators. Unfortunately, their signatures can be mimickedby different sources of background. These originate either from radioactivecontaminations within the scintillator or the surrounding materials, or they arecaused by external radiation penetrating the detector.The dominating internal background source is 14 C which is intrinsic to the scintillator.14 C decays through a β − decay and has a half-life of 5730 years. Its naturalabundance on earth is of the order of 10 −12 . The β − decay has an end point energyof 156.475 keV. Despite the small abundance of 14 C, neutrino detection below thisthreshold is not possible.The decay of 14 C is not the only challenge when building a liquid scintillatordetector. Since it is virtually impossible to create a 100% pure scintillator, allbackground created by natural radioactivity and the abundances of radioactive elementshave to be understood and effectively reduced. The main internal backgroundsources besides 14 C are 238 U, 232 Th and 40 K as well as 222 Rn and its daughters.Furthermore, muons from the cosmic radiation create radionuclides by spallationprocesses with long half-lives. Despite the Čerenkov muon veto, the decay of theseradionclides cannot be simply vetoed since they occur long after the incident particlehas passed the detector. Due to the statistic process of radioactive decays, thetime after which the decay takes place cannot be directly correlated to the muon.The most prominet example for this cosmogenic background in liquid scintillatoris the production of 11 C, which is a β + emitter with a half-life of approximately20 minutes. As will be shown in Section 4.3, a high radio purity has been achievedfor Borexino. It will also be shown how cosmogenic events can mostly be controlled.54