  The KArlsruhe TRItium Neutrino (KATRIN) experiment is a next generation,
model independent, large scale tritium beta-decay experiment to determine the
effective electron anti-neutrino mass by investigating the kinematics of
tritium beta-decay with a sensitivity of 200 meV/c2 using the MAC-E filter
technique. In order to reach this sensitivity, a low background level of 0.01
counts per second (cps) is required. This paper describes how the decay of
radon in a MAC-E filter generates background events, based on measurements
performed at the KATRIN pre-spectrometer test setup. Radon (Rn) atoms, which
emanate from materials inside the vacuum region of the KATRIN spectrometers,
are able to penetrate deep into the magnetic flux tube so that the alpha-decay
of Rn contributes to the background. Of particular importance are electrons
emitted in processes accompanying the Rn alpha-decay, such as shake-off,
internal conversion of excited levels in the Rn daughter atoms and Auger
electrons. While low-energy electrons (< 100 eV) directly contribute to the
background in the signal region, higher energy electrons can be stored
magnetically inside the volume of the spectrometer. Depending on their initial
energy, they are able to create thousands of secondary electrons via subsequent
ionization processes with residual gas molecules and, since the detector is not
able to distinguish these secondary electrons from the signal electrons, an
increased background rate over an extended period of time is generated.
