Background simulations and detector design for the KATRIN experiment

The Karlsruhe Tritium Neutrino Experiment (KATRIN) aims to measure directly the mass of the neutrino, an open question in physics. The experiment will measure the integrated tritium beta-decay electron energy spectrum near the 18.6 keV endpoint, where the shape is most sensitive to neutrino mass. In order to reach the proposed sensitivity to neutrino mass of 0.2 eV (90% CL) the detector backgrounds in the endpoint energy region must be limited to 1 mHz. Extensive Geant4 simulations of the KATRIN detector region have identified the largest contributions to the background and guided the detector design. For this experiment, at the surface of the earth, the major backgrounds will be cosmic ray induced photons as well as betas and high-energy gammas from natural radioactivity. Cosmic rays and their secondaries can be vetoed with an active shield. Careful material selection can reduce natural radioactivity, and a passive shield can mitigate radioactivity-induced backgrounds. Post-acceleration of electrons emerging from the KATRIN spectrometer can raise the signal energy to lower background regions. Armed with an understanding of the major background mechanisms, the detector design has been optimized to reduce the total background to the 1 mHz goal.