Background due to stored electrons following nuclear decays in the KATRIN spectrometers and its impact on the neutrino mass sensitivity

The KATRIN experiment is designed to measure the absolute neutrino mass scale with a sensitivity of 200 meV at 90% C.L. by high resolution tritium β-spectroscopy. A low background level of 10−2 counts per second (cps) at the β-decay endpoint is required in order to achieve the design sensitivity. In this paper we discuss a novel background source arising from magnetically trapped keV electrons in electrostatic retarding spectrometers. The main sources of these electrons are α-decays of the radon isotopes 219,220Rn as well as β-decays of tritium in the volume of the spectrometers. We characterize the expected background signal by extensive MC simulations and investigate the impact on the KATRIN neutrino mass sensitivity. From these results we refine design parameters for the spectrometer vacuum system and propose active background reduction methods to meet the stringent design limits for the overall background rate.