The Katrin experiment and the pre-spectrometer at reduced retarding potential

The KArlsruhe TRItium Neutrino experiment, KATRIN[21] will determine the mass of the ν ¯ e with a sensitivity of 0.2 eV (90% C.L.) via a measurement of the β -spectrum of gaseous tritium near its endpoint of 18.57 keV. An ultra-low background of a few mHz and an energy resolution of 0.93 eV are among the requirements to reach the sensitivity. These demands are fulfilled with the main spectrometer being of the MAC-E filter type and which is equipped with a 650 m 2 , two-layered and quasi-massless wire electrode presented in this article. We also report on a recent background related test experiment: the investigation of the possibility to reduce the retarding voltage of the pre-spectrometer in order to diminish a Penning trap leading to increased background.