Background investigations with passive transverse energy filters at KATRIN

The measurement of the absolute mass scale of neutrinos with an unprecedented sensitivity of 0.2 eV/c2 is the key goal of the KATRIN experiment. This requires a detailed understanding of background processes in the large main spectrometer. Currently, the measured background level exceeds the design value by more than one order. An initial model assigned background events to originate from Rydberg H-states generated by the decay of traces of surface-implanted Pb-210. Highly-excited Rydberg states from the inner spectrometer surface are long-lived and can be ionised by thermal radiation. The resulting low-energy electrons on the meV-scale are then accelerated by the retarding potential, thus they only possess a very small transverse energy, which is in contrast to signal electrons. In a first step we have performed measurements with a passive transverse energy filter (pTEF) implemented as a micro-structured honeycomb gold plate. In this talk we present the measured transmission of background electrons through the pTEF and compare results at different magnetic field values with the initial and refined background models.
This work is supported by the Helmholtz Association (HGF), the Ministry for Education and Research BMBF (05A17PM3, 05A17PX3, 05A17VK2, and 05A17WO3), the Helmholtz Alliance for Astroparticle Physics (HAP), and the Helmholtz Young Investigator Group (VH-NG-1055).