Fig. 1: Illustration of the 70-m-long KATRIN beamline.

The main components are labelled. The transport of β-electrons and magnetic adiabatic collimation of their momenta p are illustrated. a–f, View into the tritium source depicts three systematic effects: molecular excitations during β-decay (a), scattering of electrons off the gas molecules (b) and spatial distribution of the electric potential in the source U_src(r, z) (c). The view into the spectrometer illustrates the main background processes arising from radon decays inside the volume of the spectrometer^47,48,49 (d), highly excited Rydberg atoms sputtered off from the structural material via α-decays of ²¹⁰Po (refs. ^44,45,46) (e) and positive ions created in a Penning trap between the two spectrometers⁵⁷ (f). Low-energy electrons, created in the volume as a consequence of radon decays or Rydberg-atom ionizations, can be accelerated by qU_ana towards the focal-plane detector, making them indistinguishable from signal electrons⁶⁶.