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The KATRIN experiment will determine the effective electron anti-neutrinomass with a sensitivity of 200 meV/c$^2$ at 90% CL. The energy analysis oftritium $\beta$-decay electrons will be performed by a tandem setup ofelectrostatic retarding spectrometers which have to be operated at very lowbackground levels of $<10^{-2}$ counts per second. This benchmark rate can beexceeded by background processes resulting from the emanation of single$^{219,220}$Rn atoms from the inner spectrometer surface and an array ofnon-evaporable getter strips used as main vacuum pump. Here we report on a theimpact of a cryogenic technique to reduce this radon-induced background inelectrostatic spectrometers. It is based on installing a liquid nitrogen cooledcopper baffle in the spectrometer pump port to block the direct line of sightbetween the getter pump, which is the main source of $^{219}$Rn, and thesensitive flux tube volume. This cold surface traps a large fraction ofemanated radon atoms in a region outside of the active flux tube, preventingbackground there. We outline important baffle design criteria to maximize theefficiency for the adsorption of radon atoms, describe the baffle implementedat the KATIRN Pre-Spectrometer test set-up, and report on its initialperformance in suppressing radon-induced background.

Impact of a cryogenic baffle system on the suppression of radon-induced background in the KATRIN Pre-Spectrometer