  The KATRIN experiment will determine the effective electron anti-neutrino
mass with a sensitivity of 200 meV/c$^2$ at 90% CL. The energy analysis of
tritium $\beta$-decay electrons will be performed by a tandem setup of
electrostatic retarding spectrometers which have to be operated at very low
background levels of $<10^{-2}$ counts per second. This benchmark rate can be
exceeded by background processes resulting from the emanation of single
$^{219,220}$Rn atoms from the inner spectrometer surface and an array of
non-evaporable getter strips used as main vacuum pump. Here we report on a the
impact of a cryogenic technique to reduce this radon-induced background in
electrostatic spectrometers. It is based on installing a liquid nitrogen cooled
copper baffle in the spectrometer pump port to block the direct line of sight
between the getter pump, which is the main source of $^{219}$Rn, and the
sensitive flux tube volume. This cold surface traps a large fraction of
emanated radon atoms in a region outside of the active flux tube, preventing
background there. We outline important baffle design criteria to maximize the
efficiency for the adsorption of radon atoms, describe the baffle implemented
at the KATIRN Pre-Spectrometer test set-up, and report on its initial
performance in suppressing radon-induced background.
