Simulation and measurement of the suppression of radon induced background in the KATRIN experiment

Short-lived radon isotopes, such as ²¹⁹Rn or ²²⁰Rn, are a serious source of background for the measurement of the neu-trino mass with the KATRIN experiment. Most of the radon emanates from the main vacuum pumps of the KATRIN Main Spec-trometer, which consist of 2000 m of Non-Evaporable Getter (NEG) strips. This paper describes a method to suppress the radon rate with liquid-nitrogen-cooled baffles in front of the NEG-pumps in the ultra-high vacuum chamber and compares simulations with measured data. The effectiveness of the method depends both on the half-life of the radon isotopes, and on the temperature of the cryogenic baffles, which affects their sojourn time on the cold surface. The measurements with the Main Spectrometer showed that the radon suppression with cold baffles works sufficiently well, so that the remaining background is no longer dominated by radon decays.