Paper VT-TuM6
Simulation and Measurement of the Tritium Retention in the Beamline of the KATRIN Experiment

Tuesday, October 23, 2018, 9:40 am, Room 203B

The objective of the KATRIN neutrino experiment is the determination the effective mass of electron anti-neutrinos with an unprecedented sensitivity of 0.2 eV/c² by measuring the energy spectrum of b-electrons from tritium decays close to the endpoint of the spectrum. The decays take place in a high-intensity windowless gaseous tritium source (WGTS). The electrons are guided by strong magnetic fields through a beamline with super-conducting solenoids to the huge main spectrometer for energy measurement. Since only a fraction of 2·10^-13 of all electrons have a kinetic energy within 1 eV below the spectral endpoint, the sensitivity of the measurement depends on a low background rate in the spectrometer. Therefore the tritium flow from the source (10^-3 mbar) has to be reduced by at least 14 orders of magnitude before it reaches the spectrometer section. This large reduction in the 90-mm-diameter beamline is achieved with a combination of a differential pumping section (DPS), using turbo-molecular pumps (TMP) and a cryogenic pumping section (CPS) with an argon frost layer at around 3 K.

This talk describes the Test Particle Monte Carlo (TPMC) simulation of the beamline for the differential and cryogenic pumping sections both for the non-radioactive gas deuterium and for tritium with MolFlow+. Since the gas flow through the CPS is reduced by far more than 7 orders of magnitude, the simulation is done in several consecutive steps. The results are combined in the post processing of the TPMC results. In the post processing algorithm we also investigate the time dependence of the reduction factor by considering a finite sojourn time of the molecules on the cryogenic argon layer. This leads to a slow migration of deuterium in downstream direction. In a final step the half-life and migration of tritium is taken into account by a reduced sojourn time.