| AVS 62nd International Symposium & Exhibition | |
| Vacuum Technology | Tuesday Sessions |
| Session VT-TuA |
| Session: | Gas Dynamics and Modeling, Pumping and Outgassing |
| Presenter: | Joachim Wolf, Karlsruhe Institut for Technology (KIT), Germany |
| Correspondent: | Click to Email |
The objective of the Karlsruhe Tritium Neutrino experiment (KATRIN) at the Karlsruhe Institute of Technology (KIT) is the measurement of the electron neutrino mass. A central component is the Main Spectrometer (MS), where the energy of the β-electrons from tritium decay (18.6 keV) will be measured with high precision. It consists of a large ultra-high-vacuum vessel with a volume of 1240 m3. The pumping system of the MS consists of turbo-molecular pumps, a large-scale getter pump (3000 m NEG strips, St707) and three cryo-baffles at LN2 temperature, designed to maintain an ultimate pressure in the range of 10-11 mbar.
The NEG strips, as well as the stainless steel walls are known to emanate small amounts of radon atoms, increasing the intrinsic background rate, which would limit the sensitivity for the neutrino mass. The cryogenic copper baffles are expected to capture most of the radon, before it decays in the main volume. However, radon does no not stick to the cold surface indefinitely. There are two possibilities, it either desorbs after a limited residence time (depends on desorption enthalpy and baffle temperature), or it decays into polonium. In the first case, it can contribute again to the background rate.
This work reports on radon measurements with cold baffles at various temperatures and compares the results with Test-Particle Monte-Carlo (TPMC) simulations. The simulation was performed with a modified MOLFLOW+ code, where we added two new, time-dependent features, (i) a finite residence time for all adsorbing surfaces, and (ii) the half-life of the test particles. For the measurements we used two different radon isotopes with a half-life of 4 s and 56 s, respectively. By comparing measured rates with TPMC simulations for different residence times, we want to learn more about the surface conditions of the baffles (Cu, Cu2O, H2O) and the corresponding desorption enthalpies.
This work has been supported by the German BMBF (05A14VK2).