The Karlsruhe Tritium Neutrino experiment (KATRIN) is a large vacuum system with an overall length of about 70 m which aims to measure the electron neutrino mass from the beta-decay of tritium with unprecedented sensitivity of 0.2 eV/c2. An international collaboration is constructing the experiment in the European Tritium Laboratory on the site of Forschungszentrum Karlsruhe, Germany.
It is a next generation tritium decay experiment scaling up the size and precision of previous experiments by an order of magnitude. Ultrafine spectrometric analysis of the energy distribution of the decay electrons at the endpoint of the spectrum at 18.57 keV is the key to derive the neutrino mass.
Most of the decays take place in the 10 m long tube of the gaseous tritium source, where a permanent tritium gas flow provides a constant decay rate. While the electrons are magnetically guided to the spectrometer section, tritium gas has to be significantly reduced along the beamline before entering the spectrometer. This will be achieved by means of a modular differential pumping system, comprising turbomolecular and cryogenic pumping sections, with an expected overall flow rate reduction in the order 1010 . In order to avoid any negative influence of residual gas on the background rate, the large main spectrometer vessel (1250 m3) and the attached silicon detector are operated under UHV/XHV conditions, aiming at an ultimate total pressure of below 10-9 Pa and a wall outgassing rate below 10-13 Pam3/scm2.
The paper starts with a description of the experimental idea behind KATRIN. The emphasis will then be on details of the pumping concept for how to achieve the target values in terms of tritium flow rate reduction and ultimate pressures. Achievements are highlighted and critical issues are discussed. The manufacturing of most KATRIN components is under way, some of them are already installed on site. Thus, the paper completes with an overview of the status of the KATRIN vacuum system components. The start of the measurements is expected for 2010.
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