| Summary: | The Karlsruhe Tritium Neutrino (KATRIN) experiment aims at measuring the effective electron neutrino mass with a sensitivity of 0.2 eV/c<sup>2</sup>, i.e., improving on previous measurements by an order of magnitude. Neutrino mass data taking with KATRIN commenced in early 2019, and after only a few weeks of data recording, analysis of these data showed the success of KATRIN, improving on the known neutrino mass limit by a factor of about two. This success very much could be ascribed to the fact that most of the system components met, or even surpassed, the required specifications during long-term operation. Here, we report on the performance of the laser Raman (LARA) monitoring system which provides continuous high-precision information on the gas composition injected into the experiment’s windowless gaseous tritium source (WGTS), specifically on its isotopic purity of tritium—one of the key parameters required in the derivation of the electron neutrino mass. The concentrations <i>c</i><sub>x</sub> for all six hydrogen isotopologues were monitored simultaneously, with a measurement precision for individual components of the order 10<sup>−3</sup> or better throughout the complete KATRIN data taking campaigns to date. From these, the tritium purity, <i>ε</i><sub>T</sub>, is derived with precision of <10<sup>−</sup><sup>3</sup> and trueness of <3 × 10<sup>−3</sup>, being within and surpassing the actual requirements for KATRIN, respectively.
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