The Karlsruhe Tritium Neutrino (KATRIN) experiment aims at measuring the effective electron neutrino mass with a sensitivity of 0.2 eV/c2, 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 cx for all six hydrogen isotopologues were monitored simultaneously, with a measurement precision for individual components of the order 10-3 or better throughout the complete KATRIN data taking campaigns to date. From these, the tritium purity, εT, is derived with precision of <10-3 and trueness of <3 × 10-3, being within and surpassing the actual requirements for KATRIN, respectively.

Center for Experimental Nuclear Physics and Astrophysics and Department of Physics University of Washington Seattle WA 98195 USA

Departamento de Química Física Aplicada Universidad Autónoma de Madrid Campus de Cantoblanco 28049 Madrid Spain

Department of Electrical Engineering and Information Technology University of Applied Sciences Fulda Leipziger Str 123 36037 Fulda Germany

Department of Physics and Astronomy University of North Carolina Chapel Hill NC 27599 USA

Department of Physics Carnegie Mellon University Pittsburgh PA 15213 USA

Department of Physics Faculty of Mathematics and Natural Sciences University of Wuppertal Gaußstr 20 42119 Wuppertal Germany

Department of Physics Technische Universität München James Franck Str 1 85748 Garching Germany

Institut für Kernphysik Westfälische Wilhelms Universität Münster Wilhelm Klemm Str 9 48149 Münster Germany

Institut für Physik Humboldt Universität zu Berlin Newtonstr 15 12489 Berlin Germany

Institute for Data Processing and Electronics Hermann von Helmholtz Platz 1 76344 Eggenstein Leopoldshafen Germany

Institute for Nuclear and Particle Astrophysics and Nuclear Science Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA

Institute for Nuclear Physics Hermann von Helmholtz Platz 1 76344 Eggenstein Leopoldshafen Germany

Institute for Nuclear Research of Russian Academy of Sciences 60th October Anniversary Prospect 7a 117312 Moscow Russia

Institute of Experimental Particle Physics Wolfgang Gaede Str 1 76131 Karlsruhe Germany

IRFU CEA Université Paris Saclay 91191 Gif sur Yvette France

Laboratory for Nuclear Science Massachusetts Institute of Technology 77 Massachusetts Ave Cambridge MA 02139 USA

Max Planck Institut für Kernphysik Saupfercheckweg 1 69117 Heidelberg Germany

Max Planck Institut für Physik Föhringer Ring 6 80805 München Germany

Nuclear Physics Institute of the CAS v v i CZ 250 68 Husinec Řež Czech Republic

Project Process and Quality Management Hermann von Helmholtz Platz 1 76344 Eggenstein Leopoldshafen Germany

Triangle Universities Nuclear Laboratory Durham NC 27708 USA

Tritium Laboratory Karlsruhe Hermann von Helmholtz Platz 1 76344 Eggenstein Leopoldshafen Germany

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