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Accurate calibration of the laser Raman system for the Karlsruhe Tritium Neutrino Experiment

Accurate calibration of the laser Raman system for the Karlsruhe Tritium Neutrino Experiment

Magnus Schlösser, Simone Rupp, Hendrik Seitz, Sebastian Fischer, Beate Bornschein, Tim M. James, Helmut H. Telle 


Journal of Molecular Structure, in press, Available online, doi:10.1016/j.molstruc.2012.11.022,

Date: 23 November 2012

The Karlsruhe Tritium Neutrino (KATRIN) experiment aims to measure the neutrino mass via high-precision electron spectroscopy of the tritium β-decay with a sensitivity of mν = 200 meV/c2 (90% C.L.). This can only be achieved if systematic uncertainties are minimized. An important parameter is the isotopic composition of the tritium gas used as the gaseous β-electron source, which is measured inline by Raman spectroscopy. The KATRIN experiment requires a measurement trueness of better than 10% of said composition; to achieve this, accurate calibration of the Raman system for all hydrogen isotopologues (H2, HD, D2, HT, DT, T2) is required. Here we present two independent calibration methods, namely (i) a gas sampling technique, which promises high accuracy, but which is difficult to apply to tritiated species; and (ii) an approach via theoretical Raman signals (theoretical intensities plus spectral sensitivity), which in principle includes all six isotopologues. For the latter method we incorporated ab initio off-diagonal matrix elements of the polarizability from the literature; these have been verified by depolarization measurements. The system’s spectral sensitivity was determined by a NIST-traceable SRM2242 luminescence standard. Both methods exhibited their individual merits and difficulties, but in cross calibration proved to be successful: a comparison for the non-radioactive isotopologues (H2, HD, D2) yielded agreement to better than 2% for the relative Raman response function. This is within the estimated (dominant) uncertainty of the theoretical Raman signal approach of about 3%. Therefore, one can be confident that, when using this approach, the trueness requirement of 10% for the KATRIN-relevant species (T2, DT, D2 and HT) will in all likelihood be exceeded..