Calibration of a Laser-Raman-System using gas samples of all hydrogen isotopologues for KATRIN

Abstract

The Karlsruhe Tritium Neutrino (KATRIN) experiment aims to measure the effective electron anti-neutrino mass via high-precision spectroscopy of the energy spectrum of the β-electrons from tritium decay near the 18.6 keV endpoint. In order to achieve the design sensitivity of m(νₑ) = 0.2 eV (90% C.L.), KATRIN uses a high-luminosity Windowless Gaseous Tritium Source (WGTS), which is temperature and activity stabilised on the part-per-mille level. Due to technical reasons, the molecular T₂ inside the WGTS will always contain residues of other hydrogen isotopologues (H₂, HD, D₂, HT and DT). As the gas composition influences the shape of the β-spectrum, it must be considered in the neutrino mass analysis and hence continuously monitored. At the Tritium Laboratory Karlsruhe (TLK), tritium-compatible Laser Raman systems (LARA) were developed in order to meet the performance requirements of KATRIN. For quantitative composition analysis, the system- and isotopologue-specific response function must be obtained. The main focus of this work is the direct experimental validation of the KATRIN-relevant calibration factors for the radioactive isotopologues T₂, HT and DT. This was done by the design and construction of the Tritium Hydrogen Deuterium (TRIHYDE) experiment, which is capable of providing accurate gas samples of all six hydrogen isotopologues in chemical equilibrium, using a manometric method. Detailed investigations of the initial sample purity and the reaction kinematics of the radio-induced self-equilibration, e.g. T₂ + HD $\rightleftharpoons$ HT + DT, were carried out. It was shown that the experimentally derived calibration factors and theoretical values based on ab initio calculations agree within 2% for all six isotopologues; thus validating the already excellent accuracy of the source gas composition monitoring of the WGTS. In addition, a double fold reduction of the calibration uncertainty for the homonuclear isotopologues was achieved. In summary, using the TRIHYDE facility it was possible, for the first time, to prepare accurate gas samples of all six hydrogen isotopologues with tritium content on a technical scale, thus giving a valuable tool to further expand tritium analytics to in-situ calibration, characterisation and development of existing and forthcoming methods.