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

Abstract

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 m ν = 200 meV/c 2 (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 (H 2 , HD, D 2 , HT, DT, T 2) 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

Accurate depolarization ratio measurements for all diatomic hydrogen isotopologues

“…Therefore, we can conclude that indeed the corrected depolarization ratios of the hydrogen isotopologues agree with the theoretical values of LeRoy to a 1σ confidence level. This also means that we can be rather confident that in future calibration measurements of the KATRIN LARA system, the theoretical values derived from LeRoy's ab initio calculations can be used as input parameters in the line strength functions, with the 1σ uncertainties of the depolarization ratio measurements propagated through to the polarizability tensors.…”

“…Measured (observed raw data) r obs and corrected r SP0SA-cor depolarization ratios for Q 1 (J" = 0) of all hydrogen isotopologues, for l = 532 nm laser excitation. The errors in the last digits are given in brackets H 2 HD HT D 2 DT T 2 r obs 0.01048 (4) 0.01153 (1) 0.00601 (5) 0.01137 (2) 0.02291 (5) 0.00339 (24) r SP0SA-cor 0.00000 (60) 0.00000 (50) 0.00000 (60) 0.00000 (50) 0.00000 (60) À0.00300 (1000) the KATRIN LARA system [20] , the theoretical values derived from LeRoy's ab initio calculations can be used as input parameters in the line strength functions, with the 1s uncertainties of the depolarization ratio measurements propagated through to the polarizability tensors.…”

“…The corrected depolarization ratios of the hydrogen isotopologues have been found to agree with the theoretical values of LeRoy to better than 5% for nearly all of the measured Q 1 (J”) lines, with 1σ confidence level. This enables one to use the theoretical polarizability tensor elements derived by LeRoy as input parameters in the line strength functions for future calibration measurements of the KATRIN LARA system . Equally, such data should be invaluable for the accurate analysis of the hydrogen isotopologue‐composition in other contexts: it has been suggested that Raman spectroscopy could become a key methodology for the accountancy of tritium in the fusion fuel cycle of ITER…”

“…This enables one to use the theoretical polarizability tensor elements derived by LeRoy as input parameters in the line strength functions for future calibration measurements of the KATRIN LARA system. [20] Equally, such data should be invaluable for the accurate analysis of the hydrogen isotopologue-composition in other contexts: it has been suggested that Raman spectroscopy could become a key methodology for the accountancy of tritium in the fusion fuel cycle of ITER. [21] We also compared our measured and corrected depolarization values with other experimental values from the literature.…”

First operation of the KATRIN experiment with tritium

“…In order to maintain a constant gas flow, an equivalent small amount of DT-D 2 gas mixture was injected into the loop from the buffer vessel. At all times the gas composition was monitored by a Laser Raman spectroscopy system [20,21]. The gas circulation was maintained without interruption for the 13 days, which was the complete duration of the FT campaign.…”