Raman spectroscopy for KATRIN: monitoring of continuous tritium gas flows and tritium-graphene interactions

The KATRIN experiment aims to measure the effective electron anti-neutrino mass with an unprecedented sensitivity of 200 meV by examining the shape of the energy spectrum of the electrons emitted during the β-decay of molecular tritium near the kinematic endpoint of 18.6 keV. In order for the KATRIN experiment to reach its ultimate goal, all sources of systematic uncertainties need to be understood and quantified.

One of these contributions stems from the source gas compositions in the Windowless Gaseous Tritium Source (WGTS). An ideal source would contain only pure tritium, T$_2$. Due to technical reasons, the source gas will always contain residues of other hydrogen isotopologues (DT, D$_2$, HT, HD, and H$_2$).
For the monitoring of the tritium gas composition, a Laser Raman (LARA) system is employed at the KATRIN experiment. The suitability of the KATRIN-LARA system for the accurate gas composition measurement has been demonstrated in studies for over a decade. However, the final demonstration that the LARA system can achieve the metrological performance (trueness, precision, and accuracy) continuously and reliably over KATRIN’s full operation time of several years, can only truly be demonstrated with the passing of operation time. ... mehr

One particular aspect of the LARA system is the intensity calibration using a Standard Reference Material (SRM), supplied by National Institute of Standards and Technology (NIST); its stability over the period of several years had yet to be demonstrated. This topic is addressed in this work, and it was shown that the LARA system could be operated over the more than five years of the KNM1-11 campaigns thus far, within the bounds of the KATRIN requirements. On average, the achieved metrological performance surpassed the KATRIN requirements by at least a factor of 1.5.

Beyond the state-of-the-art KATRIN molecular tritium source, atomic tritium sources are a subject of increasing scientific interest. In addition to gaseous atomic tritium sources, tritium chemically bound to graphene has been suggested as a candidate for (quasi)-atomic solid-state tritium sources and targets.
In the research work presented here, samples of graphene-monolayer on a SiO$_2$/Si substrate were tritiated, for the first time. For this, chemisorption of tritium atoms was exploited, which were generated naturally (via β-decay of T$_2$ and subsequent thermalization of the daughter tritium atoms, T, in the gas environment). Two methods were employed for the monitoring of the successful tritiation of graphene, namely (i) in-situ sheet resistance measurements using the Van der Pauw method; and (ii) ex-situ spatially-resolved sample characterization, using a Confocal Raman Microscope (CRM). This CRM instrument was designed, set up and characterized in collaboration with partners from the Universidad Autónoma de Madrid. It was demonstrated that the CRM, in its current configuration, was well suited for first, semi-quantitative investigations of tritium-exposed graphene samples, with a total sample activity of up to 1×10$^{10}$ Bq.

In summary, the work described in this thesis advances the use and understanding of Raman spectroscopy for the KATRIN experiment. For one, the successful long-term operation of the tritium gas composition monitoring of the molecular KATRIN source was demonstrated. Second, a new tritium-compatible Raman system, i.e., a confocal Raman microscope was established. This was successfully used for spatio-chemical studies of radioactive / tritiated solid-state tritium sources / targets, initially using graphene as the carrier material.