SPONSORED: RAMAN SPECTROSCOPY


Raman spectroscopy at the Tritium Laboratory Karlsruhe


The precise analysis of tritium-containing gases by Raman spectroscopy has become routine and is now used in the Katrin experiment, in process monitoring and control, and in R&D projects


T


he KArlsruhe TRItium Neutrino mass (Katrin) experiment is one of


the flagship experiments at the Karlsruhe Institute for Technology (KIT). Using high resolution spectroscopy of the β-decay kinematics of a molecular, T2


-


source, Katrin provides a model- independent access to the neutrino- mass scale with a target sensitivity of 0.2 eV (90% CL) after five years of measurement time. Katrin is hosted at the Tritium


Laboratory Karlsruhe (TLK), which provides the full infrastructure for tritium handling, i.e. preparing the required high-purity T2


gas,


transporting it for injection into a gaseous tritium source, and recuperating it from the circulation for reprocessing. T2


-gas can never be produced in


ultra-pure form as it always contains varying amounts of the other hydrogen isotopologues, at the percent level. Thus, the source gas composition needs to be monitored in real time. In particular, the relative concentrations of the species DT and HT need to be considered in the neutrino mass analysis, due to their impact on the shape and endpoint of the general β-energy spectrum. Laser Raman spectroscopy


– Lara – has been selected as the method of choice for this monitoring task, since first, it constitutes a non-invasive and fast in-line measurement technique;


and second, unambiguous identification and, with suitable calibration, quantification for all gas constituents. Within the framework of Katrin, this Lara system has to be extremely reliable and provide high measurement sensitivity for all species concentrations, with the required precision. The concept of the Katrin Lara monitoring system follows that of traditional Raman spectroscopy of gases, in which the Raman light is collected at a right angle to the laser excitation axis (i.e. 90⁰ scattering geometry). However, the setup had to specifically ascertain that the gas circulation loop of Katrin was not affected, while at the same time augmenting the sensitivity and precision achievable with conventional Lara measurement setups. In particular, the gas sampling cell had to be compliant with tritium radioactivity requirements, and the optical windows and their coatings needed to be as resistant as possible against β-radiation damage. The cell is mounted in- line in the primary loop for tritium circulation. Consequently, the cell is situated within the tritium loop glove box. Access for laser radiation and Raman light collection is through anti-reflection coated windows in a bespoke 'appendix' extension. Further modifications to standard Lara systems for gas monitoring


Raman system at the tritium gas-mixing facility


include the implementation of double-pass laser excitation; and quantitative Raman light intensity calibration, required for high- precision composition monitoring. A continuous wave (CW) green


(λL = 532 nm) DPSS laser (Laser Quantum, finesse) is utilised in the current Katrin Lara system, with its output power set to PL = 3 W. This laser was selected because it matched the stringent requirements for the Katrin Lara system: measurements needed to be sensitive and repeatable for species


“The concept of the Katrin Lara monitoring system follows that of traditional Raman spectroscopy of gases”


concentrations to a level lower than 10-3


. The finesse laser met these


requirements, with its very high power stability of <0.1% RMS. Note that the finesse pure or axiom would be suitable alternatives; both deliver high power and a very low RMS noise level of 0.03%. Furthermore, the alignment-critical setup of the cell within the glove box benefitted from the excellent long-term laser pointing stability of <2 μrad/⁰C. The Katrin Lara system is now


Raman concentration monitoring of T2 during part of a Katrin run www.electrooptics.com | @electrooptics


in nearly constant operation since early 2019, when full operation and neutrino mass data taking commenced. Individual Katrin measurement runs typically have a duration of up to three months, with short servicing periods in between. During these periods Raman data are taken continuously. Molecular concentrations are extracted from the recorded spectra in real time and transferred to the Katrin database.


An example for such data is


shown in the graph (below) for the T2


concentration. The particular


data were recorded during the challenging period of late spring 2020, during which the Lara system (and the rest of the Katrin experiment) had to run mostly unattended. From the graph it is evident that, concentration


over time, the T2


slowly changes (and that of the other isotopologues as well). This is a consequence of the dynamic fresh gas feed/used gas extraction in the loop circulation. The sudden jumps are synchronous with new gas batches provided by the TLK T2-processing facility, which may differ in isotopic composition. During the aforementioned service periods, the finesse laser was checked, and recalibrated, to continue its high level of performance (according to specifications, small drops in laser output power do occur over time). This straightforward recalibration task was carried out by a Laser Quantum engineer, via the remote access link of the laser controller. Besides the Katrin Lara system, our Raman group runs further Lara systems, at the TLK and the Universidad Autónoma de Madrid; all incorporate 532 nm models of the Laser Quantum gem and opus series. The systems are used at the tritium transfer facility for batch composition protocols; at the tritium gas-mixing facility; and for tritium gas monitoring and R&D, e.g. to follow the evolution of β-induced reactions generating molecular contaminants. All these applications require reliable laser operation over month(s) at a time. EO www.laserquantum.com


June 2021 Electro Optics 17


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