Demonstration of tritium adsorption on graphene.

In this work, we report on studies of graphene exposed to tritium gas in a controlled environment. The single layer graphene on a SiO2/Si substrate was exposed to 400 mbar of T2, for a total time of ∼55 h. The resistivity of the graphene sample was measured in situ during tritium exposure using the van der Pauw method. We found that the sheet resistance increases by three orders of magnitude during the exposure, suggesting significant chemisorption of tritium. After exposure, the samples were characterised ex situ via spatio-chemical mapping with a confocal Raman microscope, to study the effect of tritium on the graphene structure (tritiation yielding T-graphene), as well as the homogeneity of modifications across the whole area of the graphene film. The Raman spectra after tritium exposure were comparable to previously observed results in hydrogen-loading experiments, carried out by other groups. By thermal annealing we also could demonstrate, using Raman spectral analysis, that the structural changes were largely reversible. Considering all observations, we conclude that the graphene film was at least partially tritiated during the tritium exposure, and that the graphene film by and large withstands the bombardment by electrons from the ß-decay of tritium, as well as by energetic primary and secondary ions.

Versatile Confocal Raman Imaging Microscope Built from Off-the-Shelf Opto-Mechanical Components.

Confocal Raman microscopic (CRM) imaging has evolved to become a key tool for spatially resolved, compositional analysis and imaging, down to the µm-scale, and nowadays one may choose between numerous commercial instruments. That notwithstanding, situations may arise which exclude the use of a commercial instrument, e.g., if the analysis involves toxic or radioactive samples/environments; one may not wish to render an expensive instrument unusable for other uses, due to contamination. Therefore, custom-designed CRM instrumentation-being adaptable to hazardous conditions and providing operational flexibility-may be beneficial. Here, we describe a CRM setup, which is constructed nearly in its entirety from off-the-shelf optomechanical and optical components. The original aim was to develop a CRM suitable for the investigation of samples exposed to tritium. For increased flexibility, the CRM system incorporates optical fiber coupling to both the Raman excitation laser and the spectrometer. Lateral raster scans and axial profiling of samples are facilitated by the use of a motorized xyz-translation assembly. Besides the description of the construction and alignment of the CRM system, we also provide (i) the experimental evaluation of system performance (such as, e.g., spatial resolution) and (ii) examples of Raman raster maps and axial profiles of selected thin-film samples (such as, e.g., graphene sheets).

Accurate Reference Gas Mixtures Containing Tritiated Molecules: Their Production and Raman-Based Analysis.

Highly accurate, quantitative analyses of mixtures of hydrogen isotopologues-both the stable species, H2, D2, and HD, and the radioactive species, T2, HT, and DT-are of great importance in fields as diverse as deuterium-tritium fusion, neutrino mass measurements using tritium ß-decay, or for photonuclear experiments in which hydrogen-deuterium targets are used. In this publication we describe a production, handling, and analysis facility capable of fabricating well-defined gas samples, which may contain any of the stable and radioactive hydrogen isotopologues, with sub-percent accuracy for the relative species concentrations. The production is based on precise manometric gas mixing of H2, D2, and T2. The heteronuclear isotopologues HD, HT, and DT are generated via controlled, in-line catalytic reaction or by ß-induced self-equilibration, respectively. The analysis was carried out using an in-line intensity- and wavelength-calibrated Raman spectroscopy system. This allows for continuous monitoring of the composition of the circulating gas during the self-equilibration or catalytic evolution phases. During all procedures, effects, such as exchange reactions with wall materials, were considered with care. Together with measurement statistics, these and other systematic effects were included in the determination of composition uncertainties of the generated reference gas samples. Measurement and calibration accuracy at the level of 1% was achieved.

Custom-built light-pipe cell for high-resolution infrared absorption spectroscopy of tritiated water vapor and other hazardous gases.

We present a new custom-built cell for high-resolution absorption spectroscopy of hazardous gases. The use of an aluminum light-pipe enables sensitive detection due to the small tube diameter and an increased particle density in the interaction volume for a limited analyte amount in the cell, while avoiding additional surfaces such as mirrors. To demonstrate this, we have used the cell to measure tritiated water isotopologues (HTO and traces of T2O) for which spectroscopic data is scarce, due to the challenge of performing spectroscopy of these highly radio-chemical aggressive substances. For this purpose, the new cell also features the efficient inline-production of tritiated water. In this paper we present the concept of the light-pipe cell and demonstrate its performance with a high-resolution absorption spectrum of gaseous HTO generated inside of this cell.

Radio-frequency ion deflector for mass separation.

Electrostatic cylindrical deflectors act as energy analyzer for ion beams. In this article, we present that by imposing of a radio-frequency modulation on the deflecting electric field, the ion transmission becomes mass dependent. By the choice of the appropriate frequency, amplitude, and phase, the deflector can be used as mass filter. The basic concept of the new instrument as well as simple mathematic relations are described. These calculations and further numerical simulations show that a mass sensitivity is achievable. Furthermore, we demonstrate the proof-of-principle in experimental measurements, compare the results to those of from a 1 m linear time-of-flight spectrometer, and comment on the mass resolution of the method. Finally, some potential applications are indicated.

Relative Intensity Correction of Raman Systems with National Institute of Standards and Technology Standard Reference Material 2242 in 90°-Scattering Geometry.

The U.S. National Institute of Standards and Technology (NIST) has certified a set of Standard Reference Materials (SRMs) that can be used to accurately determine the spectral sensitivity of Raman spectrometers. These solid-state reference sources offer benefits such as exact reproduction of Raman sampling geometry, simple implementation, and long-term stability. However, a serious drawback of these SRMs is that they are certified only in the backscattering (180°) configuration. In this study, we investigated if and how SRM 2242 (applicable for 532 nm) can be employed in a 90°-scattering geometry Raman system. We found that the measurement procedure needs to be modified to comply with the certified uncertainty provided by NIST. This requires a change in the SRM illumination that is possible only if we polish the side surfaces. In addition, we need to account for the polarization configuration of the Raman system by choosing the appropriate polarization of the excitation beam. On top of that, the spatial inhomogeneity of the luminescence light needs to be taken into account, as well as its behavior while traveling through the SRM bulk. Finally, we show in a round-robin test that the resulting uncertainty for the quantification of Raman spectra using the modified technique is on the order of ±1.5 percentage points.

Automated quantitative spectroscopic analysis combining background subtraction, cosmic ray removal, and peak fitting.

An integrated concept for post-acquisition spectrum analysis was developed for in-line (real-time) and off-line applications that preserves absolute spectral quantification; after the initializing parameter setup, only minimal user intervention is required. This spectral evaluation suite is composed of a sequence of tasks specifically addressing cosmic ray removal, background subtraction, and peak analysis and fitting, together with the treatment of two-dimensional charge-coupled device array data. One may use any of the individual steps on their own, or may exclude steps from the chain if so desired. For the background treatment, the canonical rolling-circle filter (RCF) algorithm was adopted, but it was coupled with a Savitzky-Golay filtering step on the locus-array generated from a single RCF pass. This novel only-two-parameter procedure vastly improves on the RCF's deficiency to overestimate the baseline level in spectra with broad peak features. The peak analysis routine developed here is an only-two-parameter (amplitude and position) fitting algorithm that relies on numerical line shape profiles rather than on analytical functions. The overall analysis chain was programmed in National Instrument's LabVIEW; this software allows for easy incorporation of this spectrum analysis suite into any LabVIEW-managed instrument control, data-acquisition environment, or both. The strength of the individual tasks and the integrated program sequence are demonstrated for the analysis of a wide range of (although not necessarily limited to) Raman spectra of varying complexity and exhibiting nonanalytical line profiles. In comparison to other analysis algorithms and functions, our new approach for background subtraction, peak analysis, and fitting returned vastly improved quantitative results, even for "hidden" details in the spectra, in particular, for nonanalytical line profiles. All software is available for download.

In-line calibration of Raman systems for analysis of gas mixtures of hydrogen isotopologues with sub-percent accuracy.

Highly accurate, in-line, and real-time composition measurements of gases are mandatory in many processing applications. The quantitative analysis of mixtures of hydrogen isotopologues (H2, D2, T2, HD, HT, and DT) is of high importance in such fields as DT fusion, neutrino mass measurements using tritium ß-decay or photonuclear experiments where HD targets are used. Raman spectroscopy is a favorable method for these tasks. In this publication we present a method for the in-line calibration of Raman systems for the nonradioactive hydrogen isotopologues. It is based on precise volumetric gas mixing of the homonuclear species H2/D2 and a controlled catalytic production of the heteronuclear species HD. Systematic effects like spurious exchange reactions with wall materials and others are considered with care during the procedure. A detailed discussion of statistical and systematic uncertainties is presented which finally yields a calibration accuracy of better than 0.4%.