Heinz Maier-Leibnitz Zentrum

SARA is the system of plastic scintillators coupled with silicon photomultipliers that will take part in the AEgIS experiment at CERN, measuring the time-of-flight of antihydrogen as it falls through a moiré deflectometer. Its development focused on simplicity, versatility and economy of the design and was supported by both physical tests and numerical simulations. The instrument structure pairs the utilization of the scintillators as structural components with custom made 3D printed corner elements and the electronics allows selection between coincidence discrimination made on each scintillator and made between different scintillators.

Abstract Coincidence Doppler Broadening Spectroscopy (CDBS) is a positron annihilation technique that allows the measurement of electron momenta within a sample. In CDBS both of the electron-positron annihilation gamma quanta are measured in coincidence. As a result, the Doppler broadened 511 keV photo peak can be measured with much lower background contribution as compared to conventional DBS. Compared to DBS, CDBS not only allows the analysis of open volume defects, but also the chemical surroundings of the positron annihilation site. Recent upgrades of the CDB Spectrometer at the NEutron induced POsitron source MUniCh (NEPOMUC) allow high-statistics bulk measurements in between reactor cycles, using a 22 Na positron source. Accurate projections of the material specific electron momentum distribution can be obtained in this way. Using un-moderated positrons from a 22 Na source, the implantation depth of the positrons is high enough for bulk measurements. Several reference samples, including metals and semiconductors like Al, Cu, Si and more, as well as, AlCu alloys in various states were measured with high statistics, i.e. at least 10 7 counts in the coincidence spectrum. The data is analyzed using in house developed software [1] to project the momentum distribution without distorting the data. The projections are subsequently compared to ab-initio calculations using ratio-curves.

Abstract Solvent organization at solid–liquid interfaces dictates nanoparticle stability, catalysis, and self‐assembly. Classical physical‐chemistry theories commonly treat solvent quality as an additive continuum variable—an assumption increasingly questioned at the nanoscale but rarely experimentally quantified. Here, we address this challenge by combining small‐angle neutron scattering (SANS) with Monte Carlo real‐space reconstruction, allowing direct, molecular‐level quantification of interfacial solvent structures under native solution conditions. In mixed solvents, we uncover discrete sub‐nanometer solvent clusters whose abundance and spatial organization evolve non‐linearly with solvent composition. These solvent clusters preferentially infiltrate nanoparticle ligand coronas, forcing ligand shells to deform from spherical symmetry into anisotropic ellipsoids—behavior that classical continuum core–shell models fail to describe. Interestingly, maximal solvent clustering, anisotropic deformation, and preferential solvent uptake coincide precisely at the bulk azeotropic composition, directly linking bulk solvent microstructure with nanoscale interfacial symmetry breaking. By turning a long‐standing theoretical prediction into a quantitative measurement, this methodology establishes a versatile route for probing and ultimately tailoring solvation effects at a wide range of soft and hybrid interfaces.

Abstract The Institute of Space Systems is currently developing a deorbit module based on thermal arcjet technology to allow fast orbit decay at end-of-life, with a focus on megaconstellation satellites. By employing additive manufacturing with tungsten, improved nozzle geometries can lead to a gain in overall performance. However, reproducibility is an ongoing concern for additively manufactured parts. Together with the Heinz Maier-Leibnitz Zentrum (MLZ) and the Budapest Neutron Centre of the Centre for Energy Research, a study was conducted scanning additively manufactured arcjet nozzles prior to and after standardized operation via neutron computed tomography. The results show a drop in performance over time, which can be related to changes in the constrictor geometry. Furthermore, cavities created during manufacturing can significantly influence operation.

Thermal neutron detection in neutron scattering science is a challenging endeavour due to a limited number of technologies which are sensitive to these weakly interacting particles. While many improvements to conventional detectors are underway at various facilities, there is a great opportunity to make a leap in performance by combining the spatial resolution benefits of imaging and the temporal resolution and background discrimination of event-driven radiation detectors. This idea has been realized by using a neutron sensitive scintillator read out by a single-photon sensitive camera based on a Timepix3 ASIC. We demonstrate how such data-driven imaging sensors can enable unprecedented performance in neutron reflectometry using the ASTERIX instrument at the Los Alamos Neutron Scattering Center. Several samples were measured with both the new and a conventional He detector systems. The results from this work demonstrate that these imaging based systems can satisfy performance parameters for the future QIKR reflectometer to be built at the Second Target Station at Oak Ridge National Laboratory. Further improvements to the detector are already underway which will allow streamlined and expedited experiments. We demonstrate at least a two order of magnitude increase in detection rate at an acceptable dead time and introduce a new way of tuning the detector efficiency using light collecting optics to accommodate highly intense direct beams which cannot be measured with any current detectors without severe attenuation. This will allow measurements of complete reflectometry profiles and using a single sample measurement combined with fewer direct beam calibration measurements on QIKR and potentially other reflectometers.

Abstract Lithium (Li) plating on graphite is a significant degradation mechanism in Li-ion batteries. While numerous experimental techniques have been used to study Li plating in laboratory cells, investigations of commercial high-energy cells often rely on electrochemical methods. Here we present and classify various methods for detecting Li plating on a commercial A123 pouch cell. In a round robin study across multiple battery research laboratories, Li-plated graphitic electrode material was analyzed using electrochemical, microscopic, and spectroscopic methods capable of detecting metallic Li deposits. After cell opening, their overall distribution on the anode surface was examined using a flatbed scanner to ensure comparability of the samples. Optical and electron microscopy provided detailed surface and, in combination with a focused ion beam, subsurface structure and morphology. Spectroscopic methods confirmed the presence and onset of plated Li with varying sensitivity. Moreover, spectroscopic and imaging techniques were combined correlatively where possible. Availability and measurement duration of each technique was compared. Optical methods are fast and easy to use; thus, they are recommended for most samples, with spectroscopic confirmation reserved for reference samples. This multimodal study demonstrates a range of methods that can be used alone or in combination to qualitatively or quantitatively detect Li-plating.

Abstract Positive nitrogen‐vacancy centers (NV+) in diamond are predicted to exist in conjunction with neutral (NV0) and negative (NV−) centers. However, the existence of NV+ has only been indirectly inferred through a shift of the Fermi level. Evidence of NV+ coexisting with NV0 and NV– in diamond has not yet been observed. In this paper, positron annihilation spectroscopy in combination with in situ light illumination is applied, in order to investigate the presence of NV+ centers in nitrogen implanted and subsequently annealed diamond. Switching of NV+ to NV0 centers is observed with a threshold photon energy of 1.234(8) eV. In complete darkness, a decay of NV0 centers with a decay time of 1.73(22) h can be detected. In conclusion, previously converted NV0 centers are metastable and partially decay in darkness, leading to the reformation of NV+ centers.

Imaging the features of a sample using Positron Annihilation Spectroscopy (PAS) is currently achieved by rastering, i.e. by scanning the sample surface with a sharply focused positron beam. However, a beam of arbitrary shape (sculpted beam) would allow the application of more versatile single-pixel imaging (SPI) techniques. I introduce the design of a microelectronic device employing a 2D array of Zener diodes as an active positron moderator, capable of sculpting positron beams with resolution. The re-emitted positrons are accelerated towards the sample through a miniaturized electrostatic lens system and reaching resolution. The fast switch-on () and switch-off () time of the device would enable state-of-the-art positron annihilation lifetime spectroscopy (PALS) and PAS imaging with high spatial and temporal resolution.

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