[en] Electron tomography using energy loss and X-ray spectroscopy in the electron microscope continues to develop in rapidly evolving and diverse directions, enabling new insight into the three-dimensional chemistry and physics of nanoscale volumes. Progress has been made recently in improving reconstructions from EELS and EDS signals in electron tomography by applying compressed sensing methods, characterizing new detector technologies in detail, deriving improved models of signal generation, and exploring machine learning approaches to signal processing. These disparate threads can be brought together in a cohesive framework in terms of a model-based approach to analytical electron tomography. Models incorporate information on signal generation and detection as well as prior knowledge of structures in the spectrum image data. Many recent examples illustrate the flexibility of this approach and its feasibility for addressing challenges in non-linear or limited signals in EELS and EDS tomography. Further work in combining multiple imaging and spectroscopy modalities, developing synergistic data acquisition, processing, and reconstruction approaches, and improving the precision of quantitative spectroscopic tomography will expand the frontiers of spatial resolution, dose limits, and maximal information recovery. - Highlights: • Quantitative EELS and EDS tomography will benefit from model-based approaches. • Model-based methods incorporate knowledge of the specimen and signal generation. • Advanced reconstruction algorithms permit tomography from a few spectrum images.

[en] In this paper we propose a new joint model for the reconstruction of tomography data under limited angle sampling regimes. In many applications of tomography, e.g. electron microscopy and mammography, physical limitations on acquisition lead to regions of data which cannot be sampled. Depending on the severity of the restriction, reconstructions can contain severe, characteristic, artefacts. Our model aims to address these artefacts by inpainting the missing data simultaneously with the reconstruction. Numerically, this problem naturally evolves to require the minimisation of a non-convex and non-smooth functional so we review recent work in this topic and extend results to fit an alternating (block) descent framework. We perform numerical experiments on two synthetic datasets and one electron microscopy dataset. Our results show consistently that the joint inpainting and reconstruction framework can recover cleaner and more accurate structural information than the current state of the art methods. (paper)

[en] Strain engineering is used to obtain desirable materials properties in a range of modern technologies. Direct nanoscale measurement of the three-dimensional strain tensor field within these materials has however been limited by a lack of suitable experimental techniques and data analysis tools. Scanning electron diffraction has emerged as a powerful tool for obtaining two-dimensional maps of strain components perpendicular to the incident electron beam direction. Extension of this method to recover the full three-dimensional strain tensor field has been restricted though by the absence of a formal framework for tensor tomography using such data. Here, we show that it is possible to reconstruct the full non-symmetric strain tensor field as the solution to an ill-posed tensor tomography inverse problem. We then demonstrate the properties of this tomography problem both analytically and computationally, highlighting why incorporating precession to perform scanning precession electron diffraction (SPED) may be important. We establish a general framework for non-symmetric tensor tomography and demonstrate computationally its applicability for achieving strain tomography with SPED data. (paper)

[en] Highlights: • NIST and NPL activity standards for ²²⁴Ra are demonstrated to be in accord. • The absolute emission intensity for the 241 keV γ ray is evaluated as I_γ = 4.011(16) per 100 disintegrations of ²²⁴Ra. • The ²²⁴Ra half-life is evaluated as T_1/2 = 3.6313(14) d. The national metrology institutes for the United Kingdom (UK) and the United States of America (USA) have compared activity standards for ²²⁴Ra, an α-particle emitter of interest as the basis for therapeutic radiopharmaceuticals. Solutions of ²²⁴RaCl₂ were assayed by absolute methods, including digital coincidence counting and triple-to-double coincidence ratio liquid scintillation counting. Ionization chamber and high-purity germanium (HPGe) γ-ray spectrometry calibrations were compared; further, a solution was shipped between laboratories for a direct comparison by HPGe spectrometry. New determinations of the absolute emission intensity for the 241 keV γ ray (I_γ = 4.011(16) per 100 disintegrations of ²²⁴Ra) and of the ²²⁴Ra half-life (T_1/2 = 3.6313(14) d) are presented and discussed in the context of previous measurements and evaluations.

[en] Highlights: • Gaseous fission products have been produced through neutron irradiation of highly-enriched uranium at NPL. • A high-resolution beta-gamma coincidence detection system has been used to measure the gaseous fission products. • This paper compares the expected isotopic activity ratios from nuclear decay-ingrowth calculations with measurements. • The ¹³⁵Xe half-life was determined through measurement and is in excellent agreement with the literature value. Gaseous fission products have been produced via thermal neutron irradiation of a highly-enriched uranium target and extracted using a custom gas processing system for measurement on a prototype, high-resolution β − γ coincidence detection system. The gas was extracted and measured in two stages in order to measure the prompt and β⁻–delayed fission products. This paper presents an overview of the system used to produce gaseous fission products, and the results of the advanced coincidence spectrometry techniques used to identify and quantify decays from the radionuclides produced, including the noble gases ⁸⁵Kr, ^85mKr, ⁸⁸Kr, ¹³³Xe, ¹³⁵Xe, ^133mXe and ^135mXe, as well as ¹³³I and ⁸⁸Rb. The measurements were validated by determination of the nuclear decay half-lives, specifically for the ground state decay of ¹³⁵Xe, which was found to be 9.15(49) hours and consistent with the literature value. This work demonstrates the UK capability to produce gaseous radionuclides for quality assurance and calibration purposes in Radionuclide Laboratories supporting the Comprehensive Nuclear-Test-Ban Treaty (CTBT).

[en] Highlights: • A novel quantification approach is proposed for STEM tomography. • 3D high-resolution quantitative element maps are reconstructed. • HAADF-EDS bimodal tomography shows advantages compared with EDS-STEM tomography. Electron tomography has been widely applied to three-dimensional (3D) morphology characterization and chemical analysis at the nanoscale. A HAADF-EDS bimodal tomographic (HEBT) reconstruction technique has been developed to extract high resolution element-specific information. However, the reconstructed elemental maps cannot be directly converted to quantitative compositional information. In this work, we propose a quantification approach for obtaining elemental weight fraction maps from the HEBT reconstruction technique using the physical parameters extracted from a Monte Carlo code, MC X-ray. A similar quantification approach is proposed for the EDS-STEM tomographic reconstruction. The performance of the two quantitative reconstruction methods, using the simultaneous iterative reconstruction technique, are evaluated and compared for a simulated dataset of a two-dimensional phantom sample. The effects of the reconstruction parameters including the number of iterations and the weight of the HAADF signal are discussed. Finally, the two approaches are applied to an experimental dataset to show the 3D structure and quantitative elemental maps of a particle of flux melted metal-organic framework glass.