The data contained within the Project IPAD database comprises searchable metadata and results from a wide range of analytical techniques, performed on particulate samples via a global network of collaborating laboratories and scientific facilities. Alongside the growing volume of particulate data comprising the database, serving to provide crucial information on the multiple reactor accident and potential decommissioning strategies, the architecture of the database itself continues to evolve to facilitate enhanced data searchability with greater filtering and refinement, in addition to including fields to document the results arising for new experimental techniques. The original Project IPAD included researchers from; (1) University of Bristol, (2) Japan Atomic Energy Agency, (3) Kyoto University, (4) Ibaraki University, (5) Osaka University, (6) University of Sheffield, (7) University of Tsukuba (8) Keio University, and (9) Tokyo University of Science. However, access to the database has subsequently been made available to all other interested/involved academic institutions and research organisations. The project and database implementation/delivery (to the necessary security, regulatory and data management requirements) also benefited from the support and collaboration of the University of Bristol Cabot Institute, UK Science and Technology Facilities Council (STFC), Engineering and Physical Sciences Research Council (EPSRC) and Amazon Web Services (AWS). The above partners and organisations brought to the project the necessary knowledge required to deliver a high-quality database, with the research institutions contributing their existing particle and analysis results databases.
The differing methodologies utilised to identify, isolate and extract the sub-mm radioactive particulate(s) from their containing matrix as well as subsequently obtain analysis results on such microscopic material using the wide range of experimental techniques is described in the extensive literature associated with the various works and studies11,20. These publications are listed in their entirety on the Project IPAD website. As subsequently discussed, outputs associated with/resulting from the analysis of a specific sample are additionally linked to that particle (via DOI and/or URL) using its enhanced metadata. The inventory of literature sources that underpins the results contained within the database currently stands at over 140 publications – each analysing an average of 5 particulate samples. Such works utilise a complimentary combination of both non-destructive and destructive analysis techniques21,22. Resulting from the large number of multi-national organisations, laboratories and facilities contributing to the portfolio of analysis results; a range of instruments, experimental setups and data output formats are associated/produced.
These analysis methods typically comprise the first techniques to be undertaken and serve to provide initial information on the particulate material – namely its radioactivity, form/structure and elemental composition, all without any damage to the sample or removal of material. This is achieved through conventional materials science and radiation counting techniques, which are foundational techniques of such characterisation laboratories worldwide.
Following the particulates isolation from the bulk (e.g. soil, aerosol filter, dust), the first methodology applied to the sample is gamma-ray (γ) spectroscopy. Through the detection and quantification of the gamma-ray photons emitted by the sample, typically using a shaped crystal of cryogenically cooled high-purity germanium (HPGe), the contributing radionuclides can be determined alongside their relative abundances23. For such Fukushima-derived material, the decay-corrected 134Cs/137Cs activity ratio has been shown to represent a crucial indicator of the materials specific reactor provenance12, following modelling of the differing core burn-up scenarios24.
After determining if the decay-corrected (to March 2011) activity ratio of 134Cs/137Cs is either <1 (Unit 1) or >1 (Unit 2 or Unit 3) and therefore the particles likely emission source (most particulates >10 μm and contained within IPAD are attributed to have been released from reactor Unit 1), subsequent non-destructive testing is performed within the scanning electron microscope (SEM). Typically equipped with energy dispersive spectroscopy (EDS) detectors, the SEM (using various integrated detector options) is capable of producing images of the surface of particulate samples at nm spatial resolution – with EDS affording complimentary surface compositional characterisation at 0.1 wt% levels of detection25.
While γ-ray spectroscopy, SEM and EDS together constitute the primary non-destructive characterisation methods, further techniques are also applied to such sub-mm fallout particulate – with their results similarly contained within the IPAD platform. In contrast to SEM and EDS analysis, which utilise a highly focused beam of electrons to examine a material, x-rays can also be used to study a sample. Whether employing laboratory or ‘brighter’ and more intense synchrotron x-ray sources, such x-ray techniques include; x-ray diffraction (XRD) – to determine the constituent phase chemistry; x-ray tomography (XRT) – obtaining a series of absorption contrast images which when combined produce a 3D reconstruction of the particle; x-ray fluorescence (XRF) – examining the characteristic x-ray energies emitted to elucidate whole particle or point elemental composition and x-ray absorption spectroscopy (XAS) to derive co-ordination chemistry, oxidation states and bonding (including x-ray absorption near edge structure, XANES, and extended x-ray absorption fine structure, EXAFS)21,22.
Further non-destructive techniques applied to such fine scale, yet highly radioactive, particles include Raman spectroscopy for compositional analysis, and proton-induced x-ray emission (PIXE), an additional form of spectroscopy using an incident proton beam (rather than x-rays or electrons) to induce an x-ray emission (through a mechanism analogous to XRF and EDS) via which to study the particles composition. Less common non-destructive techniques that have been surpassed by modern alternatives include both alpha (α) particle spectroscopy and the similar beta (β) particle spectroscopy. Similar to γ-ray spectroscopy, both methodologies also examine and quantify the specific energy of the emitted radiation, albeit in this instance the subatomic α and β particles, to determine the radioisotope from which it was emitted as well as the associated specific activity.
In support of the aforementioned non-destructive analysis techniques, a number of additional methodologies that require the consumption of some, or all, of the sample exist. Unlike the formerly described methods of sample characterisation, such destructive techniques typically afford highly accurate compositional and/or isotopic information on the material, utilising mass spectrometry methods. A mainstay of high-accuracy (and spatial) isotopic analysis is secondary ion mass spectrometry (SIMS), whereby ejecta species produced following a samples ablation and fragmentation with a primary ion beam are isotopically analysed for their mass/charge ratio26. Additional techniques applied to quantify the isotopic composition of such samples include; inductively coupled plasma – mass spectrometry (ICP-MS) and thermal ionisation mass spectrometry (TIMS) – whereby prior sample preparation permits for their injection into an Ar carrier plasma for similar high-precision atomic analysis27,28.
These more mainstream and widely adopted techniques are supported by an increasing number of more novel methodologies to quantify a particulates isotopic composition. Three dimensional-atom probe tomography (3D-APT) is one such approach29, as is resonance ionisation mass spectrometry (RIMS)30 – both of which utilise extremely small volumes of sample material (<107 atoms) while affording a low limit of detection as well as atomic-scale isotopic mapping in the case of 3D-APT. A further technique that requires the destructive analysis of a (small) portion of the sample as part of the preparation phase is transmission electron microscopy (TEM). In a process similar to creating an ultra-fine 3D-APT filament, a focused ion beam (FIB) instrument is used to produce a thin (<100 nm) ‘foil’ of material through which a high energy beam of electrons can pass. While unable to derive isotopic information on the sub-sampled portion of the particle, TEM is capable of obtaining nm-resolution compositional/phase information in addition to crystallographic and structural data on the thin sample slice.