Deconvolution of fission-track length distributions and its application to dating and separating pre- and post-depositional components

To enable the separation of pre- and postdepositional components of the length distribution of (partially annealed) horizontal confined fission tracks, the length distribution is corrected by deconvolution. Probabilistic least-squares inversion corrects natural track length histograms for observational biases, considering the variance in data, modelization, and prior information. The corrected histogram is validated by its variance–covariance matrix. It is considered that horizontal track data can exist with or without measurements of angles to the c axis. In the latter case, 3D histograms are introduced as an alternative to histograms of c-axis-projected track lengths. Thermal history modelling of samples is not necessary for the calculation of track age distributions of corrected tracks. In an example, the age equations are applied to apatites with predepositional (inherited) tracks in order to extract the postdepositional track length histogram. Fission tracks generated before deposition in detrital apatite crystals are mixed with post-depositional tracks. This complicates the calculation of the post-sedimentary thermal history, as the grains have experienced different thermal histories prior to deposition. Thereafter, the grains share a common thermal history. Thus, the extracted post-depositional histogram without inherited tracks may be used for thermal history calculation.

Digital geological map and geochronological database of the Cenozoic cover of the southern Mesa Central province, Mexico

The digital geological map of the southern sector of the Mesa Central province is presented, covering an area of ~41 000 km2 in central Mexico. This first interactive map is a compilation of the geological maps available in the literature and the unpublished maps done by our work team. The map focuses on the Cenozoic stratigraphy, supported by a geochronological database of 261 isotopic ages derived from K-Ar, U-Pb, Ar-Ar, and fission tracks methods. The interactive map displays the lithostratigraphic and chronostratigraphic units and the major and second-order Cenozoic faults. Map construction considered lithostratigraphy and geochronological correlation criteria and the main unconformities. Integrating these data in a single digital map allows a regional vision of the southern Mesa Central, making the map a valuable work tool to better understand the Cenozoic geologic evolution of central Mexico.

Confined fission-track revelation in apatite: how it works and why it matters

We present a new model for the etching and revelation of confined fission tracks in apatite based on variable along-track etching velocity, vT(x). Insights from step-etching experiments and theoretical energy loss rates of fission fragments suggest two end-member etching structures: constant-core, with a central zone of constant etching rate that then falls off toward track tips; and linear, in which etching rates fall linearly from the midpoint to the tips. From these, we construct a characterization of confined track revelation that encompasses all relevant processes, including penetration and widening of semi-tracks etching in from the polished grain surface, intersection with and expansion of confined tracks, and analyst selection of which tracks to measure and which to bypass. Both etching structures are able to fit step-etching data from five sets of paired experiments of fossil tracks and unannealed and annealed induced tracks in Durango apatite, supporting the correctness of our approach and providing a series of insights into the theory and practice of fission-track thermochronology. Etching rates for annealed induced tracks are much faster than those for unannealed induced and spontaneous tracks, impacting the relative efficiency of both confined track length and density measurements and suggesting that high-temperature laboratory annealing may induce a transformation in track cores that does not occur at geological conditions of partial annealing. The model quantifies how variation in analyst selection criteria, summarized as the ratio of along-track to bulk etching velocity at the etched track tip (vT/vB), likely plays a first-order role in the reproducibility of confined length measurements. It also accounts for and provides an estimate of the large proportion of tracks that are intersected but not measured, and it shows how length biasing is likely to be an insufficient basis for predicting the relative probability of detection of different track populations. The vT(x) model provides an approach to optimizing etching conditions, linking track length measurements across etching protocols, and discerning new information on the underlying structure of fission tracks.

AI-Track-tive: open-source software for automated recognition and counting of surface semi-tracks using computer vision (artificial intelligence)

A new method for automatic counting of etched fission tracks in minerals is described and presented in this article. Artificial intelligence techniques such as deep neural networks and computer vision were trained to detect fission surface semi-tracks on images. The deep neural networks can be used in an open-source computer program for semi-automated fission track dating called “AI-Track-tive”. Our custom-trained deep neural networks use the YOLOv3 object detection algorithm, which is currently one of the most powerful and fastest object recognition algorithms. The developed program successfully finds most of the fission tracks in the microscope images; however, the user still needs to supervise the automatic counting. The presented deep neural networks have high precision for apatite (97 %) and mica (98 %). Recall values are lower for apatite (86 %) than for mica (91 %). The application can be used online at https://ai-track-tive.ugent.be (last access: 29 June 2021), or it can be downloaded as an offline application for Windows.

Localization of actinide-bearing particles in sediment samples from the Fukushima restriction zone

<p>The Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 released significant quantities of radionuclides into the environment. Ten years after the accident, questions still remain, particularly about the processes that led to the partial core meltdown of reactors 1 and 3. So far, some answers have been provided by the investigation of particles containing caesium (Martin et al., 2020) and sometimes uranium (Ochiai et al., 2018). Indeed, the composition of particles, which were produced and spread at the time of the reactor explosion, reflect the conditions that prevailed in the reactor. Accordingly, the objective of the current research is to develop a method for specifically locating actinide-bearing particles in sediment samples collected in the vicinity of FDNPP. To identify and locate such particles, three already existing methods have been upgraded, including 1) the method of fission tracks already used in the field of non-proliferation studies, 2) the autoradiography through the use of imaging plates that are currently employed in the context of the localization of particles containing radio-caesium and the dismantling of nuclear facilities (Haudebourg and Fichet, 2016), and 3) a real time autoradiography method through the use of the BeaQuant® instrument which has been developed for detecting radioactive particles in biology and geosciences.</p><p>In this study, a sediment sample collected nearby FDNPP, which may contain particles containing both radio-caesium and actinides, was selected. This sample was dried and sieved to 63 µm before being processed according to the different analysis protocols.  A quality control sample containing only uranium oxide particles was also analysed, as these particles are devoid of gamma-emitters.</p><p>The first results of this comparison of autoradiography methods for the detection of actinide-bearing particles in Fukushima samples will be presented. The method of fission tracks was particularly efficient for detecting both natural and anthropogenic uranium.</p><p>The next steps of this study will be to implement this method identified as optimal to isolate and characterise a larger number of particles released by FDNPP. The full characterization of these particles (size, morphology, elemental and isotopic compositions) will provide novel insights to determine their origin and to improve our understanding of their formation processes within the reactors and anticipate their fate in the environment.</p><p>References:</p><p>Haudebourg, R., Fichet, P., 2016. A non-destructive and on-site digital autoradiography-based tool to identify contaminating radionuclide in nuclear wastes and facilities to be dismantled. J. Radioanal. Nucl. Chem. 309, 551–561. https://doi.org/10.1007/s10967-015-4610-7</p><p>Martin, P.G., Jones, C.P., Cipiccia, S., Batey, D.J., Hallam, K.R., Satou, Y., Griffiths, I., Rau, C., Richards, D.A., Sueki, K., Ishii, T., Scott, T.B., 2020. Compositional and structural analysis of Fukushima-derived particulates using high-resolution x-ray imaging and synchrotron characterisation techniques. Sci. Rep. 10, 1636. https://doi.org/10.1038/s41598-020-58545-y</p><p>Ochiai, A., Imoto, J., Suetake, M., Komiya, T., Furuki, G., Ikehara, R., Yamasaki, S., Law, G.T.W., Ohnuki, T., Grambow, B., Ewing, R.C., Utsunomiya, S., 2018. Uranium Dioxides and Debris Fragments Released to the Environment with Cesium-Rich Microparticles from the Fukushima Daiichi Nuclear Power Plant. Environ. Sci. Technol. 52, 2586–2594. https://doi.org/10.1021/acs.est.7b06309</p><p> </p>

Age distribution of horizontal fission tracks

Equations for the distribution of age versus length of partially annealed horizontal fission tracks in apatite is presented. Probabilistic least–squares inversion corrects natural track length histograms for observational biases considering the variance of data, modelization, and prior information. The corrected histogram is validated by its variance–covariance matrix. It is considered that horizontal track data can be with or without measurements of angles to the c–axis. In the last case, 3D–histograms are introduced as an alternative to histograms of c–axis projected track lengths. Thermal history modeling of samples is not necessary for track age distribution calculation. In an example the age equations are applied to apatites with pre–depositional (inherited) tracks, to extract the post–depositional track length histogram. Fission tracks generated before deposition in detrital apatite crystals are mixed with post–depositional tracks. This complicates the calculation of the post– sedimentary thermal history as the grains have experienced different thermal histories until deposition. Thereafter the grains share a common thermal history. The extracted post–depositional histogram without inherited tracks may be used for thermal history calculation.