Mapping 238U decay chain equilibrium state in thin sections of geo-materials by digital autoradiography and microprobe analysis
Introduction
In sub-surface sediments, soils and rocks, actinides are naturally produced in the decay chains of 238U, 235U and 232Th. Human activity linked to the nuclear industry also produces artificial actinides such as 239Pu. Radioisotopes found in these series are alpha or beta emitters. The present work focuses on natural material and subsequent U mine tailings, and only the 238U series is discussed here.
In a natural system, any decay chain will be found in secular equilibrium in a mineral, if the system (mineral) is closed (no exchange with its environment) for long enough. This time is estimated to be ten half-life of the daugther radioisotope, which has the longest half-life in the series, except for 238U. For the 238U series, this equilibrium time is nearly 2 My and is related to the half-life of 234U (2.455 × 105 years). Decay chains can also be found in disequilibrium if any geological process, such as alteration (weathering or hydrothermalism), opens the system (Contreras et al., 2015, Štrok and Smodiš, 2010, Van Orman et al., 2006). Uranium mining activity provokes isotopic fractionation by the chemical extraction of U with acidic solutions. In this case, the daughter elements will be separated from U because of their different geochemical behaviour.
The intermediate radioisotopes of the 238U series may be considered to cause environmental and health issues (Contreras et al., 2015, Štrok and Smodiš, 2010, Van Orman et al., 2006; Morishita et al., 2014; Wertheim et al., 2010; Yamamoto and Ishibashi, 2015) because (1) they are present in very small amounts (ultra-trace), (2) they have a high radioactivity and (3) some of them are chemically toxic. Mainly because of the ultra-trace nature of intermediate radioisotopes, it is difficult to separate and identify them, and sequential leaching is the most used technique at present (Blanco et al., 2004, Schultz et al., 1998, Tessier et al., 1979). Moreover, there is a current inability to locate them accurately in the intact sample at a microscopic scale. This remains a challenging task because locating these radioisotopes using the mineralogy of the material would help to understand their mobility in the environment.
Uranium can be considered as a trace element in the environment, whereas 226Ra, which is a radioisotope of the 238U series (T1/2 = 1600y), is an ultra-trace element (in the range of ppb-ppt or less). A mass content of one ppb or one ppt of 226Ra is equal to 1 µg/kg or 1 ng/kg, respectively. Efforts have been recently made to check the mobility of this radioisotope because of the presence of 226Ra anomalies in the environment (Sill, 1987). The fate of radium in the environment is constrained by its adsorption onto mineral surfaces (Ames et al., 1983, Reinoso-Maset and Ly, 2016, Sajih et al., 2014, Robin et al., 2017) and by co-precipitation reactions (Curti et al., 2010, Gnanapragasam and Lewis, 1995, Lestini et al., 2013). Radium is an alkaline earth metal, whereas uranium is a multi-oxygen state refractory actinide element. Both of these elements are incompatible, but they have a significantly different geochemical behaviour (Bourdon et al., 2003).
If it is easy to detect the bulk radioactivity produced by these daughter radioisotopes, it is more difficult to study their mobility because their accurate location in the material is largely unknown. However, new autoradiographic techniques seem able to meet this crucial need. Digital autoradiography (DA) based on a micro-pattern gas detector (MPGD) technology is employed in biological and medical fields for the quantitative mapping of beta emitters (Donnard et al., 2009a, Donnard et al., 2009b, Donnard et al., 2009c). The use of DA has recently been extended to the quantitative mapping of alpha emissions in geo-materials (Sardini et al., 2016). In order to locate and identify the equilibrium state of 238U series in rock sections and hard materials, that paper suggests the investigation of spatial correlation between alpha emission mapping and the elemental chemical mapping of uranium by microprobe.
This present work presents a new approach for mapping the alpha emitting radioisotopes of 238U series in geological sections using an alpha autoradiograph and aims to determine the spatial distribution of the equilibrium state of the 238U series. This proposed goal is performed by comparing experimental and theoretical estimates of alpha emissions. Three independent measuring techniques are combined together with image processing: (1) elemental chemical mapping, (2) DA of alpha particle emissions and (3) conventional alpha spectrometry. First, the methodology is applied to samples at secular equilibrium and then to samples of ground ore and fresh tailings obtained from the chemical extraction of U from ore (Cominak mine, Niger) as previously described by Déjeant et al., 2014, Déjeant et al., 2016.
Section snippets
Materials
The two sets of samples used in this work were polished thin sections. The first set comprised two uranium-rich rocks, which had already been used by Sardini et al. (2016) for studying the capability of Beaver™ to locate and count alpha emissions. One set of samples came from the Shea Creek (SC) area (Athabasca basin, Canada, explored by AREVA). The rock is highly silicified and altered and contains mostly quartz and clay minerals such as illite. Accessory minerals such as zircon,
Elemental chemical mapping
The result of the microprobe analyses is a pixel-by-pixel map of uranium (wt%) for each sample. The sizes of the presented uranium maps are the same as the ROI of the α-maps (see above). By using the ImageJ software, the U (wt%) content was converted into a theoretical alpha emission at equilibrium (cps/mm2). For the ore and fresh tailing samples, a threshold was applied according to the detection limit of the U analyses. By using the Si elemental map, pure quartz was selected. For both
Equilibrium state study
The work presented here deals with the challenge of characterising the spatial distribution of the 238U series equilibrium state at the scale of geological thin sections. Presently, there is a specific need for better understanding whether, in a given material, the 238U series is at equilibrium or not, and to determine which mineral phase(s) control this state. To date, the main methodological problem is that the daughter radio elements of the 238U series are not detectable in a section by any
Conclusion
A new method that combines quantitative digital autoradiography (Beaver™), elemental chemical mapping and bulk alpha spectrometry developed by Sardini et al. (2016) was tested to precisely identify and locate the radioactive equilibrium state in rock thin sections. This work showed the potential of the approach as a new tool for locating the equilibrium state at the thin section scale. It was able to identify both equilibrium and disequilibrium at a micrometric scale in the same rock, bringing
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