M.T. Crespo

A review of electrodeposition methods for the preparation of alpha-radiation sources

This paper addresses an approach to the theory and practice of electrodeposition processes of alpha-emitting nuclides. Some of the main contributions made to this field are reviewed, including the rotating disk electrode technique. Also, several interpretations concerning the electrodeposition process as well as a number of practical recommendations are included in the study.

On the determination of in environmental and geological samples by α-spectrometry using as yield tracer

Abstract A method for the determination of 226 Ra in environmental and geological samples by α-spectrometry is described. The method is based on the use of 225 Ra , in radioactive equilibrium with its parent 229 Th , as yield tracer. This tracer permits the incorporation of 226 Ra determination into the radiochemical procedure commonly used for the determination of U and Th isotopes in the same sample. Estimation of 225 Ra in the α-spectra is made via measurement of its α-emitting daughters, 217 At being the most suitable to this purpose. In order to achieve it, several requirements need to be fulfilled in the radiochemical procedure, which will be discussed.

Evolution of Chemical Species during Electrodeposition of Uranium for alpha-Spectrometry by the Hallstadius Method

The morphology and composition of uranium alpha sources with co-deposited platinum have been investigated by scanning electron microscopy (SEM) with energy dispersive X-ray fluorescence analysis (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) studies. Combined SEM and EDX measurements reveal the effect of porous platinum on the morphology of the sources which in turn affects their alpha-spectral resolution. The XPS analysis suggests that the presence of platinum initially increases the concentration of hydroxyl species in the deposits, which then act as centres for subsequent preferential uranium precipitation. XPS and XAFS analysis also provide for first time an indication of oxidation states of uranium present in the sources prepared by the Hallstadius method. These results are in line with Hansens theory of electrodeposition of actinides.

Uranium isotopic distribution in the mineral phases of granitic fracture fillings by a sequential extraction procedure

In order to study the recent rock-water interaction processes in the E1 Berrocal site, a sequential leaching method has been applied to granitic fracture fillings to obtain the U isotopic distribution in the mineral phases of these samples. Based on the mineralogical composition of these materials, six dissolution steps have been chosen to extract U as exchangeable cation, from carbonates, amorphous Fe-oxyhydroxides, labile resistates and highly insoluble resistates. In this way, the processes involved in the rock-water interaction phenomena, mainly dissolution, precipitation, coprecipitation and adsorption can be distinguished and even approximately dated.

U-series in Fe-U-rich fracture fillings from the oxidised cap of the “Mina Fe” uranium deposit (Spain): implications for processes in a radwaste repository

Abstract Within the framework of the ENRESA (Spain) natural analogue programme, the U-ore deposit of “Mina Fe” is being studied as a natural analogue of radioactive spent fuel behaviour. In this context, the knowledge of the role played by fracture minerals as scavengers of certain analogue elements, mainly U, and the establishment of the time scale of the rock-water interaction processes controlling the uptakes or losses of U in the system are two relevant objectives. Fracture-infill materials from the site have first been mineralogically characterised, then the upper part of the U-series determined in both bulk samples and U-rich leachates obtained by sequential leaching. Uranium-series of the bulk samples indicate that most of the fractures remained as closed systems in the last 1.6 Ma, while in other fractures water/rock interaction processes affecting the upper part of the U-series have been identified. These processes indicate recent or rapid U accumulation or losses ( 102 ka) or 234U+230Th recoil gain. The apparently random distribution in depth of fractures where these processes occurred corroborates the different hydraulic behaviour of fractures, as a result of their varying degree of sealing. Uranium concentrations and 234U/238U ARs of the leachates obtained with Morgans solution and 6N HCl indicate that minerals dissolved with these reagents (U(IV/VI) oxyhydroxides and goethite+clays, respectively) are responsible for the retention of almost all of the U in the bulk samples. Furthermore, the 234U lost by the U minerals dissolved with Morgans solution seems to be fixed onto goethite–clay mixtures, the intersticial water being the vehicle for the isotopic transfer, which in turn is a recent or recent-past process.

High-resolution alpha-particle spectrometry of 238U

The alpha-particle emission probabilities associated with the three main alpha transitions of (238)U were measured by high-resolution alpha-particle spectrometry. Highly enriched (238)U material was used and its isotopic composition characterised by mass spectrometry. Source production through electrodeposition was optimised to reconcile conflicting demands for good spectral resolution and statistical precision. Measurements were performed at IRMM and CIEMAT for 1-2 years in three different set-ups. A new magnet system was put into use to largely eliminate true coincidence effects with low-energy conversion electrons. Finally the accuracy and precision of the relative emission probabilities for the three transitions - 77.01 (10)%, 22.92 (10)% and 0.068 (10)%, respectively - have been improved significantly.

U and Th series disequilibrium in unaltered and hydrothermally-altered granites from the El Berrocal site (Spain): Weathering effects

Abstract Within the framework of analysis of the safety assessment of high level radioactive waste repositories, the migration of natural radionuclides in a fissured granitic environment, the El Berrocal site, is being studied in Spain. In order to determine the weathering effects on the natural radionuclides of the site, the top parts of the 238 U and 232 Th decay series have been measured in bulk samples of unaltered and differently altered granites by α-particle spectrometry. The results have established the recent geochemical processed involving 238 U, 234 U, 230 Th, 232 Th and 228 Th such as: 234 U recoil loss, bulk U assimilation or 234 U recoil gain, equilibrium states, bulk U leaching without fractionation, bulk uranium assimilation with activity ratio near unity, and loss or gain of 228 Ra ( 228 Th) as a function of the hydrochemical characteristics of the percolating water. These results have been interpreted in terms of the different degrees of weathering according to literature data.

The characterisation of an intertidal sediment from the Cumbrian coastline

The characterisation of an intertidal sediment, containing enhanced levels of radioactivity, collected from the Cumbrian coast of the U.K. by the Laboratory of the Government Chemist for the Department of the Environment is described. A wide variety of radiochemical and instrumental techniques were used including α-spectrometry, β-counting, liquid scintillation counting, γ-spectrometry, X-ray spectrometry and mass spectrometry. To derive mean values of the activity concentrations of radionuclides in the material, use was made of the technique developed by Woods and Munster. This technique first critically evaluates data for technical suitability. Then, to avoid excessive manipulation of data, calculation of the weighted (the statistical weighting is the inverse of the squared uncertainties, limited so that no one result contributes more than 50%) and unweighted means is performed, along with a χ2 test. If the value of χ2/(n−1) 2 determinations, 3 with 2 determinations and 2 with single determinations. Emphasis is placed where data appeared to be discrepant (e.g. 40K and 137Cs), where widely different measurement techniques were employed (e.g. the actinides), where there were analytical “difficulties” (e.g. uranium) or where the radionuclide is not commonly measured (e.g. 155Eu, 237Np and 241Pu). The material is now available to laboratories as a reference material and it is anticipated that it will be used to validate existing procedures and to research new methods of analysis.

Lessons learned from nuclear decay data measurements in the european metrology research programme ‘MetroFission’ (June 2013)

Nuclear decay data measurements were performed in the frame of a EURAMET project. Two specific goals were addressed: (1) High-precision measurements of 238U alpha-particle emission probabilities and (2) experimental characterization of the shape of beta spectra using cryogenic detectors. In the paper, specific lessons learned from the metrological work are highlighted. This includes effects of source preparation on the quality of alpha and beta spectrometry, the influence of exchange effects on beta spectra and the use of a magnet system to eliminate coincidence effects by conversion electrons in alpha-particle spectra.