J.-C. Dran

Hydrated-layer formation during dissolution of complex silicate glasses and minerals

Abstract The characteristics of aqueous dissolution of three complex silicate glasses (rhyolitic, basaltic, and simulated nuclear-waste glass) have been investigated by analytical techniques based on energetic ion beams, which provide depth profiles of various elements including hydrogen, on the altered surface of the materials. The two main corrosion features of this type of glass have been extensively documented: the build-up of a hydrated layer and the surficial accumulation of transition and heavy elements. Comparison with the corrosion behavior of ion-bombarded silicate minerals and study of the influence of solution temperature (in the range 60–200°C) and chemistry (effect of high sodium concentration) allow a better understanding of the corrosion mechanism. It is shown that hydration involves two different processes simultaneously, namely ion exchange between hydrogen and glass network modifiers and permeation of molecular water, their respective importance depending markedly on temperature and solution chemistry. Heavy element accumulation is explained by the precipitation of hydroxides or complex hydrosilicates, the latter possibly resulting also from ion exchange with major cations of primary hydrosilicates.

Development of an external beam nuclear microprobe on the Aglae facility of the Louvre museum

Abstract The external beam line of our facility has been recently equipped with the focusing system previously mounted on a classical nuclear microprobe. When using a 0.1 μm thick Si3N4 foil for the exit window and flowing helium on the sample under analysis, a beam spot as small as 10 μm is attainable at a distance of 3 mm from the window. Elemental micromapping is performed by mechanical scanning. An electronic device has been designed which allows XY scanning by moving the sample under the beam by steps down to 0.1 μm. Beam monitoring is carried out by means of the weak X-ray signal emitted by the exit foil and detected by a specially designed Si(Li) detector cooled by Peltier effect. The characteristics of external beams of protons and alpha particles are evaluated by means of resonance scanning and elemental mapping of a grid. An example of application is presented, dealing with elemental micro-mapping of inclusions in gemstones.

The albite-water system: Part III. Characterization of leached and hydrogen-enriched layers formed at 300°C using MeV ion beam techniques

Abstract Samples of albite feldspar were dissolved at 300C and 170 bars for periods up to 24 h in flow-through reactors at acid, neutral, and basic pH conditions. Three MeV ion beam techniques, Resonant Nuclear Reaction Analysis (RNRA), Rutherford Backscattering Spectrometry (RBS), and Elastic Recoil Detection Analysis (ERDA) were employed to obtain elemental depth profiles and information on the composition of the near-surface region after dissolution. Based on the anti-correlative trends of the H and Na profiles obtained by RNRA, Na loss and H permeation are coupled by an ion exchange process in acidic and neutral pH solutions. At basic pH conditions, the evidence is ambiguous as to whether there is a limited degree of ion exchange between aqueous cations and Na, as based on RBS spectra and Na RNRA profiles. The recorded depths of H permeation and Na leaching range from a maximum at acid pH (H permeation exceeding ∼10,000A, Na leaching ∼20,000A) to a minimum at basic pH (no H enrichment, Na leaching depths of several hundredA). The composition of the leached/H-enriched region is a function of pH. This is postulated to be primarily a function of two factors: the H ion concentration gradient between the solution and the solid, which directly controls the pH-dependence of the ion exchange couple H+ (or H3O+) ← Na+ and secondly, the speciation of Al -OH and t.sbnd; Si -OH groups created by hydrolysis reactions and the subsequent preferential release of Al within the leached/H-enriched zone. Based on the ratios of H uptake to Na loss at acid and neutral pH, which range between 0.7 and 2.5, it is not possible to distinguish between H+, H2O, and H3O+ species permeating into the structure. Free water may be created within the leached/ H-enriched structure via recondensation (repolymerization) reactions of adjacent Si&-OH groups. Excess H concentration profiles potentially provide indirect evidence for recondensation reactions at depths

Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution

Surface layers on the French borosilicate nuclear waste glass, R7T7, corroded in MgCl2 solution were studied to determine the composition, structure and stability of crystalline phases. The characteristics of the phases constituting the surface layer varied with the parameter SV × t, the glass surface area (S) to solution volume (V) ratio, times time (t). At low SV × t values ( 98% of the neodymium released from the glass were precipitated in the surface layer. In the 463 day experiment, 86% of the neodymium in the surface layer was in solid solution with powellite, the rest with saponite. Uranium was contained exclusively in saponite. High SV ratios, typical of disposal conditions for vitrified high-level radioactive waste, favor retention of actinides in fairly insoluble corrosion products. Observation of similar corrosion products on natural glasses as on nuclear waste glasses lend support to the hypothesis that the host phases for actinides observed in the laboratory are stable over geological periods of time.

Structural dependence of crystalline silicate hydration during aqueous dissolution

Abstract By using MeV ion beam analytical techniques which provide the depth profiles of incorporated hydrogen and major constituent elements, we show that, alike glasses, crystalline silicates can form surficial hydrated layers upon aqueous dissolution at temperatures up to 200°C. The occurrence of such layers depends on both crystal structure and chemical composition, tridimensional-network silicates containing easily exchangeable alkalis being the most suitable materials. This phenomenon could have important implications for the geochemical cycle of certain trace elements, since highly hydrolyzable ones tend to accumulate in such hydrated layers.

An external milli-beam for archaeometric applications on the AGLAE IBA facility of the Louvre museum

Abstract External beam lines have been built on numerous IBA facilities for the analysis of works of art to avoid sampling and vacuum potentially detrimental to the integrity of such precious objects. On the other hand, growing interest lies on microprobe systems which provide a high lateral resolution but which usually work under vacuum. Until recently, the AGLAE facility was equipped with separate external beam and microprobe lines. The need of a better spatial resolution in the external beam mode has led us to combine them into a single system which exhibits numerous advantages and allows the analysis of small heterogeneities like inclusions in gemstones or tiny components of composite samples. The triplet of quadrupole lenses bought from Oxford is used to focus the beam. By using a 0.75 μm thick Al foil as the exit window, blowing a helium flow around the beam spot and reducing the window-sample distance below 3 mm, a beam size of about 30 μm can be reached. The experimental setup includes two Si(Li), a HPGe and a Si surface barrier detectors for the simultaneous implementation of PIXE, NRA and RBS. The full description of this device is given as well as a few applications to highlight its capability.

EVIDENCE OF IONIZATION ANNEALING UPON HELIUM-ION IRRADIATION OF PRE-DAMAGED FLUORAPATITE

Abstract In order to study the behaviour of fluorapatite towards irradiation due to incorporated alpha-emitters, single crystals of this mineral have been bombarded with 220 keV Pb ions, simulating alpha recoil nuclei, and subsequently irradiated with He ions. The defect concentration measured by RBS associated with channeling steadily decreases upon He-ion irradiation. By changing the energy of the incident He ions in the range 0.3–3.2 MeV, we have shown that this defect annealing phenomenon can be ascribed to the electronic energy loss, while the use of B ions which have slightly higher electronic energy loss in the target induces the opposite effect, i.e., increases the defect concentration.

Ion beam analysis with external beams: Recent set-up improvements

Accelerator-based analytical techniques using external beams are ideally fitted to the study of works of art because of their fully non-destructive character. However, accurate quantitative analysis is not straightforward, due in particular to difficult beam monitoring. Significant improvements have been progressively made on the external beam line of the IBA facility of the Louvre museum in order to increase the accuracy and to conduct combined analyses with different IBA techniques.

Use of ion beam techniques for studying the leaching properties of lead implanted silicates

Abstract Na and H depth profiling has been performed on samples of soda-lime glass and albite subjected to Pb ion implantation and leaching, by means of resonant nuclear reactions. Before leaching, the implanted albite samples show a particular Na depth profile with a marked depletion at the range of the incident Pb ions but an unchanged surface concentration. For soda-lime glass, a Na surface depletion has been noted, which increases with the ion dose. After leaching, the compositional change in albite is limited to the implanted layer, whereas in soda-lime glass it proceeds at much larger depths and exhibits a linear time dependence for short leach times. In addition Pb implantation is found to drastically increase H penetration in vitreous silica. Such ion induced effects are discussed with reference to the use of ion implantation to simulate internal irradiation due to α-decay.

Micro-PIXE analysis of an ancient Egyptian papyrus: Identification of pigments used for the “Book of the Dead”

Abstract This paper reports a study of colours and inks of an ancient Egyptian papyrus using an external proton microprobe in PIXE mode. Representing the Book of the Dead, this papyrus is dated from the 19th dynasty, New Kingdom (c.1295–1186 BC). Elemental maps were obtained by moving the papyrus under a fixed focused external beam using a motorised support. The maps were compared to photographic pictures taken in visible light. Inks used in the hieroglyph text appeared to be based on carbon (black) and iron oxide (red). Coloured drawings illustrating the text showed a wider palette: hematite, ochre, orpiment, Egyptian blue, verdigris. Most intriguing was the observation in several parts of the drawing of a whitish pigment containing strontium. Deposits of strontium-rich minerals (e.g. strontianite, celestite) have been identified in Egypt. The exact nature and the archaeological implications of this pigment have still to be determined. Finally, fine powder and coarse grains of arsenic oxide were observed, probably remaining from an early preservation treatment against insect attacks after excavation.