Assembling of a low energy ion beam analysis facility and use of Nuclear Microprobe techniques in geological studies

In about four decades, ion beam analysis (IBA) techniques have evolved to become well established analytical techniques for routine analysis in archaeological, bio-medical, geological and environmental sciences. They basically make use of the different interactions of a high energy ion beam (of few MeV/amu) with the specimen matter so they pressupose the use of a particle accelerator. In laboratories where an accelerator is available, setting up an IBA facility can be, for many reasons, a good choice.

In conditions when neither enough analytical facilities, nor skilled personnel are available, the possibility of creating a research facility, which is accessible to multidisciplinary users allowing at the same time a manpower build up can be regarded as a worthwhile investment. At the present work, special emphasis was put in these two issues in developing a low energy IBA facility at the Eduardo Mondlane University in Maputo, Mozambique.

By using an ion beam focused down to micrometer scale and some scanning mechanism with synchronised detection of the spectra of each pixel, digital elemental maps of the analysed specimen can be generated. This feature confers microscopic ability to the IBA techniques which are then known as Nuclear Microprobe (NMP) techniques. In this work, both particle induced X-ray emission (PIXE) and ion beam induced luminescence, or just ionoluminescence (IL) were used for geochemical studies. The possibility of rapid absolute quantification of elements in the ppm level by PIXE, combined with the yet higher sensitivity of IL method to transition metals and REE activators, in the absence of quenching phenomena, allow for a synergic use of the two methods in geological applications with enhanced sensitivity.

IL and PIXE were combined for studying Rare Earth Elements (REE) distribution in apatite minerals and ion beam induced damage in inorganic material in general with emphasis to synthetically grown zircon crystals doped with REE. Due to the sensitivity of IL to changes in chemical bonding in the material, beam damage effects can be studied even at low integrated doses, through wavelength shift or fading of the induced light.

Micro PIXE technique was used for studying line profile concentrations of trace elements in pyrite grains and of elements used as geothermometers. Geothermometry allowed to assess the cooling rates in iron meteorites and the mineralisation conditions in metamorphic rocks, attempting to describe the tectonic history of the terranes, with application in petrologic studies and geological prospecting.