A. Stephan

PIXE microprobe for geoscience applications

Abstract Microprobe analysis with PIXE (proton induced X-ray emission) is a well known analytical method with detection limits at the ppm level. It is frequently used for geoscience applications. To improve the versatility and reliability, the Bochum microprobe facility has been reconstructed incorporating a new data acquisition system, a beam dose monitor, and a new slit system.

Diagenetic history of sedimentary carbonates: Constraints from combined cathodoluminescence and trace element analyses by micro-PIXE

Abstract Trace element data for a succession of cements can contribute to the reconstruction of the diagenetic history of sediments. In the case of carbonate cements, cathodoluminescence (CL) microscopy has been successfully utilized for qualitative description of different cementation environments. In order to interpret correctly CL colour, trace element data other than Mn and Fe are often required. Due to frequent microscale zonation of trace elements in natural calcites and due to their low threshold concentration for CL activation, microsampling techniques with high spatial resolution and low detection limits are required. In this case study, trace element concentrations have been correlated with the CL behaviour of carbonate cements using the Bochum proton microprobe (PIXE) and a hot cathode luminescence device. The CL patterns, including the fine zonation, correlate well with the trace element distribution.

Micro-PIXE and quantitative cathodoluminescence spectroscopy: Combined high resolution trace element analyses in minerals

Abstract We combined high resolution Cathodoluminescence (CL)-spectroscopy and micro-PIXE to study the correlation of the activator concentration and the CL-intensity. Based on these results the Quantitative High Resolution Spectral analysis of Cathodoluminescence (QHRS-CL) is developed. Micro-PIXE and the new method (QHRS-CL) have been used to investigate trace elements in minerals. Using micro-PIXE and related methods the crystal lattice site and charge state of the analysed elements cannot be determined. This can be analysed exactly by using QHRS-CL. So the combination of micro-PIXE and QHRS-CL is a powerful tool for analysing trace element concentration above 100 ppb, the charge state and the lattice site of these elements in crystal structures.

Diagenetic alteration of calcitic fossil shells: Proton microprobe (PIXE) as a trace element tool

Abstract Trace element contents (Mn, Fe, Sr) of a set of Lower Carboniferous brachiopod shell fragments have been measured using the Bochum proton microprobe. The resulting data are in good agreement with ICP measurements of the same shells and serve as a quantitative criterion to constrain the preservation state of the sample material for oxygen isotope analyses. They thus help to interpret correctly the isotope data. Linescans and elemental maps yield information on the lateral distribution of trace elements.

High resolution rare-earth elements analyses of natural apatite and its application in geo-sciences: Combined micro-PIXE, quantitative CL spectroscopy and electron spin resonance analyses

Abstract The rare-earth element (REE) distribution in natural apatite is analysed by micro-PIXE, cathodoluminescence (CL) microscopy and spectroscopy and electron spin resonance (ESR) spectroscopy. The micro-PIXE analyses of an apatite crystal from Cerro de Mercado (Mexico) and the summary of 20 analyses of six francolite (conodonts of Triassic age) samples indicate that most of the REEs are enriched in apatite and francolite comparative to average shale standard (NASC). The analyses of fossil francolite revealing the REE-distribution not to be in balance with the REE-distribution of seawater and fish bone debris. Strong inhomogenous lateral REE-distribution in fossil conodont material is shown by CL-mapping and most probably not being a vital effect. Therefore, the resulting REE-signal from fossil francolite is the sum of vital and post-mortem incorporation. The necessary charge compensation for the substitution of divalent Ca by trivalent REE being done by different kind of electron defects and defect ions.

Investigation of the resolution affecting parameters of a nuclear microprobe using a solenoid lens

Abstract The spatial resolution of a nuclear microprobe system may in practice be limited by a number of factors such as beam energy spread, system misalignment and imperfections of the final lens. A superconducting solenoid as final lens promises to have theoretically good ion optical properties. The experimental determination of relevant parameters of the solenoid lens installed at the Bochum heavy ion microprobe and of the incident ion beam is presented. With RBS and STIM the lateral dimension of the focal spot has been determined, while NRRA was used to measure the energy spread of the ion beam. The effect of alternating stray magnetic fields could be detected and was reduced. It was shown that mechanical vibrations present in the beam line setup have negligible influence on the focus quality. The shadow projection method allowed to investigate the effect of minor imperfections of the final lens. Introducing an electrostatic stigmator reduced the astigmatism of the beam spot significantly. A focus of 0.6 μm×0.7 μm could be established at a current of 100 pA.

A superconducting solenoid as probe forming lens for microprobe applications

Abstract An improved nuclear microprobe system for applications in material science has been designed at the Dynamitron Tandem Laboratory of the University of Bochum. A superconducting solenoid as probe forming lens allows a wide range of projectile masses and energies. We describe the expected performance of the new system calculated by ray tracing and first experiments with the new lens system. The effects of chromatic, spherical and mechanical aberrations, including misalignment and beam scanning, were determined. The calculations show that a very high degree of axial symmetry of the focusing coil is of main importance to avoid parasitic aberrations. This demands extreme accuracy in the fabrication approaching the technical limits.

Structural point defects in “Iceland spar” calcite

Abstract Trace element concentrations by micro-PIXE, cathodoluminescence (CL) emission spectra and electron spin resonance (ESR) spectra of Mn 2+ in “Iceland spar” calcite have been measured. The average rare earth elements (REE) abundances of the Iceland spar calcite revealed a concave shape with positive Eu and Tb anomalies. All samples show comparable average REE abundances compared to average chondrites standard. The REE signal in hydrothermal solution seems to be similar for the different locations and age of formation although the absolute REE concentration in the solution was certainly different. The CL-properties of investigated Iceland spar varied from orange to green. The orange luminescence is based on Mn 2+ in Ca-position of calcite while this uncommon green luminescence is most likely attributed to UO 2 2+ complex ions associated with electron–hole centres.

Investigation of manganese in salt- and freshwater pearls

Abstract The trace element distribution in natural and cultured pearls is analysed by micro-PIXE, cathodoluminescence (CL) microscopy and spectroscopy and electron spin resonance (ESR) spectroscopy to develop a new method to distinguish cultured from natural pearls. These different kinds of pearls can be identified by their manganese content and its distribution in aragonite and calcite structure, respectively. Manganese content of natural freshwater pearls from Persian Gulf was compared to that of natural freshwater pearls from the Mississippi river (USA). Moreover manganese content of tissue-graft freshwater pearls from Chansu (China) was compared to that of natural freshwater pearls from the Mississippi river (USA). It was proved that the Chinese freshwater tissue-graft cultured pearls generally contain domains of calcite, emitting orange Mn 2+ -activated CL which are almost absent in the natural freshwater pearls from the Mississippi river. Freshwater pearls showing much higher Mn concentrations build in calcitic and aragonitic parts compared to saltwater pearls.

Optimization of material and shape for nuclear microprobe apertures

Abstract In a nuclear microprobe the beam (usually 1–3 MeV protons) is focused on the target to a lateral spot size of micrometer dimensions, using a focusing lens and at least two beam-defining apertures. Apart from other affecting parameters, the lateral resolution also depends on the scattering and energy loss of the projectiles transmitting the region near the edge of the aperture, the “transparency zone”. The depth of this zone is influenced by the shape, material, and surface roughness of the aperture. This influence was investigated quantitatively for 3 MeV protons using both theoretical and experimental studies. The theoretical studies included the multiple scattering theory as well as a ray-tracing simulation. The experiments were performed using the Bochum nuclear microprobe together with the STIM technique. The results of the studies showed that W, WC, Pt, and Mo are suitable aperture materials. From ray-tracing simulations it turned out that the effect of aperture surface roughness on beam quality is strongly dependent on the specific material. The cylindrical slit geometry was found to be most promising for nuclear microprobe apertures.