K. Traxel

Collimation of ion beams to micrometer dimensions

Abstract A collimator has been constructed to produce high-quality ion beams with diameters and angular divergences as small as 1 μm and 1 μrad, respectively. The slit scattering could be effectively suppressed by using properly shaped and manufactured collimator slits. The quality of the collimator was investigated for collimator diameters between 20 μm and 1 μm using 1 MeV H + and H 3 + beams. A design for a proton scanning microprobe making use of this collimator system and a small quadrupole doublet is discussed.

The volatile element enrichment of chondritic interplanetary dust particles

Abstract Interplanetary dust particles (IDPs) collected in the stratosphere provide new information on the chemical and isotopic composition of material in the vicinity of the Earth and thus possibly of asteroids and comets. The available data on major, minor and trace element abundances, mainly obtained with pixe and syxfa , suggest the presence of at least two groups of chondritic IDPs which differ significantly in their volatile element content. The ubiquitous enrichment of volatile lithophile, siderophile, and chalcophile elements is most plausibly explained by atmospheric contamination and thus the question arises of the factual relationship of all properties of interplanetary dust particles to early solar system processes.

The Proton Microprobe: A Powerful Tool for Nondestructive Trace Element Analysis

A proton microprobe capable of focusing proton beams with energies up to 6 million electron volts to a spot size of 2 x 2 square micrometers has been used for chemical analysis of small grains of minerals in lunar samples by proton-induced x-ray emission. The proton microprobe is preferable to the electron microprobe for analyzing trace elements whose concentrations are below the detection limit of the latter and for analyzing objects with numerous major and trace elements with a wide range of atomic numbers. Application of the proton microprobe to biological samples is feasible.

The Heidelberg proton microprobe

Abstract All proton microprobes in operation up to now are based on the same principle: a collimated beam is focussed by a magnetic lens. The small dimension and the special shape of the collimators are most characteristic of the Heidelberg proton microprobe. The collimators are responsible for the small halo of the beam. The two singlets allow fast adjustment of the setup. The small aperture of the lens — 5 mm in diameter — makes high precision of the mechanical parts necessary. It is demonstrated that deterioration of the pole pieces of the order of 30 μm causes a dramatic change of the beam focus. A system for imaging the target surface will partially replace the optical microscope. To produce the pictures a fast scanning procedure as known from scanning electron microscopes is used. For this the intensity of the secondary electrons emitted after proton bombardment from the target surface is displayed on a screen (as a function of x and y coordinates) simultaneously with an x/y scan of the beam over the sample. The good quality of the pictures is demonstrated. Problems arising with this method are discussed. Due to the frequencies (∼ 1 Hz) for producing pictures from the sample surface, the beam can be focussed rapidly and accurately. The Ca distributions in sea urchin eggs (egg diameter 50–100 μm) are shown as an example for scans through targets with variable thicknesses. The analysis of the data takes into account the energy loss of the proton penetrating the target and the absorption of X-rays in the sample itself.

Focussing of proton beams to micrometer dimensions

Abstract A microbeam facility able to focus 2 MeV protons to a size of 2 × 2 μm2 at a beam intensity of 100 pA is described.

The new Heidelberg proton microprobe: The success of a minimal concept

Abstract The new proton microprobe is characterized by modular mounting resulting in easy maintenance, low cost personal computer (PC) system and several new features. The most important of these being the design of the lens, its location inside the vacuum, and an elaborate system for beam monitoring. The PC system allows for all kinds of standard data acquisition, beam and target positioning and an automated and quick routine for beam profile analysis. The ionoptical properties fulfil the expectations, which is not true for the brightness delivered from a Penning ion source. Focusing to a submicron beam spot results therefore in a beam current of ≈1 pA only. Tuning to 100 pA enlarges the spot to 3 × 3 μm2.

Low-dose nuclear microscopy as a necessity for accurate quantitative microanalysis of biological samples

Abstract Some aspects of quantitative analysis of biological samples – including a review of the relevant literature – are discussed. Special emphasis is put on the accuracy of trace elemental results despite severe sample damage which occurs during analysis. The occuring mass loss can be corrected by combining quantitative STIM and PIXE. It is also possible to characterize the volatility of most of the minor and trace elements of the sample. The comparison between the presented model and matrix determination with RBS, a widely used method, definitely demonstrates that the concentrations obtained using RBS correction only may be too high by a factor of two. We recommend at least one low-dose analysis to do correct quantification.

SUBCELLULAR TRACE ELEMENT DISTRIBUTION IN GEOSIPHON PYRIFORME

Geosiphon pyriforme is a unique endosymbiotic consortium consisting of a soil dwelling fungus and the cyanobacterium Nostoc punctiforme. At present this symbiosis becomes very interesting because of its phylogenetic relationship to the arbuscular mycorrhizal (AM) fungi. Geosiphon pyriforme could be an important model system for these obligate symbiotic fungi, which supply 80–90% of all land plant species with nutrients, in particular phosphorous and trace elements. Combined PIXE and STIM analyses of the various compartments of Geosiphon give hints for the matter exchange between the symbiotic partners and their environment and the kind of nutrient storage and acquisition, in particular related to nitrogen fixation and metabolism. To determine the quality of our PIXE results we analysed several geological and biological standards over a time period of three years. This led to an overall precision of about 6% and an accuracy of 5–10% for nearly all detectable elements. In combination with the correction model for the occurring mass loss during the analyses this holds true even for biological targets.

Surface profiling of trace elements across pre-carious lesions in teeth

Abstract Measurements have been made of the surface distribution of trace elements across naturally occurring pre-carious lesions in human dental enamel, and the healthy enamel adjacent to the lesions. The dominant result of this study was the observation of a strong enhancement of fluorine within the lesion area. The fluorine concentration varied markedly within the lesion itself, showing fine structure at about 100 micron intervals.

A METHOD FOR CRYOSECTIONING OF PLANT ROOTS FOR PROTON MICROPROBE ANALYSIS

By building up a cryo-preparation laboratory a method was developed which allows us to prepare cryo sections of fully differentiated plant-root material, according to the requirements of the proton microprobe. Since cryo-preparation is the only possible method delivering reliable elemental distributions, this step was a fundamental for all our further investigations in plant root physiology. As an example, PIXE-investigations of a root section of a heavy metal hyper-accumulating plant Brassica juncea L. will be presented.