Oxford Scanning Proton Microprobe Research Articles

The Incorporation of Laser Ablation for Sampling into a Proton X-ray Emission System

ABSTRACTThe variation with depth of the composition of corrosion layers on buried metal objects can provide the archaeologist with valuable information relating to the burial conditions of the object. In some cases these layers can be very thick and so, normally destructive methods such as sampling are used to characterise the metal and corrosion layers when possible. We have developed a system for the use of a microfocused high power pulsed Nd:YAG laser to ablate the corrosion layer in a series of controlled steps while monitoring the composition of the exposed surface using focused PIXE and RBS in the external beam facility of the Oxford Scanning Proton Microprobe. With proper manipulation of the data we are able to calculate the values for trace elements at various depths in the corrosion layer. It is hoped that this system will lead to a better understanding of corrosion properties.

The new external beam facility of the Oxford scanning proton microprobe

This paper describes the development of a high spatial resolution external beam facility on one of the beamlines of the Oxford scanning proton microprobe tandem accelerator. Using a magnetic quadrupole doublet to focus the beam through the Kapton exit window a beam diameter of <50 μm full width at half maximum (fwhm) can be achieved on a sample located at 4 mm from the exit window. The facility is equipped with two Si–Li X-ray detectors for proton-induced X-ray emission (PIXE) analysis of light and trace elements respectively, a surface barrier detector for Rutherford backscattering spectrometry (RBS) analysis and a HP-Ge detector for γ-ray detection. The mechanical and beam-optical design of the system is described.

The study of thick corrosion layers on archaeological metals using controlled laser ablation in conjuction with an external beam microprobe

The variation with depth of the composition of corrosion layers on buried metal objects can provide the archaeologist with valuable information relating to the burial conditions of the object. In some cases these layers can be varied in thickness (up to 1 mm) and so normally destructive methods such as sampling are used to characterise the metal and corrosion layers when possible. This paper describes the use of a microfocused high power pulsed Nd:YAG laser to ablate the corrosion layer in a series of controlled steps while monitoring the composition of the exposed surface using focused proton-induced X-ray emission (PIXE) and RBS in the external beam facility of the Oxford scanning proton microprobe (SPM). The region of the surface removed by the laser is typically less than 500 μm in diameter and so by comparison with other sampling techniques, the effect on the appearance of an object is minimal. Analysis of a spot this size requires a focused proton beam. The modification to the facility to allow the precise registration of the laser beam and the ion beam are described and preliminary results on corroded bronzes are presented.

Nuclear microbeam study of advanced materials for fusion reactor technology

The Oxford scanning proton microprobe was used to study SiC fibres, SiC/SiC ceramic composites and Be pebbles, which are some of the most important materials for fusion technology. For the SiC materials, although the results reveal a high degree of homogeneity and purity in the composition of the fibres, some grains containing heavy metals were detected in the composites. Rutherford backscattering analysis further allowed establishing that at least some of these grains are not on the surface of the material but rather distributed throughout the bulk of the SiC composites. The two different types of Be pebbles analysed also showed very different levels of contaminants. The information obtained with the microbeam analysis is confronted with the one resulting from the broad beam PIXE and RBS analysis.

RECENT BIOMEDICAL APPLICATIONS OF THE OXFORD SCANNING PROTON MICROPROBE

The Oxford Scanning Proton Microprobe continues to be used in the field of trace element measurement in biological systems, exploiting the unique advantages of sensitive, quantitative trace element analysis using PIXE, high spatial resolution and the long penetrating power of MeV protons. This paper outlines a number of recent applications which highlight these advantages. These include: (a) Analysing the distribution of metals in the pupae of leaf-cutting ants to determine the storage sites and transport mechanism of metals used to harden the edges of the mandibles. (b) A study of the distribution of zinc in the retina of rats to determine the role of zinc in light and dark adaptation of the eye. (c) The analysis of crystals of proteins and other large organic molecules prepared for structure determination using x-ray diffraction. These often contain metal atoms, and the identity and concentration of the metal is an important diagnostic for determining the nature of the protein and the quality of the crystallisation. The crystals are normally small (~100μm) and so microPIXE is being used to characterise them. This technique has wide ranging applications, including qualitative and quantitative identification of metals in reaction centres, in active sites and in metal binding proteins, and of DNA or RNA bound to proteins.

Bone growth and bone development in the presence of implants or after induced leg-lengthening studied using the Oxford Scanning Proton Microprobe

To respond to varying environmental demands the bone tissue in the body is under continual reconstruction throughout life. It is known that metallic elements are important for maintaining normal bone structure, but their roles are not well understood. More information about the effects of metal excess or deficiency is needed to help in the development of metallic bone implants and to improve the treatment of bone fractures and defects. The Oxford Scanning Proton Microprobe (SPM) is being applied in two studies involving metal ions in bone: (1) bone regrowth and bonding to titanium bone implants may be influenced by diffusion of Ti ions into the bone. We are using microPIXE to determine the metal ion content of bone developing in contact with implants of pure Nb, Ti and Ti alloys. (2) Bone lengthening as a surgical procedure is induced by fracturing the bone and allowing it to heal with a small gap between the fractured ends created by the use of external fixators. The gap can be slowly increased during the healing process to stimulate the production of new bone. The enzymes and other constituents of the developing bone need certain metals for their function. Using experimental animals we have studied the concentrations of the metals and whether a deficiency of trace metals limits the optimum rate of bone lengthening.

Micro-PIXE as a technique for studying nickel localization in leaves of the hyperaccumulator plant Alyssum lesbiacum

Certain terrestrial plants are able to accumulate metals such as zinc, manganese, nickel, cobalt, or copper in their above-ground biomass. The largest group of these so-called “metal hyperaccumulators” is to be found among certain species in the family Brassicaceae endemic to ultramafic soils. For example, nickel concentrations in members of the genus Alyssum can reach 3% of the leaf dry biomass. However, nickel levels in the root tissue of these plants are low, suggesting that hyperaccumulation is associated with effective metal translocation from root to shoot and sequestration of the metal in non-toxic form within the leaves. To investigate the sites of nickel localization within A. lesbiacum, leaf cross-sections were examined by nuclear microscopy using PIXE and RBS on the Oxford Scanning Proton Microprobe (SPM) with a spatial resolution of 1 μm. This paper describes the sample preparation and analysis methods and presents some preliminary results indicating that nickel is sequestered to a considerable degree within the epidermal trichomes on the leaf surface.

Nuclear microprobe analysis of atmospheric aerosol samples: Comparison with bulk measurements and analyses of individual particles

Selected aerosol filter samples, which have been analyzed by bulk analysis techniques, were examined with the Oxford scanning proton microprobe using micro-PIXE and micro-RBS. From a comparison of the micro-PIXE data for large collections of particles with the bulk analysis results, it was estimated that micro-PIXE of aerosols provides data which is accurate to within 10–20%. Loss in the course of the microprobe bombardment was examined for the elements detected by PIXE, but was generally insignificant. Examples of analyses on complex particle agglomerates are presented. For individual calcareous aerosol particles from Israel, the contributing chemical compounds were identified and quantitatively determined. Several of these particles consisted of a single compound (e.g., calcium carbonate, calcium sulphate), but mixed particles with equal molar amounts of two compounds (e.g., of calcium carbonate and calcium phosphate) were also observed. It is also indicated what further improvements in nuclear microprobe analysis are especially desirable for examinations on submicrometer sized aerosol particles.

Influence of the elemental composition of individual fly ash particles on the efficiency of the electrostatic precipitators

The Oxford Scanning Proton Microprobe has been used to determine the elemental composition of individual fly ash particles in the 1–5 μm geometrical size range, collected before and after a pilot electrostatic precipitator (3 MWe equivalent power) of a power plant burning South African coal (Amcoal). The electrostatic precipitator was working at high collection efficiency (99.8%) obtained by operating under semi-pulsed energization mode. Al, Si, P, S, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga and Sr have been always detected in particles collected simultaneously both before and after the electrostatic precipitator. In some particles also V, Ge, As and Zr resulted above the detection limits. Both at the ESP inlet and outlet one hundred particles have been analysed. According to major element concentrations, particles can be grouped into seven classes showing some preferential associations of minor and trace elements with specific matrix compositions. Most particles (about 80%) belong to classes 1 and 2 characterized by the same aluminosilicate nature with higher concentrations of several minor (P, Ca, Ti, Fe) and trace (S, Mn, Ni, Zn, Ga) elements in class 2. Within the same particle size range, the electrostatic precipitator seems to be selective with respect to particle composition: class 2 particles show a higher penetration. This result is discussed in terms of the particle composition effects on the fly ash electrical properties, which are relevant parameters for the electrostatic precipitation process.

PIXE analysis of fusarium culmorum fungus treated with chemical agents

PIXE measurements of elemental concentrations of light elements along the length of a fungal hyphae provide important data for the research into plant physiology, which other techniques are unable to due to the lack of sensitivity and or spatial resolution. The Oxford Scanning Proton Microprobe was used to measure the elemental concentrations of phosphorus, sulphur, potassium and calcium in the fungus fusarium culmorum under normal conditions. These results were compared with hyphae treated with two groups of chemical agents, A and B. Treatment with A and B changes the morphology of the hyphae. Special emphasis was thus put on the concentrations at the tip of the hyphae, as this represents the growing region. The differences between groups were established from elemental groupings.

Occurrence mode of major and trace elements in individual fly-ash particles

As the first stage of a research programme aimed at improving the efficiency of the electrostatic precipitators, PIXE and RBS analyses using the Oxford Scanning Proton Microprobe have been carried out for about one hundred fly-ash particles collected at the inlet of the electrostatic precipitators of a coal-fired power plant. These show that particles can be grouped into several classes according to matrix composition and trace element composition.

Recent developments in data acquisition and processing on the Oxford scanning proton microprobe

A new data acquisition system has been developed for the Oxford SPM facility. This is based on a single PC-compatible computer with a custom designed interface unit and permits the recording of several channels of pulsed detector data together with x and y scanning voltages, beam charge and deadtime. In addition, the system will also control the beam position allowing the generation of irregular scan areas, line scans at any angle and the selection of point analyses using the mouse. The hardware and software of the system are described and some examples of data collected using the system are presented. One problem of quantitative microanalysis using focused beam PIXE is the fact that the local matrix composition and secondary electron emission characteristics may change on a scale comparable with the beam size across the sample. This leads to uncertainties in the measured charge (especially at the low currents found in microbeams) and local self-absorption correction which can introduce significant inaccuracy into the analysis. This problem has been largely solved at Oxford by the simultaneous use of RBS analysis. The shape of the RBS spectrum yields the local matrix composition and its area gives a measure of the total charge, so that the dependence on precise charge measurement is much reduced. This “ Q factor” technique is described in detail and it is shown that once the system has been calibrated routine standardless accuracy of the order of 5–10% can be obtained.

External beam analysis of living sycamore xylem infected by pathogenic fungi

Interactions between the living xylem (sapwood) of sycamore ( Acer pseudoplatanus) and wood inhabiting fungi have been investigated using a number of techniques including conventional histochemical and biochemical methods, non-invasive NMR imaging and external beam micro PIXE analysis using a 200 μm diameter beam of 3 MeV protons from the new external beam facility on the Oxford Scanning Proton Microprobe. The site of the fungal lesion on a living tree was exposed by a fresh cut immediately prior to analysis and both longitudinal and radial profiles through the infected regions were obtained in a point-by-point fashion. Profiles of several inorganic elements were obtained which correlated well with the observed discoloration due to the infection. The new external beamline at Oxford is described and results are presented. These are discussed in relation to the investigation of anti-microbial defence mechanisms in living trees.

Nuclear microscopy for the study of coal combustion related phenomena

The Oxford Scanning Proton Microprobe has been used for the determination of major and trace element concentrations at the single particle level in fly ashes collected at the inlet and outlet of the electrostatic precipitators of a coal-fired power plant. The analytical results provide valuable information about the particle formation mechanisms upon combustion, the fly ash collection process within electrostatic precipitators and the environmental impact associated with the emission of particulate matter.

Determination of the number of ions in the Oxford electron beam ion trap

In order to determine absolute numbers of ions trapped in the Oxford electron beam ion trap (EBIT), the efficiency of a lithium-drifted silicon X-ray detector has been characterized over the energy range 1.5–15 keV using the Oxford scanning proton microprobe. Preliminary results for the number densities of neon-like barium and highly charged argon ions are presented.

Nuclear microbeam analysis of wear particles in tissue surrounding failed hip joint implants

Particulate wear debris from hip joint replacements is an important factor in determining the response of the surrounding tissue to the implants. Failed replacement joints are surrounded by a layer of fibrous tissue showing an inflammatory response to the wear debris. This reaction leads to bone resorption and the eventual failure of the prosthesis. In preliminary experiments the Oxford scanning proton microprobe has been used to study the composition of wear particles in the membranes surrounding failed implants constructed from Ti/V/Al alloy. The membranes were observed to contain 1–10 μm particles with major constituents from the alloy and also from the cement and polyethylene used in the fitting and construction of the implant. Histological staining shows that these particles are associated with areas of high macrophage activity. Individual PIXE analysis of the metal particles indicates that two populations (high Ti/low Al and low Ti/high Al) may be present. These observations will provide further information on the mechanisms of implant degradation.

The Oxford scanning proton microprobe

The Scanning proton microprobe uses a focused beam of MeV protons to give analytical information by exploiting two physical processes, particle induced X-ray emission (PIXE) and Rutherford Backscattering (RBS). Scanning transmission ion microscopy may also be included in the nuclear microscopy package to give imaging data. PIXE is particularly appropriate for biomedical research as signals from hydrogen, carbon, nitrogen and oxygen, which are the main components of a biological matrix, are not detected and therefore signals from inorganic elements with z > 10 will not be swamped by signals from the matrix. Elements with z < 10 can, however, be detected using RBS when present at higher concentrations. Apart from the biomedical field, the SPM has wide-ranging applications spanning diverse areas such as archaeology, materials science and environmental chemistry. This paper reviews the theory of ion beam analysis, the Oxford Scanning Proton Microprobe and outlines some of the work currently being carried out at Oxford.

Dislocation imaging with a scanning proton microprobe using channeling scanning transmission ion microscopy (CSTIM)

Scanning transmission ion microscopy (STIM) combined with channeling has previously been demonstrated to be able to map regions of relatively poor crystal quality due to beam induced damage [1,2]. This paper describes the implementation of the CSTIM technique on the Oxford scanning proton microprobe and its ability to image misfit dislocations at the interface of an epitaxial Si 0.85Ge 0.15 layer grown on a (001) silicon substrate. Proton energy loss maps are generated by detecting transmitted protons with the beam aligned with a major axis or plane of the crystal. The bending of the crystal lattice planes due to the presence of dislocations causes dechanneling of the beam, giving protons transmitted through these regions a greater energy loss than those transmitted through regions of good crystallinity. Groups of dislocations give rise to bands of contrast along the [110] and [110] directions. Changes in contrast on tilting the specimen are consistent with the dislocations being of the 60° type rather than edge type, in agreement with the TEM results obtained from this specimen. By imaging misfit dislocation “cross” patterns in a Si 0.95Ge 0.05 layer grown on a silicon substrate, it has been shown that the minimum number of dislocations in a group that can be imaged is approximately 5 with a beam size of about 0.3 μm.

Nuclear microscopy: biomedical applications

Recent developments in high energy ion beam techniques and technology have enabled the scanning proton microprobe (SPM) to make advances in biomedical research. In particular the combination of proton induced X-ray emission (PIXE) to measure the elemental concentrations of inorganic elements, Rutherford backscattering spectrometry (RBS) to characterise the organic matrix, and scanning transmission ion microscopy (STIM) to provide information on the density and structure of the sample, represents a powerful set of techniques which can be applied simultaneously to the specimen under investigation. This paper reviews briefly the biomedical work using the proton microprobe that has been carried out since the 2nd Int. Conf. on Nuclear Microprobe Technology and Applications held in Melbourne, 1990. Three recent and diverse examples of medical research are also presented from work carried out using the Oxford SPM. The first is a preliminary experiment carried out using human hair as a monitor for potential toxicity, using PIXE elemental mapping across the hair cross section to differentiate between elements contained within the hair and contamination from external sources. The second example is in the use of STIM to map individual cells in freeze-dried tissue, showing the possibility of the in situ microanalysis of cells and their extracellular environment. The third is the use of PIXE, RBS and STIM to identify and analyse the elemental constituents of neuritic plaque cores in untreated freeze-dried Alzheimer's tissue. This work resolves a current controversy by revealing an absence of aluminium levels in plaque cores at the 15 ppm level.

A survey of recent PIXE applications in archaeometry and environmental sciences using the Oxford scanning proton microprobe facility

The Oxford scanning proton microprobe (SPM) facility is now running on a routine basis in a wide variety of scientific programmes. This paper reviews this work with particular reference to the thick-target PIXE aspects of elemental mapping, and discusses some of the problems of using PIXE with a microbeam, especially in inhomogeneous “real” samples. The paper is illustrated with results from programmes in archaeometry and environmental sciences.