F. Watt

The Oxford submicron nuclear microscopy facility

Abstract This paper describes the unique nuclear microprobe facility now established in the University of Oxford. The system, which uses a dedicated small accelerator, operates on a regular daily basis, therefore the emphasis of the design has been on achieving reliable, high-quality performance while minimising overheads of alignment in the focusing system and off-line data processing. The beam-optical system from the ion source to the final lens is described. The final lens has been specially developed to have negligible sextupole-field contamination which allows submicron operation using simple alignment procedures. The system uses up to three beam lines with specialised target chambers. These are described briefly. The data acquisition system uses distributed processing with a VMEbus 68020 microcomputer system handling collection and on-line sorting of data from up to six detectors, a graphics workstation for operator interfacing and a mainframe computer for archiving and off-line processing of data. The components of the system communicate via Ethernet and a low-failure-rate/highthroughput communications protocol has been developed. The archiving procedures are designed to handle high volumes of data (up to 100 Mb per day) with efficient data compression, transparent recall of recent data and simple restoration of archived data.

The new Oxford scanning proton microprobe analytical facility

Abstract A purpose-built scanning proton microprobe facility has been established for multidisciplinary research at the University of Oxford. 1–3.8 MeV beams of protons with currents of up to 150 pA focused into submicron-sized probes can now be obtained for production of high-resolution PIXE and BS maps of samples from a wide range of applications. This article briefly describes the system hardware as well as results achieved during the commissioning stage of the facility. Further development work is now directed towards achieving 100 nm probes and, as part of this work, a novel quadrupole lens has been developed which, when used in a doublet or triplet system, achieves submicron resolution. Measurement of the aberrations of these quadrupoles, using the grid shadowmethod, showed negligible parasitic aberration below fifth order. The results are compared to similar measurements performed on the quadrupoles of the original probe-forming lens system.

Microcircuit imaging using an ion‐beam‐induced charge

Ionizing radiation such as photons, keV electrons, or MeV ions can generate electron‐hole pairs in semiconducting material. The high penetrating power of MeV light ions allows them to generate electron‐hole pairs from deep within intact microelectronic devices, so images can be formed of the device active areas with very little degradation of the spatial resolution of the focused MeV ion beam. Furthermore, the ion‐beam‐induced charge (IBIC) image contrast is not strongly affected by the energy loss through the overlying device layers. This article is the first to demonstrate the capability of a nuclear microprobe to generate IBIC images of the active regions of devices through the passivation and metallization layers. The effect of the carrier generation volume on IBIC resolution is assessed. The ability of IBIC to align the major crystal axes of semiconductor samples is shown, and the effect of ion‐induced damage on IBIC image contrast is considered.

The Oxford 1 μm proton microprobe

Abstract The coupled triplet configuration of the Oxford microprobe system in which spatial resolutions of 1 μm have been achieved is described together with the hardware, data collection and operation of the probe. Using the microprobe in conjunction with the technique of PIXE, two-dimensional elemental maps of mouse macrophages have been constructed and elemental concentrations down to 2×10 −17 g/μm 2 have been measured. Measurement of the iron content of the mouse cells indicates that the proton probe is capable of detecting an increase in iron within the cell after the cytoskeleton has been labelled by immunocytochemical means. Limitations in the technique of microprobe PIXE with respect to cellular biology are briefly discussed.

Biological applications of the Oxford scanning proton microprobe

Abstract The use of a scanning proton microprobe for the analysis of elements in biological hard and soft materials is discussed with examples. The full potential of the method is likely to be achieved shortly with improvement in resolution to the subcellular level.

MEV ION-BEAM LITHOGRAPHY OF PMMA

Abstract X-ray lithography mask contrast is limited by the thickness of the patterned absorber layer. This is limited by the need to use thin resist layers to maintain the high spatial resolution when fabricated using electron beam lithography (EBL) because electron scattering severely limits the minimum achievable linewidth in thick samples. A MeV proton beam suffers much less lateral scattering than a keV electron beam so higher resolutions should be possible in thick resist layers. This paper presents experimental results showing a 2.0 μm wide channel etched through ∼ 10 μm of PMMA (polymethyl methacrylate) using a focused 3 MeV proton beam. Simulations demonstrating that the minimum attainable linewidths for 3 MeV protons are ∼ 120 nm through 10 μm of PMMA are also presented.

A SURVEY OF RECENT PIXE APPLICATIONS IN ARCHAEOMETRY AND ENVIRONMENTAL SCIENCES USING THE OXFORD SCANNING PROTON MICROPROBE FACILITY

Abstract 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.

An induced papilla response in primary roots of Scots pine challenged in vitro with Cylindrocarpon destructans

Abstract Cell wall appositions (papillae) were formed in the cortical and occasionally rhizodermal cells of primary roots of Pinus sylvestris L. that had been challenged in vitro with the root pathogen Cylindrocarpon destructans (Zins.) Scholt. Histochemical studies indicated that these papillae were composed predominantly of a matrix of pectic substances which was usually impregnated with polyphenolic or lignin-like materials. Proteins were also detected in the papillae. Elemental analysis of the papillae, using proton induced X-ray emission, indicated a higher concentration of calcium relative to normal cell walls. This result may reflect the high content of pectic materials in the papillae. In contrast to certain other plant species, no evidence was found for elevated silicon levels in pine papillae. Papillae appear to be involved in the resistance of pine primary roots to C. destructans .

Nuclear microscopy of biological specimens

Recent developments in technology have enabled the scanning proton microprobe to scan at submicron spatial resolution on a routine basis. The use of the powerful combination of techniques PIXE (proton induced X-ray emission), nuclear (or Rutherford) backscattering (RBS), and secondary electron detection operating at this resolution will open up new areas in many scientific disciplines. This paper describes some of the work carried out in the biological sciences over the last year, using the Oxford SPM facility. n nCollaborations with biological scientists have drawn attention to the wealth of information that can be derived when these techniques are applied to micro-organisms, cells and plant tissue. Briefly described here are investigations into the uptake of heavy metals by the alga Pandorina morum, the structure of the diatom Stephanopyxis turris, the presence of various types of crystal structures within the cells of Spirogyra, the heavy metal uptake of a mycorrhizal fungus present in the bracken (Pteridium aquilinum) root, the role of sphagnum moss in the absorption of inorganic elements, the measurement of heavy metals in environmentally-adapted cells of the yeast Saccharomyces cerevisiae, and the elemental distribution in the growing tip of a spore from the plant Equisetum arvense, with special emphasis placed on the visual interpretation of the elemental and secondary-electron maps provided by the nuclear microscopical techniques.

Distribution of elements in the Lily pollen tube tip, determined with the Oxford scanning proton microprobe

SummaryPollen tubes ofLilium longiflorum were fixed with glutaraldehyde and investigated unsectioned with the Oxford scanning proton microprobe (SPM). Two-dimensional maps which show the distribution and concentration of phosphorus, sulphur, chlorine, potassium, calcium, iron, copper, zinc and arsenic are presented. The maps show that, within the pollen tube tip region, calcium and zinc exhibit relatively steep longitudinal concentration gradients compared to the more flat distributions of phosphorus and sulphur. Chlorine, potassium, iron and copper appear equally distributed along the tube. All elements with the expception of arsenic show the highest concentration within the cell protoplasm and not in the cell wall. Additional signals of arsenic, chlorine and potassium originate from the remaining fixative dried around the tube, containing also the free ions of the cell. The arsenic signals originate exclusively from the buffer used during fixation. The different maps are compared and discussed in relation to their significance to the pollen tubes.