Graeme Morrison

ESCA MICROSCOPY BEAMLINE AT ELETTRA

The article describes the ESCA microscopy beamline dedicated to high spatial resolution quantitative and qualitative analysis on surfaces and interfaces. The scanning microscope is constructed to work both in transmission and photoemission within the photon energy range from 200 to 1200 eV with a spatial resolution of ∼0.1 μm. A Fresnel zone plate demagnifies the photon beam to submicrometer dimensions with 109–1010 photons/s in the focus spot. A photodiode and a hemispherical electron energy analyzer are used as detectors for recording the transmitted x‐rays and emitted photoelectrons, respectively. The operation modes in photoemission give the opportunity to obtain conventional energy distribution curve spectra from a microspot or a two‐dimensional micrograph of the spatial distribution and local concentration of a selected element as the sample is mechanically scanned. For conductive specimen topography measurements of a selected surface area probed by SPEM are possible using a scanning tunnelling micr...

Scanning transmission x-ray microscopy with a configurable detector

A configurable charge-coupled-device (CCD) detector is used in a scanning transmission x-ray microscope to record the transmitted intensity distribution for every pixel in a raster scan of the sample. Real-time processing of the CCD frames gives simultaneous absorption and phase contrast image signals from a single scan. The CCD combines fast frame-transfer readout with very high sensitivity and makes use of x-ray to visible-light coupling to allow operation over a wide range of photon energies, from the oxygen K edge upwards. Tests on the Twinmic end station at the Elettra synchrotron are reported.

Scanning x‐ray microscopy using a laser‐plasma source

Previously high‐resolution soft x‐ray microscopy has only been possible with synchrotron sources. Here, the first successful attempts at using a scanning transmission x‐ray microscope with a laser‐plasma source are reported. Spatial resolutions were limited to about 650 nm by electrical noise in the detector, but single shot per pixel images were obtained of test and real specimens. The microscope was not optimized to the source since it was designed for use on the undulator beam line of a synchrotron. With an improved system, it is demonstrated that single shot per pixel imaging at high resolution (better than 50 nm) will routinely be possible.

Phase contrast and darkfield imaging in x-ray microscopy

The transmission x-ray microscope has so far been used almost exclusively to form images with absorption contrast. Methods of forming phase contrast and darkfield images are considered, particularly in the scanning transmission x-ray microscope, and the advantages and disadvantages of these methods are reviewed, particularly in the context of the new generation of synchrotron x-ray sources that will be able to provide high brightness over a much wider range of energies than is available at present.

The exploitation of multichannel detection in scanning photoemission microscopy

Sub-micrometer resolution has been achieved in photoemission microscopy thanks to the high flux and brightness of the soft X-rays provided by third generation synchrotron sources and the progress in microfabrication of focusing elements. The use of multichannel detectors in recently constructed scanning microscopes adds speed and flexibility in data acquisition. Here we present preliminary results on a study of the interactions between sub-monolayer Pd and polycrystalline Ni obtained with the scanning photoemission microscope at Elettra to illustrate the importance of multichannel data acquisition in photoemission microscopy.

Some Aspects Of Quantitative X-Ray Microscopy

A simple model of image formation in the x-ray microscope is used to investigate the relationship between contrast, signal to noise ratio, dose and image resolution. The important feature of this model is the role that the depth of field is expected to play in determining the dose to the specimen when imaging at high resolution, with the result that the dose can be expected to increase as the inverse sixth power of the resolution. Some deficiencies in this model are examined for the case of thin specimens, where linear contrast transfer theory can be applied, and it is shown that there are some advantages associated with the use of phase contrast imaging conditions. Finally, methods of producing phase contrast in the imaging microscope and the scanning microscope are compared in terms of their contrast transfer functions and the ease with which they can be realised in practice.

Active microstructured arrays for x-ray optics

The UK Smart X-Ray Optics programme is developing the techniques required to both enhance the performance of existing X-ray systems, such as X-ray telescopes, while also extending the utility of X-ray optics to a broader class of scientific investigation. The approach requires the control of the inherent aberrations of X-ray systems using an active/adaptive method. One of the technologies proposed to achieve this is micro-structured optical arrays, which use grazing incidence reflection through consecutive aligned arrays of channels. Although such arrays are similar in concept to polycapillary and microchannel plate optics, they are more flexible. Bending the arrays allows variable focal length, while flexing parts of them provides adaptive or active systems. Custom configurations can be designed, using ray tracing and finite element analysis, for applications from sub-keV to several-keV X-rays. The channels may be made using deep silicon etching, which can provide appropriate aspect ratios, and flexed using piezo actuators. An exemplar application will be in the micro-probing of biological cells and tissue samples using Ti Kα radiation (4.5 keV) in studies related to radiation induced cancers.

Sideslip estimation for articulated heavy vehicles at the limits of adhesion

ABSTRACT Various active safety systems proposed for articulated heavy goods vehicles (HGVs) require an accurate estimate of vehicle sideslip angle. However in contrast to passenger cars, there has been minimal published research on sideslip estimation for articulated HGVs. State-of-the-art observers, which rely on linear vehicle models, perform poorly when manoeuvring near the limits of tyre adhesion. This paper investigates three nonlinear Kalman filters (KFs) for estimating the tractor sideslip angle of a tractor–semitrailer. These are compared to the current state-of-the-art, through computer simulations and vehicle test data. An unscented KF using a 5 degrees-of-freedom single-track vehicle model with linear adaptive tyres is found to substantially outperform the state-of-the-art linear KF across a range of test manoeuvres on different surfaces, both at constant speed and during emergency braking. Robustness of the observer to parameter uncertainty is also demonstrated.

Spectromicroscopy of interfaces with synchrotron radiation: multichannel data acquisition

Abstract Probed length scales of sub-micrometer dimensions have been achieved in photoemission spectroscopy owing to the high flux and brightness of the soft X-rays provided by the third generation synchrotron sources and the progress in microfabrication of focusing elements for soft X-rays. The use of multichannel detectors in the recently constructed scanning photoelectron microscopes adds speed and flexibility in data acquisition. Here we present some results obtained with the scanning photoemission microscope at ELETTRA illustrating the importance of the multichannel data acquisition for the interpretation of the data.

Combined emergency braking and turning of articulated heavy vehicles

ABSTRACT ‘Slip control’ braking has been shown to reduce the emergency stopping distance of an experimental heavy goods vehicle by up to 19%, compared to conventional electronic/anti-lock braking systems (EBS). However, little regard has been given to the impact of slip control braking on the vehicle’s directional dynamics. This paper uses validated computer models to show that slip control could severely degrade directional performance during emergency braking. A modified slip control strategy, ‘attenuated slip demand’ (ASD) control, is proposed in order to rectify this. Results from simulations of vehicle performance are presented for combined braking and cornering manoeuvres with EBS and slip control braking with and without ASD control. The ASD controller enables slip control braking to provide directional performance comparable with conventional EBS while maintaining a substantial stopping distance advantage. The controller is easily tuned to work across a wide range of different operating conditions.