A.D. Smith

Apparatus for In Situ X-Ray Absorption Fine Structure Studies on Catalytic Systems in the Energy Range 1000 eV < E < 3500 eV

A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal ...

The local structure of aluminium sites in zeolites

The long range ordering and thus the average structure of crystalline zeolites can be determined by various diffraction and spectroscopic techniques. It has, however, proved difficult to establish the local structure surrounding these aluminium sites by diffraction methods. The most useful information has come from theoretical studies (M. Brandle et al., J. Chem. Phys., 1998, 109, 10379; U. Eichler et al., J. Phys. Chem. B, 1997, 101, 10035) which suggest that the Al–O distance associated with the proton is longer than other Al–O interatomic distances. Employing in situ X-ray absorption fine structure spectroscopy (EXAFS) of the aluminium edge at 1565.6 eV, we report individual bond lengths angles for the local aluminium environment of neutral and acidic zeolites. For two acidic zeolites we find that there is indeed one Al–O distance that is significantly longer than those in a neutral material. We also show that for the average T-atom positions our EXAFS results are consistent with X-ray diffraction measurements, to an accuracy of ca. 0.01 A. Changes in bond angles show how the zeolite structure distorts to accommodate Bronsted acidity.

Performance of the Daresbury synchrotron radiation source soft x‐ray double‐crystal monochromator

The double‐crystal monochromator on beam line 3.4 at the Daresbury synchrotron radiation source covers the energy region 0.8–4 keV, routinely using crystal pairs of beryl, quartz, InSb, Ge, and Si. The beam line has a unique system to cut off high‐energy radiation. The whole beam line has been built on a swinging arm, which allows the glancing angle on a chromium coated premirror to be varied between 0.5° and 3°. This not only reduces the heat load on fragile crystals, but also reduces the higher order contributions. We have made a thorough study of the alignment problems arising in the double‐crystal geometry. Crystal misalignment, Bragg angular offset, as well as heat‐load effects, can cause severe beam movements. A beam‐position monitor, which has recently been commissioned, is an invaluable diagnostic tool for these beam movements.

Modulation effects in wedge and strip anodes

We present results on the image distortion or ‘‘modulation’’ produced when a wedge and strip anode (WSA) position readout is used with a charge cloud of size comparable with that of the pattern repeat pitch. We use a standard detector consisting of a microchannel plate (MCP) intensifier and wedge and strip anode to investigate the effect of detector operating parameters on image modulation. We find that modulation depends on the MCP gain of each event and on the MCP voltage, and is minimized when the plates are run at high gains and with high rear MCP operating voltages. We compare the experimental results obtained using this detector with the response calculated numerically in a computer simulation. From the simulation it becomes apparent that for a detailed understanding of wedge and strip performance the microchannel plate charge cloud distribution must be considered. However, some of the general features of the wedge and strip anode performance are independent of the charge cloud form. The most import...

XAS studies of the effectiveness of iron chelating treatments of Mary Rose timbers

The oxidation of sulfur in marine archaeological timbers under museum storage conditions is a recently identified problem, particularly for major artefacts such as historic ships excavated from the seabed. Recent work on the Vasa has stressed the role of iron in catalysing the oxidative degradation of the wood cellulose and the polyethylene glycols used to restore mechanical integrity to the timbers. In developing new treatment protocols for the long term preservation of Henry VIII of Englands flagship, the Mary Rose, we are investigating the potential of chelating agents to neutralise and remove the iron products from the ships timbers. We have explored the use of aqueous solutions of chelating agents of calcium phytate, ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and ammonium citrate to extract the iron compounds. All of these solutions exhibit some level of iron removal; however the key is to find the most effective concentration at pH of around 7 of the reagent solution, to minimise the treatment time and find the most cost-effective treatment for the whole of the Mary Rose hull. Fe K-edge XAFS data from samples of Mary Rose timbers, before and after treatment by the chelating agents mentioned has been collected. The data collected provide valuable insights into the effectiveness of the treatment solutions.

The development of X-ray photo-emission electron microscopy (XPEEM) for valence-state imaging of mineral intergrowths

Abstract We demonstrate the potential of X-ray photo-emission electron microscopy(XPEEM) to reveal valence-state images of the spatial distribution and relative concentration of metals in specific oxidation states. Additionally, XPEEM allows X-ray absorption spectra to be extracted from chosen pixel areas of the images. Using an in-house-built XPEEM instrument we show an application of the method in providing valence-state images of complex mineral intergrowths. The image resolution achieved with this instrument of simple design was ~5 μm and reasonable quality X-ray absorption spectra were extracted from areas of ~5×5 μm. These initial results suggest that by using commercial XPEEM instruments on 3rd generation, high-brightness synchrotron sources a spatial resolution of 100 nm or better could be achieved, with the ability to extract high-quality X-ray absorption spectra from areas of 0.1 μm2. Given that standard thin sections or polished blocks can be studied by XPEEM, and that each XPEEM image records ~1000 μm2, XPEEM can be used in conjunction with other analytical methods such as EMPA and TEM-EELS/PEELS.

Application of Microfocus X-Ray Beams from Synchrotrons in Heritage Conservation

Synchrotron-based techniques are becoming increasingly important in heritage science and the aim of this article is to describe how recently developed microfocus methods can probe the elemental composition, speciation and structure at the micron level in samples from structures. Firstly an outline is given of the major techniques that are used, namely x-ray fluorescence, diffraction and absorption spectroscopy, and the information that they can provide. This is followed by a description of the experimental set-up and procedures. The application of the methods is exemplified by case studies of the degradation of three types of historic structural materials; marble, glass and ship timbers. The results of the studies and their role in developing conservation strategies are described.

A ReflEXAFS apparatus for use with soft X-rays in the sub-4-keV energy range

Abstract Grazing angle reflection EXAFS (ReflEXAFS) is a useful tool for determining the surface layer and interlayer structures of thin film samples. At X-ray energies above about 4 keV it is practicable to design a ReflEXAFS apparatus which uses standard optical micro-positioning equipment and can be operated in air. At soft X-ray energies below this atmospheric X-ray absorption becomes a significant problem and it is normal to measure EXAFS with the sample in vacuum which would generally preclude the use of commercially available goniometer stages. We describe an apparatus for soft X-ray ReflEXAFS which can be operated at either reduced atmospheric pressure or in a helium atmosphere and has been built using mostly off-the-shelf components.

Development of X-ray photoemission electron microscopy (X-PEEM) at the SRS.

The use of synchrotron radiation sources for X-ray spectroscopy is a well known and developed field. The majority of applications, however, have been limited to studies of materials containing only a single phase of the element of interest. Owing to limited availability of suitable instrumentation, the study of materials comprising intergrowths of different phases has presented difficulties in analysis. The majority of natural materials, including mineralogical samples, fall into this category. However, by applying the technique of photoemission electron microscopy (PEEM) to view the X-ray stimulated photoemission generated at an absorption edge, micro-area-selectable spectroscopy becomes possible. An instrument for X-ray PEEM (X-PEEM) is being developed at the Daresbury SRS and this paper shows how it can be used to obtain characteristic L-edge XANES spectra from finely intergrown iron oxide minerals.

A solid state detector for soft energy extended x‐ray absorption fine structure measurements

Following the success of solid state detector systems for extended x‐ray absorption fine structure studies at high x‐ray energies, there is now an increasing demand for similar devices capable of operating in the soft x‐ray energy range below about 3 keV. Recent developments in sophisticated detector fabrication techniques now make the construction of specialized devices, suitable for high quality spectroscopy in this energy range, a practicable proposition. We present the results of extensive testing of a new detector developed specifically for use in the sub‐3 keV energy range. We have measured energy resolutions of less than 125 eV full width at half maximum at sulfur and silicon Kα energies and the ability of the detector to achieve this resolution at the copper Lα line has also been shown. Finally we demonstrate the potential of this device in a study of trace dopants in bulk silicon based quantum dot glasses.