Please note, due to essential maintenance online transactions will not be possible between 02:30 and 04:00 BST, on Tuesday 17th September 2019 (22:30-00:00 EDT, 17 Sep, 2019). We apologise for any inconvenience.
To send content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send content items to your Kindle, first ensure email@example.com
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
The fundamental parameter approach for quantitative x-ray fluorescence analysis is an application of the equations derived by Sherman and by Shiraiwa and Fujino. Both derivations assume plane and smooth specimen surfaces and defined entrance and take-off angles of x-rays. Our investigation is dedicated to a more general description of measured characteristic signals. Consequently we have to deal with equations that allow us to account for the parameters mentioned below.
The European contributions to X-ray fluorescence analysis (XRF) have shown remarkable fluctuations with regard to time and can be divided into three groups, each of them covering about thirty years.
At the beginning there was the discovery of X-rays by Röntgen in 1895. He received the first Nobel Prize in physics (1901). Barkla (Nobel Prize in physics 1917) gave a description of the interaction of X–rays with matter–scattering (1904) and polarization (1906), absorption (1909) and fluorescence (1911). In 1912 the crucial experiment of Friedrich, Knipping and von Laue brought the confirmation on the nature of X–rays as part of the spectrum of electromagnetic waves (Nobel Prize in physics 1914).
X-ray photoelectron spectrometry (XPS) has been a well established surface analytical technique for approximately 20 years. Fhotoelectrons are ejected by characteristic x-radiation. In our investigations we use Alκα-radiation. The depth from which l-l/e of the measured signal comes, is restricted to a few nanometers by inelastic mean free paths of photoelectrons in solids.
We have developed an algorithm for calculating the x-ray tube continuum based on the depth distribution function (DDF) proposed by Pochou and Pichoir, extended the description of white and characteristic radiation given by Wiederschwinger et al. to the low energy range from 5 to 30 keV and compared the results frorn these algorithms to the signals obtained from algorithms using the absorption correction of Philibert and of Sewell et al. Preceding calculations the measured spectra were separated into characteristic peak spectra and into the corresponding white spectra, where the background below the peak areas was numerically interpolated.
A solution of the intensity equation of X-ray Fluorescence analysis is possible either by the assumption of an effective wave length or by a variation of the take-off angle. When the intensity of the fluorescence radiation is measured as a function of the take-off angle, the extrapolation of this curve against take-off parallel to the flat sample surface offers a simple evaluation of the intensity equation. Since the influence of the secondary excitation is also taken in to account the composition and the mass per unit area of samples can be investigated as well as mass absorption coefficients and mean wavelengths of the polychromatic primary radiation.
It is feasible to investigate the thickness of oxide layers on silicon wafers by X-radiation in the 0.1-10 nm thickness range. For example, X-ray photoelectron spectroscopy (XPS) is a well applicable technique, with information depth of a few nm. Fig. 1 presents the principle of this method. An impinging characteristic X-radiation hν (e.g. Al Kα) count rate ejects Si 2p photoelectrons from the Si-substrate (d), with count rate n2, which, on their way to the electron spectrometer, have to pass through the SiOx-interface (c), the SiO2-layer (b) and the contamination overlayer (a), whereas Si 2p photoelectrons ejected from the SiO2-layer, with count rate n2 have just to penetrate the contamination overlayer. The Si 2p electrons originating from the SiOx-interface, for the situation shown in Fig. 1, can be added to the substrate count rate.
A fundamental parameter algorithm for quantitative XFA with characteristic Kα- and Lα-radiations of the specimens is presented. This allows an application to programmable pocket calculators or a fast evaluation of measured countrates for the unknown composition of the specimen by means of PCs. For the practical utilization we need the incidence angle and the takeoff angle of the x-rays in the instrument, the high voltage V in kV which is supplied to the x-ray tube (E0) and reference measurements on either pure elements or a single reference specimen. The quality of the analytical results depends on the concentration differences of the unknown specimen and the reference specimen.
The depth range d of x-ray photoelectron spectrometry (XPS) is determined by the inelastic mean free path λ of photoelectrons . The following considerations are dedicated to a correlation between d and λ.
Sherman described the excitation of characteristic radiations by primary x-rays and by secondary excitation. The derivation has been made assuming a homogeneous sample. Criss and Birks inverted the problem from the calculation of fluorescent countrates to the quantitative XFA by means of fundamental parameters. Theoretical and instrumental developments enabled a reduction of the sample area and led to small area XFA and imaging XFA sytems. Depth profiling by means of XFA is a further development. We continue the original concept of variable take-off angle technique for the determination of film thicknesses without reference samples and apply the variation of the incidence angle to depth profiling.
We have developed an algorithm for calculating the x-ray tube continuum based on the eqidistribution proposed by Love and Scott, extended the description of white and characteristic radiation given by Wiederschwinger et al for the energy range 10 to 50 keV to the low energy range from 5 to 30 keV, and compared the results from this algorithm to those responses obtained from algorithms using the absorption correction of Pochou and Pichoir, Philibert, Sewell and Pella. The comparison to other models showed a significandy better performance by our model.
The X-ray reflection method according to Schulz is used for investigations of textures in rolled materials. The pole figures are measured either along spirals or along circles. Points of equal intensity are transposed from the record of X-ray intensity to a spiral diagram. Finally contour lines are delineated, pointing out regions of equal pole density. Three ways are known for simplification of the evaluation.
a)The results of the measurement are stored, evaluated by a computer and the pole figure is plotted. Points of equal pole density are represented by equal symbols (2 ,3 ).
b)The pole figure is recorded synchronously with the X-ray measurement along circles or spirals. Ranges of different pole density are characterized by different colors (4 ,5 ).
c)The pole figure is recorded on a photographic film along a spiral. The blackening depends on the measured countrate (6 ,7 ).
An outline on different instruments using photographic registration is given.
Two years ago a paper on a new method of X-ray fluorescence analysis was presented. This method uses a variable take-off geometry and permits a quantitative analysis without calibration curves or the knowledge of the primary X-ray spectrum. As references either chemically analysed samples or pure elements are used. In case that only primary excitation exists, under the aspect, mentioned above, the method is to be regarded as “absolute”. For secondary excitation a simple parabolic approximation has been introduced, which requires a calibration. This paper gives an outline on the progress obtained during the last two years.
The Dietary Inflammatory Index (DII)TM, which was developed to characterize the inflammatory potential of a person’s diet, has been shown to be associated with inflammatory conditions such as cancer. The present study aimed to investigate the association between DII scores and colorectal adenoma (CRA), a pre-cancerous condition.
Responses to baseline dietary questionnaires were used calculate DII scores. In a cross-sectional study design, the association between DII scores and CRA prevalence was determined in men and women separately using logistic regression models.
Ten cancer screening centres across the USA.
Participants were those included in the screening arm of the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial.
Among the 44 278 individuals included in these analyses, men with diets in the most inflammatory quartile of DII scores had higher odds of all types of CRA (advanced, non-advanced and multiple (>1)) compared with those with diets in the least inflammatory quartile of DII scores. In fully adjusted models, compared with those with DII scores in quartile 1 (least inflammatory), males with DII scores in quartile 3 (adjusted odds ratio (aOR)=1·28; 95 % CI 1·12, 1·47) and quartile 4 (aOR=1·41; 95 % CI 1·23, 1·62) were more likely to have prevalent distal CRA. Higher DII scores, representing a more inflammatory diet, also were weakly associated with a higher prevalence of CRA in women.
Implementing an anti-inflammatory diet may be an effective means of primary prevention of CRA, especially in men.
We have embarked on a survey of ROSAT PSPC archival data searching for all detected surface brightness enhancements due to sources in the innermost R ≤ 15′ of the PSPC field of view in the energy band 0.5–2.0 keV. This project is part of the Wide Angle ROSAT Pointed Survey (WARPS) and is designed primarily to measure the low luminosity, high redshift, X-ray luminosity function of galaxy clusters and groups. Accurate measurements of the high redshift XLF would allow the form of the XLF evolution to be determined via the position of the Schechter function break. This would help discriminate between luminosity and density evolution, and discriminate between different hierarchical models, e.g., those including a different mix of fundamental particles, a flat power spectrum of the initial fluctuations, and reheating of the intracluster gas at high redshifts.
The ROSAT observatory with its high spatial resolution X-ray telescope is an ideal instrument for the study of clusters of galaxies. In the first part of the mission an All Sky X-ray Survey was conducted with ROSAT. Here we present first results of observations of Virgo, Perseus and some other clusters of galaxies from the All Sky Survey. These data illustrate the capabilities of the ROSAT observatory for studies of the morphological structure and physical properties of galaxy clusters.
We report on a comprehensive study of the defect structure in GaN grown on c-oriented sapphire by gas source molecular beam epitaxy and metal organic vapour phase epitaxy. Transmission electron microscopy is used to investigate the defect structures which are dominated by threading dislocations perpendicular to the sapphire surface and stacking faults. Additionally, dislocation densities are determined. For determination of dislocation densities by x-ray diffraction we employ a model that uses the linewidth of x-ray rocking curves for this purpose. Finally, Rutherford backscattering spectrometry is performed to complement the structural investigation.
We investigate the structure, growth morphology and the related electro-optical properties of gallium nitride (GaN) films deposited on (0001) sapphire substrates by gas source molecular beam epitaxy (GSMBE) and use transmismission electron microscopy, atomic force microscopy and scanning tunneling microscopy, photoluminescence (PL) and cathodoluminescence (CL). We find two types of specimens: one type which shows a strong UV luminescence (band-to-band transition at 358nm/3.46eV) in CL and PL and only faint yellow luminescence (Gaussian shaped CL/PL peaks at around 528nm/2.35eV), specimen ‘B’, and another type, which shows a strong UV and a comparably strong yellow luminescence, specimen ‘Y’. These two types of specimens have a rough layer surface, specimen ‘Y’ even an islanded one with, facetted hexagonal islands with a width of 1-2μm at a height of 50nm. A correlation of spectrally resolved CL images to the observed defect structure shows: (i) the yellow luminescence is homogeneously distributed over the complete specimen for ‘B’ and ‘Y’ specimens. Our investigations strongly suggest the yellow luminescence to be related to screw dislocations with , which are found randomly distributed in ‘B’ and ‘Y’ specimens with a high density of 1.3·109cm−2; (ii) the strong UV luminescence in ‘Y’ specimens is located in the troughs between adjacent surface islands, where dislocations essentially in small angle grain boundaries of edge type, i.e. with or are located; (iii) in the case of the ‘B’ specimens these dislocations are randomly distributed and so is the luminescence.
GaN epitaxial layers on GaN single crystals were grown using molecular beam epitaxy with an NH3 source. The deposited layers were examined by high resolution x-ray diffraction and photoluminescence (PL) spectroscopy. We observed strong and extremely narrow (half-widths of 0.5 meV) lines related to the bound excitons. In the higher energy range we observed three strong lines. Two of them are commonly attributed to free exciton transitions A (3.4785 eV) and B (3.483 eV). Their energetic positions are characteristic of strain-free GaN material.