About VESPERS

General Introduction

VESPERS (Very powerful Elemental and Structural Probe Employing Radiation from a Synchrotron) is a bending magnet beamline at the third generation Canadian Light Source (CLS). VESPERS makes use of “white” (polychromatic) radiation to accomplish its primary analytical tasks. The x ray energy extends to over 20KV, with a broad maximum in the range from 7-12 KV. When this envelope of energies interacts with samples, intense x ray fluorescence spectra (XRF) are generated by coherent scattering processes: the K, L and M series of characteristic XRF lines that are emitted enable the detection of all chemical elements in the sample excepting those with atomic numbers from 1-12 (H-Mg): x rays from elements in this group (<1-1.5KV) are attenuated by the air sounding the sample before reaching the detector. The same envelope of polychromatic radiation enables Laue X ray diffraction (XRD) patterns to be measured for most crystalline structures that give rise to Bragg diffraction with conventional x ray sources. The polychromatic radiation used for these functions is focussed to a microscopic spot on the sample whose diameter is presently 7 microns; thus “maps” of the spatial distributions of elemental composition or crystalline phases can be generated by raster scanning of the sample position in the x ray beam. A full description of the VESPERS beamline is found on the website: vespersbeamline.com. This Guide is intended to take the Science Studio user through all aspects of the collection of XRF and XRD data on VESPERS. Later versions will cover those functions of VESPERS operation involving the use of the monochromator; this will be used to produce exciting radiation with highly defined energies, as well as with band passes of 1% and 8%.

The Measurement of X ray Fluorescence Spectra

Characteristic X ray spectral lines are emitted from the electronic energy levels of atoms after an electron has been excited (removed) from an inner energy shell. The spectral lines emitted are narrow and their energies are very characteristic of the element from which they originate. A good general introduction of the process is found on Wikipedia. Table 1 gives the energies for all transitions of the K, L and M series. Polychromatic radiation when focussed using the normal silicon optics delivers an energy range of up to ~15 KV to the sample. Considering that maximum energy transfer occurs within the envelope up to 5 KV above the XRF line, it can be seen that such optics favour XRF transitions below 13-14 KV. From the Table, it can be seen that all elements above Mg have transitions in this range, but some are more useful than others. In the near future, we will experiment with platinum-coated optics and filters that will deliver an energy envelope in the range of 15-20 KV; this should provide more flexibility in accessing some elements in some chemical matrices.

Energy from the exciting X rays is also transferred to the samples by incoherent processes. For our purposes the most important of these is Compton scattering. The energy lost in Compton scattering is element-dependant and the scatter is directional. Compton scattering provides background intensity at all energies in XRF spectra as well as broad intensity maxima at energies that are a function of the elemental composition of the sample. Such Compton “peaks” are particularly strong in samples with high concentrations of low Z elements.

The XRF radiation produced is measured by a Si detector that is capable of detecting and analysing x ray energies from <1KeV to 40keV. The detector is positioned at an angle of 45 degrees to the sample (and normal to the incident photon beam). This geometric arrangement reduces the Compton scattering contribution. The detector is positioned as close as 3 cm from the sample to reduce the effects of air attenuation of the XRF energies.

The Peakaboo XRF analysis software that is available through Science Studio has programs that can reduce the background caused by Compton scattering as well as the noise from various sources. As well, characteristic XRF lines can be identified and curve resolved from overlapping peak structures. Finally the resolved data can be mapped as a function of position on the sample.

Measurement of X ray Diffraction Patterns

Laue diffraction is generated by polychromatic x rays back-reflected from the sample; the wavelengths used and the highly coherent nature of the radiation allow the detection of minute changes to the spacing of interatomic planes in a sample. The energy of the x rays used maximises the detection of reflections from the top 50 microns of the sample. The spot pattern produced comprises reflections from many different planes in the sample crystal structure. Their placement will be characteristic of the type of unit cell, the interatomic spacing and the plane involved in the reflection. Using polychromatic radiation, it is not possible to determine any of these a priori; for this, knowledge of the energy needed to generate a particular reflection must be known. Instead, Laue is most profitably used to compare the result of an unknown with that of a reference material. Since most samples (reference and unknown) are polycrystalline with crystalline grains that are larger than the incident photon beam, some strategy will be needed to collect composite “powder patterns” from a region of the sample representing many different orientations.