Beam filter, primary

Table 8.7 T)T3ical Primary Beam Filters and Range of Use in EDXRF Systems...

Jwng-component of the tube output spectram are used to irradiate the sample, only a limited tube power is required. Since the bremsstraMung continuum not only ensures a uniform excitation of many elements, but also causes a significant scatter background to be present in the recorded EDXRF spectra, most direct-excitation systems are equipped with a set of primary beam filters to alter the tube spectram. By selection of an appropriate filter, the excitation conditions for a particular range of elements can be optimized. In order to facilitate the determination of low-Z elements, commercial systems can be either evacuated or flushed with He, thus reducing the absorption of low energy radiation and scatter. 

Fig. 4. Primary beam filters suppress the spectral lines of the tube anode material (e.g. Rh KB lines)...

Figure 11.9 The use of a primary beam filter to reduce scattered lines and background from the x-ray tube. (Reprinted by courtesy of EG G ORTEC.)...

In direct excitation systems, the source is directed toward the sample thus exciting the sample directly with the radiation from the target or isotope source (Figure 8.15). To modify the excitation or attenuate undesired parts of the excitation spectra, primary beam filters are used. To reduce the spot size and enable micro spot analysis, pinhole masks are in use as well. For micro spots below... 

Primary beam filters, beam masks, and other devices are used to modify the excitation from the source. One of the problems with using an X-ray tube is that both continuum and characteristic line radiation are generated at certain operating voltages, as seen in Figure 8.3. For many analytical uses, only one type of radiation is desired. Filters of various materials can be used to absorb unwanted radiation but permit radiation of the desired wavelength to pass by placing the filter between the X-ray source and the sample. 

Primary beam filters are used to modify the excitation spectra by making use of characteristic, selective absorption. The nature of the material and thickness of the material are the parameters used to tune the primary excitation. Filters are customized based upon the target material and application. Manual filters are inserted by the user into the beam path whereas automatic filters are generally arranged in a wheel-like fashion and controlled by the instrument software (see Figure 8.15). 

The spectrometer contains the X-ray tube, primary beam filters, collimators, analyzing crystals, and detectors. Parts of the spectrometer and the sample measurement position are generally under vacuum, but other atmospheres (purge gases) can be used based on the desired application. X-ray tubes and primary beam filters have been discussed earlier. Most commercially available WDXRF units today use end-window X-ray tubes, which are water-cooled. The window thickness is around 75 pm or less (see Figure 8.10). 

The primary beam filter wheel is equipped with a selection of absorbing foils, commonly A1 and Cu foils of various thicknesses. It is located between the tube and the sample, filtering out undesirable or interfering components of the tube radiation to increase the peak-to-background ratio. 

Figure 11 Energy-dispersive spectra of reference soil sample GXR-2, measured with Spectrace 5000 using a Rh anode tube and Pd and Cu primary beam filters to optimize excitation conditions.

Fig. 1-8. The experiment of Fig. 1-7 as proof of the presence of two components of widely differing wavelength in the secondary spectrum of tin. Calcium showrs only one component. The primary beam (curve 1), being hardest, is least absorbed. Filtering of the primary beam (1.7) is not pronounced enough to cause noticeable curvature here. (See Fig. 1-5.)...

In one method the intensity of the primary beam is diminished by several orders of magnitude through the use of a series of neutral filters, the percentage transmission of each at the wavelength X having been accurately measured. Comparison of the intensity of light scattered by the solution with the intensity of the incident beam meas-... 

Bassi et al. [70] have described IMR-MS for online gas analysis with a sensitivity of 100ppb-l ppm. A mass-selected ion source allows the use of three different primary ion beams (Xe+, Kr+ and CF3I+), covering the recombination energy range from 10.23 to 14.67 eV. For fast measurements, the change from one primary ion to another can be achieved by a Wien filter. IMR-MS allows quantitative analysis. 

Figure 1. Energy filtered experimental Si[ 110] zone axis CBED pattern. The pattern was obtained for a primary beam energy of 195.35 keV, an energy window of lOeV and an electron probe size of 1.4nm, using a Philips CM200/FEG electron microscope.

Figure 9. Energy-filtered experimental and fitted Si[l 10] CBED rocking curves for (a) a line scan along the [111] direction and (b) a line scan along the [002] direction (see Figure 1). The calculations were made for a primary beam energy of 195.35keV and a crystal thickness of 369 nm.

Absorbers are found at many synchrotron beamlines. Two different principles are realized. Tilt-absorbers are operated continuously, whereas filters on a revolving disc offer step-wise attenuation of flux. Absorbers change the spectral composition of the primary beam. Thus the utilization of an absorber during scattering experiments should be avoided. 

Fig. 1. Typical locations for CAM components, showing the photometer, 1 filter wheel, 2 monochromator, 3 shutter and aperture unit, 4 beam splitter, 5 accessories for polarized light such as a rotary analyzer and a compensator, 6 beam splitter for epi-excitation fluorescence, 7 objective lens, 8 stage, 9 substage condenser, 10 condenser aperture, 11 polarizer, 12 field aperture for photometry, 13 shutter, 14 primary illuminator, 15 arc lamp, 16 shutter, 17 monochromator, 18 filter wheel, 19 and ocular, 20.

Figure 24.2 Schematic diagram of the setup used to measure and control H2O concentration and gas temperature in the combustion region (in situ) of a forced 5-kilowatt combustor at Stanford University 1 — steel duct 2 — quartz duct 3 — A1 duct 4 — multiplexed beam 5 — tunable diode lasers 6 — data acquisition and control computer 7 — control signals 8 — primary air driver Aair sin(27r/of) 9 — fuel drivers Afuei sin(27r/of-f dfuei) 10 — demultiplexing box 11 — Si detector (ND filter) and 12 — laser beam...

PHI TRIFT IV ToF-SIMS (Physical Electronics, USA) employs three electrostatic analyzers in the ion path to filter the background and metastable secondary ions. Using liquid metal cluster ion guns (such as Aut ion beam for sputtering of sample surface) increased sensitivity compared to a Ga+ primary ion beam are obtained (www.phi.com). The application of dual primary ion guns is useful for an effective dual beam depth profiling on multi-layered samples. 

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