Primary Beam Modifiers

A cellulose filter placed between a Rh X-ray tube and the sample removes the Rh U line at 2.69 keV and allows only the continuum radiation to excite the sample. ( Thermo Fisher Scientific. www.thennofisher.com. Used with permission.) 

Filters are commonly thin metal foils, usually pure elements, but some alloys such as brass and materials like cellulose are used. Varying the foil thickness of a filter is used to optimize peak-to-background ratios. Commonly used filters for various targets are listed in Tables 8.5 and 8.6. 

Mini-X-Ag output spectrum with and without 80 mil (2 mm) aluminum (Al) filter 

Lead (Pb) XRF with and without an 80 mil aluminum (Al) filter on the mini-X-Ag X-ray tube 

Target Target K (A) Target Kp(A) Filter Element Filter K Edge (A) Foil Thicknese (nm) %Kp Absorbed 

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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). 

Figure 4.21 from reference [14] illustrates depth profiles of a 500 eV P implant in Si using Cs" (with 02 oxygen leak) and 02 primary beams with 150 eV impact energy. The instmment was a Cameca IMS Wf equipped with an rf plasma-type ion source for the 02 beam and a Cs" source with a modified ion extraction system. The SIMS profiles have been compared to HR-RBS data. The depth scale was based on the HR-RBS data and the 2 profile is the best fit to the HR-RBS profile. 

To overcome this problem, we have modified a commercial ion gun to generate a diffuse fast-atom beam [116, 117]. The ion beam neutralizer shown in Figure 7 consists of a multi-hole metal plate through which the primary ions pass. The ions are neutralized by the ion/surface interactions that occur as the beam passes through the metal aperatures and by charge-exchange reactions that occur within the gun assembly. A repeller grid is used to remove the residual ions from the neutralized beam. 

In general, quantitative analysis by EDS in EM is similar to that of XRF. The analytical methods, however, are different for two main reasons. First, the interactions between the electron beam and specimen are different from those of primary X-ray radiation. Second, an EM specimen for chemical analysis cannot be modified as in the internal standard method. For accurate quantitative analysis of EDS in EMs, separate standard samples containing the elements in the specimen to be analyzed are necessary. The standards should be measured at identical instrumental conditions to the specimen. It means that the spectra of specimen and standard should be collected under the same conditions with regard to the following parameters ... 

The primary reason FTIR has not been utilized as a point detection system for trace level contamination in water is that the opacity of water limits the pathlength (L) to 25 pm or less. Based on a minimum CL value of 0.22 pg/cm for phosmet, the minimum detection level would be approximately 10 ppm. Therefore direct detection at low ppb levels in the aqueous phase is not possible. The avenue to achieve a lower detection limit is to increase the concentration (C) in the IR beam. The work presented in this paper has focused on synthesizing and modifying adsorbent materials that would serve as the above mentioned concentrating surface. The main hmdle is to incorporate these adsorbent materials into a sampling technique that will allow trace detection from aqueous systems, while utilizing the inherent selectivity of FTIR spectroscopy. 

Fig. 7. Absorption (A) and absorption changes of NaBH -treated Rb. sphaeroides R-26 reaction centers induced by 33-ps excitation pulses at 880 nm with the measuring puise delayed by -38-ps (B), 0-ps (C) and 1.3-ns (D) relative to the center of the excitation pulse. Qx" and "Qy" refer to dipole transition moments in the porphyrin rings. The angles 0 , 45 and 90 refer to the angle between the E-vectors of the measuring and excitation beams. See text for discussion. Figure source Shuvalov and Duysens (1986) Primary electron transfer reactions in modified reaction centers from Rhodopseudomonas sphaeroides. Proc Nat Acad Sci, USA 83 1692.

An alternative design is based on electrostatic beam transport and focussing. Figure 3.4 shows schematically the Aarhus electrostatic positron beam line (Deutch et al. [3.9]). A 70mCi 8 mm diameter Na source delivers particles that are moderated in a laser-annealed W(100) foil. The transmitted slow positrons are collected and accelerated by a modified Soa immersion lens (Canter et al. [3.10]). They are then further accelerated and deflected through 90° in a cylindrical mirror analyzer and thus removed from the primary P" and 7 background of the source. The beam is then transported and focussed to a spot of 2 mm diameter on the target, where the positron intensity is 10 s ... 

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