Selective excitation and integral counting of x‐ray fluorescence (SEICXRF)

Abstract

INTRODUCTIONThe conventional x-ray fluorescence (XRF) spectrometer is based on the ionization of the atoms present in the sample using, as excitation source, either photons with several discrete energies (from radioisotope sources) or with an energy spectral distribution (from an x-ray tube or synchrotron radiation). An alternative XRF method is presented here. In this case, the atoms of the elements present in the sample are selectively excited using, for that purpose, a beam of monochromatic radiation whose energy is tuned in a sequential way to scan the desired range of energies. All the photons emerging from the sample (scattered photons and characteristic fluorescence radiation) are detected and counted in an integral way.One of the main differences between the SEICXRF 1 -3 method and the conventional 4,5 methods lies in the determination of the fluorescent radiation intensity emitted from a definite element; it is measured directly through the jump magnitude that occurs when the energy of the incident radiation is equal to the energy associated with one of the element absorption edges present in the sample. The main mechanisms that generate the characteristic photons as a function of fundamental parameters have great importance and are discussed in this paper. In addition, a specific algorithm is needed, which allows quantification of the fluorescent radiation emitted by the sample for each wavelength of the incident radiation.The effects and the criteria that have been taken into account to determine the jump magnitude corresponding to an element of interest are determined. The oscillating effect of the intensity at the absorption edge could be compensated for in most cases, and therefore it is not considered in the algorithms of the jump quantification. The spectrum is sequentially built up and its shape differs from the form of conventional XRF spectra. The proposed procedure for spectrum acquisition and elaboration brings together several advantages and inconveniences, which are discussed below. METHOD Excitation (selective excitation)The sample is excited with a monochromatic beam which is obtained by the Bragg diffraction of radiation from a primary x-ray source in a double-crystal arrangement device placed between the radiation source and the sample (see upperleft corner in Fig. 1). An x-ray tube or synchrotron 'white' radiation may be used as primary radiation source. The tilt angle of the crystal (Â) is changed in appropriate steps in order to scan the energy of the resulting monochromatic radiation in a range from 0.5 to several keV, depending on the analytical requirements, the sample nature and the crystal or multilayer structure. Detection (integral count)The detection is integral, so that all radiation emitted by the sample, no matter what its energy value, is counted. This may have some variants, for example the counting of the emitted radiation in the effective detection cone (Fig. 2) or the counting of the emitted radiation in all the emission solid angle (2 ) (Fig. 3). The latter alternat...

Atomic spectrometry update. X-ray fluorescence spectrometry

“…The entire emitted XRF signal was collected in a large solid angle, very low-resolution detector, such as a scintillation or hemispherical gas proportional counter. In the first paper of the series, Figueroa 46 described the experimental configuration, typical data and their treatment. Considering the high power and complexity of this system, the detection limits of 10, 6 and 38 mg g À1 for Fe, Mn and Ti, respectively, were less than exciting and were quoted with no measurement time so that any comparison to existing XRF approaches is difficult and the claim of the author for higher count rates and simpler spectra than EDXRF was not reflected in the quoted performance.…”