Barkla

C. G. Barkla (Edinburgh) discovery of the characteristic Rontgen radiation of the elements. 

Absorption measurements by Barkla revealed the existence of characteristic x-ray emission lines before x-ray wavelengths could be measured. There is no better evidence for the fundamental importance of the absorption process. 

Fig. 1-7. Schematic diagram of Barkla s experiment. Barkla proved that the component of the x-ray beam reaching the ionization chamber is independent of angle and characteristic of the element used as secondary emitter.

Barkla, originally interested mainly in v-ray scattering, discovered characteristic x-rays by an experimental method similar in principle to that described above. His experimental arrangement (Figure 1-7) is reminiscent of that used today in studies of the Raman effect. By using an absorber in the form of sheets (Figure 1-7) to analyze the scattered beam in the manner of Figure 1-4, he obtained results that clarified the earlier experiments described above. 

Figure 1-8 shows log-log curves calculated from Barkla s absorption-coefficient data. (A log-log plot shows most clearly what Barkla discovered.) For carbon, the wavelength distribution is virtually unchanged from that of the incident polychromatic beam, mainly scattered x-rays being detected the situation is reminiscent of Figure 1-5. The curve for calcium, on the other hand, begins with a straight line that shows the presence in the scattered beam of a relatively intense component for which k is large and sensibly constant. The curve for tin shows two such components. Barkla realized that these components are emitted, and he eventually called them K and L spectra.22 He...

Fig. 1-9. Discover " of the absorption edge. The curve shown is for the experiment of Fig. 1-7 with iron and aluminum as the absorbers and elements of increasing atomic number as samples. In this region of the spectrum, absorption by aluminum increases uniformly with X. Iron shows the sharp drop at its K edge. (After Barkla and Sadler, Phil. Mag. [6], 17, 739.)...

Barkla extended as follows the technique by which he discovered the characteristic spectra. He obtained mai for the K spectrum of each of a series of elements that as an example included iron and bracketed it in regard to atomic weight. In addition, with iron as absorber in Figure 1-7, he obtained /ape for the K spectrum of each other member of the series in position S. The plot of against mai (Figure 1-9) showed a marked decrease in gpe at a value of mai just below that characteristic... 

Bragg identified his lines with Barkla s L series by measuring their juai, and by showing that this characteristic was independent of the diffracting crystal. 

The discovery of the x-ray spectrometer changed x-ray research from a roughly quantitative to a highly precise activity. Barkla s discoveries, which owed so much to absorptiometry, were in the main confirmed, and the emphasis in x-ray research shifted from absorption to emission. 

Moseley found that each K spectrum of Barkla contains two lines, Ka and K(3, and that the L spectra are more complex. Later important work, especially by Siegbahn,38 has shown that M, N, and O spectra exist and are more complex in their turn. Relatively numerous low-intensity lines are now known to exist in all series. Fortunately, the analytical chemist can afford to ignore most of these low-intensity lines in his practical applications of x-ray methods at present. It generally suffices for him to know that x-ray spectra at their most complex are enormously simpler than emission spectra involving valence electrons, and that most x-ratr lines are satisfactorily accounted for on the basis of the simple selection rules that govern electron transitions between energy states. 

The relationship between characteristic lines and absorption edges, which Barkla appreciated (see Figure 1-9), is simplest for the K series because there is only one K energy level—hence only one K edge. There are three L and five M edges (see Figure 1-17). 

Kiistner11 has devised an interesting method in which a single filter is used sequentially in two positions to accomplish selection of a characteristic line. The method, which reminds one of Barkla s work (1.8),... 

Absorption, equation, 15 importance in detection, 44, 45 measurements by Barkla, 11-13 of x-rays, discussion, 9-20 in detectors, 44, 45 photoelectric, 18-20 and scattering as functions of wavelength, 107... 

Barkla, B.J., and Pantoja, O. 1996. Physiology of ion transport across the tonoplast of higher plants. Arum. Rev. Plant Physiol. Plant Mol. Biol. 47 159-184. 

Mariadason, J. M., Barkla, IJ. H., and Gibson, I. R. (1997). Effect of short-chain fatty acids on paraccHular permeabilitv in caco-2 intestinal epithelium model. Am. /. PfmsfflJ. 272, G705-G712,... 

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