Cross-section, absolute calibration standards

A particular strength of Equation (7) is that the intensity ratio is formed between mea-surements of the same X-ray energy in both the unknown and standard. This procedure has significant advant es First, there is no need to know the spectrometer s efficiency, a value that is very difficult to calibrate absolutely, since it appears as a multiplicative factor in both terms and therefore cancels. Second, an exact knowledge of the inner shell ionization cross section or fluorescence yields is not needed, since they also cancel in the ratio. 

In principle, one could calculate an absolute Raman cross section from the response of an instrument calibrated with a standard radiometric source. This approach is difficult but has been used to provide the cross sections in Table 2.2. If the relative response function is calibrated accurately, however, it is much simpler to determine cross sections by comparison to standards. Provided the sample positioning and optics permit quantitative Raman signal reproducibility, cross sections of liquids may be determined by comparing the response-corrected peak area to a band with known absolute cross section, such as the benzene 992 cm band. For response-corrected spectra, the ratio of the peak areas under identical experimental conditions equals the ratio of the absolute cross sections. 

The calibration of a PIXE system, i.e., the determination of sensitivity factors, which assign absolute concentration data to numbers of counts in X-ray peaks, can be performed in two different ways. First, sensitivity factors can be deduced theoretically or in a semiempirical way from calculated cross sections for X-ray excitation and from X-ray absorption data for the absorbents present between the points of emission and detection, in the actual experimental setup. Second, sensitivity factors can be deduced from measurements performed on standard samples consisting of pure elements or pure chemical compounds. The detection solid angle and the energy-dependent detector efficiency should also be determined. 

In particular, the neutron activation analyst, to whom efficiency curves may be an irrelevance, is not weU served by most spectrum analysis packages. As far as activation analysis is concerned, there is much evidence to show that absolute analysis, calculating concentrations from first principles, is much less accurate than comparative analysis. Apart from aU of the problems which derive from having to use efficiency calibration curves, there are specific problems associated with defining and measuring neutron fluxes and cross-sections which make absolute analysis not worthwhile, in my opinion (although there are those who have devoted a considerable amount of effort into developing absolute neutron activation analysis procedures who would dispute that). For that reason, almost every activation analysis involves irradiation of samples and standards. A direct comparison between them is the simplest solution. 

SAXS was performed at the beam-line JUSIFA at the synchrotron source HASYLAB, Hamburg. To detmnine the scattering cross section for the sample investigated on an absolute scale, the scattering of the sample was calibrated using a glassy carbon standard as a reference. 

A between the colliding partners, which is quite difficult to rationalize with standard electron-jump impulsive models. As the calibration of a v and Or is far from perfect (e.g. or has been calibrated as a total cross section for the removal of Ba out of a beam transferring through N2O gas and could somewhat overestimate Or) and their values appear to be that large, a renewed investigation of the absolute cross sections and the recoil distributions of reactive products [20] is highly desirable. 

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