[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The systematic portion cannot be determined and the

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the systematic portion cannot be determined and the program will default it to Zero. WinFTM V.6 can display the uncertainties for single readings and for the mean values (either on the screen or through the so-called online export func- tion) [248], namely the random and the systematic portion, or both together. Table 7.10 provides an overview. Table 7.10. Measurement uncertainty with WinFTM-V.6 Toggle “Display of the Error information Error information measurement uncertainty” for the single reading for the mean value Random portion Theoretical error estimate Confidence interval of from the measured spectrum the mean value according to t -distribution (Student- Distribution.), calculated from the standard distribution of the block Systematic portion Computed from the random Uses the error value of error of the calibration the last single reading of measurements and the the block uncertainties of the nominal values of the standards, as entered into the “Calibration Standard Set” Systematic + Root of the square sum Root of the square sum random portion of the random and the of the random and of the systematic portion systematic portion
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Methodological Developments and Applications 579 7.4.5 Instruments The foremost requirement for any instrument is that a high reproducibility of the results is ensured. To this end, a good repeatability is required, which for XRFA instruments is essentially determined by counting statistics and therefore by the X-ray source, the beam guidance and the detector. For many applications in the field of layer analysis, good knowledge of the measurement location is also necessary for a good reproducibility. Thus, next to X-ray source and detector, the beam guidance and the observation of the measurement spot are important components within the instruments (cf. Fig. 7.97). The instruments are designed such that the specimen can be viewed using a video camera. The observer has a vertical view (or at least one under a steep angle) of the sample surface, and the measurement spot is indicated by the crosshairs on the video image (Fig. 7.98). Accurate positioning using the video image is in many instances an im- portant prerequisite for correct measurements. Detectors Selection of a suitable detector type is governed by the requirements of the application. The objective is essentially good repeatability. Because the struc- tures to be analyzed are typically very small, the active measurement areas must be very small as well, leading to very low intensities of the radiation to be detected. To balance this, the detectors should operate very efficiently, and in particular cover a very large solid angle. Size of the detector window and distance from the sample play an important role in this respect. Table 7.11 offers some values as a guideline.
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