Xray Wavelengths And Atomic Number

The wavelength and energy of a given X-ray photon are inversely proportional to each other. As indicated in Section II, the basis of the X-ray fluorescence technique lies

In this expression K and s are constants that depend on the spectral series of the emission line in question. It has also been stated that not all vacancies result in the production of characteristic X-ray photons because of the competing Auger process involving internal rearrangement. The ratio of the number of vacancies resulting in the production of characteristic X-ray photons to the total number of vacancies created in the excitation process is called the fluorescent yield. The fluorescent yield is an important fundamental parameter, since it is one of the factors that will determine the absolute number of counts that an element will give under a certain set of experimental conditions. If the fluorescent yield value is high, the spectrometer sensitivity for that element will probably be high as well, and vice versa. The fluorescent yield takes a value from around unity for the high-atomic-number elements to as little as 0.01 for the low-atomic-number elements such as Na, Mg, and Al. It is mainly for this reason that the sensitivity of the X-ray spectrometric technique is rather poor for the elements of very low atomic number.

Most commercially available X-ray spectrometers have a range from about 0.2 to 20 A (60-0.6 keV), which will allow measurement of the K series from fluorine (Z = 9) to lutetium (Z = 71), and for the L series from manganese (Z = 25) to uranium (Z = 92). Other line series can occur from the M and N levels, but these have little use in analytical X-ray spectrometry.

Although in principle almost any high-energy particle can be used to excite characteristic radiation from a specimen, an X-ray source offers a reasonable compromise between efficiency, stability, and cost, and almost all commercially available X-ray spectrometers use such an excitation source. Since primary (source) X-ray photons are used to excite secondary (specimen) radiation, the technique is referred to as X-ray fluorescence spectrometry.

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