Energy of x-ray photons

Figure 3.1 Kinetic energies of X-rays (photons) and neutrons as a function of wavelength.

Measurement of the wavelength (or energy) of the characteristic X-rays emitted enables qualitative analysis to be carried out. The more difficult process of quantitative analysis requires a measurement of the number of X-ray photons of a given type that are emitted per second. 

When the accelerating voltage reaches a specific value (dependent on the nature of the target material), the electrons from the beam are capable of knocking out core-level electrons from the target material, thus giving rise to core vacancies. These are quickly filled by electrons in upper levels and this results in the emission of X-ray photons of characteristic energies which depend on the... 

The probability that the inner shell vacancy will de-excite by one or other of these processes depends on the energy level of the initial vacancy and the atomic weight of the atom. The fluorescent yield, co, is defined as the number of X-ray photons emitted per unit vacancy, and is a measure of the... 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

Fig. 1. Comparison of the four different physical processes which can be observed during the interaction of X-ray photons with matter 2 1. The two phenomena scetched below, namely photoelectron emission and Auger electron emission, can be detected and measured in a photoelectron spectrometer by determining the kinetic energy of the ejected free electrons...

As the energy of X-rays increases in the experiments of Cai et al., the attenuation in single layered DNA decreases, such that the contribution of SE from the metal to the yield of products becomes concomitantly larger. Taking only the dose imparted by the slow SE emitted from the tantalum substrate, it is therefore instructive to define from the data of Cai et al. [6] a LEE enhancement factor (LEEEF) for monolayer DNA to reflect this energy dependence. The LEEEF is defined as the ratio of the yield of products in monolayer DNA induced by the LEE (slow SE, E < lOeV) emitted from the metal substrate v.v the yield of products induced by the photons in a particular experiment. The LEEEF for 1.5 keV photons was... 

The most routinely used analysis mode is qualitative analysis. The probe is stationary (for a spatial resolution of around a cubic micrometre) or scanned at high speed (video) for analysing part of the sample surface (maximum of I x 1 millimetre, the depth of the analysed zone remaining approximately a few micrometres). A global spectrum can be acquired in a few seconds. This spectrum is a histogram representing the number of X-ray photons detected for each energy level. It is used to identify all the major and minor elements present in the interaction volume. 

All the early EDS systems were operated in a primary mode in which X-rays emitted from the X-ray tube irradiate the specimen direcdy. This had drawbacks because the X-rays could excite more photons than could be counted by the detector, because there is a maximum counting rate of photons for an energy detector. If the number of characteristic photons excited in the specimen is too great, a portion of photons may not be counted. Thus, the measured intensities may be lower than the actual emitted intensity of X-ray photons. The period during which the detector cannot respond to the number of photons correctly is called the dead time of the detector. The dead time is in the order of 0.5 x 10-7 seconds. 

The conversion constant (he) is about 2.0 x 10-23 Jems. For a wavenumber of 1000cm, the corresponding energy should be only about 2.0 x 10-20 J or 0.12 eV. Note that this is much smaller than the photon energy of X-rays, which is in the order of 10,000 eV. As shown... 

The detector resolution is the precision/repeatability with which the energy of a specific type of X-ray photons (e. g., the Mn-K line at 5.9 keV) can be determined and is therefore a measure of the capability of the detector to distinguish between X-rays of very similar energy but different origin (e. g., the As- K i line at 10.543 keV and the Pb-L i line at 10.549 keV). 

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