Synchrotron radiation emission process

For many of the analytical techniques discussed below, it is necessary to have a source of X-rays. There are three ways in which X-rays can be produced in an X-ray tube, by using a radioactive source, or by the use of synchrotron radiation (see Section 12.6). Radioactive sources consist of a radioactive element or compound which spontaneously produces X-rays of fixed energy, depending on the decay process characteristic of the radioactive material (see Section 10.3). Nuclear processes such as electron capture can result in X-ray (or y ray) emission. Thus many radioactive isotopes produce electromagnetic radiation in the X-ray region of the spectrum, for example 3He, 241Am, and 57Co. These sources tend to produce pure X-ray spectra (without the continuous radiation), but are of low intensity. They can be used as a source in portable X-ray devices, but can be hazardous to handle because they cannot be switched off. In contrast, synchrotron radiation provides an... 

For analytical purposes. X-rays are generated in four ways (I) by bombardment of a metal target with a beam of high-energy electrons. (2) by exposure of a substance to a primary beam of X-rays to generate a secondary beam of X-ray fluorescence, (3) by use of a radioactive source whose decay process results in X-ray emission, and (4) from a synchrotron radiation source. Only a few laboratories in the United States have facilities to produce X-rays from synchrotron radiation. For this reason, wc will consider only the first three sourcc.s. 

Nuclear excitation and nuclear resonant scattering with synchrotron radiation have opened new fields in Mossbauer spectroscopy and have quite different aspects with the spectroscopy using a radioactive source. For example, as shown in Fig. 1.10, when the high brilliant radiation pulse passed through the resonant material and excite collectively the assemblies of the resonance nuclei in time shorter than the lifetime of the nuclear excited state, the nuclear excitons are formed and their coherent radiation decay occurs within much shorter period compared with an usual spontaneous emission with natural lifetime. This is called as speed-up of the nuclear de-excitation. The other de-excitations of the nuclei through the incoherent channels like electron emission by internal conversion process are suppressed. Synchrotron radiation is linearly polarized and the excitation and the de-excitation of the nuclear levels obey to the selection rule of magnetic dipole (Ml) transition for the Fe resonance. As shown in Fig. 1.10, the coherent de-excitation of nuclear levels creates a quantum beat Q given by... 

Figure 3.17 Measured far-IR intensity for the BESSY coherent synchrotron radiation (CSR) source, compared with mercury arc and Globar conventional thermal sources. In the picture the turn-on of the CSR source below 2cm is a real effect of the CSR emission process, while the drop off at the low frequency end is due to a combination of diffraction losses in the optical path of the beamline and to contributions of optical components in the interferometer. (Reprinted from ref. 102.)...

The excitation of atoms with suitable radiation causes elimination of electrons from the inner shell (K, L, M). The electrons from the outer shells drop to the free positions and the process is accompanied by emission of electromagnetic radiation with energy up to 120 KeV (X-rays), characteristic for the respective atom (element). The excitation can be induced by X-ray, gamma ray, or accelerated particles (protones, electrons etc.). The emitted radiation can be used for analysis either after wavelength or energy dispersion. These possibilities determine the existence of several versions of X-ray fluorescence methods (XRF) wavelength, energy dispersive, proton induced, total reflection and synchrotron XRF. 

NEXAFS spectroscopy basically does not require the most sophisticated apparatus to be performed but a source of tunable radiation as that dispensed by a photon factory or synchrotron plant. The experimental station for the study of macromolecular materials requires a UHV system and a detector apparatus for counting the emitted electrons. The primary process in NEXAFS is the core electron excitation into an appropriate final state empty molecular orbital. After excitation, the whole system undergoes relaxation and this can occur through two main decay processes secondary or Auger electron emission and fluorescence emission. Mostly, the detector for NEXAFS uses a simple channeltron tuned for a specific Auger energy or tuned to collect the whole secondary electrons resulting from the relaxation process fluorescence detector are also relatively common alternatively, for sample insulator the measurement of the drain current from the conductor sample holder is often measured examples are displayed in Fig. 4.4. Measurements can be performed on gas, solid and recently liquid state [3]. 

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