Detectors source energies

The construction of a TXRF system, including X-ray source, energy-dispersive detector and pulse-processing electronics, is similar to that of conventional XRF. The geometrical arrangement must also enable total reflection of a monochromatic primary beam. The totally reflected beam interferes with the incident primary beam. This interference causes the formation of standing waves above the surface of a homogeneous sample, as depicted in Fig. 4.1, or within a multiple-layered sample. Part of the primary beam fades away in an evanescent wave field in the bulk or substrate [4.28],... 

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

CUORE belongs to the type (b) detectors (source is part of the detector). The detection of the DBD is done by means of direct measurement of the energy of the electrons emitted in the process. 

Source and detector selection are interrelated, where the output of the source is matched to the sensitivity range of the detector. However, the exact nature of the source is also dependent on the type of sample(s) under consideration, the intended optical geometry, the type of measurement technique, and the final desired performance. The bottom line is that adequate source energy must reach the detector to provide a signal-to-noise performance consistent with the required precision and reproducibility of the measurement. This assumes that the detector is well matched, optically and performance-wise, and is also capable of providing the desired performance. 

Most detector systems require that the IR beam be modulated, where the source energy is adequately differentiated in the measured signal from the ambient background. One of the traditional approaches is to use some form of mechanical chopper , usually in the form of a rotating sector wheel, which modulates the beam by blocking the radiation in one or more sectors during a rotation. Note that this is not a requirement for FTIR systems where the interferometer naturally modulates the beam. 

Until recently, nearly all forms of analytical measurement in the mid-IR and NIR spectroscopic regions involved detection at a single detector element where all of the transmitted or reflected source energy was focused and measured. This... 

Some of the optical components (e.g., windows, lens, and mirrors) inside the detector might require cleaning or replacement after several years of use. Indicators for the need to service these optical items are low source energy or low sensitivity performance even after a new lamp has been installed. Occasionally, the monochromator might need adjustment to restore wavelength accuracy. These procedures are best performed by a factory-trained specialist. 

As a result of incomplete energy deposition and the statistical nature of the events that take place in the detector, the shape of the pulse-height distribution is different from that of the source energy spectrum. In other words, two spectra are involved in every measurement ... 

Figure 12.4 The pulse height spectrum obtained from the source spectrum of Fig. 12.3, in the absence of statistical effects in the detector (perfect energy resolution).

The resulting interference between the beams depends on the optical path difference or retardation. When the fixed and moving mirror are equidistant from the beamsplitter, the retardation is zero. Neglecting losses, all the source energy reaches the detector at this point. The variation in intensity as the moving mirror is translated contains the spectral information retrieved by the FT. 

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