Detection Techniques and Electronic Equipment

Besides the radiation detectors also the detection technique and the optimum choice of electronic equipment are essential factors for the success and the accuracy of spectroscopic measurements. This subsection is devoted to some modern detection techniques and electronic devices. 

Compared with the conventional analog measurement of the anode current, the photon-counting technique has the following advantages  

The upper limit of the counting rate depends on the time resolution of the discriminator, which may be below 10 ns. This allows counting of randomly distributed pulse rates up to about 10 MHz without essential counting errors. 

The lower limit is set by the dark pulse rate [4.144]. With selected low-noise photomultipliers and cooled cathodes, the dark pulse rate may be below 1 per second. Assuming a quantum efficiency of r] = 0.2, it should therefore be possible to achieve, within a measuring time of 1 s, a signal-to-noise ratio of unity even at a photon flux of 5 photons/s. At these low photon fluxes, the probability p(N) of N photoelectrons being detected within the time interval At follows a Poisson distribution 

Many spectroscopic investigations require the observation of fast transient events. Examples are lifetime measurements of excited atomic or molecular states, investigations of collisional relaxation, and studies of fast laser pulses (Chap. 11). Another example is the transient response of molecules when the 

The boxcar integrator measures the amplitudes and shapes of signals with a constant repetition rate integrated over a specific sampling interval At. It records these 

At very low incident radiation powers it is advantageous to use the photomultiplier for counting single photoeleetrons emitted at a rate n per second 

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Since the Raman effect is intrinsically quite weak, relatively high-power lasers and sophisticated optical and electronic equipment are required to detect the Raman scattered photons. This accounts for the lengthy time that elapsed in the development of Raman spectroscopy. It experienced a rebirth in the 1960s with the invention of laser and its use as a light source. However, from the 1980s the advances in optoelectronics, particularly the development of compact lasers, detectors and efficient optical filters allowed lower cost, integrated instmments to be produced commercially. Consequently, Raman spectroscopy has been adopted as a routine method in many fields because it has been shown to be simpler and faster than alternative techniques. 

Electron Beam Techniques. One of the most powerful tools in VLSI technology is the scanning electron microscope (sem) (see Microscopy). A sem is typically used in three modes secondary electron detection, back-scattered electron detection, and x-ray fluorescence (xrf). AH three techniques can be used for nondestmctive analysis of a VLSI wafer, where the sample does not have to be destroyed for sample preparation or by analysis, if the sem is equipped to accept large wafer-sized samples and the electron beam is used at low (ca 1 keV) energy to preserve the functional integrity of the circuitry. Samples that do not diffuse the charge produced by the electron beam, such as insulators, require special sample preparation. 

This is the most useful technique for screening pesticides since it has wide applicability and sensitivity, and utilises equipment which is readily available in most laboratories. Over 95% of all pesticides may be chromatographed intact or as a simple derivative in some cases there is a clearly defined decomposition product although quantification may be difficult if the extent of decomposition is not reproducible. The sensitivity of the method is high using a flame ionisation detector when specific detectors are used, e.g. electron capture, alkali flame ionisation, or flame photometric detectors, even lower concentrations in body fluids may be detected. 

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