Other Detector Parameters

Achievement of a very high at the desired is usually the primary demand on a photon detector, but other detector performance parameters can be important also. Listed in Table 4.2 are the various detector parameters and the requirements generally placed on them by the most advanced systems. 

Response time Very fast in some systems 

Operating temperature High enough for mechanical or electrical cooling 

Electrical power dissipation Low per element for large arrays 

Ambients Insensitive to extraneous radiation in some systems 

Electrical power dissipation Cost per element Electronics Ambients 

Cover most of infrared spectrum Highest possible (BLIP) 

Very fast in some systems High enough for mechanical or electrical cooling lOO s of detector elements Low per element for large arrays Low in large arrays Integration with detector array Insensitive to extraneous radiation in some systems 

The second most important parameter in many applications is response time. It can be limited by a variety of factors, such as carrier lifetimes within the detector material, the sweepout time of photoexcited carriers, and the RC time constant of the detector and/or its associated circuitry. Detailed analysis of response times goes beyond the scope of this chapter. 

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Responsivity, noise, and many other detector parameters are traditionally measured while the detectors are flooded with uniform irradiance. A few specialized characteristics - modulation transfer function (MTF) and crosstalk - require a focused spot or specific pattern on the detector of interest - or even on some small part of a detector. Focused tests are more difficult than flood tests it is not trivial to obtain the small spot size normally required, nor to determine the actual spot shape and size. We discuss the hardware needed in Section 9.3.7. Focused testing is discussed in Section 10.5.2. 

A sensor is only one component of a cryogenic detector. In the simplest case, a detector consists of an absorber (for example absorber of energy) and a sensor (for example a thermometer like a TES). Nevertheless, other physical parameters than energy and temperature may be involved in a cryogenic detector. For example, in a cryogenic gravitational antenna (see Section 16.2) the absorber is the cooled bar, whereas the sensors is SQUID-capacitor system. 

Having optimised the efficiency of a chromatographic separation the quality of the chromatography can be controlled by applying certain system suitability tests. One of these is the calculation of theoretical plates for a column and there are two other main parameters for assessing performance peak symmetry and the resolution between critical pairs of peaks. A third performance test, the peak purity parameter, can be applied where two-dimensional detectors such as diode or coulometric array or mass spectrometry detectors are being used. The reproducibility of peak retention times is also an important parameter for controlling performance. 

Other important parameters in providing successful GC are the column packing, temperature conditions, and selection of a detector as specific to the analyte as possible. Maximum resolution of the halocar-bons is achieved with an 8-ft X 0.1-in. i.d. column of Carbopack-B coated with 1% SP-1000. The initial temperature of 45 °C is held for 3 min and then programmed at 8 °C/min to 220 °C. An organohalogen detector (OHD) is used. The aromatics are best resolved with a 6-ft X 0.085-in. i.d. column of Supelcoport coated with 5% SP-1200 plus 1.75 Bentone-34. They are measured with a photoionization detector. The temperature conditions are as follows 50 °C for 2 min then programmed at 6 °C/min to 90 °C. A 10-ft X 2-mm i.d. Porapak-QS (80-100 mesh) column at a temperature of 110 °C for 1.5 min and rapidly heated to 150 °C is now used for acrolein and acrylonitrile. This method employs a flame ionization detector (FID). 

The intrinsically low intensity of Raman scattering strongly influences both the sensitivity and penetration depth of SORS and its variants. Dominant noise components (photon shot noise or thermal/dark count [1]) can be minimised relative to signal by increasing absolute signal levels. In many Raman systems, collection optics, laser power and other relevant parameters are usually maximised for optimum performance of the system current detectors (CCD devices), for example, have detection efficiencies approaching 100%. Typically, acquisition time provides the only straightforward means available... 

Biosensors. Sensors are required to adequately monitor bioreactor performance. Ideally, one would like to have online sensors to minimize the number of samples to be taken from the bioreactor and to automate the bioreactor process. Most bioreactors have autoclavable pH and dissolved oxygen (D.O.) electrodes as online sensors, and use offline detectors to measure other critical parameters such as glucose and glutamine concentration, cell density, and carbon dioxide partial pressure (pC02). An online fiber-optic-based pC02 sensor is commercially available and appears to be robust.37 Probes are also commercially available that determine viable cell density by measuring the capacitance of a cell suspension. Data from perfusion and batch cultures indicate that these probes are reasonably accurate at cell concentrations greater than 0.5 X 106 cells/mL.38,39... 

The pressure sensitivity of a detector is extremely important as it is one of the detector parameters that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector sensitivity or minimum detectable concentration together with those other characteristics that depend on detector sensitivity. Certain detectors are more sensitive to changes in pressure than others. The katherometer detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive as can the LC refractive index detector. Careful design can minimize the effect of pressure but all bulk property detectors will tend to be pressure sensitive. 

A typical EGD-EGA apparatus cannot be described because of the wide variety of detectors that are employed. If certain restrictions are placed on the latter, plus other operating parameters, a general type apparatus can be depicted, such as is illustrated in Figure 8.22. The pyrolysis unit may be a simple furnace or other thermal analysis device such as a thermobalance, DTA, DSC, or so on. A temperature programmer provides the rate of... 

Flow injection analysis (FIA) is based on the injection of a liquid sample into a moving, nonsegmented continuous carrier stream of a suitable liquid. The injected sample forms a zone, which is then transported toward a detector that continously records the absorbance, electrode potential, or other physical parameter as it continuously changes due to the passage of the sample material through the flow cell [1, 153]. 

Whereas, in principle, simple experiments with tracers and one for each solute (explained below) allow the determination of e, e, and the component specific H from the experimentally determined pt <-> the other model parameters cannot be simply extracted from the second moment. Dispersion (D x), liquid film mass transfer (kfiijn), diffusion inside the particles (Dapp,pore)> and adsorption kinetics (kads) contribute in a complex manner jointly to the overall band broadening as described by Ot < (Equation 6.136). Therefore, an independent determination of these four parameters is not possible from Equation 6.136 only. In principle, additional equations could be obtained from higher moments (Kucera, 1965 Kubin, 1965). However, as the effect of detector noise on the accuracy of the moment value strongly increases the higher the order of the moment, a meaningful measurement of the third, fourth, and fifth moments is practically impossible. Equation 6.136 is thus not directly suited for parameter determination, but... 

The standard has to be affected by any change in the experimental conditions, particularly coming from the laser source and detector characteristics but should be nearly or totally independent of the concentration (or any other sensor parameter). We can distinguish between external and... 

The system is coupled to the central unit of a computer to ensure coordination of injection of the sample and collection of the data coming from the three detectors. Software has been developed to assist the analysis given from the detectors and to calculate the different average molecular weights and the polymolecularity index of the analysed species. Other interesting parameters... 

A computational study has been carried out on several plutonium-fueled and mixed-lueled critical assemblies of small to medium size to verify the plutonium cross sec-tlons of the twenty-six groiq> set recently produced at Argonne National Laboratory. Plutonium worths in some larger uranium-fueled reactors have been computed to ascertain the effect of softer spectra on the comparisons. Other reactor parameters included in the study were detector-response ratios and U-235,U-238 and B-10 central clanger coefficients, as well as effective delayed-neutron fraction and prompt-neutron lifetime. 

Noise equivalent BRDF (NEBRDF) the root mean square (rms) noise fluctuation, or the standard deviation of the detector signal, expressed as equivalent BRDF. Note Measurement precision is limited by the acceptable signal-to-noise ratio with respect to these fluctuations. It should be noted that although the detector noise is independent of 0, the NEBRDF will increase at large values of 0, because of the 1/cos 0 factor. Measurement precision can also be limited by other experimental parameters as discussed in Section 2.2. 

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