Detector output

Due to the pulsed radiation output of the LINAC the detectors and the detector electronics have to handle very high counting rates in very short periods. Therefore the detectors have to work in a mode, where the detector output is integrated for one or several beam pulses. For that purpose the crystals are coupled to photo- diodes. Their currents are read out and analysed by the electronic board, which has been developed for this special application. 

Figure Bl.15.5. Effect of small-amplitude 100 kHz field modulation on the detector output current. The static magnetic field is modulated between the limits and The corresponding detector current varies between the limits 1 and/. The upper diagram shows the recorded 100 kHz signal as a fiinction of B. After [3].

Signal conditioning. The process of altering the relationship of a transducer (ion or neutral detector) output with respect to time or other parameters (frequency, voltage, or current). 

To circumvent this need for calibration as well as to better understand the separation process itself, considerable effort has been directed toward developing the theoretical basis for the separation of molecules in terms of their size. Although partially successful, there are enough complications in the theoretical approach that calibration is still the safest procedure. If a calibration plot such as Fig. 9.14 is available and a detector output indicates a polymer emerging from the column at a particular value of Vj, then the molecular weight of that polymer is readily determined from the calibration, as indicated in Fig. 9.14. 

Polydisperse polymers do not yield sharp peaks in the detector output as indicated in Fig. 9.14. Instead, broad bands are produced which reflect the polydispersity of synthetic polymers. Assuming that suitable calibration data are available, we can construct molecular weight distributions from this kind of experimental data. An indication of how this is done is provided in the following example. 

The basic premise of this method is that the magnitude of the detector output, as measured by hj for a particular fraction, is proportional to the weight of that component in the sample. In this sense the chromatogram itself presents a kind of picture of the molecular weight distribution. The following column entries provide additional quantification of this distribution, however. 

Figure 14. Effect of Sensor Volume on Detector Output...

As previously stated, GPC is the method of choice for studying polymer degradation kinetics. The GPC trace, as given by the detector output, does not provide the true MWD due to various diffusion broadening processes inside the different parts of the equipment. The first step is to correct for instrument broadening if a precise evaluation of MWD is desired. Even with the best columns available, this correction may change the MWD significantly as can be visualized... 

It is seen that the value of (a) can be obtained from the slope of the curve relating the log (detector output)to the log (solute concentration) and an example of such a curve is shown in figure 1. 

The curves relating detector output to solute concentration for detectors having different response indexes are shown in figure 2. 

The pressure sensitivity of a detector will be one of the factors that determines the long term noise and thus can be very important. It is usually measured as the change in detector output for unit change in sensor-cell pressure. Pressure sensitivity and flow sensitivity are to some extent interdependent, subject to the manner in which the detector functions. The UV detector, the fluorescence detector and the electrical... 

Both the sensing device of the LC detector and the associated electronics can be temperature sensitive and cause the detector output to drift as the ambient temperature changes. Consequently, the detecting system should be designed to reduce this drift to a minimum. In practice the drift should be less than 1% of FSD at the maximum sensitivity for 1°C change in ambient temperature. 

As long as in the presence of negative velocity values, the absolute value of Af does not exceed the carrier frequency/q, i.e., fg> A/1, the resulting frequency of the detector output signal correctly preserves the directional information (sign) of the velocity vector. In the case of a vibrating object where v(t)=v the bandwidth of the modulated hetero-... 

Equations (33) and (34) demonstrate that the motion quantities 5 (displacement) and v (velocity) are encoded in phase and frequency modulation of the detector output signal, purely referenced to the laser wavelength A. Tobeableto recover the time histories s(t) and v t) from the modulated detector signal, adequate phase and frequency demodulation techniques, or both, are utilized in the signal decoder blocks of a laser vibrometer. 

Radiation detector output signals are usually weak and require amplification before they can be used. In radiation detection circuits, the nature of the input pulse and discriminator determines the characteristics that the preamplifier and amplifier must have. Two stages of amplification are used in most detection circuits to increase the signal-to-noise ratio. 

The radiation detector is located some distance from the readout. A shielded coaxial cable transmits the detector output to the amplifier. The output signal of the detector may be as low as 0.01 volts. A total gain of 1000 is needed to increase this signal to 10 volts, which is a usable output pulse voltage. There is always a pickup of noise in the long cable run this noise can amount to 0.001 volts. 

The log n amplifier converts the detector output signal to a signal proportional to the logarithm of the detector current. 

For vapour detection there are three aspects that are modelled sensitivity, response time, and regeneration. The sensitivity determines at which concentration level the detector will respond. The theoretical detector output (alarm or no alarm) is calculated by comparing the input data (concentration, relative humidity) with empirical detector display outputs, obtained during controlled exposure laboratory experiments. The response time determines how long it takes before the detector actually shows the response and it depends on the concentration level. The regeneration time determines how long it takes, after a positive detection, before the detector can do a new measurement. 

For HPLC, the injector is a valve. In the charge position, a 50- jL syringe is used to fill the sample loop that holds a specific volume of sample solution. The valve is switched to the run position, and the eluent carries the sample out of the sample loop and into the column (auto samplers are also available for HPLC). A recording of the detector output is automatically started at the time of injection and produces a chromatogram of the separated components. 

Fingerprint analysis a direct-injection GC/FID analysis in which the detector output—the chromatogram—is compared to chromatograms of reference materials as an aid to product identification. 

Figure 8. Plots of computer generated representation of the input code, x(t), detector output, y(t), and final correlo-gram output, < (r), for a sample containing 18 and 82% concentration of two components. Reproduced with permission from Ref. 9, copyright 1973, "American Chemical Society."...

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