Dead time collection

The dead time is typically 3-5 ms. so stopped flow is not quite as fast as continuous flow, but it requires less than a milliliter of each solution per run. Methods have been described for measuring the dead time " " these are based upon standard reactions whose kinetic behavior is well known. The error introduced by collecting data before mixing is complete can be corrected." ... 

Scattered radiation. In a transmission experiment, the Mossbauer sample emits a substantial amount of scattered radiation, originating from XRF and Compton scattering, but also y-radiation emitted by the Mossbauer nuclei upon de-excitation of the excited state after resonant absorption. Since scattering occurs in 4ti solid angle, the y-detector should not be positioned too close to the absorber so as not to collect too much of this unwanted scattered radiation. The corresponding pulses may not only uimecessarily overload the detector and increase the counting dead time, but they may also affect the y-discrimination in the SCA and increase the nonresonant background noise. 

The simulations [30] showed that the model satisfactorily reproduced the results collected over the physical mixture upon NO step addition in the presence of oxygen. Both the oxidation of NO to N02 and the dead time for the breakthrough of NO are reasonably simulated by the model, as indeed expected according to nitrate route adsorption. 

E represents the combined collection and detection efficiency of the system and F the intrinsic photon-economy of the technique. The factor 77 accounts for the subtractive noise, tA is the dead-time of the detector and Q the count rate of the system. 

The time resolution of the instrument is governed not only by the pulse width but also by the electronics and the detector. The linear time response of the TAC is most critical for obtaining accurate fluorescence decays. The response is more linear when the time during which the TAC is in operation and unable to respond to another signal (dead time) is minimized. For this reason, it is better to collect the data in the reverse configuration the fluorescence pulse acts as the start pulse and the corresponding excitation pulse (delayed by an appropriate delay line) as the stop pulse. In this way, only a small fraction of start pulses result in stop pulses and the collection statistics are better. 

Fig. 9.5. Efficiency and speed of sorting are affected by the flow rate of cells. At high flow rates, more desired cells are lost, but the speed of collecting these desired cells increases until the loss of efficiency becomes greater than the increase in speed. Highspeed sorting, with more drops per second, increases the efficiency and decreases the time required to obtain the desired number of cells. The model from which these graphs were generated was derived by Robert Hoffman for these data, a three-drop sort envelope was used, 1% of the cells were sorted, and the electronic dead time was set at 6 ps. If one drop is sorted with each sort decision (instead of three), the theoretical efficiency of the sorting improves considerably (as does the rate of collecting the sorted cells).

This time difference, called the correlation time, is stored in a histogramming memory, and after the necessary processing time (dead time rdead) the TDC is reset and waits for the next START signal. The histogramming memory collects all individual coincidences by sorting them according to their correlation times. In this way, a time spectrum is obtained which can be transferred to a computer. 

Dead time is particularly significant for Geiger counters they have the largest dead times (up to 200 ps) because they have the most charge to collect (maximum gas multiplication, maximum size ionization avalanche), which takes time. 

Zitomer (67) was the first to describe the coupling of a thermobalance to a time-of-flight mass spectrometer and a magnetic sector mass spectrometer. This technique eliminated the practice of collecting or trapping fractions for subsequent analysis and also permitted careful control of the furnace atmosphere. One of the important features of the TG-MS system is its relatively short dead time, that is, the time between product evolution and introduction into the mass spectrometer ion source. Under proper flow conditions, this time is of the order of seconds. There is also less probability of the formation of secondary reaction that can lead to products other than those initially evolved. 

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