Discriminator levels

Integral discriminator Shaped linear pulse Logic pulse if input amplitude exceeds discriminator level... 

For the photon-counting mode, an optimal resolution can be maintained by coordinating integration time with monochromator scan rate or interval. For each Raman apparatus, careful optimization of the PM tube s high voltage and pulse discriminator levels based on S/N ratios must be performed. 

For each crystal, it is necessary to define the tube excitation conditions (voltage in kV and current in mA), the collimator type (fine slits, coarse slits), the energy discriminator levels and the type of detector used. 

An essential element in quantitative assays is the setting of the positive/negative discrimination level (eut-off value). The term negative is used as indicating the absence of specific antibodies or antigens above the background noise. 

The problems with setting the positive negative discrimination level... 

Electrical pulses from the detectors are sorted by analogue electronics to reduce the occurrence of false detection. The most common approach is to discriminate against signals that are too weak or too strong. Electrical pulses stronger than a minimum value, the lower level of discrimination, are accepted and transmitted, digitally, as a count. Pulses that are stronger than the upper discrimination level are also rejected. 

Indirect geometry spectrometers have no requirement (within the limitations implied by the use of S (Q,a>), 2.5.1) to calibrate detector efficiencies, on either continuous or pulsed sources (compare 3.4.3). Since the final energy of the neutrons never varies the detection efficiency is constant. Variations arising from differing discrimination levels ( 3.3.2) could play a significant role, except that (on low final energy instruments) all detectors follow almost the same path in Q,o ) space ( 3.4.2.3). Occasionally there is a need to calibrate the detected intensity in respect of the sample mass and standard analytical chemical techniques can be readily adapted to this circumstance. 

Figure 8.19 Particles detected are those that interact inside the detector and produce a pulse higher than the discriminator level.

Effect of electronics. The electronics of a detector affects the counter efficiency indirectly. If a particle interacts in the detector and produces a signal, that particle will be recorded only if the signal is recorded. The signal will be registered if it is higher than the discriminator level, which is, of course, determined by the electronic noise of the counting system. Thus, the counting efficiency may increase if the level of electronic noise is decreased. 

As an example, consider a counting system with electronic noise such that the discriminator level is at 1 mV. In this case, only pulses higher than 1 mV will be counted therefore, particles that produce pulses lower than 1 mV will not be recorded. Assume next that the preamplifier or the amplifier or both are replaced by quieter ones, and the new noise level is such that the discriminator level can be set at 0.8 mV. Now, pulses as low as 0.8 mV will be registered, more particles will be recorded, and hence the efficiency of the counting system increases. 

Equations 8.19 and 8.20 probably overestimate efficiency, because their derivation was based on the assumption that a single interaction of the incident photon in the detector will produce a detectable pulse. This is not necessarily the case. A better way to calculate efficiency is by determining the energy deposited in the detector as a result of all the interactions of an incident particle. Then one can compute the number of recorded particles based on the minimum energy that has to be deposited in the detector in order that a pulse higher than the discriminator level may be produced. The Monte Carlo method, which is ideal for such calculations, has been used by many investigators for that purpose. 

Another advantage of the Bonner sphere, in addition to its convenient response function, is its complete insensitivity to gammas. This is the result of relying for neutron detection on charged-particle reactions with high Q value, thus making possible the complete rejection of pulses due to gammas with the use of a proper discriminator level. 

Figure 4.14 The result of scanning the integral discriminator through a pulse height spectrum (a) The pulse height spectrum at the amplifier output, (b) The counting rate at the integral discriminator output as a function of the discriminator level voltage. (Reprinted by courtesy of EG G ORTEC.)...

A second discriminator level Vu has been added to the lower-level discriminator Vl normally used in the integral mode. In the window mode the pulse height selector produces a standard output logic pulse only if the amplifier pulse amplitude... 

Next the multichannel analyzer must add one count to memory location Nc. This is carried out during the memory cycle time [Fig. 4.28(e)]. The memory location whose address or channel number is Nc is identified and its present contents are read. If memory location Nc presently contains m counts, the number m + 1 is written back in. At the end of the memory cycle the multichannel analyzer is free to process another amplifier pulse. If a pulse is already present above the lower-level discriminator level at the linear gate input, the linear gate remains closed until the pulse drops below the discriminator threshold. At this point the linear gate opens and the multichannel analyzer is ready to process the next amplifier pulse that exceeds the lower-level discriminator threshold. The measurement process is repeated on a pulse-by-pulse basis to build up the energy spectrum histogram in memory. 

As described above, the TAG functions to determine the time interval between the excitation pulse and the subsequent fluorescence photon arriving at the detector. The MCA consists of an ADC, a memory consisting of channels for storing data, and data input and output facilities. A standard instrument incorporates lower and upper discriminator levels and two modes of data collection pulse height analysis mode for displaying fluorescence decay profiles ( 1000 channels) and multichannel scaHng mode to bin the data into given time increments. Data are usually displayed on an oscilloscope or on a computer terminal. 

In order to test whether the products are discriminated, it is possible to run a multivariate analysis on variance (MANOVA) or a discriminant analysis on the rotated data after GPA (i.e. the (product subject) x GPA axes table), using product as a dependent variable. The general discrimination level can then be estimated using multivariate F-ratio (Wilks A,). Note that pair-wise differences can also be tested in a multivariate way, which may be useful for decision making. Alternatively, confidence ellipses can be calculated and drawn on the sensory map (Husson et al, 2005), as in Fig. 6.3. 

Liking scores were submitted to ANOVA with sample, session and subject as factors for each group in order to test discrimination and inter-session repeatabiUty. Results showed a decrease in the discrimination level with age and dependency, but significant differences between the products were still observed for the elderly group (Young people = 100.71 p < 0.001 = 0.82 p = 0.54 Independent... 

Luminescence lifetimes measured using pulsed laser excitation involve either direct detection of emission decays with time or a technique known as time-correlated single photon counting (TCSPC). The latter technique involves repeated measurement of the delay time between the excitation pulse and the arrival of an emitted photon packet above a given discrimination level the intensity-time decay profile accumulates over many millions of excitation pulses. The TCSPC experiment has the advantage that much better signal to noise can be obtained relative to the direct capture of the luminescence decay. 

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