Pulse discrimination

Figure 5.6. Diagram of a low-energy, high-angle electron-impact spectrometer. (A) Electron gun (B) monochromator (180° spherical electrostatic energy selector) (C) electron optics (D) scattering chamber (E) analyzer (180° spherical electrostatic energy selector) (F) electron multiplier (G) amplifier and pulse discriminator (H) count-rate meter (I) multichannel scaler (J) X Y recorder (K) digital recorder. (After Kupperman et a/.<42))...

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. 

Amplifier 2 Pulse discriminator 3 Keithley counter 4 Keithley multimeter 6 Personal Computer with lEC Bus interface 7 Plotter... 

Ionization chamber The first radon measurements were made with various electrometers, which were primitive ion chambers. Most of the improvements in ion chambers over the years have been in the associated devices or electronics. Since the early applications of the ion chambers, the most important problem to be solved was the discrimination of the radon signal from that of the y-radiation. The earlier system with paired chambers allowed automatic subtraction of background. One chamber, filled with air and sealed, responded to external y-radiation. This response was subtracted from that of the chamber containing the sample. The present generation of ion chambers is based on fast pulse measurement rather than total ionization. In this case, pulse discrimination eliminates any jS- or y-response. 

Figure Bl.10.2. Schematic diagram of a counting experiment. The detector intercepts signals from the source. The output of the detector is amplified by a preamplifier and then shaped and amplified friitlier by an amplifier. The discriminator has variable lower and upper level tliresholds. If a signal from the amplifier exceeds tlie lower tlireshold while remaming below the upper tlireshold, a pulse is produced that can be registered by a preprogrammed counter. The contents of the counter can be periodically transferred to an online storage device for fiirther processing and analysis. The pulse shapes produced by each of the devices are shown schematically above tlieni.

A successful modification to the technique involves delayed pulsed-field extraction which allows discrimination between zero and near-zero kinetic energy electrons. About 1 ps after the laser pulse has produced photoelectrons, a small voltage pulse is applied. This has the effect of amplifying the differences in fhe velocities of fhe phofoelecfrons and allows easy discrimination befween fhem as a resulf of fhe differenf times of arrival af fhe defector. In fhis way only fhe elections which originally had zero kinetic energy following ionization can be counted to give fhe ZEKE-PE specfmm. 

Fig. 10-2. Threshold contrast in distinguishing an object from its surroundings. The eye responds to an increment in light intensity by increasing the number of signals (pulses) sent to the brain. The detection of threshold contrast involves the ability to discriminate between the target (1) and the brighter background (/ + A/). Source Gregory, R. L., Eye and Brain "The Psychology of Seeing." Weidenfeld and Nicolson, London, 1977.

For the amplifier pulse to be recognized in the ADC, it must exceed the lower level set by a discriminator, which is used to prevent noise pulses from jamming the converter. Once the pulse is accepted it is used to charge a capacitor that is discharged through a constant current source attached to an address clock typically... 

Considerable confusion results because the terms pulse-height analysis, pulse-height discrimination, and pulse-height selection are sometimes used loosely or interchangeably. We shall follow The International Dictionary of Physics and Electronics, D. Van Nostrand Company, Inc., Princeton, N. J., 1950, pages 715 and 710, where the instruments for these techniques are described as follows ... 

Discrimination by adjustable path length in gas-flow Geiger Counters, 258 Discriminators, see Pulse-height discriminator Pulse-height selection Distribution of sample in spot test, 228 Divergence of x-ray beam in slit system, 111, 112... 

Reactions involving a transfer of a proton or a hydrogen atom are an extremely common type of ion-molecule reaction and are particularly suited for study by the pulsed source technique. The secondary ion will usually occur at an m/e ratio where it is not obscured by abundant primary ions, and the product and reactant ions frequently will differ only slightly in mass, thus minimizing discrimination effects. 

Experimental exposure studies have attempted to associate various neurological effects in humans with specific trichloroethylene exposure levels. Voluntary exposures of 1 hours resulted in complaints of drowsiness at 27 ppm and headache at 81 ppm (Nomiyama and Nomiyama 1977). These are very low exposure levels, but the results are questionable because of the use of only three test subjects per dose, lack of statistical analysis, sporadic occurrence of the effects, lack of clear dose-response relationships, and discrepancies between the text and summary table in the report. Therefore, this study is not presented in Table 2-1. No effects on visual perception, two-point discrimination, blood pressure, pulse rate, or respiration rate were observed at any vapor concentration in this study. Other neurobehavioral tests were not performed, and the subjects were not evaluated following exposure. 

As with in vivo voltammetry, a variety of electrochemical techniques have been used for the stripping step. Because of its simplicity, linear sweep voltammetry has enjoyed widespread use however, the detection limit of this technique is limited by charging current. Differential pulse has become popular because it discriminates against the charging current to provide considerably lower detection limits. 

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