Single Channel Analyzer Output

Fig. 3.7 Pulse-height analysis (PHA) Function of the single-channel analyzer (SCA) and data recording by the multi-channel analyzer (MCA). The output (L) of the SCA yields a 5 V squareshaped pulse, a so-called TTL pulse for each y-pulse matching the voltage selection window. The SCA is set to select the Mossbauer pulses for the subsequent measurement...

The single channel analyzer has two dial settings a LEVEL dial and a WINDOW dial. For example, when the level is set at 2 volts, and the window at 0.2 volts, the analyzer will give an output pulse only when the input pulse is between 2 and 2.2 volts. The output pulse is usually a standardized height and width logic pulse, as shown in Figure 11. 

Since the single channel analyzer can be set so that an output is only produced by a certain pulse size, it provides for the counting of one specific radiation in a mixed radiation field. 

The single channel analyzer/discriminator produces an output only when the input is a certain pulse size. 

In pulse fluorometry, we take advantage of the fact that the amplitudes of the output pulses of the TAC are proportional to the times of arrival of the fluorescence photons on the photomultiplier. Selection of a given height of pulse, i.e. of a given time of arrival, is electronically possible (by means of a single-channel analyzer) and allows us to record the fluorescence spectra at a given time t after the excitation pulse. This is repeated for various times. The method described above for phase fluorometry can also be used in pulse fluorometry. 

The block scheme of a Mossbauer spectrometer. A, absorber CR, cryostat with temperature controller TC (optional, for low-temperature measurements) S, source moved by velocity transducer VT of driving unit DR FG, function generator VC, velocity calibrator (optional) LI, laser interferometer (optional) DET, detector HV, high-voltage power supply PA, preamplifier AM, amplifier SCA, single channel analyzer MCA, multichannel analyzer and PC, computer, OP, output... 

The output of the single channel analyzer is a digital pulse or count of 5-10 volts in magnitude and indicates the acceptance of an x-ray photon of the appropriate energy. These digital pulses may be passed to a ratemeter to indicate count rate, to a scaler/timer to measure total counts, to the photoplotter on a rectilinear scanner to produce pictures, or to a computer to record digital information. 

To be useful in different applications, the output signal from a single channel analyzer frequently contains both amplitude and timing information. Circuits related to each one of these... 

The stretcher, for Instance, accepts input signals, and its output is inspected by three blocks stretcher/input interrogation logic, single channel analyzer and zero crossing discrimination. 

The detector signal is conditioned through a single channel pulse height analyzer whose output pulses are fed to a scaler-timer in the single crystal controller. 

Trend outputs consist of a continuous electrical signal [0 to 10 volts or 4 to 20 mini (inA)] from Ihe processor. As many as 30 to 40 such outputs may be available from a single processor. Each output represents Ihe concentration of a particular component in one of the sample streams on a given analyzer sealed lo some convenient range. Component identity and scale factors for each output channel are user-assigned from the processor keyboard. 

Modern XPS spectrometers employ a lens system on the input to the CHA, which has the effect of transferring an image of the analyzed area on the sample surface to the entrance slit of the analyzer. The detector system on the output of the CHA consists of several single channeltrons or a channel plate. Such a spectrometer is illustrated schematically in Fig. 2.6. 

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