Pulse-height analysis mode

Many multichannel analysers have a facility known as pulse-height analysis, and a typical system employing an analyser in this mode is shown in Fig. 2.3. Several detailed descriptions are available [25-27]. 

A reference waveform identical to that supplying the drive is given a d.c. shift such that it is always positive. When a y-ray which has passed through the absorber is detected, the waveform is sampled so that a pulse is produced which has a voltage maximum characteristic of the instantaneous velocity of the source at the time of emission (see Fig. 2.2d). An analogue-to-digital 

Other advantages are as follows the use of a 40-Hz scanning frequency results in a very small amplitude of motion which reduces geometry distortions to negligible proportions for most nuclides and minimises errors from non-linearities in the voltage/velocity correspondence of the pick-up coil. Half-life corrections with short-lived sources are also unnecessary, and high velocities can be achieved at relatively small amplitudes. One particular system accumulates two spectra simultaneously using a common input to the analyser [28]. 

This presents a difficulty if the data are to be analysed by computer and it reduces the feasible accuracy of measurements. 

Both these objections can be overcome by using a time-mode system and spectrometers based on this latter principle have now virtually replaced those operating in the pulse-height analysis mode. 

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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. 

Because ihe excitation is not instantaneous, the observed decay curve I(t) obtained with the analyser operating in pulse height analysis mode is a convolution of the excitation pulse profUe P(t) with the true decay of the sample G(t) viz. 

To measure an energy spectrum of a radioactive source means to record the pulse-height distribution produced by the particles emitted from the source, which is achieved with the use of an instrument called the multichannel analyzer (MCA). Multichannel analyzers are used in either of two different modes the pulse-height analysis (PHA) mode or the multichannel scaling (MCS) mode. 

In energy dispersive instruments, all the characteristic X-rays from the specimen are first detected by a high resolution solid state detector. The detector output consists of electrical pulses of amplitude proportional to the energy of the detected photons. These pulses, once linearly amplified, are electronically sorted according to their amplitude by means of a pulse height analyzer. An advantageous consequence of this detection mode is that analysis of all chemical elements can be performed simultaneously. Since it eliminates the use of crystal goniometers and the X-rays are not spatially dispersed, this type of spectrometer is also sometimes referred to as nondis-persive. 

Pulsed amperometric detection is used for the analysis of carbohydrates and other nonchromophoric molecules such as alcohols, aldehydes, and amines. In the DC mode the products of the oxidation reaction of those compounds poison the surface of the working electrode, and further analyte oxidation is inhibited. This results in peaks that decrease rapidly in height. To maintain a stable and active electrode surface, alternatively positive and negative potentials are repeatedly pulsed. 

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