The Multichannel Analyzer

The multichannel analyzer (MCA) records and stores pulses according to their height. Each storage unit is called a channel. 

Introduction to the Theory of Error, Addison-Wesley, Reading, Mass., 1957. 

Bevington, P. R., Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969. 

Statistical Methods in Nuclear Material Control, TID-26298, U.S. Atomic Energy Commission, 1973. 

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Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

Cu (5.1 min). Instrumental radioassay was performed with a similar nuclear counting system as for the airborne gunshot residues (Ref 17) described above with the addition of a programmable computer coupled to the multichannel analyzer for data processing. Using these procedures, it was possible to detect Ba levels above 2 x 10 g/cm and Sb levels above 1.5 x 10" g/cm of floor surface... 

The voltage pulse produced by the TAC is fed to the multichannel analyzer (MCA), and is stored in a specific channel according to its amplitude, and hence time, post-excitation. The probability of a single photon event being counted is high soon after excitation and decreases with time. Repetitive operation of the TAC produces a probability histogram for the detection of fluorescence photons, which is identical to the fluorescence decay curve. 

Most laboratories involved in radiation measurements now use personal computers and commercially available software for the analysis of y-ray spectra. Some of these programs allow the user to control the multichannel analyzer (MCA), calibrate the detector for various geometries, and provide analysis results. The programs are easy to use and do not require the user to be an expert in y-ray spectrometry. 

Assuming that our absorber contains enough Fe57 to produce an effect of 2.5% we can estimate the time of measurement if we know the counting rate N per sec of the 14.4 keV /-radiation (which depends on the activity of the source and on the geometry of the experimental set-up), and if we limit the accuracy of the measured effect to 5%. Under this condition we need N counts in each of the 400 channels of the multichannel analyzer and from Fig. 6 we see that the total error of the measured effect is given by 2 ]/lV/0.025 N = 0.05, leading to N=2.5 106 counts per channel. With a source activity of 100 mCi, a source detector distance of about 12 cm, a detector window of 2.54 cm diameter, and an absorber with effective thickness of about 0.5 g/cm2... 

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. 

Modern counting equipment is also rather highly priced a system with a high-resolution Ge(Li) or HP-Ge detector and a 4000 channel analyzer costs from US 45 000 to US 60 000 depending on the detection efficiency and resolution of the detector and the data reduction capabilities of the multichannel analyzer. A sample changer to count automatically and computer for data reduction push up the expenses further. 

Figure 4.22 The analysis of the shaping amplifier output pulse heights into the x-ray energy spectrum, (a) A multiple-trace oscilloscope picture of the pulse-shaping amplifier output for a spectrum containing Mn and Ag K x-rays, (b) The analyzed pulse height spectrum as viewed on the multichannel analyzer display. Each dot represents one channel in the analyzer memory. The channel numbers have been calibrated in terms of x-ray photon energy. (Reprinted by courtesy of EG G ORTEC.)...

Since deadtimes in this type of spectrometer are quite long ( 60 fis), the system must normally operate with deadtime losses in the 10 to 60% range. Consequently, most multichannel analyzers are equipped with an electronic means of deadtime correction, such that the observed spectrum represents the true number of photons arriving at the detector during the period of data accumulation. In addition to the ability to display the spectrum on a cathode-ray tube or television monitor, the analyzer can usually drive an X-Y plotter to produce a permanent copy. Alternatively, the contents of the analyzer memory can be printed as the number of counts in each channel, listed by channel number. Most quantitative fluorescence spectrometers include a personal computer with approximately 2-6 megabytes of memory plus some form of mass storage. In such a system the computer may control specimen presentation, the excitation conditions, and data accumulation in the multichannel analyzer. At the end of data acquisition for each specimen the computer analyzes the spectrum in the multichannel analyzer, computes the raw element intensities, corrects for interelement effects, and computes the concentration of each element. 

Figure 4.28 Signals occurring during the pulse height measurement process in the multichannel analyzer [23],...

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. 

Both the amplifier pulse and the multichannel analyzer contribute to the system deadtime. The deadtime caused by the multichannel analyzer for each analyzed pulse is the sum of the rundown time and the memory cycle time, that is. 

Figure 4.30 demonstrates the case where the second photon is detected before the slow amplifier pulse from the first photon reaches peak amplitude. Thus, the amplitudes of both pulses are distorted, and both events must be rejected. If the second photon arrives after the multichannel analyzer has sensed the peak amplitude of the first pulse and closed its linear gate, then the second pulse will not distort the measured amplitude of the first pulse. In this case, only the second pulse must be rejected. Consequently, the multichannel analyzer only rejects an event that it otherwise would have analyzed if the inhibit pulse occurs before peak amplitude is determined. Peak amplitude determination is marked by the closing of the linear gate in the analog-to-digital converter. 

Figure 4.36 Output versus input rate for a typical energy-dispersive spectrometer with pileup rejection. The input rate is the true counting rate at the detector. The output rate is the measured counting rate in the multichannel analyzer memory for events not distorted by pileup. Shaping time constants of 2,6, and 10 fis have been used. (Reprinted by courtesy of EG G ORTEC.)...

The discussions above show that each component of the energy spectrum as a poissonian distribution, and the sum of any number of components also has a poissonian distribution. This also means that the i-th component can be considered to be defined by a channel at energy Ei in the multichannel analyzer memory. If the ni counts in each of s adjacent channels are summed to arrive at the total counts within a peak associated with a particular characteristic x-ray line, then the number of counts in the peak can be reported as... 

Figure 4.49 Definition of deadtimes in the multichannel analyzer system on energy-dispersive spectrometers. (Reprinted by courtesy of EG G ORTEC.)...

Applying the general methods for calculating deadtime losses with cascaded deadtimes, the expected mean rate of counting undistorted pulses in the multichannel analyzer memory is... 

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