Pulse-height analysis spectrum

Nondestructive testing, 68, 290, 291 Nondispersive spectrum analysis, see Pulse-height analysis. 

Fig. 3.18 Pulse-height analysis (PHA) spectrum (or energy spectrum) for Co/Rh Mossbauer source radiation backscattered nonresonantly and/or resonantly from aluminum and stainless steel plates. Data were obtained with Si-PIN diodes with sensitive area of 1 cm per diode and a thickness of 400 pm (from [36, 46])...

The alternative approach to detection and analysis incorporates a solid state detector and a multichannel pulse height analysis system. The crystals used are of silicon (of the highly pure intrinsic type), or the lithium drift principle (p. 463 etseq.) is utilized. All emitted radiations are presented to the detector simultaneously and a spectrum is generated from an electronic analysis of the mixture of voltage pulses produced. Chapter 10 contains a more detailed account of pulse height analysis and solid state detectors. Production of an X-ray spectrum in this way is sometimes known as energy dispersive analysis ofX-rays (EDAX) and where an electron microscope is employed as SEM-EDAX. 

Modem pulse height analysers essentially contain dedicated digital computers which store and process data, as well as control the display and operation of the instrument. The computer will usually provide spectrum smoothing, peak search, peak identification, and peak integration routines. Peak identification may be made by reference to a spectrum library and radionuclide listing. Figure 10.15 summarizes such a pulse height analysis system. 

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 y-ray spectrometry, a mixture of y-ray emitting radionuclides can be resolved quantitatively by pulse-height analysis. The analysis is based on the fact that the pulse heights (in volts) produced by a phototube are proportional to the amounts of y-ray energy arriving at the scintillation detector. The amplification of the pulse is proportional to the voltage applied to the phototube, which can be adjusted so that the entire y-spectrum can be examined. 

A gamma-ray spectrum is produced nondispersivdy by pulse-height (multichannel) analysis using scintillation or semiconductor detectors. Resolving power, typically - 100 at 100 keV and 700 at 2 MeV, is quite modest compared with that achievable in other spectral regions, but is sufficient to identify nuclides unambiguously. 

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.)...

The escape peaks for the Si(Li) detector are generated by the same mechanism as in the gas-flow proportional counter and the NaI(Tl) scintillation detector. Section 4.2.3 should be consulted for a more detailed description of this mechanism. The Si(Li) detector has two important differences. First, since the detector is composed primarily of silicon, it is the escape of the silicon K x-rays that causes the escape peaks in the spectrum. Second, the pulse height spectrum is used for elemental analysis and usually contains a large number of lines. Each of these parent lines will have an escape peak associated with it. The escape peak from an element with high concentration can interfere with the analysis of trace amounts of an element of lower atomic number. 

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