Pulse-height analysis

Because n is proportional to E, we can find from Eq. (7-3) the energy or wavelength dependence of the resolution  

Although Eqs. (7-3) and (7-4) are useful for rough qualitative arguments, they do not include the substantial effect of electronic noise in Si(Li)-FET counters. A better estimate of resolution in such counters is given by 

Circuits (1) and (2) may be used with ordinary diffractometers to increase the peak/background ratio of diffraction lines. They are by no means necessary quite adequate diffraction patterns can be obtained from a wide variety of specimens with no other discriminator than a Kp filter. Circuit (3) is required only in x-ray spectroscopy (Chap. 15), in a very special kind of diffractometry (Sec. 7-10), and with a position-sensitive proportional counter. Any one of these circuits is more effective, the better the resolution of the counter with which it operates. 

Both Fi and V2 are adjustable. If the window (F2 — Fj) is made quite small, say 0.5 volt, and the baseline Fj continuously varied from low values to high values, with (Fj — Fi) constant, then a narrow window can be traversed across the voltage range of pulse heights if the counting rate is measured at each setting 

An analyzer can markedly reduce the background of a diffraction pattern, chiefly by excluding short-wavelength white radiation. For example, examining the 111 line from silicon powder with copper radiation and a xenon-filled proportional counter, Parrish [7.8] found the peak/background ratio to be 57 without an analyzer and 146 with one. To achieve this almost three-fold improvement, the analyzer window was centered on the center V (Fig. 7-13) of the Cu Ka distribution and made wide enough to accept about 90 percent of it. 

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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 ... 

The apparatus as modified for x-ray emission spectrograph is also shown in Figure 11-1. The proportional counter may be used alone (pulse-height analysis Section 2.13) or a curved-crystal spectrometer can be employed to achieve better resolution. Analytical results were comparable to those quoted above, but localization of the area analyzed was considerably less sharp than the micron-diameter spot achieved in differential absorptiometry. 

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

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...

In summary, pulse-height analysis (PHA) prior to a Mossbauer measurement is an essential step in tuning a Mossbauer spectrometer. PHA allows the adjustment of the y-detection system to the Mossbauer photons and the reduction of noise by rejecting nonresonant background radiation. 

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 signals from the detectors are amplified to create a voltage pulse with amplitude proportional to the energy of the charged particle. Data acquisition, storage and display is effected by an MCA providing pulse-height analysis. 

The instruments used in X-ray emission spectrometry reflect the principles set out in Chapter 7. Radiation characteristic of the specimen is produced by electron or radiation bombardment. Monochromatic radiation is then presented to the detector by a diffraction device or by use of a series of narrow bandpass filters. Alternatively pulse height analysis (p. 465) can be applied to a series of pulses which have been generated with a size proportional to the radiation energy. Typical X-ray spectrometry arrangements are shown in Figures 8.40 and 8.41. 

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. 

Schematic layout of a nuclear spectrometer with a computer to provide pulse height analysis and data processing.

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

Saros, M. T., R. J. Weber, J. J. Marti, and P. H. McMurry, Ultrafine Aerosol Measurement Using a Condensation Nucleus Counter with Pulse Height Analysis, Aerosol Sci. Techno , 25, 200-213 (1996). 

Figure 18.21 Schematic diagram of a simple pulse height analysis system for nuclear spectroscopy. (From Wang et al., 1975.)...

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