Electronic pulse counters

Gas Ionization Counters A common gas ionization counter is the Geiger-Muller counter where the electronic pulses derived from the ionization process are registered as counts. The instrument can be adjusted to detect only radiation with a desired penetrating power. 

This is considered in more detail in 10.7.5. It is a method which involves a modified electronic cell counter, i.e. a Coulter counter linked to a pulse height analyser and an electronic cell sorter. It is capable of separating cells at a rate of about 50,000/min into a... 

The three-compartment source was attached to the analyzer tube of a 6-inch radius 60° sector magnetic deflection mass spectrometer. Differential pumping was used between the source and analyzer regions. The ion detector was a 14-stage electron multiplier coupled to both a vibrat-ing-reed electrometer and a pulse counter (38). The electrometer was connected to a strip-chart recorder and the counter to a printer. This arrangement allowed any range of e/m to be scanned or a given peak to be monitored. 

Resolution in a semiconductor detector EDXRF system is a function of both the detector characteristics and the electronic pulse processing. The energy resolution of semiconductor detectors is much better than either proportional counters or scintillation counters. Their excellent resolution is what makes it possible to eliminate the physical dispersion of the X-ray beam without the energy resolution of semiconductor detectors, EDXRF would not be possible. 

Use of the LS counter for alpha-particle spectral analysis is discussed in Section 8.3.2. Source preparation is simpler, but energy resolution is worse than with the solid-state detector. Special source preparation and electronic pulse-shape selection can improve resolution. 

FIGURE 9.9 XRF of nominal lead shot measured at Virginia Commonwealth University using a Kevex Quantex ISI-130 SEM-EDX electron microscope with XRF attachment and a 20 keV excitation beam. The y-axis is in counts because the intensity was measured with an internal pulse counter and the x-axis is given directly in kiloelectron volt as reported with the software associated with the instrument. The spectrum was run by James Spivey at VCU. The XRF spectmm of the lead shot reveals it is mainly lead but contains other metals as well. Note the presence of poisonous As and Pb. Thanks are due to Rhonda Stroud of the Naval Research Laboratory for interpretation of the spectrum and the assignment of the peak between As and Pb at 1.740 keV as due to a K line from Si. 

Intensity measurements are simplified when a detector always gives one electrical pulse for each x-ray quantum absorbed the detector remains linear so long as this is true. For low intensities, when the rates of incidence upon the detector are low, the Geiger counter fulfills this condition. As this rate increases above (about) 500 counts per second, the number of pulses per second decreases progressively below the number of quanta absorbed per second. This decrease occurs even with electronic circuits that can handle higher counting rates without appreciable losses. 

In the phosphor-photoelectric detector used as just described, the x-ray quanta strike the phosphor at a rate so great that the quanta of visible light are never resolved they are integrated into a beam of visible light the intensity of which is measured by the multiplier phototube. In the scintillation counters usual in analytical chemistry, on the other hand, individual x-ray quanta can be absorbed by a single crystal highly transparent to light (for example, an alkali halide crystal with thallium as activator), and the resultant visible scintillations can produce an output pulse of electrons from the multiplier phototube. The pulses can be counted as were the pulses-from the proportional counter. 

The tube of Figure 2-2 can be operated as an ionization chamber, as a proportional counter, or as a Geiger counter. The tube output differs radically from one case to another. Because of these differences, the electronic circuitry associated with the tube must also be different for each case if the pulses from the tube are to be reliably selected and counted. In particular, the circuitry will have to differ in characteristics such as stability, amount of amplification, and time of response. In all cases, linear amplification (amplifier output always proportional to tube output) is desirable. 

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