3.5.6. Single-Channel Analyzers & Pulse Height Analysis

Recall that the amplitude of the pulse from gas-flows detectors is proportional to the energy of the incident X-ray photon. This permits the electronic circuitry to select for a specific X-ray photon by excluding anything but the desired amplitude. A single-channel analyzer (SCA) selects pulses of interest and outputs pulses suitable for the counting electronics. The SCA can be set to reject pulses of lower energy than a certain threshold (baseline) and higher than the baseline plus a "window." Sometimes, these discrimination values are expressed as "lower" and "upper" limits, E_L and E_U, rather than as a baseline and window (Figure 3.5.6). Selection of desired pulse energy is termed pulse-height analysis, and SCAs are commonly called "Pulse Height Analyzers" (PHAs).

Figure 3.5.6. Schematic representation of pulse height analyzer behavior. (a) Main amplifier output; (b) single channel analyzer output with EL = 5 V and EU = 7V. Pulses I and III are rejected (after Goldstein et al. 1981).

When both baseline and window are in use, the PHA is in differential mode. Differential mode is used to eliminate the interference from higher order peaks of other undesired elements which also satisfy Bragg's Law. In addition to the desired wavelength, an analyzing crystal will also diffract 2l (2^nd order), 3l (3^rd order), 4l, etc. However, only the desired wavelength has the energy, E, of interest (the 2^nd order peak has 2E, the 3^rd, 3E, etc.). Setting the window to screen out these higher energies insures that only photons with the correct intensities are counted.

As an alternative, the PHA can be set in integral mode with the window wide open to accept all pulses greater than a given baseline. The use of integral mode is recommended in most cases to limit the problems associated with a narrow window caused by drift in the detector electronics, and changes in P-10 gas pressure and room temperature. The significance of the high-order interference depends on how much of the interfering element is in the sample and the effective intensity of its X-ray line. Obviously, in trace element work, where the peaks of interest are very small, very careful evaluation of interferences is required.

The user must consider three possibilities:

• If the X-ray of interest has energy exceeding E_c of Ar (2.96 keV), escape peaks can be a problem but multiorder diffractions are not. For example, the Ca-Ka (Ar) escape peak overlaps P-Ka, but the baseline may be used to eliminate the escape peak and the PHA run in integral mode.
• If the X-ray energy is less than E_c of Ar and greater than about 30 times the ionization potential of Ar (30 x 26.4 eV = 0.8 keV), there are neither escape peaks nor higher-order diffractions. Again, integral mode is appropriate.
• If the X-ray energy is less than 0.8 keV, there are likely to be wavelength shifts and higher-order peaks can cause significant interferences. Differential mode, with carefully selected baselines and windows is appropriate.

 

In summary, PHA settings must be chosen to optimize several (often conflicting) requirements: optimizing signal-to-noise ratio, avoiding escape peaks, avoiding interferences, and minimizing potential problems with drift. In either differential or integral mode, the baseline is used primarily to eliminate noise produced by electron vibration in the amplification circuitry.