An Overview of XRF Basics
2. Instrumentation
2.5 Electronic Pulse Processing
The pulses produced in the detectors by X-rays are processed and counted by subsequent electronic processing. The flow counter's signals are electronically amplified in a preamplifier, shaped and further processed as voltage pulses in a main amplifier (sine amplifier) and discriminator. After the photomultiplier, the scintillation counter's signals are fed directly into a main amplifier and discriminator.
2.5.1 The Discriminator
Depending on the application, higher order peaks or other sources of interference appear in the pulse height distribution, i.e. the detector's energy spectrum with different levels of energy (see also Figs. 10a, 10b and 11a). A discriminator window is used to set a lower and an upper pulse-height threshold. Only the pulse heights that lie within these limits are counted. In this way, higher order peaks or interference radiation with pulse heights beyond the window are suppressed (see Fig. 16). Discriminating undesirable pulses reduces the background.
2.5.2 Main Amplifier, Sine Amplifier
After the preamplifier (flow counter and proportional counter), or photomultiplier (scintillation counter), the pulses are further enhanced electronically in a main amplifier. As the detectors' high voltages are set separately for each crystal, the electronic additional amplification must also be made dependent on the crystal used.
X-ray energies, for example, of 3.3 keV to 30 keV (potassium to iodine) are detectable with LiF(200). To have each one of the set peaks in the pulse height spectrum always appear in the same place (i.e. have identical pulse height), the electronic amplification must be linked to the goniometer's angle setting. This is achieved by making the main amplifier's amplification factor V for the appropriate crystal (2d value) and the selected reflection order (n) dependent on the sine of the adjustment angle:
V ∼ sin θ
This ensures that a discriminator window, once set for a crystal, will be applicable for all detectable energies. A main amplifier coupled in this way is called a sine amplifier.
2.5.3 Dead Time Correction
Spectrometer electronics need a certain amount of time to process a pulse, during which no other pulse can be registered. This period is called counter channel dead time for an individual pulse. As the pulse formation is different for the flow counter and the scintillation counter, the dead times (typically 300 to 400 ns) are also different for each detector. Total dead time is the result of an individual pulse multiplied by the pulse rate. As the measured pulses occur statistically distributed over time, the proportion of pulses occurring during the processing period of a previously registered pulse depends on the intensity of the radiation, i.e. the total dead time increases due to the increase of the intensity. This results in a non-linear rise of the measured intensity with the intensity irradiated in the detector. The greater the incident intensity, the greater the losses during measurement. Fig. 30 illustrates how the dead time is dependent on the increasing incident intensity (increasing generator current). The curve flattens out distinctly at high measured intensities.
A correction of the measured intensities is necessary to produce a linear relationship between incident and measured intensities. A dead time correction can be made in the analysis computer. Fig. 31 shows the dead time corrected measurement points. A useful correction can be obtained in tandem operation up to an incident intensity of approximately 1,200 kcps per detector. Greater intensities are not worthwhile.
For routine operation it is recommended not to exceed an intensity of approximately 400 kcps per detector.
Fig. 30: The dead time effect
Fig. 31: Dead time corrected readings
2.5.4 Line-shift Correction
Line-shift correction is only important for the flow counter and the proportional counter at high intensities. Line shifts are noticeable when a peak in the pulse height spectrum shifts to lower values at high counting rates. The reason for this is that high counting rates in the detector volume between the flow counter's cathode and counting wire build up a space charge that causes a temporary reduction of the effective high voltage and thus the reduction of gas amplification.
Shifts of pulse heights to lower values are automatically corrected by the electronics. The correction can be switched on and off in the software (SPECTRA 3000 and SPECTRAplus).