An Overview of XRF Basics
1. Fundamental Principles
1.7 Pulse Height Analysis (PHA)
1.7.1 Pulse Height Distribution
If the number of measured pulses (intensity) dependent on the pulse height is displayed in a graph, we have the "pulse height spectrum." Synonymous terms are: "pulse height analysis" or "pulse height distribution." Since the height of the pulses of voltage is proportional to the X-ray quanta's energy, it is also referred to as the energy spectrum of the counter (Fig. 10a and 10b). The pulse height is given in volts, scale divisions are in "%" (and can be stated in keV after appropriate calibration). The "%"-scale is defined in such a way (SPECTRAplus) that the peak to be analyzed appears at 100%.
|
|
If argon is used as the counting-gas component in a gas proportional counter (flow counter or sealed proportional counter), an additional peak, the escape peak (Fig. 11a), appears when X-ray energies are irradiated that are higher than the absorption edge of argon.
Fig. 11a: Pulse-height distribution (Fe) with escape peak
The escape peak arises as follows:
The incident X-ray quantum passes its energy to the counting gas thereby displacing a K electron from an argon atom. The Ar atom can now emit an Ar Kα1,2 X-ray quantum with an energy of 3 keV. If this Ar fluorescence escapes from the counter, then only the incident energy minus 3 keV remains for the measured signal. A second peak, the escape peak that is always 3 keV below the incident energy, appears in the pulse height distribution. In Fig. 10a, no escape peak appears because the incident energy of sulphur radiation (S Kα1,2) is lower than the absorption edge of argon.
When using other counting gases (Ne, Kr, Xe) instead of argon, the escape peaks appear with an energy difference below the incident energy that is equivalent to the appropriate emitted fluorescence radiation (Kr, Xe). Using neon as the counting-gas component produces no recognizable escape peak because the Ne K-radiation, with an energy of 0.85 keV, is almost completely absorbed in the counter. Also, the energy difference from the incident energy of 0.85 keV and the fluorescence yield are very small.
1.7.2 The Counter Plateau
Every counter has a high-voltage area within which it can be optimally adapted to the appropriate application (operating range). It has already been mentioned that the gas amplification must be set somewhat higher for measuring light elements than for the K-radiation of heavier elements by changing the high voltage of the gas proportional counter. The high-voltage area that can be used for the application is called the "plateau" of the counter. This applies for the gas counter as well as for the scintillation counter with an integrated photomultiplier. Generally, the counter plateau is determined by irradiating X-ray energy typical for the application into the counter and measuring the intensity under increasing high voltage.
Fig. 11b illustrates the example of a counter plateau for a gas proportional counter with Ar + 10% CH4 as counting gas and Fe Kα1 as the radiation source (Fig. 11a). The number of pulses has been applied whose pulse height (Volt) exceeds a lower electronic discriminator threshold (e.g. 100 mV). If the high voltage is too low, the electrical field strength is not sufficient for producing gas amplification; the pulse heights are too low to pass the threshold.
If the high voltage is increased in increments, at first the pulses produced by the Fe K-peak will exceed the discriminator threshold's voltage height and be registered. If the power is increased further, the escape peak will pass the threshold, too. So, by increasing the counter high-voltage the radiation source's peaks are pushed over the discriminator threshold.
After a steep increase in intensity, a relatively flat high-voltage area takes shape. This is the counter's plateau or operating range. At the end of the plateau, the intensity increases sharply again due to counter pulses that do not primarily originate from the incident source. No measurements are to be taken in this area. Fig. 11b shows a form of plateau that occurs as a result of the integral measurement of all pulses over the discriminator threshold. If the pulses are pushed over a discriminator window with a lower and upper threshold, the intensity drops once more as the peaks are pushed out of the window again.
Fig. 11b: A gas proportional counter plateau