PHA
Pulse Height Analysis (or Analyzer). The x-rays that satisfy Bragg's Law reach the detector inside the spectrometer housing, where they ionize the gas (produce photoelectrons and Ar cations) and create a small analog electronic signal. This signal passes to a pre-amp and then through an electronic circuit that shapes it into a digital pulse for input to the x-ray counter scalers. In a properly configured detection system, the bias in the detector is set so that the ionization occurs in the "proportional range", and higher order (n>1) diffractions that also satisfy Bragg's Law (and are discrete fractions of the wavelength of the desired element), will have discrete multiples of the pulse energy. Thus, if the first order peak has a peak at 2.0 volts, the 3rd order peak would have a peak at 6.0 volts. This provides one way to reduce or eliminate higher order interferences: setting a window so that the desired element's x-rays are passed through, but that the higher energy interfering x-rays are rejected. This is known as "differential" mode, whereas under "integral" mode there is no window. Also, a lower limit or baseline (e.g. 0.5 volts) is set to filter low energy electronic noise. Use of differential mode PHA should be used with care, as there are possible serious errors that can occur to the unwary (see discussion of energy shifts in the manual). Probe for EPMA offers a sophisticated interference correction which in many situation eliminates the need to use differential mode PHA (which can only reduce higher order interferences in any case).