3.6.2. Amplifier & Pulse Processor

The pulses produced by the Si(Li) detector requires amplification before they can be shaped, analyzed and measured. The preamplifier is located physically near the detector crystal to minimize stray electronic noise. The EDS preamplifier incorporates a field-effect transistor (FET) that is used to reset the pulse amplification circuitry. Both the preamplifier and FET are cooled by liquid N₂ to reduce stray noise. The resulting signal is passed on to the main amplifier.

The main amplifier provides linear, low noise amplification of the preamplifier signal. In must be able to recover quickly after processing one pulse to be ready for the next. With EDS systems this is most critical since pulses are produced rapidly. The amplifier in this case must be able to deal with what is termed pulse pileup that is caused by a second pulse reaching the amplifier before the first has been fully processed (Figure 3.6.2). This results in a total pulse that is a combination of two pulses and thus meaningless. Pulse pileup rejection circuitry, which assures that processing of one pulse is finished before accepting another, is used to avoid this. Unfortunately, the pulse pileup circuitry significantly increases the deadtime.

Figure 3.6.2. Illustration of pulse pileup. Failure to discriminate pulses 1 and 2 leads to an anomalously large pulse (1+2) being digitized and stored (after Kevex Corporation 1983).