A multichannel analyzer (MCA) is used to process the pulses from the amplifier, which cover a wide range of amplitudes. Recall that the amplitude of the pulse is proportional to the energy of the incident X-ray photon. The MCA contains an analog-to-digital converter (ADC) and as each pulse is received it is converted into a digital signal (number). Large pulses are converted into larger numbers. The resulting ADC output is used as the address of a memory location and one is added to the value at that location. In effect, the memory acts as a set of independent counters, each covering a narrow range of energy. Counts are accumulated into these "bins" or "channels" for a preset "live" time (actually counting time corrected for deadtime) and the results can then be displayed.
The EDS system on the Cameca MBX microprobe consists of a Quantuum detector system connected to an 4Pi MCA board, which is located in the G3 computer. The MCA board and has a 1024-channel memory and can accumulate up to 16,777,215 (224-1) counts in each channel. Each channel is approximately 10 eV wide. The system can be calibrated using known energies to allow the use of a set of KLM marker lines.
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Copyright 1997-2003, James H. Wittke
Last update: 01/18/2006 01:45 PM.