Important EDS Parameters EDAX Detector Geometry Deadtime & Time Constants

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Important EDS Parameters EDAX Detector Geometry Deadtime & Time Constants. Optimum Count Rate. High Energy Peaks Use a Longer time constant CPS to give 20 - 40% Deadtime Low Energy Peaks Use Longest time constant 500 - 1000 counts/sec
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Important EDS ParametersEDAX Detector GeometryDeadtime & Time ConstantsOptimum Count Rate
  • High Energy Peaks
  • Use a Longer time constant
  • CPS to give 20 - 40% Deadtime
  • Low Energy Peaks
  • Use Longest time constant
  • 500 - 1000 counts/sec
  • Throughput (faster time constant, cps to give 30 - 60% Deadtime)
  • Accelerating Voltage
  • Overvoltage = Voltage/Peak Energy
  • Typically, the accelerating voltage should be 2x the highest energy line and no more than 10 to 20 times the lowest energy line of interest.
  • 10 times for Quantitative Analysis
  • 20 times for Qualitative Analysis
  • Why should the overvoltage be at least 2 for the highest energy element?X-Ray Vol.Low overvoltage means a small, poorly excited peak and poor statistical quality in the spectrumElectron VolumeWhy should the overvoltage be less than 10 to 20 times the lowest energy peak?X-Ray Escape VolumeHigh overvoltage means a high absorption condition and a small peak and poor statsitics (again)X-Ray Generated VolumeElectron Int. Vol.Take-Off Angle
  • The take-off angle is the angle between the x-ray trajectory and the sample surface.
  • The angle is a combination of detector angle, its position, sample working distance, and sample tilt.
  • Typical angles will range from 25 - 40 degrees
  • Detector Geometry
  • Elevation Angle = The angle between the horizontal and the detector normal.
  • Intersection Distance = The distance in mm between the pole piece to where the electron beam intersects the detector normal.
  • Working Distance = The distace from the pole piece to the sample surface.
  • IntersectionDistance/ Working Distancescale settingElevationAngleEDAX Detector GeometryWD < IDTOA < EAIDEATilt > 0TOA > EAWD > IDTOA > EAReal Time“Clock Time”
  • Real Time = Live Time + Dead Time (Real Time = Clock Time)
  • Live Time - time when detector is alive and able to receive an x-ray event
  • Dead Time - time when the detector or preamplifier is unable to accept a pulse because it is busy processing or rejecting an event(s).
  • Stored Counts in 10 Clock Seconds (peak = 67% DT)The higher count rate does not necessarily produce more counts in the spectrum.Stored Counts in 10 Clock Seconds --using fast and slow time constantsSometimes using a faster time constant is a way to get more data in the same time. Usually a good idea when collecting maps.Deadtimes on a DX Prime SystemDeadtimes on a Phoenix System
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