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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 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 • 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 spectrum Electron Volume
Why should the overvoltage be less than 10 to 20 times the lowest energy peak? X-Ray Escape Volume High overvoltage means a high absorption condition and a small peak and poor statsitics (again) X-Ray Generated Volume Electron 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. Intersection Distance/ Working Distance scale setting Elevation Angle
EDAX Detector Geometry WD < ID TOA < EA ID EA Tilt > 0 TOA > EA WD > ID TOA > EA
Real 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 constants Sometimes using a faster time constant is a way to get more data in the same time. Usually a good idea when collecting maps.
