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Electron Microprobe Lab, University of Minnesota

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Hi Steve,

Yes, I've seen that before on our 8900.  It is dirt in the lower  
column charging, which deflects the beam as it becomes increasingly  
charged.  Then it discharges, and the beam "snaps back" to its  
starting position again.  You'll probably see it happen faster with a  
larger beam current.  The dirt is most likely in the lower column,  
probably just inside the objective lens pole piece or very near it --  
something from the chamber deposited there.  Cleaning the objective  
lens pole piece should fix that problem.

Best,
Ellery

-----------------
Ellery Frahm
Senior Research Fellow, Department of Geology & Geophysics
Manager & Principal Analyst, Electron Microprobe Lab
University of Minnesota - Twin Cities campus
Lab website: http://probelab.geo.umn.edu/
Personal: http://web.mac.com/elleryfrahm/