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April 2007

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Subject:
From:
"Hlava, Paul" <[log in to unmask]>
Reply To:
JEOL-Focused Probe Users List <[log in to unmask]>
Date:
Thu, 5 Apr 2007 11:16:31 -0600
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*

Hi All, 

	I just have to add my 2 or maybe 4 cents into this discussion.

	Cent 1.  Rastering while analyzing is the worst possible way to
do a "bulk" analysis of inhomogeneous materials due to the "flag pole
effect" (my term or I may have heard it a long time ago).  The beam is
not rastered evenly.  A delay is programmed into the raster at the
beginning of each horizontal line to compensate for hysteresis in the
scanning coils.  This produces a "flagpole" of extra dwell time at the
left side of the raster.  AND there is a longer delay at the start of
each vertical sweep for the same reason.  This produces a ball-shaped
"finial" on the flagpole (the flag portion is the evenly rastered,
rectangular portion).  The flag and pole can be seen by viewing a small
raster on a fluorescent (OK, cathodoluminescent) sample.

	Cent 2.  A defocused beam is better.  I have never been able to
determine if the beam intensity is evenly distributed.  Visual
examination of the beam on a CL material SEEMS/APPEARS to be uniform.  

	Cent 3.  A focused beam trace on a lot of points is best. I
often do 300 to 500 points on Al alloys with minor amounts of
intermetallic compounds that contain most of the alloying elements.  The
summaries of these long traces usually allow me to uniquely identify the
particular alloy.

	Cent 4.  All of these methods have one big problem.  At times
your beam is going to be on 2 or more phases at the same time.  This
means YOU are lying to your correction program (it assumes a homogeneous
target).  The severity of the inaccuracy produced is directly
proportional to the interferences of the elements in the system and the
size of the various phases.  (I often get 107 % analyses on Sn/Pb
solders.)  The bigger the grains are, the better the average becomes
because you are more often on one phase at a time.

	Chuck, I empathize with your specification problem.  We have to
work with specs at Sandia a lot.  One "funny" thing is some old specs
call for analytical techniques no longer present at Sandia.  (But I
better not get into a discussion of management shortsightedness here.)

Paul


Paul F. Hlava 
Electron Microprobe Laboratories 
Department 01822 - Materials Characterization 
505-844-1890 
[log in to unmask] 

mail to 
Sandia National Laboratories 
MS - 0886 
Albuquerque, NM 87185-0886 

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