Try turning the sample 180 degrees to see if the deflection is in the opposite direction.If so likely magnetism.
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-------- Original message --------From: John Donovan <[log in to unmask]> Date: 11/29/16 10:01 AM (GMT-08:00) To: [log in to unmask] Subject: Re: [PROBEUSERS] Weird beam deflection issue: magnetism? charging? other?
Hi Julien,
Not only can samples be magnetic, but I have observed that the
electron beam can actually "induce" magnetism in some alloys.
There is an extended discussion of these effects here:
http://probesoftware.com/smf/index.php?topic=354.0
john
On 11/28/2016 7:59 PM, Julien Allaz
wrote:
Dear probers,
I hope you all had a great Thanksgiving break. Today
a colleague brought three epoxy mounts (1’’) containing each one
5-10 mm meteorite chip. Whereas the day started well with
acceptable (large) Px analyses (100-101%), troubles arrived when
I hit a CL-active plagioclase: the beam was actually
off-centered by 10-15 um compare to my usual well-centered
position. As a result, a series of 20+ points on well-polished
and large surface yielded 103-104% total, even when using 20 nA
and 10 um beam size (2 min analysis time). Problem is that the
electron beam is perfectly centered on any other standards, and
other CL active minerals in other mounts. So… the problem
appears to be sample-related?
From there, I started wondering about possible (a)
charging (old epoxy => loose connection, plus sample is badly
polished and fractured), and (b) presence of magnetic minerals
(magnetite especially).
I have not seen any “flash”, abnormal SE intensity
(except on some cracks as expected), or image displacement, even
during prolonged beam exposure. Despite this, I opt to re-coated
a SECOND time (so TWO coating layers of C ~20-30 nm at least)
hoping it would cover the cracks & so. Additional re-coating
(after re-polishing) did not help. I also added some C-paint to
connect the sample surface directly to the sample holder. None
of this improve the behavior. The beam was still showing some
displacement. The weird thing is that on one single sample, the
beam deviation can be variable within one single sample: more or
less centered in one zone, 10 um on the right in one other and
10 um to the top-left on the other. And it is variable in each
sample, too. So… Not a charging issue?
Could magnetism be an issue? The users have actually
run some QEMSCAN, and beside some Cr-Fe oxide and ilmenite there
does not appear to have magnetite, s.str. Could other minerals
(in meteorite!) be a problem? BTW, I do NOT have access to a
demagnetizer, but I have some hope to find some (or buy one)
soon?
To double check everything, in one coating run, I
freshly re-coated all the samples (after brief re polishing with
.25 Al2O3 + ultrasonic bath), and added an other similar sample
made with a new epoxy mount (~2 month old) of another meteorite,
not the same composition / mineralogy. This one worked perfectly
(beam was centered)… Could the problem come from “old” epoxy
(how old is old???). Living in CO, any compounds like epoxy
might behave differently than on the west and east coast due to
our well-known near-absence of H2O, and the high elevation near
the mile-high city…)?
At this point I would welcome any suggestion /
opinion…
Best,
Julien
=============================
Dr. Julien Allaz
Electron microprobe manager
University of Colorado Boulder
Dept. of Geological Sciences
UCB 399
2200 Colorado Av.
Boulder, CO 80309-0399
Phone: (303) 735 2413
Cell: (413) 210 0917
Lab: (303) 492 5251
Fax: (303) 492 2606
Lab website: http://geode.colorado.edu/~jallaz/
=============================
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