Author Topic: Change in peak shape over time  (Read 3291 times)

Brian Joy

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Re: Change in peak shape over time
« Reply #15 on: February 04, 2017, 08:09:21 am »
The plot below illustrates background radiation (counts per second per nanoamp) collected at the upper background offset of Si Ka measured using TAP (P-10 gas-flow counter) on either of two diopside standards, one natural (PSU Px-1) and one synthetic (crystalline).  The background offset in every case was located at an L-value 2.8 mm above that of the peak.  Accelerating potential in each case was 15 kV.  Each point on the plot represents an average of 9 or 10 measurements on the standard.

At accelerating potential = 15 kV, a large enough fraction of backscattered electrons has energies appropriate to ionize argon such that these ionizations can contribute significantly to measured background radiation.  The effect is especially pronounced at low Bragg angle, such as for Si Ka diffracted using TAP.  Using the plot below, I can state with some confidence that the period of failure of the static filter -- apparently due to cracking of a cold solder joint at the point of attachment of the power supply to the filter -- was 20 Oct 2014 to 17 Jan 2017.

I had my reservations about the modification to the objective lens cooling system.  It brought to mind an old adage, “If it ain’t broke, don’t try to fix it.”  I guess the solder joint might have failed at some point, but by then (Oct 2014) it had held for 3.5 years.


« Last Edit: February 04, 2017, 10:53:31 am by Brian Joy »
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

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Re: Change in peak shape over time
« Reply #16 on: February 05, 2017, 12:47:42 pm »
The plot below illustrates background radiation (counts per second per nanoamp) collected at the upper background offset of Si Ka measured using TAP (P-10 gas-flow counter) on either of two diopside standards, one natural (PSU Px-1) and one synthetic (crystalline).  The background offset in every case was located at an L-value 2.8 mm above that of the peak.  Accelerating potential in each case was 15 kV.  Each point on the plot represents an average of 9 or 10 measurements on the standard.

This just goes to show that monitoring of standard intensities can provide the proverbial "smoking gun" for troubleshooting such issues. PFE users can utilize the Drift application for these purposes as described here:

http://probesoftware.com/smf/index.php?topic=575.msg3565#msg3565
The only stupid question is the one not asked!

Brian Joy

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Re: Change in peak shape over time
« Reply #17 on: June 08, 2018, 12:32:47 pm »
After speaking with the JEOL office in Peabody, Mass. and collecting some SE images with an “electron mirror,” I now have a plausible explanation for the deterioration in resolution of the three PET crystals at high Bragg angles:  the static filter is not functioning correctly, and this has probably been the case for some time.  Thus the diffracting crystals (and other parts of the spectrometers) have been bombarded on a daily basis with backscattered electrons.  This probably heated the PET crystals sufficiently that their surfaces have become damaged.  In looking at an “electron mirror” SE image that I collected in May, 2015 in order to document a problem with the BSE detector, I can clearly see the diffracting crystals.  Live and learn, I guess.

An update on this old thread:  As of this morning, JEOL has agreed to replace our three PET crystals (PETJ, PETL, and PETH) at no charge to us.  This is fantastic news and is very gracious of them.
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230