Hi all,
I'm deciding the sample holders for our new probe and am leaning heavily toward the split stage where the standards hang out in the chamber for long periods of time. I recognize needing to recoat for different accelerating voltages, but that might necessitate having different standard sets with different coat thicknesses. I mean our bread and butter is 15 kV so those standards could be installed in the chamber making it harder to repolish/recoat the standards. That said, how often do you repolish/recoat your standards?
Hi Dawn,
I'm trying to understand why one would "recognize needing to recoat for different accelerating voltages"?
You will definitely want to have *known* coating thicknesses on your standards and unknowns, if they aren't the same thickness. Because as you know, the difference in coating thickness between ones standards and unknowns will most definitely affect quantification, particularly for very low energy emission lines (differential x-ray absorption by the coatings), e.g., O ka, and also for very low overvoltage x-rays (differential electron energy loss from the coatings), e.g., Fe Ka at 10 keV.
But I don't quite see why the coating thickness would need to be different for different electron beam energies. As long as the coating thicknesses are the same, or known and specified in Probe for EPMA as described in this topic, I think it shouldn't matter:
https://probesoftware.com/smf/index.php?topic=23.0Using the Calculate Electron and X-ray Ranges dialog in CalcZAF, even a 5 keV beam penetrates almost 500 nm in carbon. Are you thinking of looking at inclusions approaching 20 nm in size (your nominal coating thickness)?
5 keV, 2.26 grams/cm^3, Electron range radius = 0.437628 umI'm probably missing something, so can you explain what you mean?
As to when to re-polish ones standard mounts, the rule of thumb in my lab is: whenever the metals start looking a bit oxidized. When they're not in the instrument, we keep our standards in a dessicator, which helps a little I think, but keeping them in the probe under vacuum should help more. Though I sometimes wonder if someone should do a study on these assumptions.
I'm thinking of when I worked at Lawrence Berkeley Lab many years ago, on stable isotope sputter targets for their particle accelerators. One of our targets for transuranic studies was 48Ca which was very expensive and would also oxidize quickly in air. At first we tried storing the targets in a vacuum cannister with a dessicant, but they still oxidized pretty quickly. So then we tried storing them under high pressure Ar (~200 psi) which seemed to really slowed down the oxidation of our 48Ca targets. Our hypothesis was that under vacuum, the remaining oxygen (or H2O?) molecules had such long mean free paths that they all eventually got to strike the surface of the calcium targets and cause oxidation, but under high pressure Ar, the mean free path would be much shorter, and that would slow down the oxidation. Well, that was the thinking anyway.
Here's a fun link:
https://www.sciencealert.com/this-calcium-isotope-is-so-rare-it-costs-500-000-for-just-2-gramsBack to EPMA, the other time we normally re-polish our standard mounts is when the carbonate or phosphate standards get too "chewed up" by the beam and we can't find any "clean" areas for standardization.