This doesn't seem correct. I wonder if they actually tried to model this themselves. I also don't understand why Casino would have produced a lower intensity for a thinner carbon coating. That doesn't make sense.
If you run CalcZAF and select the Run | Calculate Electron and X-Ray ranges menu dialog. You can specify Pure carbon, set the density, specify the thickness and calculate both the x-ray transmission efficiency and the electron energy loss. Yes, it's not as accurate as a MC simulation, but should be more than accurate enough (and a lot less subject to statistical variation!). And quicker!
So here are results for a 15 keV beam with 20 nm (0.02 um) of carbon and a 4 keV x-ray:
Electron energy transmitted at incident electron energy of 15 keV, 2.26 grams/cm^3, thickness of 0.02 um = 14.99345 keV
X-ray transmission fraction at energy 4 keV, thickness of 0.02 um (average u/p = 37.26325) = 0.9998316
And here for a 2 keV x-ray:
X-ray transmission fraction at energy 2 keV, thickness of 0.02 um (average u/p = 307.1905) = 0.9986125
Why would someone think a loss of 0.14% is worth worrying about?
Trying the same but at 6 keV electron beam energy and 4 keV x-ray we get:
Electron energy transmitted at incident electron energy of 6 keV, 2.26 grams/cm^3, thickness of 0.02 um = 5.987882 keV
And finally let's try a 10 nm carbon coat with a 2 keV x-ray:
X-ray transmission fraction at energy 2 keV, thickness of 0.01 um (average u/p = 307.1905) = 0.999306
Is a fractional percent improvement in transmission really worth all that trouble? For another thing, as you mentioned, technically one should specify the different thicknesses in carbon coating for standards vs. unknowns for the k-ratio as described here:
https://probesoftware.com/smf/index.php?topic=23.msg8997#msg8997In the example linked above the user had accidentally coated his unknown thin sections with
70 nm of carbon, and yeah, that does make a difference in this case because the overvoltage of Fe ka is quite low even at 15 keV (Eo/Ec ~ 2) and even a small reduction in electron energy is sufficient to affect the ionization efficiency!