Duane-Hunt limit

[Probeman]

When we bought the instrument in 2006 we specified that Cameca test the accelerating voltage using a high voltage reference and they did and it looked excellent at that time.  Unfortunately the installation engineer from France accidentally set the water chiller too cold and condensation shorted out the HV tank before he had even finished installing the instrument.

Recently on our Cameca SX100 if I set the keV to 15 and read the operating value back from the instrument I get something like 14.88 keV.  Now this could be just an A-D calibration issue, so I calibrated our Thermo EDS system using the Cu La and Ka lines and when I acquired a long (1000 sec) acquisition on Bi metal at 15 keV and 10 nA (to minimize coincidence counts above the accelerating voltage) I see this:



So exactly what is the Duane_Hunt limit in this spectrum?   I guess I should try again at 5 nA?

[Probeman]

Just for reference I had started the Duane-Hunt measurement at 30 nA and this is what that showed:



That is why I tried 10 nA.  Now I'll try 5 nA.

[Probeman]

Ok, here's an acquisition on Bi metal at 15 keV 5 nA for almost 2000 sec (22% deadtime).



It sure looks like the electron beam is more than 15 keV, whereas I would expect 15 keV or a smidgem less. Particularly since this is a carbon coated sample and from CalcZAF we should see a loss of about 8 eV for 20 nm of carbon and 15 keV electrons.

[Probeman]

Ok, I think I figured this out.  I again acquired a spectrum (4000 sec) on Bi metal at 15 keV, but this time at 2 nA which gives a deadtime of about 16%.



Now that the coincidence continuum x-rays are further reduced, I can convince myself that my high voltage on my gun is actually quite close to 15 keV.

So I guess the lesson is that if one wants to see the Duane-Hunt limit clearly, you should keep the deadtime below 20%.