EDS filter "turret" on Cameca probes

[John Donovan]

On most SX100 instruments you will find a small handle attached to a lever below the EDS front port.

If the instrument was configured for EDS in the front port, this "turret" will have three positions with three different apertures from small, to medium to large.

This was designed to allow older, Si(Li), EDS systems to run at high beam currents without sending the deadtime to 99%, by "stopping down" the aperture size. However, with the new SDD EDS detectors that can handle much higher count rates, this isn't actually necessary any more.

So what I have done with two different instruments (my SX51 at UCB and my Sx100 at UofO) is to remove the turret hardware and punch out the smallest aperture and replace it with a small piece of Be foil (from an old Si(Li) detector) attached with a couple spots of epoxy.

Why?

Because as I said above there is no real need to reduce the count rate to the modern SDD detector, but more importantly one can dramatically improve the sensitivity for higher energy emission lines by using a Be window as a low energy "filter" to remove the carbon, oxygen, etc photons from striking the detector. Here is a few PPT slides that demonstrate the issue:

http://epmalab.uoregon.edu/reports/Hg%20in%20Plastic%20EDS%20vs%20WDS.ppt

[neko]

Anyone know what it means if changing the aperture breaks vacuum immediately?

If not, I'll let you know if/when I figure out what just happened. 

Update 1: breaks whenever you move position, but vacuum comes back quickly no matter what position you leave it in. I'm guessing: unused, unmaintained 13 year old mechanical part is the culprit. I am going to stop touching it.

[Probeman]

Anyone know what it means if changing the aperture breaks vacuum immediately?

If not, I'll let you know if/when I figure out what just happened. 

Update 1: breaks whenever you move position, but vacuum comes back quickly no matter what position you leave it in. I'm guessing: unused, unmaintained 13 year old mechanical part is the culprit. I am going to stop touching it.

Hi Nick,
Sounds like a dried out o-ring.

But removing the aperture turret on the sx100 requires first removing the EDS and the light optics housing over the turret. But when you or your engineer gets around to it, you might consider performing this small modification to the EDS aperture assembly:
 
http://probesoftware.com/smf/index.php?topic=342.msg1789#msg1789

I did this to both my sx51 at UC Berkeley and my sx100 here at Oregon  and it can be very useful on occasion for improving EDS sensitivity.
John