EDS Physics

[Probeman]

Rick Mott at PulseTor posted this nice explanation of the Fe55 emission physics on the Microscopy listserver and I thought I would add it here (used with permission):

Dear Gary --
I'm one of the guilty parties, going back about 4 decades.

First, the emission of an Fe55 radioactive source really is the Mn K lines. It is identical to the K emission of Mn under the beam, except there is no Brehmsstrahlung background.  Instead of ionization by an electron beam, the excited Mn is created by electron capture from an Fe55 nucleus (nuclear proton grabs an electron and becomes a neutron, reducing the atomic number by 1).  It is convenient and very safe, because no other radiation is emitted (*). Sources up to 100 uCuries can be sent by regular mail with no special packaging.

Detectors are normally tested in a controlled environment "on the bench" with a source.  There are many reasons a detector might not meet spec on a column in the real world besides the detector itself.  For example, CE marking be d*mned, we've discovered that one of our Tektronix digital oscilloscopes costs about 1.5 eV if it's anywhere near the BNC cables from the detector to the pulse processor.  X-ray detectors are exquisitely sensitive EMI detectors too. You might find for that resolution varies slightly with the time of day, depending on what other equipment in the area is operating.  I've spent many happy (?) hours at various installations trying to figure out what was causing resolution degradation.  123 eV is close to the absolute limit for a silicon-based detector, and it only takes microvolts of noise to kick that up an eV or so.

Anybody else have enough gray in their hair to remember when anything better than 150 eV was a pretty decent detector?

I second the kudos to Nicholas Ritchie!   DTSA-II is an extraordinarily useful tool for all aspects of EDS analysis.   Good luck figuring out what's going on.

Rick Mott, PulseTor LLC (formerly with PGT back when woolly mammoths still walked the hills of New Jersey)

(*) Ok, all you picky nuclear physicists out there.  There are extremely low probability gammas up to 231 KeV, 5 to 7 orders of magnitude lower in intensity than the Mn K lines.  Not worth losing sleep over.

I'm one of those dinosaurs that remember 150 eV Si(Li) detectors, but my first EDS was an Oxford Ge detector that had slightly better resolution...  it was a real hassle because one had to keep it filled with LN2 even when the bias voltage was off!

[Probeman]

I was going through some old files and found a PowerPoint presentation from Dale Newbury that he gave in 2007 at one of our EPMA workshops in Eugene. Remember when we used to have these?  This is Dale at the 2008 workshop where it appears we are analyzing a "piece" of Saturn...   



Anyway, I wrote Dale and asked if it would be OK to post his PPT file to the user forum and he said that would be fine, and also sent me a more recent version of the presentation from 2014. See below for the attachments (be sure to log in first to see attachments).  It's interesting to how much the performance of these detectors changed in 7 years!

Here's another photo from the 2008 workshop:

[jrminter]

Thank you both for making these files available here.