In my carrier using both electron-based EDS and X-ray-based EDS (Micro-X-ray Fluorescence) I have experienced the 5 (reported) general types of artifact peaks. 1- Sum peaks, 2- Silicon escape peaks, 3- X-ray source peaks, 4- Compton peaks, and 5- Diffraction peaks. However, I have also experienced a 6th type of artifact peak, something I reported briefly at Microscopy/Microanalysis(1), (commonly referred to as M & M). I have also documented it in my book "Food Forensics Handbook, practice, instrumentation, case studies"(2).
Once I knew what was causing it, I put a name to it. I call it the "Ricochet Peak." Basically, it is formed by an extraneous or non-intended event. An X-ray or an electron strikes something that is not your sample. This forms an X-ray with the energy of its source. If that extraneous x-ray then bounces off of your sample and then strikes the detector (a ricochet event) it maintains the energy of its source and the x-ray detector records it as having originated from your sample. If this event is common enough, one will detect it in the spectrum and in the resulting dot map.
When I have seen this event, I was always using my E-SEM, which is mostly aluminum within the specimen chamber. So, what I see is an aluminum peak in my spectrum, when there should be no aluminum within my sample. I see this ricochet event most often when I am looking at crystalline material. What occurs is this: Some of my source electrons (or x-rays) strike a facet of the crystal. That source particle is then deflected into the sample chamber where it strikes something aluminum and forms an aluminum x-ray. If that resulting aluminum x-ray then strikes my sample at an angle that allows it to ricochet directly to the x-ray detector, it gets recorded as having originated in my sample. If this event is common enough in occurrence, I will see the dreaded aluminum peak within my EDS spectrum.
I finagled an image that demonstrates this event from a more-or-less forced situation. If interested I think I can include that contrived image in a following response. I can show the event happening in image and within the spectrum. It is a bit disconcerting to see aluminum seemingly originating from a sugar crystal X-ray dot map, the crystal is thus glowing the assigned color for aluminum.
1) St. Jeor, Var L.; "EDS-Lite, Quantitative Energy Dispersive Spectroscopy of Light Elements." Microscopy and Microanalysis, proceedings, 1 July 2017.
2) de St. Jeor, Var L.: "Food Forensics Handbook, Practice, Instrumentation, Case Studies;" (2021), copyright (2022), DEStech Publications, Inc.
Dear Var St. Jeor ,
you are right,
your 'ricochet peaks' arise from the multiple scatter capability of electrons and x-rays.
There are some means to reduce these backscatter signal(s) from the surrounding onto the detector:
a) strict collimation of the exciting beam onto the net sample surface; direct bombardment of the vicinity will enhance the ricochet peaks;
b) strict collimation of the viewing field of the detector; also here the detector only has to 'look/view' at the net sample surface;
c) rounding/chamfering of the rims/edges of the diaphragms will reduce scatter arising from them;
d) avoiding any scatterers (e.g. part of mounting supports) in the close vicinity of the sample.
During the experiments, which I have done, often the Ba K-alpha(s) have shown up.
They arose from the pigments (BaSO4) of the painting of the walls*) of the x-ray lab/cabinet. Although there was a quite large distance to the walls, in long-term scans the Ba K-alphas did show up. Only omitting a direct optical path between the detector surface and the walls could strongly reduce these lines. But often I could not completely get rid of them...
*) there was no barite concrete involved
Best regards
G.M.
Thankyou Gerard. You know, I have looked for a discussion of this type of event, but was not able to find one. It is good to know others have seen this. I wonder why this has not been included before in the listings of artifact peaks? There needs to be a warning so others know to look for it as part of there ‘good practices.’ I can usually reduce ‘ricochet‘ events by rotating or tilting my sample a bit. The flat surfaces of crystals encourage this type of artifact peaks. But your discussion is quite enlightening regarding your suggestions for fixes. Thank you again.
Dear Var St. Jeor , the artefact you described is a well known one, usually it is called stray radiation. Pretty usual for EDS/TEM and much less common for EDS/SEM (for properly installed and collimated detector).
SEM specimen chambers are usually made of steel. So, stray radiation should produce Fe peaks. Of course you can see Al peaks from specimen stubs if your specimen is small or you are observing the edge. For Environmental SEM effect more pronounced since electron "skirt" could be pretty wide.
Some links from the first page after googlins "stray radiation EDS"
Chapter Artifacts in Energy Dispersive X-Ray Spectrometry in Electro...
https://www.globalsino.com/EM/page3920.html
http://www2.arnes.si/~sgszmera1/html/procedures/11eds.html
In addition to what has been said, the stray radiation does not only show up as fluoresecence lines but also as background signal in electron and x-ray excited fluoresence spectra, but mainly in the x-ray energy regime of above a few ten keV. Here the attenuation of x-rays will be more and more dominated by the Compton scatter processes, which gradually smear the primary x-ray photon energy by multiple scatter events. Especially when dealing with tiny fluoresence peaks or measurements with respect to the monochromaticity of x-rays, such backgrounds affect the detectability of these peaks and the monochromaticity (e.g. ratio of net peak integral to total integral) in a spectrum respectively. The background stray radiation is admittedly small, but not zero; but can be minimized by appropriate shielding and beam collimation.
Valadimir, I appreciate your input and the name identification. Yes, this SEM chamber is mostly Aluminum, with steel components. And, yes, it is possible to produce this effect from the SEM stub as you have indicated. In fact, I have an EDS dot map showing this effect in image, based upon an aluminum SEM stub. My current concern is that this affect is almost never discussed as an artifact, and new people in the field may be deceived into thinking their sample contains aluminum (or iron) or that there is more aluminum present in the sample (in an attempted quant) than really exists. I will check out your references. I may seek more input once I have reviewed them.
When I was new to the field I took a course on EDS that also reviewed potential artifacts. I asked if their list covered all artifacts. They said it did. However nothing was said of this particular artifact. So, even though experienced operators “may” know about this, I have never heard it discussed in my entire carrier (book, class, or discussion). So, this is part of my attempt to bring awareness, and open discussions, and add my word of caution.
The term “Stray radiation” is a bit deceptive and does not reflect definitive cause. Such a term seems more akin to background radiation, but I do see how “Stray radiation” might be the descriptor, as in a stray cat that might show up once in a while.
My experience indicates angular flat surfaces on samples ( such as crystal facets) can encourage formation of this artifact.
Thanks for your input Valadimer.
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