XRF, the Auger effect, scattering and noise.

In summary: K-shell binding energy... is what is necessary for the electron to be ejected and for the X-Ray emission to happen.
  • #1
zappacake
4
0
Hello everyone,

I have a few questions regarding the principles behind XRF spec, as most of the sources I've consulted either don't go into enough detail and miss bits out (undergrad textbooks), or are simply beyond my current level of understanding (QM papers etc).

When, for example, a K-shell electron is ejected by an incident X-ray, a core vacancy is created, along with a high energy, unstable state. An electron from the L or M subshells fills the vacancy, emitting an X-ray of equivalent energy to the energy gap between the subshells involved. This makes perfect sence.

But, what happens after that? A gap in, say, the L-shell has now been created. If the atom is big enough, does this mean that an electron from M or N then fills THAT gap, emitting a photon, and so on until... what? Until you have an ion in its most stable state? Or does the ion capture an electron from elsewhere (say an auger electron), where applicable and return to the neutral state?

Also - let's assume the incident X-ray was (purely for illustration) 30 keV. By chance, this is the K-shell binding energy and the electron is ejected, and a Kα1 emission results. So, you have 30 keV absorbed and 30 keV emittied, but then you still have an electron gap in the L shell to be filled by other electron, which will cause loss of more energy via photons. Is the binding energy greater than any of the allowed transitions from the L or M shells to the K shell? If so, is this why further transitions are possible and energy is released?

I'm sure this is something really simple, but I've asked seems to know what happens and why, beyond the basic principles.

I'd really appreciate any assistance, and the questions RE auger effect and compton/rayleigh scattering will be asked following responses (which I'm sure you're all delighted to hear...)

Lee.
 
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  • #2
zappacake said:
Hello everyone,

I have a few questions regarding the principles behind XRF spec, as most of the sources I've consulted either don't go into enough detail and miss bits out (undergrad textbooks), or are simply beyond my current level of understanding (QM papers etc).

When, for example, a K-shell electron is ejected by an incident X-ray, a core vacancy is created, along with a high energy, unstable state. An electron from the L or M subshells fills the vacancy, emitting an X-ray of equivalent energy to the energy gap between the subshells involved. This makes perfect sence.

But, what happens after that? A gap in, say, the L-shell has now been created. If the atom is big enough, does this mean that an electron from M or N then fills THAT gap, emitting a photon, and so on until... what? Until you have an ion in its most stable state? Or does the ion capture an electron from elsewhere (say an auger electron), where applicable and return to the neutral state?

Also - let's assume the incident X-ray was (purely for illustration) 30 keV. By chance, this is the K-shell binding energy and the electron is ejected, and a Kα1 emission results. So, you have 30 keV absorbed and 30 keV emittied, but then you still have an electron gap in the L shell to be filled by other electron, which will cause loss of more energy via photons. Is the binding energy greater than any of the allowed transitions from the L or M shells to the K shell? If so, is this why further transitions are possible and energy is released?

I'm sure this is something really simple, but I've asked seems to know what happens and why, beyond the basic principles.

I'd really appreciate any assistance, and the questions RE auger effect and compton/rayleigh scattering will be asked following responses (which I'm sure you're all delighted to hear...)

Lee.

Hi Lee.
You asked: "But, what happens after that? A gap in, say, the L-shell has now been created. If the atom is big enough, does this mean that an electron from M or N then fills THAT gap, emitting a photon, and so on until... what? Until you have an ion in its most stable state?"

Answer: Yes exactly, this happens in a cascade until the atom is stable. Each vacant hole that is filled will shed an X-Ray photon. The result is an complete atom.

You asked: "Also - let's assume the incident X-ray was (purely for illustration) 30 keV. By chance, this is the K-shell binding energy and the electron is ejected, and a Kα1 emission results. So, you have 30 keV absorbed and 30 keV emittied, "

Answer: Generally the exciting energy in the form of X-Ray, electrons, alpha particles etc., to make an element XRF is chosen to be at least just above that element's K-edge, although any energy above that will work too. This energy level may be found using a K-edge calculator such as:
http://www.csrri.iit.edu/mucal.html
Here are the X-Ray statistics for Xenon (Ka=pretty close you your 30 keV hypothetical) - notice the fluorescent yield probabilities (heavily weighted towards K shell):

X-ray properties

Data for Xe; Z = 54
atomic weight = 131.300003 ; density = 5.90000022E-03
K-edge at: 34.5820007 keV
L-edges at: 5.45200014 , 5.09999990 , 4.78100014 keV
M-edge at: 1.14300001 keV
K-Alpha1,K-Beta1 at: 29.8020000 33.6440010 keV
L-Alpha1,L-Beta1 at: 4.11100006 4.42199993 keV
K,L1,L2,L3 jumps: 6.07753229 1.15944588 1.40999997 2.87899995
Fluorescence yield for K,L1,L2,L3: 0.8910, 0.0460, 0.0830, 0.0850

and from the XRF Periodic Table of the Elements:
Xe K and L shell Binding E

1567788332431.png


So somewhere above 35 keV or so. In practical terms what this means is if you have a choice of X-Ray tubes with targets of Cu, Mo, or W, you would select W based upon it's "Characteristic X-Ray" peak, with appropriate low energy filters.
1567787364447.png
Geo
 
Last edited:

1. What is XRF and how does it work?

XRF stands for X-ray fluorescence and is a non-destructive analytical technique used to determine the chemical composition of a material. XRF works by shooting high-energy X-rays at a sample, which causes the atoms in the sample to emit characteristic fluorescent X-rays. These X-rays are then measured and used to identify the elements present in the sample.

2. What is the Auger effect and how is it related to XRF?

The Auger effect is a physical phenomenon that occurs when an atom is excited by an incoming photon. When the atom emits an X-ray due to the Auger effect, it also releases an electron. In XRF, the emitted X-rays are used to identify the elements present in the sample, while the ejected electrons are measured to determine the energy of the X-rays.

3. How does scattering affect the accuracy of XRF analysis?

Scattering occurs when the X-rays used in XRF interact with the sample and are deflected in different directions. This can lead to inaccuracies in the results, as some of the emitted X-rays may not reach the detector. To minimize the effects of scattering, XRF instruments are designed with collimators and filters to limit the spread of X-rays and improve accuracy.

4. How can noise be reduced in XRF analysis?

Noise in XRF analysis can come from various sources, such as electronic interference and background radiation. To reduce noise, XRF instruments are equipped with filters, pulse processors, and amplifiers to improve the signal-to-noise ratio. Additionally, taking multiple measurements and averaging the results can also help reduce noise.

5. What are the main applications of XRF analysis?

XRF analysis has a wide range of applications, including environmental analysis, materials characterization, and quality control in various industries such as mining, pharmaceuticals, and electronics. It is also commonly used in archaeological studies to determine the composition of artifacts and in forensic science to analyze trace evidence.

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