XRF, the Auger effect, scattering and noise.

[zappacake]

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.

 

[geoelectronics]

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

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.

Geo