Why do X-rays preferentially ionize inner electrons?

[jdstokes]

Hi all,

I got asked a question recently which I really should know but wasn't able to answer to my satisfaction.

When an x-ray is emitted by an ionizing process, typically an inner electron is ejected and an electron from a higher orbit ``falls'' into its place. In the process an x-ray is emitted of energy equal to the difference between the initial and final states.

The thing I wasn't able to explain is why the incoming x-ray/electron preferentially interacts with the inner (core) electron rather than the outer electrons. Surely the outer electrons are more susceptible to ejection since less work is required to bring them to infinity, and yet it seems in most cases that the incoming photon will interact with the core electron which is shielded by the outer electrons.

Can anyone help me understand this and hopefully point me in the direction of a suitable reference?

Thanks,

James

 

[Hootenanny]

Hi James,

I've replied to something similar on a related thread in the Homework Forums [it was a multiple choice question]

Quantum transitions that result in the characteristic sharp lines of the X ray spectrum always involve;
a. the inner electron shells
b. electron energy levels that have the same principal quantum number
c. emission of beta particles from the nucleus
d. neutrons within the nucleus
e. protons within the nucleus

I put (b) for this answer because I thought the energy levels were related to the wavelength of the absorbed/emitted photons and spectrums depend on wavelength of radiation? Correct answer is (A)

Indeed, the wavelengths of the spectral lines do depend on the radiation wavelength. Consider the typical energy of an x-ray photon (10nm-0.01nm), now consider the difference in energy levels between the lower shells (n=1, n=2 etc). Do these energy levels get close together, or futher apart as n increases?

[...] what I was getting at is x-ray photons have a relatively short wavelength and are therefore very energetic. This means that xray photons are only absorbed/emitted by electrons which undergo large energy level transitions. Since, as you correctly say the difference between the energy levels decreases as $n\to\infty$, xray photons are only absorbed/emitted by electrons at the lower energy levels (since $\Delta E$ is greatest here).

Does that make sense?

I Hope this was helpful