In an atomic environment, a $\beta$-decay process competing with positron emission is electron capture, in which the nucleus absorbs one of its cloud of atomic electrons, emitting only a neutrino.

$$p+e^-\rightarrow n+\nu_e$$ Such a process often called K-capture since $e^-$most likely to come from the innermost K-shell of atomic electrons.

An outer electron replaces the "missing" electron. An x-ray, equal in energy to the difference between the two electron shells, is emitted. See the following.

Now as the outer orbital electron takes place the misplaced one, Isn't another position is vacant? Then What would happen? Is the upper once again take the place of the lower site? Then Does this process repeat again and again? Does that imply there would be many x-ray emissions?

  • $\begingroup$ I don't follow what position you mean when you say "another position is vacant". After capture there is a single position on an inner shell vacant. An outer shell electron drops down into this position, filling it. Where is your "vacant" position? $\endgroup$
    – ACuriousMind
    Commented Apr 22, 2021 at 16:55
  • $\begingroup$ I mean, if the vacancy in the atomic k-shell is filled by an electron falling from L-shell. Then they will be vacancies created in L-shell as the electron from their is jumped to k-shell. $\endgroup$ Commented Apr 22, 2021 at 17:06
  • $\begingroup$ Why did you change the title? Gammas are emitted by the nucleus as it de-excites from internal excited states. X-rays are the result of atomic electron transitions. Your text asks about x-rays. $\endgroup$
    – Bill N
    Commented Apr 22, 2021 at 17:58

1 Answer 1


Multiple X-rays are possible.

Typically, when a nucleus undergoes electron capture, it ends up in an excited state of the daughter nucleus. De-excitation of the daughter nucleus then typically ejects its energy in the form of photons. Since these photons originate from a nuclear process, they are usually referred to as gamma rays (this is a better definition than using the energy to delineate gammas from X-rays). Most of the time, multiple paths to the final state are available, which means that gammas of different energies can be produced. In addition, if the de-excitation happens in a cascade, then multiple gammas are possible from a single electron capture decay.

Then there is the relaxation of the electron shell. If that results in photons, these would be called X-rays. Indeed a cascade of X-ray energies is possible as electrons from higher-lying shells fill up the lower-lying vacancies. In addition to the emission of X-rays, Auger electrons may also be emitted. Actually, for heavy atoms, the emission of Auger electrons is more relevant than the emission of X-rays.

For some specific examples of gamma and X-ray energies check out for example the Live chart of decays from the IAEA.


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