# What un-ionizes an electron after the photo-electric effect?

In an x-ray tube, the atoms in the anode are ionized by incident electrons "knocking" their K-shell electrons out of orbit. Following this, an outer-shell electron decays to fill the vacancy. Now:

An atom remains ionized for a very short time (about $10^{-14}$ second) and thus an atom can be repeatedly ionized by the incident electrons which arrive about every $10^{-12}$ second. (NDT)

What causes the un-ionization, if there's another $10^{-12}$ second until the next incident electron arrives?

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