Photoelectric (PE) absorption of X-rays occurs when the X-ray photon is absorbed,
resulting in the ejection of electrons from the outer shell of the atom, and hence
the ionization of the atom. Subsequently, the ionized atom returns to the neutralstate with the emission of an X-ray characteristic of the atom. This subsequent
emission of lower energy photons is generally
absorbed and does not contribute to (or hinder) the
image making process. Photoelectron absorption is
the dominant process for X-ray absorption up to
energies of about 500 keV. Photoelectric absorption
is also dominant for atoms of high atomic numbers.