Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

The electron that is removed is then called a photoelectron. The incident
photon is completely absorbed in the process. Hence it forms one of the
reason for attenuation of X-ray beam as it passes through the matter.
PEA is related to the atomic number of the attenuating medium (Z), the
energy of the incident photon (E) and the physical density of the attenuating
medium (ρ) by:
PEA ∝ Z³ ·ρ / E³.
Therefore, if Z doubles, PEA will increase by a factor of 8 (because 2³ is 8)
and if E doubles, PEA will reduce by 8. As small changes in Z can have quite
profound changes in PEA this has practical implications in the field of
radiation protection and is why materials with a high Z such as lead (Z = 82)
are useful shielding materials.
Charlie Chong/ Fion Zhang
http://radiopaedia.org/articles/photoelectric-effect