In electrostatics we learn that electric field outside a metallic conductor cannot have an influence within the conductor because the dielectric constant of metals tends to infinity. By Maxwell's electromagnetic theory we know electromagnetic waves are composed of both magnetic and electric fields and for the propagation of electromagnetic waves both magnetic and electric field should exist. Then how are gamma rays able to penetrate through thin sheets of metal?
By using the formula $c=\frac{1}{\sqrt{\mu \varepsilon}}$ we get that for metals the speed of any electromagnetic wave should be zero. Then how are some electromagnetic waves blocked by metals and some like gamma rays able to penetrate through thin layers of metals?
Best Answer
At high frequencies the skin effect is so strong that the current flows only through a very thin surface layer. As the frequency increases, this layer becomes thinner making its resistance higher. As a result, metals are not good conductors at very high frequencies.
In turn, dielectrics don't have free electrons and cannot conduct low frequencies that require electrons to move between atoms. However, at high frequencies electrons don't move, but only oscillate where they are. Thus very high frequencies do not require electrons to leave their atoms, but only oscillate (in the classical sense) around their atoms. As a result, some dielectrics become conductive at very high frequencies.
For example, the visible light frequency is in hundreds of terahertz. Metals are not conductive at such frequencies, but a glass optical fiber is.
To answer your question, metals are not conductive at gamma-ray frequencies and cannot stop gamma-rays based on the electron conductivity logic. The frequency of gamma-rays is generally too high for electrons to interact with. Gamma-rays are normally emitted and absorbed by the nucleus of an atom.