When gamma radiation is absorbed by a material, such as a block of lead, several processes can occur depending on the specific characteristics of the material and the energy of the gamma rays. Here are a few possibilities:
Photoelectric Effect: In this process, the gamma ray interacts with an atom in the material and transfers all its energy to an electron, ejecting it from the atom. The electron may undergo further interactions within the material, such as ionization or excitation of other atoms.
Compton Scattering: Gamma rays can also scatter off electrons in the material through a process known as Compton scattering. In this case, the gamma ray transfers some of its energy to the electron and changes direction. The scattered gamma ray may continue to interact with other atoms, losing energy in the process.
Pair Production: If the energy of the gamma ray is sufficiently high (at least 1.02 MeV), it can interact with the atomic nucleus or an electron in the material and create an electron-positron pair. The positron may quickly annihilate with an electron, producing two gamma rays with lower energy.
In the scenario you described, where a block of lead that has previously absorbed radiation explodes, it's important to note that the absorption of gamma radiation does not make the lead radioactive itself. However, if the explosion is violent enough to cause significant disruption and dispersion of the lead material, it is possible that some previously absorbed gamma rays could be released. The released gamma rays would follow the laws of physics and interact with the surrounding environment based on the processes mentioned above.
It's worth mentioning that lead is often used as a shielding material precisely because it is effective at absorbing gamma radiation. However, if the lead block were fragmented or dispersed, it could potentially release any previously absorbed radiation, although the likelihood and extent would depend on the specific circumstances and energy levels involved.