A gamma ray can pass through a considerable amount of solid matter without interacting, but when it does interact, the results can be spectacular.
A gamma ray with extremely high energy can be absorbed by the nucleus of an atom, and for certain types of atoms, this can cause the nucleus to split into two or more fragments.
We call this Photofission.
A similar, but slightly different phenomena is Photodisintegration, which plays an important role in supernova explosions.
Here, the absorption of an extremely high energy gamma ray can cause a nucleus to enter an excited state, and then to immediately decay by knocking out one or more neutrons, protons, or alpha particles out of the nucleus.
The most common interaction for gamma rays with extremely high energy is pair production.
In pair production, as a gamma ray passes near an atomic nucleus, the nucleus recoils, and the gamma ray is transformed into a matter & anti-matter particle pair, such as an electron and a positron.
A gamma ray can’t engage in pair production by itself in empty space, because the recoil of an object such as an atomic nucleus is required for energy and momentum to both be simultaneously conserved.
All the interactions we have discussed so far are associated with gamma rays with extremely high energy levels.
For gamma rays with medium amounts of energy, the most common interaction with atoms is Compton Scattering.
Here, the gamma ray knocks an electron out of the atom, causing the gamma ray to bounce off in a different direction, with less energy than it had before.
Compton Scattering occurs much more frequently in materials with higher densities, such as lead, because there is a much higher density of electrons for the gamma ray to interact with.
Lead has a high electron density both because there are more electrons per atom, and because there are more atoms per unit volume.
For low energy gamma rays, the most common form of interaction is the photoelectric effect.
The photoelectric effect can never occur when a gamma ray interacts with a free electron in empty space.
It can occur when a gamma ray interacts with an electron bound to an atom.
In the photoelectric effect, all of the gamma ray’s energy is used to knock an electron out of an atom, and the gamma ray disappears.
Each of the different types of gamma ray interactions have a different quantum probability, depending on the type of atom the gamma ray is passing through, and depending on the energy of the gamma ray.
A gamma ray can pass through many atoms without any interaction, and gamma rays can therefore travel a considerable distance through solid matter.
Gamma Rays typically have far more energy than all other types of photons, including X-Rays.
However, what determines if we call a photon a gamma ray is not how much energy it possesses, since a very low energy gamma ray can actually have less energy than a very high energy X-ray.
We define a gamma ray as a photon which is generated by the nucleus of an atom.
Nuclei can be placed into excited energy states by being bombarded with neutrons, after which the nuclei will emit gamma rays as the nuclei return to their lower energy levels.
The available energy levels are different for different types of nuclei, and the gamma rays emitted by each type of nucleus therefore have a different set of energy levels.
By bombarding a material with neutrons, and then monitoring the energy levels of the resulting gamma rays, we can gain knowledge about the material’s chemical composition.
Also, the energy levels for the resulting gamma rays are different depending on if the nucleus absorbed a neutron, or if the neutron bounced off of the nucleus without being absorbed.
Much more information is available in the video, "Nuclear Physics."
When watching the Nuclear Physics video, please read the correction in the video description for that video.
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