2.2 Interaction of Photons
Photons (gamma rays and X-rays) have no charge and no mass, so they interact with matter in fundamentally different ways than charged particles.
1. The Photoelectric Effect
In the **photoelectric effect**, a photon transfers all of its energy to an electron in an atom. This causes the electron to be ejected from the atom, becoming a photoelectron. The photon itself disappears.
- This effect is dominant for low-energy photons.
- It is more likely to occur in materials with a high atomic number (Z), such as lead.
2. Compton Scattering
In **Compton scattering**, a photon collides with an electron (typically a free or loosely-bound electron) and transfers only part of its energy to it. The photon is then scattered in a new direction with a lower energy, and the electron recoils.
- This is the most likely interaction for mid-energy photons (from about 100 keV to a few MeV).
- The scattered photon can go on to undergo other interactions, such as another Compton scatter or a photoelectric effect.
3. Pair Production
In **pair production**, a very high-energy photon disappears and converts its energy into an electron-positron pair. This can only occur when the photon has an energy greater than the rest mass of the electron-positron pair (\(\approx 1.022 \text{ MeV}\)). The interaction happens in the strong electric field of the atomic nucleus.
- This effect is dominant for high-energy photons (\(> 1.022 \text{ MeV}\)).
- It is more likely to occur in materials with a high atomic number (Z).