EGS: Lab Activities
Photons interact with material in many ways, but the three most dominant mechanisms for energies above a few keV are:
- Photoelectric Effect
- The photon's energy is totally absorbed by the atom and an inner-shell electron is ejected. Immediately following this, an outer shell electron makes a transition to fill the gap in the inner shell and in the process emits photons (X-rays).
- Compton Scattering
- The photon scatters off an electron in an outer orbit of an atom and the energy of the incident photon is then shared between the outgoing electron and the scattered photon.
- Pair Production
- The photon "materializes" into an electron and its antiparticle, a positive electron (a positron), while in the presence of the electromagnetic field of the nucleus.
The above three mechanisms account for more than 99% of the interactions that take place between photons and matter. Which of the interactions is the most important at any given time depends on the energy of the photon and the medium in which it is traveling.
However, these interactions are quantum in nature. This means that a specific interaction is never guaranteed, it is simply more (or less) probable than the others (depending on energy and material). The EGS program simulates this quantum nature by randomly selecting which interaction, if any, occurs with the correct (average) probabilities.
In the following exercises, you will learn how to recognize these three basic interactions using the EGS computer code, which we have set up to allow you to create visual images of particle tracks in color. In particular, you will learn about:
- Interaction distances (the average distance to a collision).
- Which interaction type dominates.
- The random, but statistically determined, behavior of nature.
Now, go to:
