Interaction of Radiation with Matter

Knowledge of interactions of radiation with matter constitute key knowledge of modern physics. Modern physics is an experimental science and it is based on experiments, which provide key information for our understanding of nature. Most modern nuclear or particle experiments use a variety of sophisticated devices (detectors) for measuring and detection of sub-atomic particles. In order to be detected, a particle must leave some trace of its presence in a detector. Particles mostly deposit energy along its path. Knowledge of this interaction, how different particles deposit energy in the matter and how much energy particles deposit, is fundamental for our understanding of the problem.

Each type particle interacts in a different way, therefore we must describe interaction of particles (radiation as a flow of these particles) separately. For example charged particles with high energies can directly ionize atoms. On the other hand electrically neutral particles interacts only indirectly, but can also transfer some or all of their energies to the matter. This is the key feature of the categorization of radiation sources. They are usually categorized into two general types as follows:

  • Charged particles (directly ionizing)
    • Beta particles. Beta particles are fast electrons or positrons emitted in nuclear beta decay, as well as energetic electrons produced by any other process.
    • Heavy charged particles. Heavy charged particles are all energetic ions with mass of one atomic mass unit or greater, such as protons, alpha particles (helium nuclei) or fission fragments.
  • Neutral particles (indirectly ionizing)
    • Photon radiation (electromagnetic radiation). Photons are particles/waves (Wave-Particle Duality) without rest mass or electrical charge. Also visible light is electromagnetic radiation, but with much lower energies. The electromagnetic radiation of interest includes X-rays emitted in the rearrangement of electron shells of atoms and gamma rays that are emitted from nucleus.
    • Neutrons. Neutrons can be emitted by nuclear fission or by the decay of some radioactive atoms. Neutrons have zero electrical charge and cannot directly cause ionization.
    • Neutrinos. Neutrinos are electrically neutral, weakly interacting elementary particles, which have very low cross sections for any interaction with matter and therefore low probabilities for colliding in matter.

The design of all nuclear reactors and other nuclear systems depends fundamentally on the way in which radiation interacts with matter. This knowledge is very important for understanding of:

  • Neutron Moderation. How neutrons slow down to thermal energies.
  • Power Distribution. Where is the energy generated?
  • Reactor Power Measurement. How can we measure the reactor power and how can we control the chain reaction.
  • Radiation Shielding. How can we shield all the various types of radiation produced in the reactor core.