Nature of Interaction of Beta Radiation with Matter
Summary of types of interactions:
- Inelastic collisions with atomic electrons (Excitation and Ionization)
- Elastic scattering off nuclei
- Cherenkov radiation
- Annihilation (only positrons)
The nature of the interaction of beta radiation with matter is different from alpha radiation, even though beta particles are also charged particles. Beta particles have a much lower mass compared with alpha particles, and they reach mostly relativistic energies. Their mass is equal to the mass of the orbital electrons with which they are interacting. A much larger fraction of its kinetic energy can be lost in a single interaction than the alpha particle. The nonrelativistic Bethe formula cannot be used since the beta particles mostly reach relativistic energies. For high-energy electrons, a similar expression has also been derived by Bethe to describe the specific energy loss due to excitation and ionization (the “collisional losses”).
Moreover, beta particles can interact via electron-nuclear interaction (elastic scattering off nuclei), which can significantly change the direction of a beta particle. Therefore their path is not so straightforward. The beta particles follow a very zig-zag path through absorbing material. This resulting path of the particle is longer than the linear penetration (range) into the material.
Beta particles differ from other heavy charged particles in the fraction of energy lost by the radiative process known as the bremsstrahlung. From classical theory, when a charged particle is accelerated or decelerated, it must radiate energy, and the deceleration radiation is known as the bremsstrahlung (“braking radiation”).
There is another mechanism by which beta particles lose energy via the production of electromagnetic radiation. When the beta particle moves faster than the speed of light (phase velocity) in the material, it generates a shock wave of electromagnetic radiation known as the Cherenkov radiation.
Positrons interact similarly with matter when they are energetic. But when the positron comes to rest, it interacts with a negatively charged electron, resulting in the annihilation of the electron-positron pair.