When inhalation or ingestion of an alpha emitting radioactive material occurs, internal irradiation becomes a major concern. The alpha particles interact strongly with the surrounding internal tissues (live tissue). All of their energy is absorbed inside the body, potentially causing damage to the cells. Therefore, special precautions are taken when handling open, volatile sources of alpha emitting radio nuclides. 3.2 Beta Radiation 16
[ INTERACTION OF Β PARTICLE WITH MATTER ] Positrons, positively charge beta particles, interact with electrons from surrounding matter through the process called annihilation of radiation, producing 2 gamma rays (see chapter 2.5). Electrons are negative beta particles with relatively light mass. Their interaction with matter can be characterized as average. There are 2 main mechanisms of interaction that are important from the point of view of radiation protection. 3.2.1 Excitation and Ionization The interaction between the electric field of a beta particle and the orbital electrons of the absorbing medium leads to inelastic collisions that generate electronic excitation and ionization. Because of the continuous spectrum of beta particles, the specific ionization (the number of ion pairs created per cm of air) decreases from approximately 200 ion pairs with increase of beta energy, reaching a minimum of approximately 60 ion pairs at beta energy of 1 MeV. 3.2.2 Bremsstrahlung The second important mechanism of reducing energy of beta particles is "bremsstrahlung". When a high-speed charged particle passes through a medium, it occasionally undergoes a substantial nuclear scattering, which results in the emission of continuous electromagnetic energy called bremsstrahlung or "braking radiation". This energy is in the range of X-rays (lower electromagnetic energy than gamma rays) and becomes more energetic if the stopping material is made of heavy materials such as metals of high Z. The use of light materials reduces "bremsstrahlung". This is why light materials such as Plexiglas's are used to absorb beta radiation. When gamma emission follows beta disintegration, protection against gamma rays is also required. In this situation, we need to stop the beta particles first with light materials and then gamma and bremsstrahlung radiation with heavy material (lead or other metal). Because of the continuous energy distribution, absorption of beta particles in material is also continuous. The range, however, has a maximum value for different materials, and is related to the maximum energy of the beta particles. For example, tritium (H-3) has a low maximum beta energy (0.018 MeV) and a maximum range in air of 6 mm. On the other hand, P-32 has a higher maximum beta energy (1.71 MeV) and a maximum range in air of 7.9 m. 3.3 Gamma Radiation Gamma rays are photons (quanta of light) and have no electric charge and no rest mass.