radguide

14 Radiation for Radionuclide Users millimeter in tissue). Beta particles have a much lower specific ionization than alpha particles and, generally, a greater range. For example, the relatively energetic beta particles from 32P have a maximum range of 7 meters in air and 8 millimeters in tissue. The low energy betas from 3H, on the other hand, are stopped by only 6 millimeters of air or 6 micrometers of tissue. Gamma, x-rays, and neutrons are referred to as indirectly ionizing radiation since, having no charge, they do not directly apply impulses to orbital electrons as do alpha and beta particles. They instead proceed through matter until it undergoes a chance interaction with a particle. If the particle is an electron, it may receive enough energy to be ionized whereupon it causes further ionization by direct interactions with other electrons. As a result, indirectly ionizing radiation (e.g. gamma, x-rays, and neutrons) can cause the liberation of directly ionizing particles (e.g. electrons) deep inside a medium. Because these neutral radiations undergo only chance encounters with matter, they do not have finite ranges, but rather are attenuated in an exponential manner. In other words, a given gamma ray has a definite probability of passing through any medium of any depth. Neutrons lose energy in matter by collisions which transfer kinetic energy. This process is called moderation and is most effective if the matter the neutrons collide with has about the same mass as the neutron. Thus, water, paraffin, or other materials with a high hydrogen content are very efficient moderators. Once slowed down to the same average energy as the matter being interacted with (thermal energies), the neutrons have amuch greater chance of interacting with a nucleus. Such interactions can result in material becoming radioactive or can cause radiation to be given off. An example of the process of activation in which material becomes radioactive is the reaction: n + Na 23  Na 24 Here natural sodium with atomic number 23 becomes activated to the radioactive sodium 24 (half-life of 15 hours) by absorbing a neutron. An example of a neutron-nucleus interaction which results in radiation being given off is the reaction: n + B 10  L i 7 + a In this case, boron-10 absorbs a neutron and gives off an alpha particle, leaving a lithium-7 nucleus. This process is often used in the development of neutron detectors since the resulting alpha particle is relatively easy to detect.