radguide

13 Radiation for Radionuclide Users Neutrons Neutrons are typically produced by one of three methods. Large amounts of neutrons are produced in nuclear reactors due to the nuclear fission process. High energy neutrons are produced by accelerating deuterons and causing them to interact with tritium nuclei. The third method of producing neutrons is by bombarding beryllium with alpha particles. Neutron sources can be made using the alpha-neutron reaction on beryllium by making a mixture of powered alpha emitter and beryllium and sealing it in a metal container. Early neutron sources used radium as the alpha emitter. Modern neutron sources typically use plutonium or americium as the alpha source. The radium-beryllium (RaBe) sources were also sources of large amounts of gamma radiation while the plutonium-beryllium (PuBe) sources and the americium- beryllium (AmBe) sources only produce small amounts of very low energy gamma radiation. Thus, as neutron sources, PuBe and AmBe sources tend to be less hazardous to handle. The older RaBe sources also had a tendency to develop leaks over time and give off radon gas, one of the products of radium decay. Interaction of Ionizing Radiation With Matter As ionizing radiation moves from point to point in matter, it loses its energy through various interactions with the atoms it encounters. The rate at which this energy loss occurs depends upon the type and energy of the radiation and the density and atomic composition of the matter through which it is passing. Mechanisms of Interaction The various types of ionizing radiation impart their energy to matter primarily through excitation and ionization of orbital electrons. The term “excitation” is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom. Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state. The term “ionization” refers to the complete removal of an electron from an atom following the transfer of energy from a passing charged particle. In describing the intensity of ionization, the term “specific ionization” is often used. This is defined as the number of ion pairs formed per unit path length for a given type of radiation. Characteristics of Different Types of Ionizing Radiation Because of their double charge and relatively slow velocity, alpha particles have a high specific ionization and relatively short range in matter (a few centimeters in air and only fractions of a