Designated containers can store radioisotopes that will be administered to patients as part of a medical treatment. Isotopes are stored in lead containers that block the radiation from escaping and exposing people. These lead containers are called a "pig", a term long used for referring to castings of lead or other metals. The origin of the term is not clear, but was probably first applied to these containers by the people who made them. Penetrating Ability of Emissions The various emissions differ considerably in their ability to go through matter, known as their penetrating ability . The \(\alpha\)-particle has the least penetrating power since it is the largest and slowest emission. It can be blocked by a sheet of paper or a human hand. Beta particles are more penetrating than alpha particles, but can be stopped by a thin sheet of aluminum. Of the three basic types of emissions, gamma particles are the most penetrating. A thick lead shield is required to stop gamma emissions. Positrons represent a special case in that they annihilate when they come in contact with electrons. The collision of a positron and an electron results in the formation of two gamma emissions that go 180 degrees away from each other. Blocking of alpha particles can easily be accomplished by as little as \(10 \: \text{mm}\) plastic or paper. Beta emissions represent a somewhat different situation. The negative charge on a beta particle has the potential for activating the element being used to block the radiation. Lead and tungsten are large atoms with many protons and neutrons in their nuclei. While the beta electron may be blocked, the target material could become irradiated in the process. High-density materials are much more effective protection against gamma emissions than low-density ones. Gamma rays are usually blocked effectively by lead shielding. The thickness of the shielding will determine the effectiveness of the protection offered by the lead. Summary The penetrating ability of a radioactive emission is its ability to go through matter. The relative penetrating abilities of radioactive emissions are described.