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March 2011 Elements that contain unstable nuclei are radioactive; they are called radionuclides. They decay by releasing mostly alpha and beta particles accompanied by gamma rays. An alpha particle has low-energy, is positively charged and consists of two protons and two neutrons, i.e., a helium atom without its 2 electrons; it can be stopped by tissue paper or human skin.  A beta particle is a high-energy, negatively charged electron (negatron) or a positively charged positron; it can be stopped by a sheet of aluminum. Gamma rays are high energy, short-wavelength, electromagnetic radiation; they can be stopped by concrete or lead. The energy released by radionuclides may knock electrons out of their orbits around an atom’s nucleus. This process is called ionizing radiation. Ionizing radiation damages living tissues, leads to changes in constituents of the cell, including the DNA of chromosomes, and results in changes in structure and function of the cells and organ syst...