When an atomic nucleus undergoes nuclear decay it may end in an excited state with extra energy. A stable nucleus may also gain energy from interacting with an energetic neutron or other particle. In that state, it may undergo gamma decay to release the extra energy in the form of a very high-energy light wave known as a gamma ray.
High-energy gamma rays are among the most penetrating types of nuclear radiation. They are best shielded by materials such as lead, which have a large number of protons and electrons in each nucleus. This type of atom has a much higher probability that its collection of charged particles will absorb a high-energy gamma ray. It is largely due to the gamma radiation that the concrete shielding walls around the reactor are required to be approximately six feet thick. Gamma rays are used in sterilization of medical equipment, gamma-knife removal of tumors, and widely used in many types of medical imaging. They have also recently been observed in astronomy to find evidence of water on the planet Mars and to image specific materials created in nuclear fusion and from high energy astronomical events such as supernovae: