Nuclear physics is the study of atomic nuclei and the application of nuclear processes to technology. Nuclear physics began in about 1900 as the study of radioactive materials, such as radium and uranium.
Three major nuclear processes are (1) radioactivity, involving the decay of a nucleus; (2) fission, the breakup of a nucleus to form two smaller nuclei; and (3) fusion, the joining of two nuclei to form a larger nucleus. Both fusion and fission create the energy of nuclear weapons.
Radioactivity is the emission (giving off) of particles and energy by a nucleus. An atomic nucleus lies at the center of an atom. The simplest nucleus is that of the lightest isotope (form) of hydrogen. This nucleus consists of a single particle, called a proton, which carries a positive electric charge. All other nuclei consist of at least one proton and one neutron. A neutron has internal charges that cancel one another out, making it electrically neutral.
The location of individual protons and neutrons in a nucleus depends on their energy and other factors. The protons and neutrons of certain isotopes, called radioisotopes, will change their locations by means of radioactive decay. In one kind of decay, two protons and two neutrons leave the nucleus together as a unit known as an alpha particle. Decay occurs spontaneously—that is, no input of particles or energy is needed to trigger it.
Physicians can use radioisotopes to diagnose disease. To detect heart ailments, for example, doctors inject a radioactive substance called a tracer into a patient’s bloodstream. A camera equipped with a radiation detector then records the flow of the tracer through the heart. In another medical application, physicians treat cancer by bombarding tumors with radiation.
Other scientists make use of the fact that radioisotopes decay at an exact and uniform rate. One application, known as radiocarbon dating, uses the rate of decay of a carbon isotope to determine the age of ancient plants and animals. See Radiocarbon (Radiocarbon dating). 
Fission creates energy in a nuclear power plant. In this process, a heavy nucleus, such as a uranium nucleus, absorbs a subatomic particle, then splits into two smaller nuclei. During each fission, a tiny amount of nuclear matter changes into heat energy. Electric generators in the power plant convert the heat to electric energy.
Fusion powers the sun and other stars. In one kind of fusion reaction, two protons come together. One of them emits a positively charged particle called a positron and a neutral particle known as a neutrino. The emissions turn the proton into a neutron, so the resulting nucleus consists of a proton and a neutron. During fusion, some nuclear matter changes into heat energy. Scientists are experimenting with ways of harnessing the energy of fusion to produce electric energy.
