Nuclear Physics, the branch of physics that deals with the atomic nucleus, or inner core of the atom. Work on nuclear physics began around 1900 as the study of radioactive materials, such as uranium and radium. The British physicist Lord Rutherford is regarded as the founder of nuclear physics. In 1911, after a series of laboratory experiments, he developed the following theory:

An atom of matter consists of a heavy inner core (the nucleus) that is surrounded by lightweight particles called electrons. The electrons each have a negative electric charge, while the nucleus has a positive charge. The electrons can be pictured as orbiting the nucleus, just as the earth and other planets orbit the sun.

Experiments since 1911 have shown that the atomic nucleus is made up of positively charged particles called protons and uncharged (neutral) particles called neutrons. A proton has nearly the same weight as a neutron: about 1,840 times that of an electron. Protons and neutrons occur in many different combinations to form the nuclei of atoms. Each such combination is called a nuclide. Nuclear physicists investigate such properties of nuclides as their stability and shape, the ways in which they can vibrate and rotate, and the motions of the particles within them. These studies have helped lead to a better understanding of the structure of matter.

The protons and neutrons in the nucleus are held together by a type of force called the strong force. This force is much stronger than the electromagnetic force, which tends to push the protons apart. However, the strong force is effective only over a very short distance. Another force, called the weak force, also exists in the nucleus. This force is responsible for radioactive decaythe spontaneous transformation of one kind of nuclide into another.

There are three major types of nuclear processes: radioactivity, fission, and fusion. Radioactivity is the process of decay of a nucleus. Fission is a nuclear process in which a large nucleus splits into two or more smaller nuclei. Fusion is a nuclear process in which two nuclei combine to form a larger nucleus. The destructive energy of nuclear weapons comes from the nuclear reaction of fusion and fission. Radioactivity refers to the release of particles and energy by a nucleus of an atom. The simplest nucleus is that of the lightest isotope (form) of hydrogen. This nucleus consists of only one proton, which carries a positive electric charge. All other nuclei consist of at least one proton and one neutron. A neutron is electrically neutral due to its internal charges that cancel one another out.

The location of individual neutrons and protons in a nucleus is determined by their energy and other factors. A radioactive isotope is called a radioisotope. It changes its locations by means of radioactive decay. In one kind of decay, the nucleus emits an alpha particle, which is a group of two protons and two neutrons. This type of decay occurs spontaneously, without the influence of any particles or energy.

Scientists produce more than 900 different kinds of artificial radioisotopes for use in science, medicine, agriculture, and industry. Some are valuable because of the kind of radiation they emit. Physicians use radioisotopes to diagnose disease. Cobalt 60, for example, emits high-energy gamma rays that can be used for the same purposes as x-rays.

Other kinds of radioisotopes are used to detect heart ailments. Radioactivity of radioisotopes makes it possible to trace them with radiation detectors when, for example, they pass through a living body. Radioisotopes used for medical purpose are called tracers, or tagged elements. Another medical application of radioisotopes is treatment of cancer by bombarding tumors with radiation. Radioisotopes decay at an exact and uniform rate. Scientists use this fact to determine the age of plant and animal specimens containing carbon. This application is known as radiocarbon dating, which involves using the rate of decay of a carbon isotope. In fission, a heavy nucleus, such as a uranium nucleus, absorbs a subatomic particle, and divides into two smaller nuclei, thus releasing energy in a nuclear plant. During the process, a small amount of nuclear matter changes into heat energy, which is converted into electric energy by electric generators. Fusion is a process which powers the sun and other stars. In one kind of fusion reaction, two protons come together. One of them emits a neutral particle known as a neutrino and a positively charged particle called a positron. The emissions convert the proton into a neutron, and the resulting nucleus thus consists of a neutron and a proton. During this process, some nuclear matter turns into heat energy.