Published: 11 March, 2004

High-energy neutrons or protons can split atoms

How do you split atoms?

Atoms have a central core, called the nucleus. The nucleus consists of protons and neutrons. The number of protons is called the "atomic number". The sum of the protons and neutrons is the "atomic weight". Nuclei with the same atomic number but different atomic weights are called isotopes of an element. The elements we find on the earth have atomic numbers from 1 to 92 and atomic weights from 1 to 238.

When you hit a nucleus with a particle such as a neutron or proton several things can happen. The particle can bounce off. The particle can be absorbed. There will then usually be a relatively small nuclear change such as transforming a neutron to a proton by emitting an electron or a larger one in which the nucleus emits two protons and two neutrons. Both of these nuclear reactions change the nucleus from one element into another. But the new element is not very different in atomic number or weight from the original one.

In 1938 a German chemist, Otto Hahn, hit uranium, atomic number 92, with neutrons. He created some much lighter elements with atomic numbers between 30 and 60 and weights between 90 and 150. He was astonished by this and sent his results to a former colleague, Lise Meitner. She provided the explanation. The neutrons had split the uranium nucleus into two large fragments. They named the splitting "fission". If each split creates about two new neutrons to do more splitting, a "chain reaction" can result. Fission can only occur in very heavy elements and is likely only in some isotopes.

The phenomenon that atoms can be split this way led to nuclear power plants and the atomic bomb. In a bomb the chain reaction occurs very rapidly which leads to a very quick release of a lot of energy. In a nuclear power plant the speed of the chain reaction is controlled, and so is the rate of release of energy.

Hans A. Bethe

Hans Bethe is professor emeritus of physics at Cornell University. He has been at Cornell since 1935. He won the Nobel Prize for physics in 1967 for explaining how stars make energy. He worked on the atomic bomb during World War II and on the design of nuclear power plants after the war. He has been one of the leading advocates for the peaceful use of atomic energy and for reductions in atomic weapons.