Neutron
Neutron
The atom’s structure consists of a central, dense nucleus surrounded by a cloud of electrons. Inside the nucleus are two types of particles, neutrons (which are electrically neutral) and protons (which are positively charged). In most atoms, the nucleus is a stable, long-lasting structure; in some, it is unstable, and can break apart (fission) at an unpredictable time. When this happen, energy is released, and usually fast-moving neutrons as well.
The atom is so small that it takes several million of them in a row to span the period at the end of this sentence. The nucleus is much smaller still, about 1/100,000 of an atom’s size.
Neutrons and protons were once considered fundamental particles, but it is now known that they are made up of even smaller components called quarks, of which there are three types. Evidence for quarks comes from experiments done with very high energy particle accelerators.
Discovery
The neutron as a distinct, separate particle was discovered in l932 by a British physicist, James Chadwick (1891–1974). He was using radiation emitted from radioactive radium to bombard various materials. When he irradiated the element beryllium, he found that a very penetrating particle was produced. It could go through an inch thickness of lead. Chadwick reasoned that the new particle must be electrically neutral, because all other radiation known at that time would have been stopped by the lead. Chadwick was awarded the 1935 Nobel Prize in physics for his discovery.
New radioactive materials
Enrico Fermi (1901–1954), a young Italian scientist, constructed a neutron source according to Chadwick’s design, mixing radium and beryllium powder together in a small glass tube. Fermi’s plan was to use this neutron source for making new radioactive materials. Together with his co-workers, he was successful in producing radioactive sodium, iron, copper, gold, and many other elements. Fermi received the l938 Nobel Prize in physics for his work with neutrons.
Almost all elements found in nature can now be made radioactive. Radioactive potassium and phosphorus are used as tracers to measure how effectively plants take up fertilizer from soil. Radioactive iodine is applied in nuclear medicine to diagnose and treat thyroid problems. Radiation treatment for cancer therapy uses radioactive cobalt, which is made by irradiating ordinary cobalt with neutrons.
Nuclear energy production
When some elements, including uranium, are bombarded by neutrons, nuclear fission takes place. The nucleus breaks into two pieces with a large release of energy. In addition, several extra neutrons are emitted. These can cause more nuclei to split apart, which releases more energy and more neutrons in a chain reaction. In an atomic (i.e., fission) bomb, the chain reaction becomes an uncontrolled explosion. In a nuclear power plant, the chain reaction is maintained in a steady state by materials that absorb extra neutrons. Further, the unstable nuclei in the fuel of a normal fission power plant are too far apart to support an explosive reaction. Only a few special reactors, including certain breeder reactors, have the potential to support an atomic explosion.