II.

Characteristics

The neutron is slightly heavier than a proton and 1,838 times as heavy as the electron. It is affected by all the four fundamental forces of nature. Because it has mass, it is affected by gravitation, the force of attraction between all objects in the universe. Although the neutron has no electrical charge, it is slightly magnetic, so it is affected by the electromagnetic force, the force of attraction or repulsion between electrically charged or magnetic objects. The neutron is affected by the strong nuclear force, an attraction that binds the neutron to protons and other neutrons in the nucleus. The neutron is also affected by the weak nuclear force, an interaction among the building blocks of the neutron that causes the neutron to decay, or break apart. Isolated from nuclear matter, a free neutron decays into a positively charged proton and a negatively charged electron, releasing energy in the process (see Nuclear Energy). The average lifetime of a free neutron is just under 15 minutes.

Scientists discovered neutrons after they first discovered protons in the nuclei of atoms. For a time, physicists thought that protons and neutrons were the smallest particles inside the nucleus. However, after 1947 physicists found other elementary particles, such as the lambda particle and mesons. These particles are not found in the nucleus but can be created by nuclear reactions, changes within the nucleus that release particles. Many of these elementary particles have properties similar to those of the neutron and proton. Physicists reasoned that, since the elementary particles appear to be related to each other, they must all be composed of the same smaller building blocks, which they called quarks. At first, scientists thought that quarks were not actual objects, but in 1967 physicists used high energy beams of electrons to probe deep inside the proton and neutron and revealed evidence for tiny grainlike particles, or quarks. The neutron, like the proton, is made of three quarks. The strong nuclear force is actually a force that attracts quarks to each other to make a proton or neutron. The quarks of a neutron or proton will also attract the quarks of another neutron or proton, thus holding a nucleus together.