Neutrons are uncharged heavy particles (“hadrons”, Greek: strong, robust); they are unstable, decaying into a proton, an electron and an anti-neutrino with an average lifetime of just 886 seconds. Only in a bound system like an atomic nucleus, they are stable.
Neutrons have been identified as nuclear constituents by Chadwick in 1932. With the protons, they contribute a large fraction to the atomic mass, which is concentrated in the atomic nucleus.
One can obtain free neutrons from nuclear reactions, e.g. neutrons are emitted in a fissioning nucleus. This process is used in research reactors, where typically 1013 to 1015 Neutrons per cm2 and second are emitted from the reactor core. Less than a quarter of an hour after the neutrons have been released by fission from a nucleus, about 63% of them have already decayed. This short span in the life of neutrons is used by researchers to perform e.g. scattering experiments and activation analysis by using nuclear reactions induced by neutrons in a large variety of materials.
The following pages give some insight into the short and interesting life of neutrons. However, let us first start with their discovery...