Properties of Neutrons
Neutrons together with protons are the constituents of atomic nuclei. The neutron is slightly heavier than a proton and can therefore decay into the “lighter” particles, a proton, an electron and an anti-electron-neutrino. A neutron is only stable when bound in an atomic nucleus. The lifetime of free neutrons is about 886 seconds.
Neutrons are neutral only to the outside; they have an inner structure with a charge distribution of positive and negative charges (this is similar to the neutral atoms). It has also a spin, a kind of an inner angular momentum. The moving inner charge leads to another interesting property of neutrons: it has a magnetic dipole moment and can, just as a compass needle, adjusts its spin in a magnetic field.
These properties of neutrons, being neutral, having a magnetic moment and a spin, make them a very desirable tool in solid state physics. Neutrons are able to pass through massive layers with thicknesses of several centimeters. They can induce nuclear reactions, which can be used for a so-called activation analysis. The reaction lead to a radioactive nucleus, whose presence can easily be detected by its characteristic radiation. When the neutron beam is only scattered by a solid, it transports the information on the microstructure inside the solid body. Scattering of neutrons from research reactors is therefore used to study, the microscopic structure of crystals, or the inner mechanical strain of big mechanical parts, like the valve of a ship diesel motor. Neutrons are also useful to study the structure of magnetic layer systems, like those used in magnetic storage media (hard discs in computers).
Texts: Th. Wilpert , Ch. Schulz
