A neutron is one of the subatomic particles that make up matter. The neutron has no electric charge and a rest mass equal to 1.67493 × 10−27 kg — marginally greater than that of the proton but nearly 1839 times greater than that of the electron. The neutron has a mean square radius of about 0.8×10−15 m or 0.8 fm, and it is a spin-½ fermion.
Key properties of neutrons are summarized below:
- Mean square radius of a neutron is ~ 0.8 x 10-15m (0.8 fermi)
- The mass of the neutron is 939.565 MeV/c2.
- Neutrons are ½ spin particles – fermionic statistics.
- Neutrons are neutral particles – no net electric charge.
- Neutrons have a non-zero magnetic moment.
- Free neutrons (outside a nucleus) are unstable and decay via beta decay. The decay of the neutron involves the weak interaction and is associated with a quark transformation (a down quark is converted to an up quark).
- The mean lifetime of a free neutron is 882 seconds (i.e., the half-life is 611 seconds).
- A natural neutron background of free neutrons exists everywhere on Earth. It is caused by muons produced in the atmosphere, where high-energy cosmic rays collide with particles of Earth’s atmosphere.
- Neutrons cannot directly cause ionization. Neutrons ionize matter only indirectly.
- Neutrons can travel hundreds of feet in the air without any interaction. Neutron radiation is highly penetrating.
- Neutrons trigger nuclear fission.
- The fission process produces free neutrons (2 or 3).
- Thermal or cold neutrons have wavelengths similar to atomic spacings. They can be used in neutron diffraction experiments to determine the atomic and/or magnetic structure of a material.