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 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).
- Mean lifetime of a free neutron is 882 seconds (i.e. half-life is 611 seconds ).
- A natural neutron background of free neutrons exists everywhere on Earth and 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 air without any interaction. Neutron radiation is highly penetrating.
- Neutrons trigger the nuclear fission.
- The fission process produces free neutrons (2 or 3).
- Thermal or cold neutrons have the wavelengths similar to atomic spacings. They can be used in neutron diffraction experiments to determine the atomic and/or magnetic structure of a material.