section_path: Properties/Spin
section_name: Spin
paragraph_text: Spin
  
  The neutron is a spin 1 / 2 particle, that is, it is a fermion with intrinsic angular momentum equal to 1 / 2 ħ, where ħ is the reduced Planck constant. For many years after the discovery of the neutron, its exact spin was ambiguous. Although it was assumed to be a spin 1 / 2 Dirac particle, the possibility that the neutron was a spin 3 / 2 particle lingered. The interactions of the neutron's magnetic moment with an external magnetic field were exploited to finally determine the spin of the neutron. In 1949, Hughes and Burgy measured neutrons reflected from a ferromagnetic mirror and found that the angular distribution of the reflections was consistent with spin 1 / 2. In 1954, Sherwood, Stephenson, and Bernstein employed neutrons in a Stern–Gerlach experiment that used a magnetic field to separate the neutron spin states. They recorded two such spin states, consistent with a spin 1 / 2 particle.
  
  As a fermion, the neutron is subject to the Pauli exclusion principle; two neutrons cannot have the same quantum numbers. This is the source of the degeneracy pressure which counteracts gravity in neutron stars and prevents them from forming black holes.