section_path: The "free" neutron
section_name: The "free" neutron
paragraph_text: The "free" neutron
  
  "Free" neutrons or protons are nucleons that exist independently, free of any nucleus.
  
  The free neutron has a mass of 939 565 413.3 eV/ c, or 939.565 4133 MeV/ c. This mass is equal to 1.674 927 471 × 10 kg, or 1.008 664 915 88 Da. The neutron has a mean-square radius of about 0.8 × 10 m, or 0.8 fm, and it is a spin-½ fermion. The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields. The neutron has a magnetic moment, however, so it is influenced by magnetic fields. The specific properties of the neutron are described below in the Intrinsic properties section.
  
  Outside the nucleus, free neutrons undergo beta decay with a mean lifetime of about 14 minutes, 38 seconds, corresponding to a half-life of about 10 minutes, 11 s. The mass of the neutron is greater than that of the proton by 1.293 32 MeV/ c, hence the neutron's mass provides energy sufficient for the creation of the proton, electron, and anti-neutrino. In the decay process, the proton, electron, and electron anti-neutrino conserve the energy, charge, and lepton number of the neutron. The electron can acquire a kinetic energy up to 0.782 ± 0.013 MeV.
  
  Still unexplained, different experimental methods for measuring the neutron's lifetime, the "bottle" and "beam" methods, produce different values for it. The "bottle" method employs "cold" neutrons trapped in a bottle, while the "beam" method employs energetic neutrons in a particle beam. The measurements by the two methods have not been converging with time. The lifetime from the bottle method is presently 877.75 s which is 10 seconds below the value from the beam method of 887.7 s
  
  A small fraction (about one per thousand) of free neutrons decay with the same products, but add an extra particle in the form of an emitted gamma ray:
  
  Called a "radiative decay mode" of the neutron, the gamma ray may be thought of as resulting from an "internal bremsstrahlung " that arises from the electromagnetic interaction of the emitted beta particle with the proton.
  
  A smaller fraction (about four per million) of free neutrons decay in so-called "two-body (neutron) decays", in which a proton, electron and antineutrino are produced as usual, but the electron fails to gain the 13.6 eV necessary energy to escape the proton (the ionization energy of hydrogen), and therefore simply remains bound to it, forming a neutral hydrogen atom (one of the "two bodies"). In this type of free neutron decay, almost all of the neutron decay energy is carried off by the antineutrino (the other "body"). (The hydrogen atom recoils with a speed of only about (decay energy)/(hydrogen rest energy) times the speed of light, or 250 km/s.)