section_path: Properties/Mass
section_name: Mass
paragraph_text: Mass
  
  The mass of a neutron cannot be directly determined by mass spectrometry since it has no electric charge. But since the masses of a proton and of a deuteron can be measured with a mass spectrometer, the mass of a neutron can be deduced by subtracting proton mass from deuteron mass, with the difference being the mass of the neutron plus the binding energy of deuterium (expressed as a positive emitted energy). The latter can be directly measured by measuring the energy (B d {\displaystyle B_{d}}) of the single 2.224 MeV gamma photon emitted when a deuteron is formed by a proton capturing a neutron (this is exothermic and happens with zero-energy neutrons). The small recoil kinetic energy (E r d {\displaystyle E_{rd}}) of the deuteron (about 0.06% of the total energy) must also be accounted for.
  
  The energy of the gamma ray can be measured to high precision by X-ray diffraction techniques, as was first done by Bell and Elliot in 1948. The best modern (1986) values for neutron mass by this technique are provided by Greene, et al. These give a neutron mass of:
  
  The value for the neutron mass in MeV is less accurately known, due to less accuracy in the known conversion of Da to MeV/ c:
  
  Another method to determine the mass of a neutron starts from the beta decay of the neutron, when the momenta of the resulting proton and electron are measured.