Editing Neutron (section)

=== 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 (<math>B_d</math>) of the single {{val|2.224|u=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 (<math>E_{rd}</math>) of the deuteron (about 0.06% of the total energy) must also be accounted for.
: <math>m_n= m_d - m_p + B_d - E_{rd}</math>

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.<ref>{{cite journal |last1= Greene |first1= GL |display-authors= etal |year= 1986 |title= New determination of the deuteron binding energy and the neutron mass |journal= [[Physical Review Letters]]|volume= 56 |issue= 8|pages= 819–822 |doi=10.1103/PhysRevLett.56.819 |bibcode=1986PhRvL..56..819G |pmid=10033294}}</ref> These give a neutron mass of:
: ''m''<sub>neutron</sub> = {{physconst|mn_Da}}
The value for the neutron mass in MeV is less accurately known, due to less accuracy in the known conversion of [[Dalton (unit)|Da]] to MeV/''c''<sup>2</sup>:<ref name="Byrne_NNM"/>{{rp|18–19}}
: ''m''<sub>neutron</sub> = {{physconst|mnc2_MeV|unit=MeV/c2}} 

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.