Abstract
The decay of the free neutron into a proton, electron, and antineutrino is the prototype semileptonic
weak decay and is the simplest example of nuclear beta decay. It played a key role in the early
Universe as it determined the ratio of neutrons to protons during the era of primordial light element
nucleosynthesis. Neutron decay is physically related to important processes in solar physics and
neutrino detection. The mean neutron lifetime has been the subject of more than 20 major
experiments done, using a variety of methods, between 1950 and the present. The most precise
recent measurements have stated accuracies approaching 0.1%, but are not in good agreement as
they differ by as much as 5 sigma using quoted uncertainties. The history of neutron lifetime
measurements is reviewed and the different methods used are described, giving important examples
of each. The discrepancies and some systematic issues in the experiments that may be responsible
are discussed, and it is shown by means of global averages that the neutron lifetime is likely to lie in
the range of 880–884 s. Plans and prospects for future experiments are considered that will address
these systematic issues and improve our knowledge of the neutron lifetime.