a proton can turn into a neutron through nucleosynthesis.
a proton can turn into a neutron through nucleosynthesis.
how can a proton turn into a neutron?
nucleosynthesis
usually unstable nuclides are clearly either "neutron rich" or "proton rich", with the former undergoing beta decay and the latter undergoing electron capture (or more rarely, due to the higher energy requirements, positron decay). however, in a few cases of odd-proton, odd-neutron radionuclides, it may be energetically favorable for the radionuclide to decay to an even-proton, even-neutron isobar either by undergoing beta-positive or beta-negative decay. an often-cited example is the single isotope (29 protons, 35 neutrons), which illustrates three types of beta decay in competition. copper-64 has a half-life of about 12.7 hours. this isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay. this particular nuclide (though not all nuclides in this situation) is almost equally likely to decay through proton decay by positron emission (18%) or electron capture (43%) to , as it is through neutron decay by electron emission (39%) to .
beta decay
the question of whether the neutron was a composite particle of a proton and an electron persisted for a few years after its discovery. in 1932 harrie massey explored a model for a composite neutron to account for its great penetrating power through matter and its electrical neutrality, for example. the issue was a legacy of the prevailing view from the 1920s that the only elementary particles were the proton and electron. the nature of the neutron was a primary topic of discussion at the 7th solvay conference held in october 1933, attended by heisenberg, niels bohr, lise meitner, ernest lawrence, fermi, chadwick, and others. as posed by chadwick in his bakerian lecture in 1933, the primary question was the mass of the neutron relative to the proton. if the neutron's mass was less than the combined masses of a proton and an electron (), then the neutron could be a proton-electron composite because of the mass defect from the binding energy. if greater than the combined masses, then the neutron was elementary like the proton. the question was challenging to answer because the electron's mass is only 0.05% of the proton's, hence exceptionally precise measurements were required.
discovery of the neutron