Discovery of Neutrons

In 1930, the German physicists Walther Bothe and Herbert Becker discovered that bombarding beryllium with alpha particles produced a highly penetrating, uncharged radiation, one able to pass through thick lead and completely unaffected by electric or magnetic fields; it was initially, mistakenly, thought to be a form of gamma radiation. In 1932, James Chadwick showed conclusively that these were not electromagnetic waves at all, but rather a new, previously unknown particle - electrically neutral, with a mass just slightly greater than the proton's - which he named the neutron. The discovery reaction is written

where $^{1}_{0}n$ denotes the neutron.

Neutrons are stable inside a nucleus, but a free neutron - one that has left the nucleus - is unstable: it undergoes its own beta decay (as noted in section 8.6.2), emitting an electron and an antineutrino to become a proton, with a half-life of about 13 minutes. …