During the early 1920s a number of investigators, including Harkins in
the United States, Orme Masson in Australia, and Rutherford and his
assistant James Chadwick in England, seriously considered the
possibility that a neutral particle might exist in nature, possibly
formed by the very close association of a proton and an electron.
However, strenuous efforts to produce such particles by combining
protons and electrons were unsuccessful.
During these years the new technique of bombarding all kinds of matter
with alpha particles to see what would happen was widely exploited, and
it gradually became clear that in a few instances a peculiar and highly
penetrating kind of radiation was produced. In 1932, Chadwick succeeded
in showing that the peculiar radiation must consist of a stream of
particles, each weighing about the same as a proton but having no
electrical charge.
The name “neutron” for a possible neutral particle of this type was
suggested by Harkins in the United States in 1921. Much evidence now
exists that the neutron is a fundamental particle in its own right and
that it should not be thought of merely as a particle formed by a very
close association between a proton and an electron.
The new particle discovered by Chadwick was destined to play a totally
unexpected role, not only in the history of atomic science but also in
the fate of nations. It immediately outmoded a previous concept of the
nucleus that pictured it as a cluster of protons approximately half of
which were neutralized by electrons crowded into the nucleus. A nucleus
is now thought of as containing just protons and neutrons.
The neutron was also greeted by nuclear workers as a practically perfect
kind of bullet. Unlike charged alpha particles, uncharged neutrons can
approach a charged nucleus completely unopposed. It is physically
impossible for any kind of container to hold a swarm of free neutrons;
they seep right through its walls.