Some Particles Have No Electric Charge
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.