The amount of neutral matter present in the nuclei
of these isotopes, expressed in units of mass of a proton, is approximately
equal to one and two, respectively This fact indicates that the nuclei of
atoms contain, in addition to protons, neutral matter in multiples of a
basic unit This hypothesis was verified in 1932 by James Chadwick
who observed emission of neutral radiation when beryllium nuclei were
bombarded with alpha-particles (a-particles are helium nuclei, to be
discussed in a later section) It was found that this neutral radiation
could knock out protons from light nuclei such as those of helium, carbon
and nitrogen