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 The only neutral radiation known at that time was photons
(electromagnetic radiation) Application of the principles of conservation
of energy and momentum showed that if the neutral radiation consisted
of photons, the energy of photons would have to be much higher than is
available from the bombardment of beryllium nuclei with a-particles