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 The clue to this puzzle, which Chadwick satisfactorily solved, was to
assume that the neutral radiation consists of a new type of neutral
particles called neutrons From conservation of energy and momentum,
he was able to determine the mass of new particle ‘as very nearly the
same as mass of proton’