Training and the nuclear problem
Chadwick studied radioactivity with Ernest Rutherford at Manchester and subsequently worked with Hans Geiger in Germany. Internment during the First World War interrupted his career. After returning to Britain he resumed research in Cambridge, completed his doctorate in 1921 and became assistant director of the Cavendish Laboratory in 1923. His later achievement rested on experience with radiation and particle detection.
The relation between nuclear charge and nuclear mass already posed a problem. Rutherford had suggested a neutral constituent, but a proposal was not experimental identification, and early ideas about such a constituent were not identical to the later understanding of the neutron. Chadwick's distinctive contribution was to bring discriminating observations to bear on the issue.
References: [2]
A puzzling radiation
Bothe, Becker and other researchers found that alpha-particle bombardment of beryllium generated highly penetrating radiation. It was initially interpreted as energetic gamma radiation. Irène and Frédéric Joliot-Curie then observed that this radiation could eject fast protons from hydrogen-containing material, and considered an energy-transfer mechanism analogous to Compton scattering.
Chadwick did not need to reject their observations to question their explanation. The important issue was whether the same proposed radiation could account for the responses of different targets. Penetration and absence of electric charge alone did not identify a neutron, because photons also have no charge. The competing descriptions had to be tested through their quantitative consequences.
Reading the evidence through recoils
His apparatus included a small ionisation chamber connected to an amplifier, allowing a charged particle's passage to be registered through the ionisation it produced. He examined recoil particles from hydrogen and several other materials. His short 1932 report also credited collaboration with Norman Feather in observing recoil tracks in an expansion chamber. The evidence was thus obtained through secondary charged particles, rather than by directly seeing a neutral projectile.
Hydrogen supplied one constraint; heavier targets supplied others. An extremely energetic photon interpretation chosen to explain the fast protons did not produce enough predicted recoil in nitrogen to account for what was observed. Comparing targets therefore did more than add examples. It tested whether a single physical account could explain several results while retaining conservation of energy and momentum.
References: [1]
Why the neutron interpretation worked
A neutral projectile with a mass close to that of a proton could transfer energy to nuclei in collisions in a way consistent with the recoil observations. Chadwick also considered directional differences in the ranges of ejected protons and the energy available in the beryllium reaction. These provided additional difficulties for the proposed gamma-ray account.
The point was not that light can never transfer momentum to a massive particle. It was that the particular photon explanation required energies and recoil behaviour incompatible with the evidence and reaction constraints. This distinction matters because an oversimplified account can turn a sound experimental argument into an incorrect general claim.
The February 1932 note was cautiously entitled Possible Existence of a Neutron. More extensive work followed later that year. Establishing evidence for a neutral nuclear particle did not simultaneously settle every detail of its nature or internal structure. The historical achievement should be stated precisely rather than expanded to encompass all subsequent neutron physics.
Career and consequences
Chadwick received the 1935 Nobel Prize in Physics and in the same year became a professor at the University of Liverpool. The university identifies his appointment as an important stage in the development of its nuclear and particle research. During the Second World War he worked on the Manhattan Project, a later part of his career rather than the purpose or completed outcome of the original 1932 discovery.
The neutron helped researchers distinguish the sources of nuclear mass and charge and offered a new means of investigating nuclei. Subsequent discoveries and technologies, including fission and reactors, required many further contributions. Chadwick died in 1974. His most characteristic achievement was the disciplined comparison of explanations: experimental agreement across several targets mattered more than the familiarity of the initial interpretation.