Particle that broke the war defence

James Williams

James Williams examines the career of one of the nuclear physicists who made an important breakthrough in atomic structure - with awesome consequences for war and peace power and kickstarted the arms race ere dropped on Japan to end the Second World War.

Hiroshima and Nagasaki were at the forefront of the commemorations of VJ (Victory in Japan) day this August. But the documentary evidence of the production of the world's first atomic bombs has also brought forward the names of many scientists who worked on their production, not least that of J Robert Oppenheimer, the director of the Manhattan Project, which created the bombs in the closing years of the Second World War.

One name that has not figured large, however, is that of James Chadwick, the man who discovered the existence of neutrons, the sub-atomic particles that ultimately led to the feasibility of atomic bombs.

Chadwick was a native of Manchester and his early education was undertaken there, including a spell at Manchester University. At the age of 21 he travelled south to Cambridge where he worked closely with another famous nuclear scientist, Ernest Rutherford.

In 1911 Rutherford proposed the famous solar system model of the atom. Chadwick was encouraged by him to pursue his interest in atomic structure as the theories that had been put forward to explain it were largely unsatisfactory.

It was known that there was a nucleus and that it must have a positive charge but it had only been the identification of the existence of the electron in 1897 by J J Thompson that had prompted this.

While at Cambridge, Chadwick managed to secure a scholarship that allowed him to travel to Europe, working at some of the leading physical laboratories of the day. Unfortunately, while he was in Germany, war was declared. He was not allowed to return to England and was interned as an enemy alien until the end of the war.

In 1919 he returned to Cambridge to resume his interrupted work on atomic structure. In that same year, Rutherford concluded that protons carry away the internal energy of the nucleus. This was also the first time that a nucleus had been artificially transmuted.

Unfortunately, no more real progress was made on atomic structure until Chadwick's discovery in 1932. He had been investigating the intense radiation given off by beryllium when it was bombarded by alpha particles. He recognised that some secondary particles were being given off that could displace protons from nearby hydrocarbon wax. After many complex calculations Chadwick announced the discovery of a new nuclear particle with no charge: the neutron.

Here at last was an important breakthrough in atomic structure with the existence of a particle that could be used to investigate the structure of other atoms. As it carries no charge, the neutron is not deflected by electrons and protons and, as its mass is slightly greater than that of a proton, it can dis-place particles from the nucleus, enabling scientists to make deductions about its composition.

By the start of the Second World War, the implications of the impact of neutrons on the nucleus and the subsequent release of energy were well understood. Slowed-down neutrons, released during the fission of Uranium 235 (U235), initiate fission in additional uranium nuclei, creating a chain reaction if a sufficient or critical mass of uranium nuclei is available.

In theory, a small amount of U235 could be used to transform a large amount of U238 into explosive plutonium 239 with both beneficial (nuclear reactors), and devastating (nuclear bombs), consequences. The bomb that was dropped on Hiroshima was a uranium bomb, the one dropped on Nagasaki three days later used plutonium.

For his work leading to the discovery of the neutron Chadwick received the Nobel prize for Physics in 1935. Ten years later the year that saw Chadwick's neutron effectively end the war in the Pacific he received a knighthood. He continued his academic career, becoming Professor of Physics at Liverpool University.

In terms of progress in science, his single discovery was a breakthrough that undoubtedly advanced the understanding of atomic structure enough to make atomic weapons possible and also led to the theoretical possibility of other chargeless particles.

James Williams is head of science at The Beacon School, Banstead, Surrey

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