As an enemy alien, Chadwick was interned by the Germans, but allowed to set up a laboratory in the stables of his civilian internee camp outside Berlin. He remained there throughout the war, doing his research, before returning to Britain in Working again with Rutherford, who had by this time moved to Cambridge University, Chadwick helped his mentor achieve the first artificial nuclear transformation. They also achieved the transmutation of other light elements by bombarding them with alpha particles, while pressing ahead with research into the basic structures of the atomic nucleus.
They assume that this could be a Compton effect between a gamma which they consider to have an energy of about 50 million electron volts a very high energy for the time and hydrogen. In the case of a proton, the kinematics of the collision is such that the photon must have such a high energy for communicating a kick to protons such as the one observed by Frederic and Irene Joliot-Curie. In the s, there was not known sources of gamma of 50 million electron volts Typically, the energies of gamma emitted by the nuclei are less than one million electron volts In , once published these results, J.
Chadwick in England made a test confirming those results and went further. By accurately measuring the energy of the ejected nuclei, he could say that the "ultra penetrating radiation" cannot be a gamma ray, of high energy, but must be composed of particle of mass 1 and of electric charge 0: this is the neutron.
All three teams had worked with the devices they have developped, but also with their own knowledge and wereimmersed in the tradition of their laboratories. It is not surprising that it was in the Cambridge laboratory, led by Rutherford, that the neutron was discovered.
Chadwick had well remembered that Rutherford, indeed, had put forward a year-old hypothesis of neutral particle about as heavy as a proton In the s, it was assumed that the nucleus was composed of protons accompanied by some electrons compensating some of the positive electric charges.
Physicists immediately give up their image of a nucleus made up of protons accompanied by captive electrons whose negative electric charge compensated for the positive charge of the protons. The Joliot-Curies in their laboratory.
Courtesy of the American Institute of Physics. They found that this radiation ejected protons from a paraffin target. This discovery was amazing because photons have no mass. However, the Joliot-Curies interpreted the results as the action of photons on the hydrogen atoms in paraffin. They used the analogy of the Compton Effect, in which photons impinging on a metal surface eject electrons.
We now know that gamma photons do not have enough energy to eject protons from paraffin. He not only bombarded the hydrogen atoms in paraffin with the beryllium emissions, but also used helium, nitrogen, and other elements as targets. By comparing the energies of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral component with a mass approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, , issue of Nature.
You can read his lecture as he received his Nobel prize.
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