James Chadwick and the neutron
It took nearly twelve years for someone to come along and explain what was missing. On February 27, 1932, James Chadwick published his paper on the "Possible Existence of a Neutron", a subatomic particle with no electric charge but with the same mass as the proton. In his Nobel Lecture, on December 12, 1935, Chadwick acknowledged that his experiment and his conclusions were mostly pieced together with the findings of many colleagues. He starts by saying that it was Rutherford who first said it would be rather impossible for an alpha particle to penetrate the nucleus of large atoms due to its strong repelling force, unless this field was somehow diluted by other particles within the nucleus. He then acknowledges that it was the Joliot-Curie power-couple (Irène and Frédéric) who first noticed that paraffin wax would eject more radiation than it was receiving when exposed to beryllium radiation (something that would later have world-changing consequences). Chadwick also pointed out that Norman Feather was one of the first to notice that this special radiation was more penetrating than simple alpha particles—they can “pass easily through thicknesses of matter, e.g. 10 or even 20 cm lead. But a proton of the same velocity as this particle is stopped by a thickness of 0.25 mm of lead”. Chadwick concluded that all these phenomena he had collected “could be readily explained on this assumption, that the radiation consists of particles of mass 1 and charge 0, or neutrons.” He had found the stealth bomber of the Atomic world.
Source: Chadwick, J. Possible Existence of a Neutron. Nature 129, 312 (1932).and Nobel Lectures, Physics 1922-1941, Elsevier Publishing Company, Amsterdam, 1965
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!