<P> In 1931, Walther Bothe and Herbert Becker in Giessen, Germany found that if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced . Since this radiation was not influenced by an electric field (neutrons have no charge), it was thought to be gamma radiation . The radiation was more penetrating than any gamma rays known, and the details of experimental results were difficult to interpret . The following year Irène Joliot - Curie and Frédéric Joliot in Paris showed that if this unknown radiation fell on paraffin, or any other hydrogen - containing compound, it ejected protons of very high energy . This observation was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis . In Rome, the young physicist Ettore Majorana suggested that the manner in which the new radiation interacted with protons required a new neutral particle . </P> <P> On hearing of the Paris results in 1932, neither Rutherford nor James Chadwick at the Cavendish Laboratory in Cambridge were convinced by the gamma ray hypothesis . Assisted by Norman Feather, Chadwick quickly performed a series of experiments showing that the gamma ray hypothesis was untenable . He repeated the creation of the radiation using beryllium, used better approaches to detection, and aimed the radiation at paraffin following the Paris experiment . Paraffin is high in hydrogen content, hence offers a target dense with protons; since neutrons and protons have almost equal mass, protons scatter energetically from neutrons . Chadwick measured the range of these protons, and also measured how the new radiation impacted the atoms of various gases . He found that the new radiation consisted of not gamma rays, but uncharged particles with about the same mass as the proton; these particles were neutrons . Chadwick won the Nobel Prize in Physics for this discovery in 1935 . </P> <P> The year 1932 was later referred to as the "annus mirabilis" for nuclear physics in the Cavendish Laboratory, with discoveries of the neutron, artificial nuclear disintegration by the Cockcroft--Walton particle accelerator, and the positron . </P> <P> Given the problems of the proton--electron model, it was quickly accepted that the atomic nucleus is composed of protons and neutrons, although the precise nature of the neutron was initially unclear . Within months after the discovery of the neutron, Werner Heisenberg and Dmitri Ivanenko had proposed proton--neutron models for the nucleus . Heisenberg's landmark papers approached the description of protons and neutrons in the nucleus through quantum mechanics . While Heisenberg's theory for protons and neutrons in the nucleus was a "major step toward understanding the nucleus as a quantum mechanical system," he still assumed the presence of nuclear electrons . In particular, Heisenberg assumed the neutron was a proton--electron composite, for which there is no quantum mechanical explanation . Heisenberg had no explanation for how lightweight electrons could be bound within the nucleus . Heisenberg introduced the first theory of nuclear exchange forces that bind the nucleons . He considered protons and neutrons to be different quantum states of the same particle, i.e., nucleons distinguished by the value of their nuclear isospin quantum numbers . </P>

When was the existence of the neutron confirmed