NUTRON

Discovery of the neutron 

The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century. The century began with Ernest Rutherford and Thomas Royds proving that alpha radiation is helium ions in 1908^[1][2] and Rutherford's model for the atom in 1911,^[3] in which atoms have their mass and positive charge concentrated in a very small nucleus.^[4] The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick in 1932.

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.^[19][20] 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.^[21] 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.^[22]
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.^[9] Chadwick had searched for Rutherford's neutron by several experiments throughout the 1920s without success. 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.^[23] 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.^[24][25] Chadwick won the Nobel Prize in Physics for this discovery in 1935.^[26]