Collision of α particles with light atoms. I. Hydrogen
Ernest Baron Rutherford(1871-1937)
 By 1919, Ernest Rutherford had achieved great contributions in nuclear physics. He started with his research on the nature of radiated rays, and as Curie had, distinguished two kinds of rays, a positively charged particle stream, and a negatively charged particle stream. He named these alpha and beta rays. When a third type of radiated rays, which was not affected by a magnetic field was discovered by Villard in 1900, he immediately confirmed that these rays were the electromagnetic waves in which the wave-length was one hundred thousand times shorter than that of visible light. He also verified that the mass of alpha particles was seven thousand times larger than that of the beta particles and demonstrated that these particles constituted the nucleus of helium in 1903.
 From 1902, Rutherford investigated the phenomenon of radioactive decay in collaboration with Soddy, one of his pupils and the discoverer of isotopes. They investigated the rate of radioactive disintegration in all known radioactive elements, and determined the time in which their radioactivity was reduced to half. This velocity which they called a “half-life,” varied between several deciseconds and several hundred million years. In this manner, a radioactive element is disintegrated by radiation and is transmuted into another element and finally becomes a stable element like plumbum, which is no longer radioactive. They distinguished two series of disintegration of radionuclides or that of radioactive transformation, a series derived from uranium and a series derived from thorium, and clarified the process of both series. In 1908, Rutherford was awarded the Nobel Prize for all of these contributions.
 Rutherford continued the study of the alpha particle and observed that the scattering of alpha particles is caused when a particle is shot into gold leaf. Almost all the alpha particles penetrated without changing their courses, but a few particles at a ratio of one to twenty thousand changed their courses greatly or were sometimes repelled strongly. He conjectured this was because of a positively charged particle which had a large mass in the atom and proceeded in 1911 to establish a model of atomic structure in which the electrons rotated around a nucleus which constituted almost the entire mass of the atom. Finally as reported in the above paper, he succeeded in liberating the nuclei of hydrogen atoms from nitrogen by colliding alpha particles with nitrogen atoms. Through this he not only confirmed his theory of radioactive decay and his model of atomic structure, but also achieved the artificial transmutation of an element for the first time in history. It was a great step toward nuclear fission.