Rutherford , Ernest

Rutherford , Ernest, first baron Rutherford of Nelson

(1871–1937) New Zealand physicist

Rutherford, who was born at Nelson in New Zealand, was certainly the greatest scientist to emerge from that country; he can also fairly be claimed to be one of the greatest experimental physicists of all time. His career almost exactly spans the first great period of nuclear physics, a field he did much to advance and which he dominated for so long. This period stretches from the detection of radioactivity by Henri Becquerel in 1896 to the discovery of nuclear fission by Otto Hahn in 1938, the year after Rutherford's death. He came from a fairly simple background, the fourth of twelve children, the son of a man variously described as a farmer, wheelwright, and miller. He was educated at Canterbury College, Christchurch, and in 1895 won a scholarship to Cambridge University, England.

In New Zealand Rutherford had done some work on high-frequency magnetic fields. At Cambridge, working under J.J. Thomson, he first continued this research, and then in 1896 began to work on the conductivity of air ionized by x-rays. In 1898 he moved to become professor at McGill University in Canada. This was a good appointment for Rutherford in two respects. McGill had one of the best-equipped physics laboratories in the world and, in particular, there was a good supply of the then very costly radium bromide. The other main gain for Rutherford in Montreal was the presence of Frederick Soddy, an Oxford-trained chemist with whom he entered into a most rewarding eighteen-month collaboration, from October 1901 to April 1903, during which time they produced nine major papers laying the foundations for the serious study of radioactivity.

When Rutherford began working on radioactivity at the end of the century little was known about it apart from the result of Pierre and Marie Curie that it was not limited to uranium alone but was also a property of thorium, radium, and polonium. Rutherford's first important advance, in 1899, was his demonstration that there were two quite different kinds of emission, which he referred to as alpha and beta rays. The first kind had little penetrating power but produced considerable ionization while the beta rays (electrons) were as penetrating as x-rays but possessed little ionizing power. To find out exactly what they were took Rutherford the best part of a decade of careful experimentation but, long before he had the final answer, he had used their existence to work out with Soddy a daring theory of atomic transmutation. In 1900 Rutherford showed that a third type of radiation, undeflected by magnetic fields, was high-energy electromagnetic radiation. He called this radiation ‘gamma rays’.

Rutherford also began to investigate the radioactive element thorium, which in addition to alpha, beta, and gamma rays also emits a radioactive gas that he called ‘emanation’. He showed that the emanation decayed in activity at a particular rate, losing half its activity in a fixed period of time (the half-life). Rutherford and Soddy began an intensive investigation of thorium compounds and showed that a more active substance, thorium X, was present. They eventually came to the view that the emanation was produced from the thorium X, which in turn came from the original thorium. In other words, there was a series in which chemical elements were being changed (transmuted) into other elements. Rutherford and Soddy published their theory of a series of transformations in 1905. Rutherford later published a book with the title The Newer Alchemy (1937). Soddy went on to continue this work, eventually introducing the idea of isotopes.

Rutherford directed his attention to the alpha radiation emitted in radioactive decay, proving that it consisted of helium atoms that have each lost two electrons. He continued to study alpha radiation, moving to the University of Manchester, England, in 1907. At Manchester Rutherford and Hans Geiger invented the Geiger counter in 1908. Here too Geiger and E. Marsden in 1910, at Rutherford's suggestion, studied the scattering of alpha particles passing through thin metal foils. The particles were detected by a screen coated with zinc sulfide, which gives brief flashes of light (scintillations) when hit by high-energy particles.

Geiger and Marsden found that most of the particles were deflected only slightly on passing through the foil but that a small proportion (about 1 in 8000) were widely deflected. Rutherford later described this as “quite the most incredible event that has ever happened to me in my life … It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” To make sense of the results, Rutherford published in 1911 a model of the atom in which he suggested that almost all the mass was concentrated in a very small region and that most of the atom was ‘empty space’. This was the nuclear atom (although Rutherford did not use the term ‘nucleus’ until 1912). He also produced a theoretical formula giving the numbers of particles that would be scattered by a nucleus at different angles. The idea of the nuclear atom was developed further by Niels Bohr.

After the end of World War I, which Rutherford spent working for the Admiralty on sonic methods of detecting submarines, he moved in 1919 to take the Cavendish chair of physics and the directorship of the Cavendish Laboratory at Cambridge University. It was there that he announced in 1919 his third major discovery, the artificial disintegration of the nucleus. Following some earlier experiments of Marsden he placed an alpha-particle source in a cylinder into which various gases could be introduced. At one end of the cylinder a small hole was covered with a metal disk through which some atoms could escape and register their presence on a zinc sulfide screen. The introduction of nitrogen produced highly energetic particles, which turned out to be hydrogen nuclei (that is, protons). The implications were not lost on Rutherford who concluded that “the nitrogen atom is disintegrated under the intense forces developed in a close collision with the swift alpha particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus.” Occasionally, it was later shown, “the alpha particle actually enters the nitrogen nucleus…breaks up…hurling out a proton and leaving behind an oxygen nucleus of mass 17.” Rutherford had thus succeeded in bringing about the first transmutation, although when described in nuclear terms it seems a simple enough process. With James Chadwick he went on to show between 1920 and 1924 that most of the lighter elements emitted protons when bombarded with alpha particles.

By his work in 1911 and 1919 Rutherford had shown that not only does the atom have a nucleus but that the nucleus has a structure from which pieces can be knocked out and by which other particles can be absorbed. It was this work which virtually created a whole new discipline, that of nuclear physics. Rutherford received the Nobel Prize for chemistry in 1908.