Rutherford, Ernest

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RUTHERFORD, ERNEST

Ernest Rutherford, the central figure in the science of radioactivity and the founder of its extension, nuclear physics, was born in Brightwater, near Nelson, on the southern island of New Zealand, on August 30, 1871. He died in Cambridge, England, on October 19, 1937. Rutherford's parents, seeking economic opportunity, were part of the mid-nineteenth-century migration from the British Isles. Before her marriage, his mother was a teacher, while his father pursued a variety of jobs, from cutting railroad ties to flax farming.

Young Ernest, one of a dozen siblings, won a scholarship to Nelson College, an excellent secondary school, and a further award in 1889 to Canterbury College in Christchurch, one of the few institutions of higher education in New Zealand. He received his B.A. degree in 1892, won another scholarship for his M.A. (1893), and stayed still another year for his bachelor of science degree (1894). The Canterbury faculty, although quite few in number, offered Rutherford solid grounding in mathematics and the sciences and, better yet, inspired in him a love of physical investigation.

For the B.Sc. degree, he magnetized iron by high-frequency electrical discharges. James Clerk Maxwell had predicted the existence of radio waves, Heinrich Hertz had found them less than a decade before, and Rutherford was one of a number of people to construct detectors of these waves. Seen as a promising scientist, he won a scholarship that required him to

attend another institution; he chose Cambridge University because the director of its Cavendish Laboratory, Joseph John Thomson, was the leading authority on electromagnetic phenomena.

When Rutherford arrived in England in September 1895, he became part of the first class of "research students" (today called graduate students, but the doctoral degree would not be awarded there for another quarter-century) admitted by new university regulations. He continued to improve his wireless wave detector, increasing its range. Thomson was so impressed that he invited Rutherford to collaborate with him in studying the recently discovered X rays. Without the foresight that wireless would become big business, and also lacking the entrepreneurial skills of a Guglielmo Marconi, Rutherford accepted his professor's offer.

Thomson had spent years studying the discharge of electricity in gases. X rays provided another means of making a current flow readily. Thomson and Rutherford determined that equal numbers of positively and negatively charged particles were formed and advanced a theory of ionization. In 1897, Thomson asserted that the negative particles were smaller than atoms, and soon they were called electrons.

Since the radiation that Henri Becquerel found issuing from uranium was at first thought to be X rays, it was only natural that Rutherford would test its ability to ionize air. Indeed, this property became a major indication of a source's strength. In 1898, Gerhard C. Schmidt and Marie Curie independently found thorium was similarly "radioactive," and Curie and her associates soon detected other sources, named polonium and radium. Testing uranium radiation's ability to penetrate metal foil, Rutherford found one component that was easily stopped and another that passed through some layers of foil. These he named alpha and beta, respectively, "for convenience."

In 1898, Rutherford was appointed as a full professor of physics at McGill University in Montreal. Blessed with a wealthy patron, the laboratory was probably the best equipped in the Western Hemisphere. Even better, the department chairman, who wanted a research star, willingly took over some of Rutherford's teaching duties. Rutherford now began a careful study of thorium and soon found that it evolved a radioactive gaseous product, which he called emanation (now called thoron). Significantly, some radioactive bodies maintained a steady level of activity, while others exhibited a rise or fall. These latter each changed over a period unique to it, which was soon called its half-life. This measure served as a means of identifying sources which contained too few atoms to be determined by ordinary chemical tests.

Always adept at drawing others into his investigations, Rutherford and a chemical colleague, Frederick Soddy, in 1902 recognized that freshly prepared thorium increased in activity at the same rate as a constituent found in its ore, thorium X, decreased. They reasoned that a genetic relationship existed, with the parent decaying into a daughter product, which also decayed if it was radioactive. The several active bodies could be arranged in decay series, which ultimately would end in inactive products. Since each product was regarded as an element, what they proposed was atomic transmutation—alchemy—which had been driven out of scientific chemistry centuries before. Yet, few challenged this new idea of unstable atoms, for the evidence fit into the theory exceedingly well. For this explanation of radioactivity, Rutherford received the 1908 Nobel Prize in Chemistry, an indication of this subject's position on the borderline between physics and chemistry.

In 1907, desiring to be closer to the centers of scientific activity, Rutherford accepted the director-ship of the physics laboratory at Manchester University, the best in Britain after Cambridge. While in Canada he had shown that the alpha ray was a positively charged particle but could not decide if it was hydrogen or helium. Availing himself of the university's skillful glassblower, who constructed a tube that alphas could enter but not leave, Rutherford's student Thomas Royds in 1908 showed that alphas produced the spectrum of helium.

With his assistant, Hans Geiger, Rutherford developed a means of visually counting the flashes of light made by alpha particles striking a scintillating screen. Geiger later extended this valuable measure of a source's strength to electrical and then electronic counting. At McGill, Rutherford had observed a certain fuzziness when experimenting with alphas, which he supposed was due to slight scattering when they hit an object. To explore this phenomenon further, in 1909 he asked an undergraduate, Ernest Marsden, to allow alphas from a naturally decaying source to strike a foil target and measure the scattering. Most hit the detector without any deflection, many were bent through small angles, but some were, surprisingly, turned more than ninety degrees. Rutherford's (embellished) reaction was that "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you."

Two years later Rutherford could explain what happened. The atom, whose diameter was of the order of 10^-8 centimeters, was not a solid billiard ball-like object or J. J. Thomson's popular "plum pudding" model of a sphere of positive electrification studded with a geometric array of electrons. Instead, it was mostly empty space, with the atom's mass concentrated in a tiny nucleus measuring 10^-12 centimeters, with electrons orbiting at a distance. When an alpha came near enough to a charged nucleus for electrostatic forces to act, the alpha could be deflected from its original path in this single encounter.

The discovery of the nuclear atom turned few heads at first. But Niels Bohr, who had visited Rutherford's laboratory in 1911 and absorbed the excitement of the new concept, showed its implications in1913. He explained radioactivity as a nuclear phenomenon, chemical reactions as belonging to outer electrons, and spectral lines as jumps by electrons from one orbit to another. Still more, he incorporated the new quantum ideas to explain that only certain orbits were permitted, a major consolidation of atomic physics. Another former student, Henry Gwyn-Jeffreys Moseley, at the same time explained that the regular sequence of X-ray spectral lines as one went through the periodic table of elements was due to the regular increase of positive charge on the nucleus. Thus, the periodic table was organized upon atomic number, not atomic weight, as previously believed. And still another Manchester alumnus, Kasimir Fajans, added to the significance of the nucleus when he devised the group displacement laws to show how alpha and beta decay transformed radioelements from one box to another of the periodic table.

With his laboratory largely empty during World War I, Rutherford spent some time on submarine-detection apparatus but had the opportunity to pursue something curious that Marsden had found. When alpha particles traveled in a tube filled with hydrogen gas, scintillations were observed on a screen placed beyond the alphas' range. Obviously, an alpha-hydrogen collision sent the latter particle flying toward the screen. But scintillations that looked like those from hydrogen were also seen when the tube was filled with nitrogen. In 1919, Rutherford explained that this was not an elastic collision but an induced nuclear reaction. The alpha and nitrogen (he concluded) transformed into hydrogen (a proton) and oxygen. Along with the explanation of radioactivity and the discovery of the nucleus, this induced nuclear reaction confirmed the view that Rutherford was the greatest experimental physicist since Michael Faraday.

Also in 1919, Rutherford succeeded Thomson as director of the Cavendish Laboratory, continuing it as the preeminent research center and source of physics professors for the British Commonwealth. With James Chadwick, he showed that other light elements succumbed to alpha-induced transformations. However, heavier elements, with larger positive charges on their nuclei, resisted close encounters with alphas. By the late 1920s, Rutherford encouraged engineers to overcome insulation breakdown and other technical problems to build high-voltage apparatus that could accelerate copious quantities of protons toward a target. If the voltage (or energy) were high enough, the projectile might overcome (or tunnel through) the potential barrier around the nucleus and cause a nuclear reaction. This was accomplished in 1932 by John Douglas Cockcroft and Ernest Thomas Sinton Walton in the Cavendish Laboratory, who also used for the first time Albert Einstein's equation E = mc² in this interpretation of the nuclear reaction. Whereas in natural radioactivity elements spontaneously transmuted into other elements, and in his 1919 experimental arrangement, Rutherford had produced artificial transmutation by natural means, this accelerator experiment involved artificial transmutation produced artificially.

Rutherford's leadership of the Cavendish Laboratory was burnished still more in 1932, when Chadwick discovered the neutron, an uncharged particle able easily to reach atomic nuclei and cause reactions. While Rutherford believed correctly that "harnessing the energy of the atom" was unlikely, given the known reactions and equipment, the phenomenon of neutron-induced nuclear fission was discovered shortly after his death, and both reactors and bombs followed. Curiously, Rutherford was associated also with the other process for constructing nuclear weapons, fusion, when he and colleagues achieved a fusion reaction in 1934.

As one of the world's leading scientists, Rutherford was awarded many honors, including knight-hood, then a peerage, the Order of Merit, and presidency of the Royal Society and of the British Association for the Advancement of Science. Not politically active, he nonetheless was drawn into public positions in the 1930s when one of his colleagues, Peter Kapitza, was prevented from returning to Cambridge after a visit to his home in the Soviet Union, and when he accepted the presidency of the Academic Assistance Council, an organization created to help refugee scientists from Hitler's Germany. Rutherford, thus, was one of the last scientists largely able to devote himself single-mindedly to exploring nature. The next generation could not avoid questions of science and social responsibility, politics, and national security.