Résumé

Rutherford – la découverte du noyau atomique

Il y a un siècle, en mai 1911, le Philosophical Magazine publiait un article d'Ernest Rutherford considéré maintenant comme historique. Rutherford avait analysé les résultats de la diffusion de particules alpha, obtenus deux ans auparavant par ses collègues de l’Université de Manchester, Hans Geiger et Ernest Marsden. Rutherford avait compris que les grands angles de diffusion mesurés signifiaient que la totalité de la charge positive de l’atome devait nécessairement se concentrer en un noyau central minuscule. Comme l’explique John Campbell, le hasard a joué un grand rôle dans cette découverte.

After three degrees and two years of research at the forefront of the electrical technology of the day, Ernest Rutherford left New Zealand in 1895 on a Exhibition of 1851 Science Scholarship, which he could have taken anywhere in the world. He chose the Cavendish Laboratory at the University of Cambridge because its director, J J Thomson, had written one of the books about advanced electricity that Rutherford had used as a guide in his research. This put the right man in the right place at the right time.

Initially, Rutherford continued his work on the high-frequency magnetization of iron, developing his detector of fast-current pulses to measure the dielectric properties of materials at high frequencies and hold briefly the world record for the distance over which electric "wireless" waves were detected. "JJ" appreciated Rutherford’s experimental and analytical skills, so he invited Rutherford to participate in his own research into the nature of electrical conduction in gases at low pressures.

Within five months of Rutherford’s arrival at the Cavendish Laboratory, the age of new physics had commenced. Wilhelm Röntgen’s discovery of X-rays was swiftly followed by Henri Becquerel’s announcement on radioactivity in January 1896. Rutherford capitalized on the new forms of ionizing radiation in his attempts to learn what it was that was conducting electricity in an ionized gas. He soon changed to trying to understand radioactivity itself and with his research determined that two types of rays were emitted, which he called "alpha" and "beta" rays.

Thomson continued mainly studying the ionization of gases. Less than two years after Rutherford’s arrival he had carried out a definitive experiment demonstrating that cathode rays were objects a thousand times less massive than the lightest atom. The electronic age and the age of subatomic particles had begun, though mostly unheralded. Rutherford was a close observer of all of this and became an immediate convert to – and champion of – subatomic objects. Beta rays were quickly shown to be high-energy cathode rays, i.e. high-speed electrons.

For Rutherford, however, there was no future at Cambridge. After only three years there he – as a non-Cambridge graduate – was not yet eligible to apply for a six-year fellowship, so in 1898 he took the Macdonald Chair of Physics at McGill University in Canada. (Cambridge changed its rules the following year.) From then on, the world centre of radioactivity and particle research was wherever Rutherford was based.

At McGill, he showed that radioactivity was the spontaneous transmutation of certain atoms. For this he received the 1908 Nobel Prize in Chemistry (CERN Courier December 2008 p19 and March 2009 p46). He also demonstrated that alpha particles were most likely helium atoms minus two electrons, and he dated the age of the Earth using radioactive techniques. In studying the nature of alpha particles and by being the first to deflect them in magnetic and electric fields in beautifully conceived experiments, Rutherford observed that a narrow beam of alphas in a vacuum became fuzzy either when air was introduced into the beam or when it was passed through a thin window of mica.

Return to England

With blossoming international scientific fame, Rutherford was regularly offered posts in America and elsewhere. He accepted none because McGill had superb laboratories and support for research, but he was wise enough to let the McGill authorities know of each approach; they increased his salary each time. However, Rutherford also wished to be nearer the centre of science, which was England, where he would have access to excellent research students and closer contact with notable scientists. His desire was noted. Arthur Schuster, being from a wealthy family, said he would step down from his chair at Manchester University provided that it was offered to Rutherford, and in 1907 Rutherford moved to Manchester

At Manchester University Rutherford first needed a method of recording individual alpha particles. He was an expert in ionized gases and had been told by John Townsend, an old friend from Cambridge, that one alpha particle ionized tens of thousands of atoms in a gas. So, with the assistant he had inherited, Hans Geiger, the Rutherford-Geiger tube was developed.

Many labs at the time were studying the scattering of beta particles from atoms. People at the Cavendish Laboratory claimed that the large scattering angles were the result of many consecutive, small-angle scatterings inside Thomson’s "plum pudding" model of the atom – the electrons being the fruit scattered throughout the solid sphere of positive electrification. Rutherford did not believe that the scattering was multiple, so once again he had to quantify science to undo the mistaken interpretations of others.

Geiger was given the task of measuring the relative numbers of alpha particles scattered as a function of angle over the few degrees that Rutherford had measured photographically at McGill. However, photography could not register single particles. Nor was the Rutherford-Geiger detector suitable for "quickly" measuring particles scattered over small angles; it was not sensitive to the direction of entry of the alpha particle and all that they observed was the "kick" of a spot of light from a galvanometer. Yet one of the reasons for developing the Rutherford-Geiger tube had been to determine whether or not the spinthariscope invented by William Crookes did, indeed, register one flash of light for every alpha particle that struck a fluorescing screen.

So, Geiger allowed monochromatic alpha particles in a vacuum tube to pass through a metal foil and onto a fluorescing plate that formed the end of the tube. A low-power microscope, looking at about a square millimetre of the plate, allowed the alphas to be counted. It was tiring work, waiting half an hour for the eye to dark adapt, then staring at the screen unblinking for a minute before resting the eye. It is said that Rutherford often cursed and left the counting to the younger Geiger.

Another of Geiger’s duties was to train students in radioactivity techniques and it was Rutherford’s policy to involve undergraduates in simple research. So, when Geiger reported to Rutherford that a young Mancunian undergraduate was ready to undertake an investigation, Rutherford set Ernest Marsden the task of seeing if he could observe alpha particles reflected from metal surfaces. This seemed unlikely, but, on the other hand, beta rays did reflect.

Marsden used the same counting system as Geiger, but had the alpha source on the same side of the metal as the fluorescing screen, with a lead shield to prevent alphas from going directly to the screen (figure 1). When he reported that he did see about 1 in 10,000 alphas scattered at large angles, Rutherford was astonished. As he later famously recalled: "It was as if a 15-inch naval shell had been fired at a piece of tissue paper and it bounced back."

Geiger and Marsden published their measurements in the May 1909 issue of the Proceedings of the Royal Society, but the study laid fallow for more than a year, while Geiger continued obtaining more accurate results for his small-angle scattering from different materials and various thicknesses of foils. It is said that one day Rutherford went in to Geiger’s room to announce that he knew what the atom looked like. In January 1911 Rutherford was able to write to Arthur Eve in Canada: "Among other things, I have been interesting myself in devising a new atom to explain some of the scattering results. It looks promising and we are now comparing the theory with experiments."

The nuclear atom

On 7 March 1911 Rutherford spoke at the Manchester Literary and Philosophical Society. Two other speakers followed him: one spoke on "Can the parts of a heavy body be supported by elastic reactions only?", the other showed a cast of the "Gibraltar Skull". A reporter from The Manchester Guardian was present and in the edition of 9 March (p3) succinctly paraphrased Rutherford: "It involved a penetration of the atomic structure, and might be expected to throw some light thereon." Rutherford had asked Geiger to test experimentally his theory that the alpha scattering through large angles varied as cosec4(φ/2). He concluded that the central charge for gold was about 100 units, that for different materials the number was proportional to NA2 (where N was the number of atoms per unit volume and A the atomic weight), and that large-angle scattering (hyperbolic paths) was independent of whether the central charge is positive or negative. The reporter concluded: "…we were on the threshold of an enquiry which might lead to a more definite knowledge of atomic structure."

Rutherford’s talk was published in the Proceedings of the Manchester Literary and Philosophical Society (Rutherford 1911a) and more fully in the Philosophical Magazine for May (Rutherford 1911b). In the latter, he acknowledged Hantaro Nagaoka’s mathematical consideration of a "Saturnian" disc model of the atom (Nagoaka 1904), stating that essentially it made no difference to the scattering if the atom was a disc rather than a sphere.

The nuclear atom created no great stir among scientists and the public at the time. Three nights after his announcement, Rutherford addressed the Society of Industrial Chemists on "Radium". The nuclear atom was not mentioned by Sir William Ramsay in his opening address to that year’s meeting of the British Association, although his reported claims of various discoveries caused Schuster – who had stepped down to attract Rutherford to Manchester – to write a letter to The Manchester Guardian stating which of those were discovered by Rutherford.

Rutherford’s busy life continued as normal: accepting a Corresponding Membership of the Munich Academy of Sciences; giving talks on all manner of subjects but the nuclear atom; refuting several claims of cold fusion that came from Ramsay’s laboratory; motoring in the car recently purchased with the money that had accompanied his Nobel prize; and being involved with many organizations, including being a vice-president of both the Manchester Society for Women’s Suffrage and the Manchester Branch of the Men’s League for Women’s Suffrage. (At Canterbury College in New Zealand, his landlady and future mother-in-law was one of the stalwarts who in 1893 had obtained the vote for women in New Zealand.)

Rutherford’s Nobel Prize in Chemistry of 1908 was too recent for physicists to nominate him again for a prize. It was to be 1922 before he was next nominated, unsuccessfully. There have been 27 Nobel prizes awarded for the discovery of, or theories linking, subatomic particles but there was never one for the nuclear atom (CERN Courier March 2009 p46). However there was a related one. At the end of 1911 Rutherford was the guest of honour at the Cavendish Annual Dinner, at which he was, not surprisingly, in fine form. The chairman, in introducing him, stated that Rutherford had another distinction: of all of the young physicists who had worked at the Cavendish, none could match him in swearing at apparatus.

Rutherford’s jovial laugh boomed round the room. A young Dane, visiting the Cavendish for a year to continue his work on electrons in metals, took an immense liking to the hearty New Zealander and resolved to move to Manchester to work with him. And so it was that Niels Bohr received the 1922 Nobel Prize in Physics for "his services in the investigation of the structure of atoms and of the radiation emanating from them". He had placed the electrons in stable orbits around Rutherford’s nuclear atom.