ERNEST RUTHERFORD

The Artificial Transmutation of the Elements

Ernest Rutherford was born of a Scottish father and an English mother in Nelson, New Zealand, in 1871. His father was a small farmer and something of a general engineer, and his mother was a school teacher. He won a scholarship to Nelson College for his secondary education. He excelled at school, particularly in mathematics. Another scholarship took him to Canterbury College at Christchurch, then one of the constituent colleges of the University of New Zealand, in 1889. He took his M.A. in 1893 with a double First in Mathematics and Mathematical Physics. He had already begun research work into magnetism, and in 1894 to 1895 he developed a detector for radio waves.

In 1895 he was awarded an 1851 Exhibition Scholarship to Cambridge, where he worked under J. J. Thomson, in the Cavendish Laboratory. His first studies in Cambridge were in collaboration with Thomson, on the ionization effects of X-rays. Then, in 1898, he turned to the exploration of the phenomenon of radioactivity, the emission of radiation from the natural breakdown of elementary substances.

He was offered the chair of physics in McGill University in Montreal in 1898. Not only did this move give him a laboratory of his own, but put him in the financial position to marry Mary Newton, to whom he had become engaged while at Christchurch. Here he began the astonishingly fruitful collaboration with the eccentric Frederick Soddy, who supplied the necessary chemical expertise, in their joint investigation of the properties of radioactive materials. With Soddy, Rutherford formulated the atomic disintegration theory of radioactivity in 1902. He was elected a Fellow of the Royal Society in 1903 and awarded the Rumford Medal in 1904.

In 1907 he returned to Britain as Professor of Physics at the University of Manchester. He immediately attracted around him a group of very talented younger men. He was awarded the Nobel Prize for chemistry in 1908. In 1909, in collaboration with Geiger and Marsden, he carried out the experiments that suggested that atoms consisted of heavy nuclei surrounded by orbiting electrons. At first this discovery was not widely recognized, but it began a very fruitful period of collaboration between Niels Bohr and Rutherford, in the course of which Bohr sketched out the quantum theory of fundamental particles and their interactions.

During the First World War Rutherford worked on problems of submarine detection, but at the same time managed to continue his major researches. The discovery of the artificial disintegration of elements and their forced transmutation came in 1919, the experiment to be described in this section. In 1919 Rutherford finally returned to Cambridge, succeeding J. J. Thomson as Director of the Cavendish Laboratory. Here he worked with Chadwick on systematic studies of the artificial disintegration of the elements, and it was here that with Oliphant and Hunter he produced the first nuclear fusion, the creation of atoms of a heavier element by fusing the atoms of a lighter one.

He was awarded the Order of Merit in 1925 and elevated to the Peerage in 1931. He died in Cambridge in 1937.

The state of knowledge before Rutherford's experiment

In trying to set out the history of the problem of the transmutation of the elements a great deal depends on what one takes the term `elements' to mean. In antiquity the distinction between compounds and elements, as we know it, was not clearly drawn. Historically the most important distinction was rather between the metals and non‑metals. But there was a doctrine of elements. They were thought of as the.basic principles which, in combination, formed the familiar substances of the earth's crust, such as metals, organic materials, stones and so on. There were generally thought to be four of these elements or principles, which existed in different proportions in different substances. The basic principles were indestructible, and most scientists of antiquity thought that they could not be transformed into one another.

However, from the time of Alexandria's dominance of the scientific culture of the Mediterranean world, say from about 300 BC, a growing number of students of nature came to think that ordinary substances could be transformed into one another. Food could be transformed into flesh, ice into water, ore into metal and so on. This idea, well grounded in common experience, was generalized to include all substances, including the metals. So it was thought that by suitable manipulations lead or tin could be transmuted into gold, iron or anything else. This project had a double significance. By being connected with the signs of the Zodiac, the metals had been related to astrological theories and were thought to have powers of a rather special kind. So gold, as the supposedly most perfect metal, began to assume an importance over and above its role in the economic systems of the time. To find a way of transmuting common metals into gold would then not only be of some economic advantage (even in the ancient world not everyone had fully grasped the folly of inflation), but it would also open up the technical possibility of creating other perfect substances, for instance the perfect medicine, the panacea.

Chemists, in this tradition, believed that the metals, like all other substances, were formed from different proportions of the four basic elements. They supposed that if they could find out the proportions in baser substances they could add to or take away from the amounts of the elements which were out of balance, so to speak, and so modify the substance. If they could hit on the perfect balance, then they would have created gold. There were mathematical theories derived from some of the simple properties of natural number sequences, such as magic squares, which suggested that some proportions were well grounded mathematically. The research programme based on these theories, which we call `alchemy', was a total failure. But in the course of trying to do the impossible, alchemists discovered a great many useful chemical reactions and preparations.

Some time between the Renaissance and the end of the nineteenth century, the whole idea of transmuting the elements, now thought of as the most elementary amongst the ordinary substances of nature, had fallen into disrepute. The exact story of the development and spread of this opinion is not really known. The scientists of the seventeenth century, who, like Robert Boyle, believed that material substances were made of different structures of basically similar corpuscles, had no difficulty with the idea of transmutation, though they had a lordly contempt for alchemical theories and attempts at transmutation based upon them. After all, if Boyle had been able to break down a substance into an undifferentiated broth of its basic constituents and recombine them in a different arrangement, he would have transmuted one substance into another. Whatever the history of the dogma that transubstantiation was impossible, it was very well entrenched by the end of the nineteenth century.

Rutherford's first contribution to the business was his theory of radioactivity. Becquerel had noticed that some minerals gave out `rays' which made the mineral fluorescent, and which would blacken photographic plates. The Curies were isolating the active constituents from the mineral. But the theory of radioactivity was primitive. Most thought it a form of fluorescence, a chemical reaction producing light. Only Rutherford seemed to have grasped how fundamental a process was going on when a radioactive substance gave out rays. He proposed the novel hypothesis that the source of the rays was a disintegration of the very atoms themselves. Disintegration would lead to the fission of the atom into smaller atoms, which would necessarily be atoms of a different substance. This idea explained why radium, a heavy metal, decayed through various intermediate substances into lead, a somewhat lighter metal than radium, atom for atom. So far as anyone knew, this disintegration was confined to the very heavy elements, and it was spontaneous.

The first artificial transmutation of an element

There is a widespread myth that scientists do experiments to test hypotheses. This implies that a hypothesis is first formulated, somehow, and then various consequences are drawn from it which are put to the test. If they fail, the hypothesis is to be rejected, and if they pass, then it can be accepted for the moment as plausible. Rutherford's experiment is not like this. It was not a test at all. The experiment, designed originally to explore the strength of the impetus imparted to products of the disintegration of atoms, revealed a surprising effect. Rutherford had the wit to formulate a hypothesis to fit the unexpected effect; a hypothesis which developed directly out of and extended the ideas he had already introduced to explain natural radioactivity.

The original experiment involved the use of a source of a-particles, common products of the disintegration of heavy atoms (now known to be nuclei of helium atoms), a chamber into which different gases of different stopping powers could be introduced, and a screen which would detect particles which had either passed right through the gas in the chamber or had been emitted by collisions between a‑particles and molecules of the enclosed gas.

When this apparatus is equipped with a Radium‑C source to produce a‑particles and filled with air, there appear `scintillations on the screen far beyond the range of the a‑particles' emitted at the source. At first sight they seemed to Rutherford similar to the `swift H [hydrogen] atoms produced by passing a‑particles through hydrogen'. When an a‑particle hits a hydrogen atom it gives it a'shove', projecting it with very high velocity and long range. When oxygen or carbon dioxide (i.e. constituents of air other than nitrogen) were introduced into the apparatus the scintillations due to long‑range particles were much reduced. `A surprising effect was noticed, however, when dried air was introduced.' Instead of the number of longrange scintillations being reduced it was increased by a large amount, and what is more, they were of very long range (19 cm). To what could they be due?

The experimental arrangement.

Rutherford began a long series of subsidiary experiments to answer this question. Each experiment was designed to eliminate a possible source for the mysterious fast particle. There are some swift oxygen and nitrogen atoms produced by collisions with a‑particles, but these have a range of about 9 cm only. By using a screen between the chamber and the detector screen, which had a stopping power greater than 9 cm of air, `these atoms are not completely stopped'. He showed that the anomalous effect was not due to water vapour since it still occurred with carefully dried air. It was not due to dust particles since carefully filtered air produced the same effect. But if it were due to the nitrogen of the air, in some way, then the long‑range particles should continue to be produced and perhaps even increase, if nitrogen from some chemical source was introduced. And that is exactly what happened. The increase was precisely what would be expected as the amount of nitrogen is increased from 80 per cent as in atmospherical air, to 100 per cent.

'The results so obtained show that the long‑range scintillations obtained from air must be ascribed to nitrogen.' But the next step was to show that they are due to collisions with a‑particles. This could be presumed if there were any evidence that they were due 'to collisions of a‑particles with atoms of nitrogen throughout the volume of the gas'. One obvious test would be to change the pressure of the gas. If the number of scintillations decreased directly proportional to the decrease in the pressure of the gas then this would be good evidence. Further, Rutherford showed that the range of the expelled atom that produced the scintillation was proportional to the range of the expelling atom. When a target molecule was hit which was further from the source of a‑particles, the expelled particle would go correspondingly further. This simple reasoning clearly implied that the effect must have been due to collisions.

The hypothesis‑test stage, so often proffered as the whole of science, comes last in this piece of work. All the evidence derived from common‑sense examination of the effect points to the theory that the long‑range scintillations are due to hydrogen atoms, and that they must come from within nitrogen atoms, as the product of an artificial disintegration, leaving behind transmuted atoms of a lighter element.

But first one must be sure that the long‑range particles are indeed hydrogen atoms. The degree to which a moving particle is deflected in a magnetic field is proportional to the ratio of charge to mass, a very characteristic ratio, for particular kinds of atoms. So if the long‑range particles were deflected by the same amount as hydrogen atoms similarly prepared, and by the same magnetic field, it could be presumed that they were indeed hydrogen atoms. Rutherford did get an effect of the right order, but the `numbers involved were too small' for him to be satisfied with the experiment as a proof. But everything added up to the near certainty that that was what the longrange particles were. Now for the interpretation of the experiment.

`. . . we must conclude', says Rutherford (p. 586), `that the nitrogen atom is disintegrated under the intense forces de‑

veloped in close collision with a swift a‑particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus.' But this interpretation must not be just fudged up ad hoc to explain the effect ‑ it must be able to be seen as a natural extension of theories already well established.

Rutherford goes on to show how the new effect fits in. 'Considering the enormous intensity of the forces brought into play, it is not so much a matter of surprise that the nitrogen atom should suffer disintegration as that the a‑particle itself escapes disruption into its constituents.' From nitrogen Rutherford had been able to produce another element, hydrogen, by his active manipulation of the necessary equipment.

With characteristic prescience he went on to suggest the research programme that has dominated nuclear physics ever since. `. . . if a‑particles ‑ or similar projectiles ‑ of still greater energy were available for experiment, we might expect to break down the nuclear structure of many of the lighter atoms.'

Nuclear research after Rutherford

This programme was able to be fulfilled in two ways. By the 1930s a new kind of ray had been discovered, the cosmic ray. There were particles coming into our atmosphere from outer space with immense energies. By exposing photographic plates to the cosmic radiation in balloons rising high above the heavier, denser parts of the atmosphere it was possible to record collisions between these very powerful projectiles and atoms of the air. All kinds of new particles, supposedly constituents of ordinary atoms, have been found. But this technique, though fruitful when it comes off, is quite happenchance. One may wait about quite a while for a cosmic ray collision, and when it comes one may not have a collision involving the kind of energies which one wants.

The second line of development was the design and construction of artificial accelerators. The basic principle is simple. A charged particle like an electron or a proton is attracted by an electric field of suitable polarity, and so is accelerated. By arranging a sequence of electric field generators, which are switched off as the particles pass into the grasp of the next field, huge accelerations are achieved.

The path of a beam in an accelerator.

To make the equipment more compact, another effect is often employed. In a magnetic field charged particles tend to be drawn into a curved path. By arranging a suitable magnetic field a stream of particles can be made to go in a spiral or circular path; with electric fields to accelerate them particles can be made to go round and round, up to a million times, within the circular core of the apparatus until they are discharged at a target with terrific energy. With these machines the exploration of the structure of the nucleus of the atom has been carried on with great success in recent years.

It is sometimes argued that science does not accumulate knowledge but lurches from one world view to another. This can hardly be true in the short term. Rutherford's experiments and his interpretation of the anomalous effects as the disintegration of complex atomic structures by collision with projectiles depend on his general acceptance of Thomson's interpretation of the phenomena of gas discharge. Without the idea that there are subatomic electrically charged material projectiles, Rutherford could have not made his discovery.