Nobel Lecture, December 12, 1935
Artificial Production of Radioactive Elements
It is a great honour and a great pleasure to us that the Swedish Academy of Sciences has awarded us the Nobel Prize for our work on the synthesis of radio-elements, after having presented it to Pierre and Marie Curie in 1903, and to Marie Curie in 1911, for the discovery of the radio-elements.
I would like here to recall the extraordinary development of radioactivity, this new science which had its origin, less than forty years ago in the work of Henri Becquerel and of Pierre and Marie Curie.
It is known that the efforts of chemists of the last century established as a fundamental fact the extreme solidity of the atomic structures which go to make up the ninety-two known chemical species. With the discovery of the radio-elements, physicists found themselves for the first time confronted with strange substances, minute generators of radiation endowed with an enormous concentration of energy; alpha rays, positively charged helium atoms, beta rays, negatively charged electrons, both possessed of a kinetic energy which it would be impossible to communicate to them by human agency, and finally, gamma rays, akin to very penetrating X-rays. Chemists had no less astonishment as they recognized in these radioactive bodies, elements which had undergone modifications of the atomic structure which had been thought unalterable.
Each emission of an alpha or beta ray accompanies the transmutation of an atom; the energy communicated to these rays comes from inside the atom. As long as they continue to exist, radio-elements have well-defined chemical properties, like those of ordinary elements. These unstable atoms disintegrate spontaneously, some very quickly, others very slowly, but in accordance with unchanging laws which it has never been possible to interfere with. The time necessary for the disappearance of half the atoms, called the half-life, is a fundamental characteristic of each radio-element; according to the substance the value of the half-life varies between a fraction of a second and millions of years.
The discovery of radio-elements has had immense consequences in the knowledge of the structure of matter; the study of the materials themselves, and the study of the powerful effects produced on atoms by the rays they emit occupy scientific workers of numerous great research institutes in all countries.
Nevertheless, radioactivity remained a property exclusively associated with some thirty substances existing naturally. The artificial creation of radio-elements opens a new field to the science of radioactivity and so provides an extension of the work of Pierre and Marie Curie.
After the discovery of the spontaneous transmutations of radio-elements, the achievement of the first artificial transmutations is due to Lord Rutherford. Fifteen years later, by bombarding with alpha rays certain of the lighter atoms, nitrogen and aluminium for example, Lord Rutherford demonstrated the ejection of protons, or positively charged hydrogen nuclei; this hydrogen came from the bombarded atoms themselves: it was the result of a transmutation. The nature of the nuclear transformation could be firmly established: the aluminium atom, for example, captures the alpha particle and is transformed, after expelling the proton, into an atom of silicon. The amount of matter transformed cannot be weighed and the study of radiation alone has led to these conclusions.
In the course of recent years various artificial transmutations of different types have been discovered; some are produced by alpha rays, others by protons or deuterons, hydrogen nuclei of weight 1 or 2, others by neutrons, neutral particles of weight 1 about which Professor Chadwick has just spoken. The particles expelled when the atom explodes are protons, alpha rays or neutrons.
These transformations constitute true chemical reactions which act upon the innermost structure of the atom, the nucleus. They can be represented by simple formulae as Monsieur Joliot will be telling you in a moment.
I shall now speak to you of the experiments which have led us to obtain by transmutation new radioactive elements. These experiments have been made together by Monsieur Joliot and me, and the way in which we have divided this lecture between us is a matter of pure convenience.
In our study of the transmutations with emission of neutrons produced in the light elements irradiated with alpha rays, we have noticed some difficulties in interpretation in the emission of neutrons by fluorine, sodium, and aluminium. Aluminium can be transformed, by the capture of an alpha particle and the emission of a proton, into a stable silicon atom. On the other hand, if a neutron is emitted the product of the reaction is not a known atom.
Later on, we observed that aluminium and boron, when irradiated by alpha rays do not emit protons and neutrons alone, there is also an emission of positive electrons. We have assumed in this case that the emission of the neutron and the positive electron occurs simultaneously, instead of the emission of a proton; the atom remaining must be the same in the two cases.
It was at the beginning of 1934, while working on the emission of these positive electrons that we noticed a fundamental difference between that transmutation and all the others so far produced; all the reactions of nuclear chemistry induced were instantaneous phenomena, explosions. But the positive electrons produced by aluminium under the action of a source of alpha rays continue to be emitted for some time after removal of the source. The number of electrons emitted decreases by half in three minutes.
Here, therefore, we have a true radioactivity which is made evident by the emission of positive electrons.
We have shown that it is possible to create a radioactivity characterized by the emission of positive or negative electrons in boron and magnesium, by bombardment with alpha rays. These artificial radio-elements behave in all respects like the natural radio-elements.
Returning to our hypothesis concerning the transformation of the aluminium nucleus into a silicon nucleus, we have supposed that the phenomenon takes place in two stages: first there is the capture of the alpha particle and the instantaneous expulsion of the neutron, with the formation of a radioactive atom which is an isotope of phosphorus of atomic weight 30, while the stable phosphorus atom has an atomic weight of 31. Next, this unstable atom, this new radio-element which we have called “radio-phosphorus” decomposes exponentially with a half-life of three minutes.
We have interpreted in the same way the production of radioactive elements in boron and magnesium; in the first an unstable nitrogen with a half-life of II minutes is produced, and in the second, unstable isotopes of silicon and aluminium.