The natural and artificial disintegration of the elements An address by Professor Sir Ernest Rutherford — Background and Themes

THE NATURAL AND ARTIFICIAL DISINTEGRATION OF THE ELEMENTS

Professor Sir ERNEST RUTHERFORD

Kt., D. Sc., LL. D., Ph. D., D. Phys., F. R. S.

ON THE OCCASION OF THE CENTENARY CELEBRATION OF THE FOUNDING OF

THE FRANKLIN INSTITUTE

AND THE INAUGURATION EXERCISES OF THE BARTOL RESEARCH FOUNDATION SEPTEMBER 17, 18, 19, 1924

THE FRANKLIN INSTITUTE

THE NATURAL AND ARTIFICIAL DISINTEGRATION OF THE ELEMENTS

_By_ Professor Sir ERNEST RUTHERFORD, Kt., D. Sc.,

LL. D., Ph. D., D. Phys., F. R. S.

IT is not my intention in this paper to give a detailed account of the natural disintegration of the radio elements or of the methods employed to effect the artificial disintegration of certain light elements. I shall assume that you all have a general knowledge of the results of these investigations, but I shall confine myself to a consideration of the bearing of these results on our knowledge of the structure of the nuclei of atoms.

There is now a general agreement that the atoms of all elements have a similar electrical structure, consisting of a central positively charged nucleus surrounded at a distance by the appropriate number of electrons. From a study of the scattering of _α_ particles by the atoms of matter and from the classical researches of Moseley on X-ray spectra, we know that the resultant positive charge on the nucleus of any atom, in terms of the fundamental unit of electronic charge, is given numerically by the atomic or ordinal number of the element, due allowance being made for missing elements. We know that with few exceptions all nuclear charges, from 1 for the lightest atom, hydrogen, to 92 for the heaviest element, uranium, are represented by elements found in the earth. The nuclear charge of an element controls the number and distribution of the external electrons, so that the properties of an atom are defined by a whole number, representing its nuclear charge, and are only to a minor degree influenced by the mass or atomic weight of the atom.

This minute but massive nucleus is, in a sense, a world of its own which is little, if at all, influenced by the ordinary physical and chemical forces at our command. In many respects, the problem of nuclear structure is much more difficult than the corresponding problem of the arrangement and motions of the planetary electrons, where we have a wealth of available information, both physical and chemical, to test the adequacy of our theories. The facts known about the nucleus are few in number and the methods of attack to throw light on its structure are limited in scope.

It is convenient to distinguish between the properties assigned to the nucleus and the planetary electrons. The movements of the outer electrons are responsible for the X-ray and optical spectra of the elements and their configuration for the ordinary physical and chemical properties of the element. On the other hand, the phenomena of radioactivity and all properties that depend on the mass of the atom are to be definitely assigned to the nucleus. From a study of the radioactive transformations, we know that the nucleus of a heavy atom not only contains positively charged bodies but also negative electrons, so that the nuclear charge is the excess of positive charge over negative. In recent years, the general idea has arisen that there are two definite fundamental units that have to do with the building up of complex nuclei, viz., the light negative electron and the relatively massive hydrogen nucleus which is believed to correspond to the positive electron.

This view has received very strong support from the experiments of Aston on Isotopes in which he has shown that the masses of the various species of atoms are represented nearly by whole numbers in terms of O = 16. From the general electric theory, it is to be anticipated that the mass of the hydrogen nucleus in the nucleus structure will be somewhat less than its value 1.0077 in the free state on account of the very close packing of the charged units in the concentrated nucleus. From Aston's experiments, it appears that the average mass of the hydrogen nucleus, or proton as it is now generally called, is very nearly 1.000 under these conditions. We should anticipate that the whole number rule found by Aston would hold only to a first approximation, since the mass of the proton must be to some extent dependent on the detailed structure of the nucleus. In the case of tin and xenon Aston has already signalized a definite departure from the whole number rule, and no doubt a still more accurate determination of the masses of the atoms will disclose other differences of a similar kind.

While our present evidence indicates that the proton and electron are the fundamental constituents of the nucleus, it is very probable that secondary combining units play a prominent part in nuclear constitution. For example, the expulsion of helium nuclei from the radioactive bodies indicates that the helium nucleus of mass 4 is probably a secondary unit of great importance in atom building. On the views outlined, we should expect the helium nucleus of charge to be built up of four protons and two electrons. The loss of mass in forming this nucleus indicates that a large amount of energy must be liberated during its formation. If this be the case, the helium nucleus must be such a stable structure that the combined energy of four or five of the swiftest _α_ particles would be necessary to effect its disruption. Such a deduction is supported by our failure to observe any evidence of disintegration of the swift particle itself, whether it is used to bombard matter or whether the _α_ particle is used to bombard other helium atoms.

On these views, we should anticipate that the nucleus of radium of atomic number 88 and atomic weight 22.6 contains in all 226 protons of mass 1 and 138 electrons. While this gives us the numerical relation between the two fundamental units, we have, at present, no definite information of their arrangement in the minute nuclear volume, nor of the nature and magnitude of the forces that hold them together. We should anticipate that many of the protons and electrons unite to form secondary units, _e. g._ helium nuclei, and that the detailed structure of the nucleus may be very different from that to be expected if it consists of a conglomeration of free protons and electrons.

It is thus of great importance to obtain definite evidence of the nature and arrangement of the components of the nucleus and of the forces that hold them in equilibrium. We shall now consider some of the lines of evidence which throw light on the actual dimensions of the nucleus and the law of force operative in its neighborhood; the structure and modes of vibration of the nucleus, together with the effects observed when some light nuclei are disintegrated by bombardment with _α_ particles.

DIMENSIONS OF THE NUCLEI AND THE LAW OF FORCE

The conception of the nucleus atom had its origin in 1911 in order to explain the scattering of an _α_ particle through a large angle as the result of a single collision. The observation that the _α_ particle is in some cases deflected through more than a right angle as the result of an encounter with a single atom first brought to light the intense forces that exist close to the nucleus. Geiger and Marsden showed that the number of particles scattered through different angles was in close accord with the simple theory which supposed that, for the distance involved, the _α_ particle and nucleus behaved like charged points, repelling each other according to the law of the inverse square. The accuracy of this law has been independently verified by Chadwick, so that we are now certain that in a region close to the nucleus the ordinary laws of force are valid.

These scattering experiments also gave us the first idea as to the probable dimensions of the nuclei of heavy atoms, for it is to be anticipated that the law of the inverse square must break down if the _α_ particle approaches closely to or actually enters the nuclear structure. This variation in the law of force would show itself by a difference between the observed and calculated numbers of _α_ particles scattered through large angles. Geiger and Marsden, however, observed no certain variation even when the _α_ particles of range about 4 cms. were scattered through 100° by a gold nucleus. In such an encounter, the closest distance of approach of the _α_ particle to the center of the nucleus is about 5 x 10^-12 cm., so that it would appear that the radius of the gold nucleus, assumed spherical, could not be much greater than this value.

There is another argument, based on radioactive data, which gives a similar value for the dimensions of the radius of a heavy atom. The _α_ particle escaping from the nucleus increases in energy as it passes through the repulsive field of the nucleus. To fix a minimum limit, suppose the _α_ particle from uranium, which is the slowest of all _α_ particles expelled from a nucleus, gains all its energy from the electrostatic field. It can be calculated on these data that the radius of the uranium nucleus cannot be less than 6 x 10^-12 cm. This is based on the assumption that the forces outside the nucleus are repulsive and purely electrostatic. If, as seems not unlikely, there also exist close to the nucleus strong attractive forces, varying more rapidly than an inverse square law, the actual dimensions may be less than the value calculated above.

At this stage of our knowledge it is of great importance to test whether the law of force breaks down for the distance of closest approach of an _α_ particle to a nucleus. This can be done by comparing the observed with the calculated number of _α_ particles scattered through angles of nearly 180°. It seems almost certain that the inverse square law must break down when swift _α_ particles are used. This can be seen from the following argument. If an _α_ particle, of the same speed as that ejected during the transformation of uranium, is fired directly at the uranium nucleus, _it must penetrate into the nuclear structure_. If a still swifter _α_ particle is used, _e. g._ that from radium C, which has about twice the energy of the uranium _α_ particle, it is clear that it must penetrate still more deeply into the nuclear structure. This is based on the assumption that the field due to a nucleus is approximately symmetrical in all directions. If this is not true, it may happen that only a fraction of the head-on collisions may be effective in penetrating the nucleus. It is hoped soon to attack this difficult problem experimentally.

We have so far dealt with collisions of an _α_ particle with a heavy atom. We know, however, from the results of Rutherford, Chadwick and Bieler that in a collision of an _α_ particle with the lightest atom, hydrogen, the law of the inverse square breaks down entirely when swift particles are used. Not only are the numbers of H nuclei set in swift motion much greater than is to be expected in the simple-point nucleus theory, but the change of number with the velocity of the _α_ particle varies in the opposite way from the simple theory. Such wide departures between theory and experiment are only explicable if we assume either that the nuclei have sensible dimensions or that the inverse square law of repulsion entirely breaks down in such close collisions. If we suppose the complexity in structure and in laws of force is to be ascribed to the _α_ particle rather than to the hydrogen nucleus, Chadwick and Bieler, as the result of a careful series of experiments, concluded that the _α_ particle behaved as if it were a perfectly elastic body, spheroidal in shape with its minor axis 4 x 10^-13 cm. in the direction of motion and major axis 8 x 10^-13 cm. Outside this spheroidal region the forces fell off according to the ordinary inverse square law, but inside this region the forces increased so rapidly that a particle was reflected from it as from a perfectly elastic body. No doubt such a conception is somewhat artificial, but it does serve to bring out the essential points involved in the collision, viz., that when the nuclei approach within a certain critical distance of each other, forces come into play which vary more rapidly than the inverse square. It is difficult to ascribe this break-down of the law of force merely to the finite size or complexity of the nuclear structure or to its distortion, but the results rather point to the presence of new and unexpected forces which come into play at such small distances. This view has been confirmed by some recent experiments of Bieler in the Cavendish Laboratory in which he has made, by scattering methods, a detailed examination of the law of force in the neighborhood of a light nucleus like that of aluminum. For this purpose he compared the relative number of _α_ particles scattered within the same angular limit from aluminum and from gold. For the range of angles employed, viz., up to 100°, it is assumed that the scattering of gold follows the inverse square law. He found that the ratio of the scattering in aluminum compared with that in gold depended on the velocity of the _α_ particle. For example, for an _α_ particle of 3.4 cms. range, the theoretical ratio was obtained for angles of deflection below 40° but was about 7 per cent lower for an average angle of deflection of 80°. On the other hand, for swifter particles of range 6.6 cms. a departure from the theoretical ratio was much more marked and amounted to 29 per cent for an angle of 80°. In order to account for these results he supposes that close to the aluminum nucleus an attractive force is superimposed on the ordinary repulsive forces. The results agreed best with the assumption that the attractive force varies according to the inverse fourth power of the distance and that the forces of attraction and repulsion balanced at about 3.4 x 10^-13 cm. from the nuclear center. Inside this critical radius the forces are entirely attractive; outside they are repulsive.

While we need not lay too much stress on the accuracy of the actual value obtained or of the law of attractive force, we shall probably not be far in error in supposing the radius of the aluminum nucleus is not greater than 4 x 10^-13 cm. It is of interest to note that the forces between an _α_ particle and a hydrogen nucleus were found to vary rapidly at about the same distance.

It thus seems clear that the dimensions of the nuclei of light atoms are small, and almost unexpectedly small in the case of aluminum when we remember that 27 protons and 14 electrons are concentrated in such a minute region. The view that the forces between nuclei change from repulsion to attraction when they are very close together seems very probable, for otherwise it is exceedingly difficult to understand why a heavy nucleus with a large excess of positive charge can hold together in such a confined region. We shall see that the evidence from various other directions supports such a conception, but it is very unlikely that the attractive forces close to a complex nucleus can be expressed by any simple power law.

A study of the long series of transformations which occur in uranium and thorium provides us with a wealth of information on the modes of disintegration of atoms, but unfortunately our theories of nuclear structure are not sufficiently advanced to interpret these data with any detail. The expulsion of high speed _α_ and _β_ particles from the radioactive nucleus gives us some idea of the powerful forces resident in the nucleus, for it can be estimated that the energy of emission of the _α_ particle is in some cases greater than the energy that would be acquired if the _α_ particle fell freely between two points differing in potential by about 4 million volts. The energies of the _β_ and _γ_ rays are on a similar scale of magnitude.

Notwithstanding our detailed knowledge of the successive transformation of the radio-elements, we have not so far been able to obtain any definite idea of their nuclear structure, while the cause of the disintegration is still a complete enigma. In comparing the uranium, thorium, and actinium series of transformations, one cannot fail to be struck by the many points of similarity in their modes of disintegration. Not only are the radiations similar in type and in energy, but, in all cases, the end product is believed to be an isotope of lead. This remarkable similarity in the modes of transformation is especially exemplified in the case of the "C" bodies, each of which is known to break up in at least two distinct ways, giving rise to branch products. For example, thorium C emits two types of _α_ rays, 65 percent of range 8.6 cms. and 35 per cent of range 4.8 cms., and in addition some _β_ rays.

In order to explain these results, it has been suggested that a fraction of the atoms of thorium C break up first with the expulsion of an _α_ particle and the resulting product then emits a _β_ particle. The other fraction breaks up in a reverse way, first expelling a _β_ particle, while the subsequent product emits an _α_ particle. Similar dual changes occur in radium C and actinium C, although the relative number of atoms in each branch varies widely for the different elements.

This remarkable similarity between the "C" bodies is still further emphasized by the recent discovery of Bates and Rogers that both radium C and thorium C give rise in small numbers to other groups of _α_ particles, some of them moving at very high speeds.