NATURAL AND ARTIFICIAL DECOMPOSITION OF ELEMENTS1

Sir Ernest Rutherford.

In this article I shall not dwell in detail either on the natural transformations of radioelements or on the methods by which the artificial decomposition of certain light elements is accomplished. I believe that all of you are familiar with the general results of these investigations2, and therefore I shall confine myself to considering the role of these results in the knowledge of the structure of the atomic nucleus.

At the present time it is generally accepted that the atoms of all elements have an analogous structure and consist of a central positively charged nucleus, surrounded at a certain distance by a corresponding number of electrons. From the study of the scattering of $\alpha$-particles by matter and from Moseley’s classical investigations of X-ray spectra, we know that the total positive charge of the nucleus, expressed in charges equal in absolute magnitude to the electronic charge, is numerically equal to the atomic, or ordinal, number of the element, i.e. to the number of the place occupied by the element in the periodic system. We know that, with a few exceptions, all nuclear charges from 1—corresponding to the lightest atom, hydrogen—to 92—corresponding to the heaviest element, uranium—are represented by elements occurring on the earth. The charge of the nucleus of an element determines the number and distribution of the external electrons, so that the properties of the atom depend chiefly on a certain whole number, its nuclear charge, and only to an insignificant degree—on the atomic weight of this atom.

This nucleus, negligible in size but massive, is a world enclosed within itself, and if the ordinary physical and chemical forces, locat—

...available to us and can exert any influence upon it, then this influence is negligible. The problem of the structure of the atomic nucleus is in many respects considerably more difficult than the corresponding problem of the distribution and motion of the planetary electrons. In the latter case we possess an enormous number of facts which make it possible to check the correctness of our theories, whereas the number of facts known concerning the nucleus is insignificant, and the methods permitting the study of its structure are limited.

Among the properties of the elements it is convenient to distinguish properties dependent on the nucleus from properties determined by the planetary electrons. The motion of the outer electrons determines the X-ray and optical spectra of the elements, and the configuration of these electrons—the ordinary physical and chemical properties of the elements. On the other hand, the phenomena of radioactivity and all properties dependent on the mass of the atom must definitely be attributed to the nucleus. The study of radioactive transformations has shown us that the nucleus of heavy atoms contains not only positively charged bodies, but also negative electrons, so that the charge of the nucleus is the excess of positive charge over negative. In recent years the view has become strengthened that there exist in general two basic structural units which take part in the construction of complex nuclei: these are the light negative electron and the relatively massive hydrogen nucleus, which, it is believed, is a positive electron.

This view received very strong support in Aston’s experiments on isotopes, which showed that the masses of different types of atoms are expressed by numbers very close to integers, if \(O = 16\) is assumed. Proceeding from the general electrical theory of the structure of matter, it could be predicted that, owing to the very close grouping of charged units in the nucleus, the mass of a hydrogen nucleus entering into the structure of some other nucleus would be somewhat less than the value it has in the free state \((1.0077)\). From Aston’s experiments it appears that, under these conditions, the average mass of the hydrogen nucleus, or, as it is now called, the proton, is very close to \(1.000\). We may foresee that the “whole-number rule” found by Aston will be valid only as a first approximation, since the mass of the proton may depend to some extent on the peculiarities of the structure of the nucleus. In the case of tin and xenon Aston has already established a definite departure from the rule of whole numbers, and there can be no doubt that a considerably more precise determination of masses will reveal a whole series of similar departures.

While our present knowledge definitely indicates that the proton and the electron are the principal components of the nucleus, it seems highly probable that secondary, more complex structural...

units. Thus, for example, the emission of a helium nucleus by radioactive bodies indicates that a helium nucleus, with mass equal to four, is probably a secondary unit of atomic structure of the highest importance. From the point of view just outlined, we must expect that the helium nucleus with charge \(+2e\) is built up of four protons and two electrons. The loss of mass that occurs in the formation of this nucleus indicates that a large amount of energy must have been released in the process. If this is so, then the helium nucleus must possess so strong a structure that the combined energy of four or five of the fastest \(\alpha\)-particles would be required to bring about its destruction. This conclusion is supported by our failure to observe any signs of disintegration even by the fastest \(\alpha\)-particles, regardless of whether they were used to bombard matter or whether the helium atoms themselves were bombarded by \(\alpha\)-particles.

From this point of view we must conclude that the nucleus of radium, with atomic number 88 and atomic weight 226, contains in all 226 protons of mass 1 and 138 electrons. Thus we have an idea of the numerical ratio of the two principal structural units. At present, however, we still have no definite information either about the arrangement of these units in the atom’s nucleus, which is negligible in size, or about the nature and magnitude of the forces that hold them together. We must expect that some of the protons and electrons combine to form secondary units, for example helium nuclei, and that the details of the structure of the nucleus may differ greatly from what would be expected if the nucleus were a simple conglomerate of free protons and electrons.

It is therefore of the highest importance to obtain definite information about the nature and arrangement of the components of the nucleus and about the forces that hold them in equilibrium. We shall now consider some paths leading to knowledge of the true dimensions of the nucleus and of the law of the forces acting in its immediate vicinity; alongside this we shall dwell on the study of the structure and character of the oscillations of the nucleus, as well as of those phenomena that are observed in the disintegration of certain nuclei by bombardment with \(\alpha\)-particles.

Size of the Nucleus and Law of Force.

The idea of the nuclear structure of the atom arose in 1911 and was intended to explain the scattering of \(\alpha\)-particles through large angles as the result of single collisions. The fact that \(\alpha\)-particles in some cases are deflected through angles greater than a right angle as the result of a collision with only one atom first showed what intense forces act in the immediate vicinity of the nucleus. Geiger and

Marsden showed that the number of particles scattered through various angles agrees very well with a simple theory based on the assumption that, for the distances under consideration, the $\alpha$-particle and the nucleus behave as charged points repelling one another according to the law of inverse proportionality to the square of the distance. The strict applicability of this law has, moreover, recently been confirmed by Chadwick, so that we may now definitely assert that, in the region immediately surrounding the nucleus, the ordinary law of force holds.

These scattering experiments gave the first idea of the probable dimensions of the nuclei of heavy atoms, for it was to be expected that the law of inverse proportionality to the squares of the distances should break down when an $\alpha$-particle comes very close to, or even partly penetrates into, the structure of the nucleus. A change in the law of force, in turn, should have shown itself in a discrepancy between the calculated and the observed number of particles scattered through large angles. Meanwhile Geiger and Marsden observed no such discrepancies even in cases where $\alpha$-particles with a range of about $4$ cm were scattered by the nucleus of gold through $100^\circ$. In such collisions the distance of closest approach of the $\alpha$-particle to the center of the nucleus is approximately $5 \cdot 10^{-12}$ cm, whence it follows that the radius of the gold nucleus, if it is regarded as spherical, cannot substantially exceed this value.

There is still another method of approximate calculation. This method rests on radioactive data and leads to a value for the radius of the nucleus of a heavy atom close to the preceding one. An alpha-particle, on leaving the nucleus, increases its energy by traversing the repulsive field of the nucleus. To establish a lower limit, let us suppose that the $\alpha$-particle of uranium obtains all its energy at the expense of the electrostatic field. On the basis of these data one can calculate that the radius of the uranium nucleus cannot be less than $6 \cdot 10^{-12}$ cm. This calculation is based on the assumption that the forces outside the nucleus are repulsive and purely electrostatic. If—as is by no means implausible—there also exist near the nucleus intense attractive forces, varying more rapidly than inversely as the square of the distance, then the true dimensions of the nucleus may be smaller than the value calculated above.

In the present state of our knowledge it is highly important to verify whether the simple law of force is in fact violated at the closest approaches of an $\alpha$-particle to the nucleus. Such a verification can be carried out by comparing the observed and calculated number of $\alpha$-particles scattered through angles close to $180^\circ$. It seems beyond doubt that the law of inverse proportionality to the square of the distance must break down if experiments are performed with fast $\alpha$-particles. This can be seen from the following simple argument.

If an \(\alpha\)-particle with the same velocity as that with which the \(\alpha\)-particles of uranium are emitted is directed straight at the uranium nucleus, then it must penetrate into the interior of the nucleus itself.

If one takes a considerably faster \(\alpha\)-particle—for example, an \(\alpha\)-particle of radium C, whose energy is approximately twice the energy of the uranium \(\alpha\)-particle—then it is clear that it must penetrate much farther into the interior of the nucleus. This conclusion is based on the assumption that the field of the nucleus is approximately symmetric in all directions. If the latter is not true, then it may happen that only some of the central impacts will lead to penetration into the depth of the nucleus. We hope in the near future to approach this difficult problem experimentally.

So far we have been dealing with collisions of an \(\alpha\)-particle with heavy atoms. We know, however, from the experiments of Rutherford, Chadwick, and Bieler, that in the collision of an \(\alpha\)-particle with the lightest atom—with the hydrogen atom—the inverse-square law is completely violated if the experiments are performed with fast \(\alpha\)-particles. Not only is the number of H-nuclei set into rapid motion much greater than could have been expected according to the simple theory of point nuclei, but the change in the number of H-nuclei as a function of the velocity of the \(\alpha\)-particles occurs in a direction opposite to that predicted by the simple theory. Such a considerable discrepancy between theory and experiment can be explained only by assuming either that nuclei have appreciable dimensions, or that the inverse-square law of variation of the repulsion is completely violated in such close collisions. Let us suppose that the complexity of the structure and of the law of action of the force is to be ascribed to the \(\alpha\)-particle, and not to the hydrogen nucleus. Chadwick and Bieler, as the result of a series of careful experiments, concluded that in such a case the \(\alpha\)-particle must behave like a perfectly elastic body of spheroidal form, with a minor axis of \(4 \cdot 10^{-13}\) cm directed along the motion, and a major axis of \(8 \cdot 10^{-13}\) cm. Outside this region the forces vary according to the usual law—in inverse proportion to the square of the distance; inside it the forces increase so rapidly that the particle is repelled as from a perfectly elastic body.

Of course, such a conception is somewhat artificial; nevertheless it conveys in its essential features the picture of the collision, and in particular the fact that, when nuclei approach one another closer than a certain limiting distance, forces develop which vary much more rapidly than in inverse proportion to the square of the distance. It is difficult to ascribe this violation of the law of force action solely to the finite dimensions or the complexity of the structure of the nucleus, or to its distortion; the experiments point rather to the appearance of new and unexpected forces which develop at such small distances. This conception is confirmed by some new experiments of Bieler, performed

in the Cavendish Laboratory. Bieler, using the scattering method, made a detailed study of the law of the action of the force near a light nucleus, namely near the nucleus of aluminum. For this purpose he compared the relative number of $\alpha$-particles scattered within one and the same solid angle by aluminum and by gold. For the interval of angles investigated (up to $100^\circ$) it was assumed that scattering by gold follows the law of inverse proportionality to the square of the distance. Bieler found that the ratio of scattering in aluminum to scattering in gold depends on the velocity of the $\alpha$-particle. Thus, for example, for an $\alpha$-particle with a range of $3.4\ \mathrm{cm}$ the theoretical ratio was obtained for angles less than $40^\circ$, but it turned out that the ratio for the mean scattering angle of $80^\circ$ was only $7\%$ smaller. On the other hand, for faster $\alpha$-particles with a range of $6.6\ \mathrm{cm}$, the deviations from the theoretical ratio are expressed much more sharply and reach $29\%$ for an angle of $80^\circ$. To explain these results, Bieler assumed that near the aluminum nucleus an attractive force is superposed on the ordinary repulsive force. The results agree well with the assumption that the attractive force varies inversely as the fourth power of the distance, and that the forces of repulsion and attraction balance at a distance of $3.4 \cdot 10^{-13}\ \mathrm{cm}$ from the center of the nucleus. Within this critical radius the forces become exclusively attractive; outside it—exclusively repulsive.

Although we cannot make any special claims for the accuracy of the figure obtained or for the strictness of the proposed law of the attractive force, we shall probably not be greatly mistaken if we suppose that the radius of the aluminum nucleus does not exceed $4 \cdot 10^{-13}\ \mathrm{cm}$. It is interesting to note that the forces of interaction between an $\alpha$-particle and the hydrogen nucleus undergo a rapid change beginning at approximately this same distance.

Thus it is clear that the dimensions of the nucleus in the light elements are small, and in the case of aluminum one may even say—unexpectedly small, if we recall that this negligible volume contains 27 protons and 14 electrons. The supposition that the forces of interaction between nuclei change from repulsion to attraction when they approach very closely appears quite plausible; otherwise it is extremely difficult to imagine how a heavy nucleus with a large excess of positive charge could be held within a limited space. We shall see that a whole series of other facts supports this view; however, it is unlikely that the attractive forces near a complex nucleus could be expressed by any simple power law.

Facts from the Field of Radioactivity

The study of the long series of transformations to which uranium and thorium are subject gives us an enormous amount of information about the modes of disintegration of atoms; unfortunately, our theories of the structure of the nucleus are still insuffi—

precisely developed to interpret these facts in any detail. The emission of high-velocity $\alpha$- and $\beta$-particles gives some idea of the powerful forces at work in the nucleus, for the energy of emission of an $\alpha$-particle in some cases exceeds that which this particle would acquire by freely traversing a potential difference of about four million volts. The energies of $\beta$- and $\gamma$-rays are of the same order of magnitude.

Although we have studied in detail the successive transformations of the radio-elements, we are still quite unable to draw a definite picture of the structure of the nucleus of radioactive substances, and the causes of their decay remain mysterious to us. Comparing the transformation series of uranium, thorium, and actinium, one cannot fail to be struck by the similarity of these transformations. In all cases not only are the radiations analogous in their character and energy, but the final products of decay everywhere are isotopes of lead. This remarkable analogy of the transformations is especially sharply expressed in the case of the “$C$-products,” each of which decays by at least two different paths, giving rise to a branching of the series. For example, thorium $C$ emits two types of $\alpha$-rays—65% with a range of 8.6 cm and 35% with a range of 4.8 cm—and, in addition, also $\beta$-rays.

To explain this fact it was suggested that part of the atoms of thorium $C$ decay, first emitting an $\alpha$-particle, and the product thereby obtained then emits a $\beta$-particle. The other part decays by the reverse path, first emitting a $\beta$-particle and only then an $\alpha$-particle. An analogous double transformation occurs in radium $C$ and actinium $C$, although the relative number of atoms in each branch varies sharply for the different elements.

This remarkable analogy among the $C$-products has been still further emphasized by the recent discovery of Bates and Rogers, who showed that both radium $C$ and thorium $C$ give—besides those already noted—additional groups of $\alpha$-particles which move with very considerable velocities.

It has often been noted that the radioactive properties of the $C$-products depend rather on the atomic number, i.e., on the nuclear charge, than on the atomic weight. Let us focus our attention on radium $C$ and thorium $C$, which are best known. Both substances have nuclear charge 83, but the atomic mass of radium $C$ is 214, and that of thorium $C$ is 212. Thus the nucleus of radium $C$ contains two protons and two electrons more than thorium $C$. If one were to suppose that the nuclei of these elements consist of a large number of charged units in ceaseless and disorderly motion, then one would have to expect that the addition of protons and electrons to a complex structure should completely change its constitution and, consequently, also the stability and the character of the transformation. In actual fact, in sharp

contradiction with this assumption, we find that both nuclei are transformed in a remarkably analogous manner. We can, however, give some explanation of such an anomaly if we suppose that the \(\alpha\)- and \(\beta\)-particles which are emitted from these elements do not enter deeply into the structure of the nucleus, but exist as satellites of a certain “core” common to both elements. If these satellites are in motion, then they may be maintained in equilibrium by attractive forces emanating from the “core,” and these forces must be the same in both cases. From this point of view, the manifestations of radioactivity should be attributed not to the core of the nucleus, but to the distribution of the satellites, which may differ somewhat in the two elements, while nevertheless displaying, in general, a significant similarity. Of course, such a theory is highly speculative. Yet it may serve as a useful working hypothesis, not only making it possible to understand the analogy in the character of the transformation of our two elements, but also directly providing a probable explanation of the emission, by one and the same element, of \(\alpha\)-particles of different velocities. There are two ways of approaching this question. We may, first of all, suppose that in disintegration a certain amount of excess energy is released, and that this energy can be imparted to any of the satellites. There is a definite probability that each given particle will receive this energy, and on this depends the number of particles in the various groups of \(\alpha\)-rays. The energy of an \(\alpha\)-particle, in the final analysis, depends on its position in the field of forces surrounding the “core” of the nucleus at the moment of emission of this \(\alpha\)-particle. On the other hand, we may suppose that one and the same \(\alpha\)-particle is always emitted, but that this particle can occupy in the atom a series of “stationary” positions, analogous to the “stationary states” of the electron in Bohr’s theory. This leads to the supposition that atoms are not identical with respect to their satellites, but that there exists a series of “excited” states of the atom, as a consequence of transformations that have previously occurred. The theory of “satellites” is useful in yet another respect. It is possible that high-frequency \(\gamma\)-rays arise not as a consequence of the motion of electrons, as is usually assumed, but as a consequence of the transfer of \(\alpha\)-particles from one energy level to another. In that case the difference in the energies of the various groups of \(\alpha\)-particles of radium C and thorium C must be related, by the quantum relation, to the frequency of the \(\gamma\)-rays. The evidence available at present is not sufficiently definite to allow this problem to be finally resolved; in any case, very precise measurements of the energies of the various groups of \(\alpha\)-particles are necessary. Owing to the relatively small number of particles in some groups, such measurements are difficult to carry out.

In discussing the theory of satellites in connection with radioactive substances, it is at first sight natural to make the following assumption: since the final products of the uranium and thorium series are

lead isotopes, then one of these isotopes might form the “core” of the nucleus. It is, however, quite possible that the radioactive process comes to an end while a certain number of remaining satellites are still present. If this is so, then the “core” may have a smaller nuclear charge and mass than lead. From certain considerations set forth below, it is evident that this core may correspond to an element close to platinum, with atomic number 77 and mass 192.

FREQUENCY OF VIBRATION OF THE NUCLEUS.

One of the most interesting and important methods for gaining knowledge of the structure of the nucleus is the study of the highly penetrating γ-rays emitted by certain radioactive substances. Gamma rays are identical in their nature with X-rays, but possess a much greater penetrating power and consist of waves of considerably higher frequency than those which can be obtained in an ordinary X-ray tube. The work of the last few years indicates quite clearly that the greater part of the γ-rays of such substances as radium \(B\) and \(C\) arises in the nucleus. Thus the determination of the frequencies of γ-rays gives us direct information about the character of the vibration of the components of the structure of the nucleus. The frequency of some of the softer γ-rays excited by radium \(B\) and \(C\) was measured directly by Rutherford and Andrade by the method of reflection from a crystal; it is difficult, however, if not altogether impossible, to determine by this method the frequencies of highly penetrating rays. Fortunately, for this purpose a new and powerful method was developed, chiefly through the work of Ellis and Mrs. Meitner (Lise Meitner). It is well known that the rays of radium \(B\) and radium \(C\) in a magnetic field give a whole spectrum, revealing the presence of a number of groups of β-rays, each of which is emitted with a definite velocity. It is clear that each of these groups of β-rays arises when the energy of a γ-ray of definite frequency is transformed into a β-ray at one or another of the electronic levels of the outer part of the atom. The energy \(\omega\), required in order to transfer an electron from one of these levels beyond the limits of the atom, is known from the study of X-ray absorption spectra. Thus the frequency of the γ-ray is determined by the quantum relation

\[ h\nu = E + \omega, \]

where \(E\) is the measured energy of the β-particle.

Since each ray may undergo transformation at any of the known energy levels of the outer part of the atom, a single γ-ray may give rise to the appearance of a known number of groups of β-rays, corresponding to transformation at the \(K, L, M\), etc., levels. Thus analysis of the β-ray spectrum enables us to fix the frequencies of the more intense rays emitted by the nucleus. The energy of the most

short waves, measured in this way by Ellis, corresponds to more than two million volts, and a number of other facts indicate that, probably, much shorter waves exist, emitted in appreciable quantity by radium C.

Artificial Disintegration of Elements

As we have seen, it is assumed that the nuclei of all atoms are built of protons and electrons, and the numbers of the one and the other can be inferred from the mass of the nucleus and its charge. At first sight it seems unexpected that the study of the transformations of radioactive elements does not make it possible to detect the individual existence of protons. Our observations show that in the long series of transformations of uranium, thorium, and actinium only electrons and helium nuclei are emitted, but not protons. One of the most obvious methods of studying the structure of the nucleus consists in finding a way to decompose it into its components. Such decomposition is carried out for us continually by nature itself in the case of the heavy radioactive elements, but similar spontaneous decomposition is unknown for ordinary, lighter elements. Since the fast \(\alpha\)-particles of radioactive substances are the most powerful of all projectiles known to us, it immediately appears quite possible that the nucleus of a light atom may be destroyed as a result of a close collision with an \(\alpha\)-particle. Owing, however, to the exceedingly small dimensions of the nucleus, one must expect that the probability of a central collision will be very small and that, consequently, if disintegration is observed at all, it will be on an extremely small scale. During the last few years Chadwick and I have definitely shown that, by bombardment with \(\alpha\)-particles, hydrogen nuclei can be torn out of the elements boron, nitrogen, fluorine, sodium, aluminium, and phosphorus. In these experiments the presence of H-nuclei was detected by the scintillation method, and their maximum velocity of ejection was estimated from the thickness of the layer of matter through which these particles can pass. The number of H-nuclei ejected, even in the most favorable case, is very small in comparison with the number of bombarding \(\alpha\)-particles: approximately one H-nucleus for a million \(\alpha\)-particles.

In these experiments the material subjected to bombardment was placed directly in front of the source of \(\alpha\)-particles, and observation of the ejected particles was made on a zinc-sulfide screen placed in a straight line at a distance of several centimeters. When radium C served as the source of \(\alpha\)-particles, the ranges of the H-nuclei liberated from the elements, expressed in cm of air, were in all these cases greater than the range of free nuclei (30 cm in air) set in motion by \(\alpha\)-particles in hydrogen. If an absorbing screen equivalent to 30 cm of air is placed in front of the zinc-sulfide screen,

air, then the results will not depend at all on the presence of free or bound hydrogen contaminating the bombarded material. Some light elements were investigated at absorptions smaller than this; however, the number of H-particles caused by hydrogen contamination of the source and of the material under test was so large that these results deserve no confidence.

In these experiments one can observe a large number of scintillations, but it is very difficult to decide whether in fact they can in part be attributed to the disintegration of the substance under investigation. The presence of particles with a long range, of the type of the \(\alpha\)-particles of radium C, complicates the question still further, since the number of such particles is, generally speaking, large in comparison with the ordinarily observed disintegration effect.

In order to avoid these difficulties, Chadwick and I developed a simple method which makes it possible with certainty to observe the disintegration of an element when the particles ejected in the process have a range of only \(7\) cm of air. This method is based on the assumption—confirmed by our earlier experiments—that the particles, the products of disintegration, are emitted in all directions relative to the incident rays. A powerful beam of \(\alpha\)-rays fell on the substance under investigation, and the liberated particles were observed on the average at an angle of \(90^\circ\) to the direction of the incident \(\alpha\)-particles. By means of screens the experiment can be arranged so that the \(\alpha\)-particles do not fall at all on the zinc-sulphide screen.

This method has many advantages. We can now discover particles with a range of \(7\) cm with the same certainty as particles with a range of \(30\) cm in our earlier experiments, since the presence of hydrogen in the bombarded substance had no effect. This latter circumstance could be shown by bombarding a paraffin screen, whereupon no particles were observed on the zinc-sulphide screen. Since the number of H-nuclei or \(\alpha\)-particles is very greatly diminished on scattering through \(90^\circ\), the results did not depend at all on the presence of H-nuclei from the source or of \(\alpha\)-particles of great range. The latter could still be detected with our arrangement of the experiment when the scattering substance was a heavy element such as gold, but they were quite imperceptible with light elements. A slight change in the arrangement allowed us to investigate gases as well as solids.

Working in this way, we found that, in addition to the elements boron, nitrogen, fluorine, sodium, aluminium, and phosphorus, which give particles with a maximum range in the forward direction between \(40\) and \(90\) cm, the following elements give particles with a range greater than \(7\) cm: neon, magnesium, silicon, sulphur, chlorine, argon, and potassium. The number of particles emitted by these elements is small in comparison with that given by aluminium, and varies between \(\frac{1}{3}\) and \(\frac{1}{20}\) of the latter. The ranges of the particles were not deter-

measured with accuracy. Neon, evidently, gives particles with the smallest range—about 16 cm under our conditions; the ranges of particles from other elements lie between 18 and 30 cm. Thanks to the kindness of Dr. Rosenhain, we were able to carry out experiments with metallic beryllium. The latter gave a weak effect: about \(\frac{1}{30}\) of the effect given by aluminum; however, we are not certain that this effect, too, is not caused by fluorine present as an impurity. The remaining elements—hydrogen, helium, lithium, carbon, and oxygen—gave no noticeable effect beyond 7 cm. It is interesting to note that, whereas carbon and oxygen gave no effect whatever, sulfur, i.e., in all probability a “pure” element with atomic mass \(4n\), gives an effect amounting to approximately \(\frac{1}{3}\) of the effect in aluminum. This clearly shows that the sulfur nucleus is not built up solely of helium nuclei, as might have been concluded from the atomic weight of sulfur, 32.07.

We made a preliminary test of the elements from calcium to iron, but without definite results, owing to the difficulty of obtaining these elements free from “active” elements, especially from nitrogen. For example, whereas electrolytic iron gave no particles beyond 7 cm, Swedish iron gave a considerable effect which, without doubt, was caused by the presence of nitrogen, for after prolonged heating in vacuum the greater part of the effect disappeared. Similar results were obtained with other elements in this region.

We observed no effect at all in the following elements: nickel, copper, zinc, selenium, krypton, molybdenum, palladium, silver, tin, xenon, gold, and uranium. Krypton and xenon were kindly supplied to us by F. W. Aston.

Investigation of light elements for particles with a range of less than 3 cm in air

A simple theory shows that when \(\alpha\)-particles are scattered by light elements, the velocity of the scattered particles depends on the angle of scattering. For example, when the bombarding \(\alpha\)-particles have a range of 7 cm, the range of \(\alpha\)-particles scattered through an angle greater than \(90^\circ\) cannot exceed: for lithium (7)—1.0 cm, for beryllium (10)—2.0 cm, for carbon—2.5 cm, for oxygen—3.2 cm, for aluminum—4.3 cm, and for gold—6.8 cm.

If we introduce an absorbing screen of sufficient thickness so as to stop particles scattered through \(90^\circ\), then we can investigate the disintegration products at ranges, for carbon, for example, exceeding 2.5 cm. In these experiments there arise certain special—

…difficulties which are absent when the thickness of the absorber exceeds 7 cm: any heavy element present in the substance under investigation in the form of an impurity may give scattered \(\alpha\)-particles with a range greater than those scattered by carbon and thus complicate the observations. Moreover, serious complications may be introduced by the “distillation” or volatilization of the active substance of the source. This is especially noticeable if the vessel containing the radioactive source is evacuated. To avoid these difficulties, we found it desirable to cover the source with a thin layer of celluloid having an absorbing power for \(\alpha\)-rays equivalent to 2–3 mm. In this way we were able to avoid serious contamination and to investigate the light elements by this method. We did not find any appreciable number of particles either from lithium or from carbon—for ranges greater than 3 cm. As for carbon, if it does in general exhibit any effect, then the number of particles must in any case be less than one-tenth the number of particles from aluminium under the same conditions. This is in complete contradiction to the results of the work of Kirsch and Pettersson, who found a large number of particles from carbon with a range of 6 cm. They also observed a weak effect in beryllium, in agreement with our experiments. No effect was detected in gaseous oxygen. Beginning with beryllium, no effect was detected in elements lighter than boron.

Under the conditions of our experiments it was clear that neither H-particles nor other particles with a range greater than 3 cm are liberated in appreciable numbers from these elements in a direction perpendicular to the flight of the bombarding \(\alpha\)-particles. This is a very discouraging result, for if only these elements are not structures of the highest degree of strength, it would have been expected that the bombarding \(\alpha\)-particles should break them up into their constituent components.

We hope to investigate this question much more thoroughly, for for the theory of the structure of the nucleus it is of the highest importance to know definitely whether light elements are disintegrated by fast \(\alpha\)-particles or not.

In discussing the results of our new and old observations, certain points of the highest interest become apparent. First of all, all the elements from fluorine to potassium inclusive are destroyed under the influence of bombardment by \(\alpha\)-rays. So far as our observations permit one to judge, there can scarcely be any doubt that the particles ejected from these elements are H-nuclei. The elements with odd atomic numbers B, N, F, Na, Al, P give particles with a large range, varying from 40 to 90 cm in the forward direction; the elements of even atomic number C, O, Ne, Mg, Si, S either give no particles at all, or give them in negligible quantity, as C and O do, or, finally, give particles of a much smaller range than the corresponding elements of odd numbers. The difference between the rang…

parts of particles from even and odd elements is expressed less sharply for elements heavier than phosphorus.

This obvious difference in the rate of ejection of \(H\)-nuclei from elements of even and odd ordinal numbers is of a high degree of interest. This distinction may be placed in parallel with other observations from an entirely different field. Harkins showed that the earth’s crust is far richer in elements of even atomic numbers than in elements of odd atomic numbers.

Aston, in studying isotopes, showed that odd elements usually have two isotopes differing in mass by two units, whereas even elements in most cases have a larger number of isotopes. This remarkable distinction between elements of even and odd numbers cannot be a mere curiosity, but at present we can only speculate as to its causes.

Velocity of Ejection of Hydrogen Nuclei.

As we have already seen, Bieler’s experiments on the scattering of \(\alpha\)-rays in aluminium and magnesium indicate that very large attractive forces act in the immediate vicinity of the nucleus. If this is in fact so, then the forces of attraction and repulsion must balance at some distance from the nucleus. Beyond this critical distance the forces acting on a positively charged body are purely repulsive. From this general conception of nuclear forces there follow certain important consequences. Suppose, for example, that as a result of a collision with a fast \(\alpha\)-particle, a hydrogen nucleus is liberated from the nucleus of some atom. Having passed through the critical sphere, it acquires energy under the action of the repulsive field. From this point of view it is clear that the energy of the charged particle after liberation from the atom cannot be less than the energy acquired in the repulsive field. Therefore we must expect that there exists a minimum velocity for the liberation of particles from the disintegrating nucleus, and the existence of such a velocity can be detected experimentally. We have obtained definite evidence for the existence of such an effect in aluminium and in sulphur, by studying the absorption of \(H\)-nuclei liberated from these elements. The number of scintillations for a thin film was found to be almost constant for absorptions between 7 and 12 cm, but it fell rapidly for greater thicknesses. This is precisely what could be expected from the viewpoint outlined. There is no doubt that this limiting velocity varies for different elements; however, a large number of experiments is required in order to fix this limit accurately. On the basis of these results one can make a rough estimate of the potential of the field at the critical surface, and such an estimate gives

about three million volts for aluminium. For sulfur a somewhat larger number is obtained.

These results show with striking clarity how small the nucleus is, for it may be calculated that the critical surface cannot be farther than \(6 \cdot 10^{-13}\) cm from the center of the nucleus. This conclusion regarding the critical distance is in excellent agreement with the results obtained by Bieler from observations on the scattering of \(\alpha\)-particles.

Next—one further important conclusion. It is clear that an \(\alpha\)-particle directed at the nucleus cannot penetrate beyond this critical surface, and consequently cannot bring about the destruction of the nucleus, if its velocity does not exceed this critical potential. In experiments carried out several years ago, we found that the number of H-nuclei liberated from aluminium falls rapidly with decreasing velocity of the \(\alpha\)-particles, and at a velocity of the latter corresponding to a range of 4.9 cm of air becomes so small that the particles cannot be detected at all. This corresponds to the energy of an \(\alpha\)-particle traversing an accelerating field of approximately three million volts, which agrees well with the value given above.

Further experiments with other elements are still needed in order to test whether, in general, this relation holds between the minimum velocity of H-nuclei and the minimum velocity of \(\alpha\)-particles required for the disintegration of the nucleus of an element; the results already available give strong hope that this relation will prove to be generally valid.

It is interesting to note that the results obtained give a certain confirmation of the nuclear theory of the atom and inspire hope that we shall be able to determine the magnitude of the critical potential for some light elements.

Evolution of Nuclei.

In conclusion I wish to make a few remarks of a more speculative character—remarks concerning the question of the origin and evolution of the elements from two fundamental units, the positive and the negative electron. It must be admitted that the data by which one might be guided—apart from atomic mass and nuclear charge—are few. It is exceedingly difficult even to imagine how more complex nuclei can arise by the successive addition of protons and electrons, for the proton must possess an enormous velocity in order to approach close to the charged nucleus. I have already considered in this article facts indicating that, in the immediate vicinity of the nucleus, enormous attractive forces act, changing very rapidly

depending on the distance. These forces should probably be attributed to the protons forming the nucleus. In such a case, it seems possible that the proton and the electron will form very close pairs, neutrons, as I have called them. The probable distance between the centers of such doublets is of the order of \(3 \cdot 10^{-13}\) cm. The forces between two neutrons must be very small, except in those cases when the neutrons approach one another to a distance of the indicated order of magnitude; it is possible that neutrons hold one another together in the same way as a group of small movable magnets forms a connected whole, being held by the forces of interaction.

In considering the question of the evolution of the elements we may, for simplicity, assume that there initially existed a dispersed mass of hydrogen, which gradually became heated as a result of compression under the action of gravitational forces. At high temperatures the gas consisted chiefly of free hydrogen nuclei and electrons; with the passage of time some of them could combine, forming neutrons, and this process was accompanied by the liberation of heat. These neutrons then gathered into nuclei of varying degrees of complexity. Further, groups of neutrons showed a tendency to form more stable combinations, such as helium nuclei with mass four, and—possibly—intermediate nuclei with masses two and three. Energy was liberated in these processes, in all probability, in the form of rapidly moving excess electrons, which proved unnecessary for the stability of the nucleus. Probably all these nuclei were radioactive, but some of them, in their transformations, could attain stable configurations representing the nuclei of some of the surviving elements.

If we suppose that helium nuclei are the basic components, liberating the greatest amount of energy in their formation, then we must, in the end, expect that some of the neutrons in a heavy nucleus will combine into helium nuclei. These helium nuclei, further, assemble and form definite systems, and it is quite possible that this grouping has an ordered character, so that the structures formed are in some respects analogous to crystals, only the distances between their individual components are much smaller than in crystals. In such a case some of the elements may consist of a “core,” built of helium nuclei in the manner of a crystal and surrounded by positively and negatively charged satellites which revolve around this “core.” Let us admit that such ordered arrangements of helium nuclei are possible; it is interesting to note that, in such a case, proceeding from a simple assumption, one can obtain the actually observed relation between atomic charges and atomic masses. Suppose that helium nuclei form a spatially centered cubic lattice, being arranged along

vertices of an elementary cube, at the center of which an electron is placed. The following table gives some possible types of groupings and the corresponding atomic masses and nuclear charges. The structure “4 . 3 . 2” denotes a rectangular arrangement in which the edges of the parallelepiped contain respectively 4, 3, and 2 helium nuclei. Thus the whole nucleus will contain twenty-four helium nuclei, and its mass will be 96; at the same time it will contain six intranuclear electrons, as a result of which the charge of its nucleus will be \(48 - 6 = 42\).

Although the agreement is far from perfect, we nevertheless undoubtedly observe a concordance between the calculated and the observed quantities. If we assume that some of these structures may be enlarged by the addition of satellites, we shall retain a certain freedom in the sense of finding masses corresponding to real elements. Such a theory is, of course, of a purely speculative character, and one can imagine, alongside helium, other basic structural components that take part in the construction of heavier nuclei. The violation of the whole-number rule for isotope masses, observed in some cases by Aston—for example, between zinc and xenon—supports this idea. From the study of the artificial decomposition of elements we have become convinced that carbon and oxygen possess a very stable structure and, probably, are built of helium nuclei. It is possible that oxygen nuclei, for example, in their turn serve as structural components of some elements following oxygen, but our information is at present still too scanty to settle this question with complete certainty.

I think, however, that from this lecture it is clear how difficult and fascinating the problem of the structure of the nucleus is. In order to be able to hope for greater successes in this field, it is essential to learn much better the forces acting in the immediate vicinity of protons and electrons; the path toward this is the detailed study of the scattering of \(\alpha\)- and \(\beta\)-rays by nuclei. Fortunately, there are several definite paths leading to the solution of the problem of the structure of the nucleus. By combining the results obtained along these paths, we may hope for steady, though rather slow, progress—on the road to the solution of one of the greatest problems of physics.

1. Address delivered at the ceremonial celebration of the centenary of the Franklin Institute in Philadelphia, printed in Journ. Frankl. Inst. 198, No. 6, p. 725. —Ed. ↩

2. The results of work on the artificial decomposition of elements may be found in the articles by E. Rutherford collected in the book: E. Rutherford, The Structure of the Atom and the Artificial Decomposition of Elements. Gosizdat. Moscow. 1923. —Ed. ↩