Part 2

Since each _γ_ ray may be converted in any one of the known electronic levels in the outer atom, a single _γ_ ray is responsible for the appearance of a number of groups of _β_ rays, corresponding to conversion in the K, L, M, etc., levels. In this way, an analysis of the _β_ ray spectrum allows us to fix the frequency of the more intense _γ_ rays which are emitted from the nucleus. The energy of the shortest wave measured in this way by Ellis corresponds to more than two million volts, while other evidence shows that probably still shorter waves are emitted in small quantity from radium C.

Ellis and Skinner have shown that the energies of these rays show certain combination differences, such as are so characteristic of the energies of the X-rays arising from the outer electrons. A series of energy levels may thus be postulated in the nucleus similar in character to the electron levels of the outer atom, and the _γ_ rays have their origin in the fall either of an electron or of an _α_ particle between these levels. This is a significant and important result, indicating that the quantum dynamics can be applied to the nucleus as well as to the outer electronic structure.

The probability of levels in the nuclear structure is most clearly seen on the satellite hypothesis, but in our ignorance of the laws of force near the core we are at the moment unable to apply the quantum dynamics directly to the problem. The outlook for further advances in this direction is hopeful, but is intimately connected with a further development of our knowledge of the laws of force that come into play close to the nucleus in the region occupied by the satellites.

ARTIFICIAL DISINTEGRATION OF ELEMENTS

We have seen that it is believed that the nuclei of all atoms are composed of protons and electrons and that the number of each of these units in any nucleus can be deduced from its mass and nuclear charge. It is, however, at first sight rather surprising that no evidence of the individual existence of protons in a nucleus is obtained from a study of the transformations of the radioactive elements, where the processes occurring must be supposed to be of a very fundamental character. As far as our observations have gone, electrons and helium nuclei, but no protons, are ejected during the long series of transformations of uranium, thorium and actinium. One of the most obvious methods for determining the structure of a nucleus is to find a method of disintegrating it into its component parts. This is done spontaneously for us by nature to a limited extent in the case of the heavy radioactive elements, but evidence of this character is not available in the case of the ordinary elements.

As the swift _α_ particle from the radioactive bodies is, by far, the most energetic projectile known to us, it seemed from the first possible that occasionally the nucleus of a light atom might be disintegrated as the result of a close collision with an _α_ particle. On account of the minute size of the nucleus, it is to be anticipated that the chance of a direct hit would be very small and that consequently the disintegration effects, if any, would be observed only on a very minute scale. During the last few years Dr. Chadwick and I have obtained definite evidence that hydrogen nuclei or protons can be removed by bombardment of _α_ particles from the elements boron, nitrogen, fluorine, sodium, aluminum and phosphorus. In these experiments the presence of H nuclei is detected by the scintillation method, and their maximum velocity of ejection can be estimated from the thickness of matter which can be penetrated by these particles. The number of H nuclei ejected even in the most favorable case is relatively very small compared with the number of bombarding _α_ particles, viz., about one in a million.

In these experiments the material subject to bombardment was placed immediately in front of the source of _α_ particles and observations on the ejected particles were made on a zinc sulphide screen placed in a direct line a few centimetres away. Using radium C as a source of _α_ rays, the ranges of penetration, expressed in terms of centimetres of air, were all in these cases greater than the range of free nuclei (30 cms. in air) set in motion in hydrogen by the _α_ particles. By inserting absorbing screens of 30 cms. air equivalent in front of the zinc sulphide screen the results were quite independent of the presence of either free or combined hydrogen as an impurity in the bombarded materials. Some of the lighter elements were examined for absorptions less than this, but, in general, the number of H particles due to hydrogen contamination of the source and the materials was so large that no confidence could be placed in the results.

In such experiments many scintillations can be observed, but it is very difficult to decide whether these can be ascribed in part to an actual disintegration of the material under examination. The presence of long-range particles of the _α_ ray type from the source of radium C still further complicates the question, since in general the number of such particles is large compared with the disintegration effect we usually observe.

To overcome these difficulties, inherent in the direct method of observation, Dr. Chadwick and I have devised a simple method by which we can observe with certainty the disintegration of an element when the ejected particles have a range of only 7 cms. in air. This method is based on the assumption, verified in our previous experiments, that the disintegration particles are emitted in all directions relative to the incident rays. A powerful beam of _α_ rays falls on the material to be examined and the liberated particles are observed at an average angle of 90° to the direction of the incident _α_ particles. By means of screens it is arranged that no _α_ particles can fall directly on the zinc sulphide screen.

This method has many advantages. We can now detect particles of range more than 7 cms. with the same certainty as particles of range above 30 cms. in our previous experiments, for the presence of hydrogen in the bombarded material has no effect. This can be shown at once by bombarding a screen of paraffin wax, when no particles are observed on the zinc sulphide screen. On account of the very great reduction in number of H nuclei or _α_ particles by scattering through 90°, the results are quite independent of H nuclei from the source or of the long-range _α_ particles. The latter are just detectable under our experimental conditions when a heavy element like gold is used as scattering material, but are inappreciable for the lighter elements.

A slight modification of the arrangement enables us to examine gases as well as solids.

Working in this way we have found that in addition to the elements boron, nitrogen, fluorine, sodium, aluminum, and phosphorus, which give H particles of maximum range in the forward direction between 40 and 90 cms., the following give particles of range above 7 cms.: neon, magnesium, silicon, sulphur, chlorine, argon, and potassium. The numbers of the particles emitted from these elements are small compared with the number from aluminum under the same conditions, varying between ⅓ and ¹⁄₂₀. The ranges of the particles have not been determined with accuracy. Neon appears to give the shortest range, about 16 cms., under our conditions, the ranges of the others lying between 18 cms. and 30 cms. By the kindness of Dr. Rosenhain we were able to make experiments with a sheet of metallic beryllium. This gave a small effect, about ¹⁄₃₀ of that of aluminum, but we are not yet certain that it may not be due to the presence of a small quantity of fluorine as an impurity. The other light elements, hydrogen, helium, lithium, carbon, and oxygen, give no detectable effect beyond 7 cms. It is of interest to note that while carbon and oxygen give no effect, sulphur, also probably a "pure" element of mass 4n, gives an effect of nearly one-third that of aluminum. This shows clearly that the sulphur nucleus is not built up solely of helium nuclei, a conclusion also suggested by its atomic weight of 32.07.

We have made a preliminary examination of the elements from calcium to iron, but with no definite results, owing to the difficulty of obtaining these elements free from any of the "active" elements, in particular, nitrogen. For example, while a piece of electrolytic iron gave no particles beyond 7 cms., a piece of Swedish iron gave a large effect, which was undoubtedly due to the presence of nitrogen, for after prolonged heating _in vacuo_ the greater part disappeared. Similar results were experienced with the other elements in this region.

We have observed no effects from the following elements: nickel, copper, zinc, selenium, krypton, molybdenum, palladium, silver, tin, xenon, gold and uranium. The krypton and xenon were kindly lent by Dr. Aston.

EXAMINATION OF LIGHT ELEMENTS FOR PARTICLES OF RANGE LESS THAN 3 CMS. OF AIR

When _α_ particles are scattered from light elements, the simple theory shows that the velocity of the scattered particles depends on the angle of scattering. For example, using bombarding _α_ particles of range 7 cms., the range of the _α_ particles scattered through more than 90° cannot be greater than 1.0 cm. for lithium (7), 2.0 cms. for beryllium (10), 2.5 cms. for carbon, 3.2 cms. for oxygen, 4.3 cms. for aluminum, and 6.8 cms. for gold.

Provided we introduce sufficient thickness of absorber to stop the _α_ particles scattered through 90°, we can examine for disintegrated particles from carbon, for example, whose range exceeds 2.5 cms. Certain difficulties arise in this type of experiment which are absent when the thickness of absorber is greater than 7 cms.; any heavy element present as an impurity will give scattered _α_ particles of range greater than those from carbon and thus complicate the observations. In addition, serious troubles may arise due to the volatilization or escape of active matter from the source. This is especially marked if the vessel containing the radioactive source is exhausted. To overcome this difficulty, we have found it desirable to cover the source with a thin layer of celluloid of 2 or 3 mm. stopping power for _α_ rays. By this procedure we have been able to avoid serious contamination and to examine the lighter elements by this method. We have been unable to detect any appreciable number of particles from lithium or carbon for ranges greater than 3 cms. If carbon shows any effect at all, it is certainly less than one tenth of the number from aluminum under the same conditions. This is in entire disagreement with the work of Kirsch and Patterson (Nature, April 26, 1924), who found evidence of a large number of particles from carbon of range 6 cms. A slight effect was observed in beryllium in accordance with our other experiments. No effect was noted in oxygen gas. Apart from beryllium, no certain effect has been noted for elements lighter than boron.

Under the conditions of our experiment, it seems clear that neither H nuclei nor other particles of range greater than 3 cms. can be liberated in appreciable numbers from these elements in a direction at right angles to the bombarding _α_ rays. This is, in a sense, a disappointing result, for, unless these elements are very firmly bound structures, it was to be anticipated that an _α_ particle bombardment would resolve them into their constituent particles.

We hope to examine this whole question still more thoroughly, as it is a matter of great importance to the theory of nuclear constitution to be certain whether or not the light elements can be disintegrated by swift _α_ particles.

In considering the results of our new and old observations, some points of striking interest emerge. In the first place, all the elements from fluorine to potassium inclusive suffer disintegration under _α_ ray bombardment. As far as our observations have gone, there seems little doubt that the particles ejected from all these elements are H nuclei. The odd elements, B, N, F, Na, Al, P, all give long-range particles varying in range from 40 cms. to 90 cms. in the forward direction, the even elements, C, O, Ne, Mg, Si, S, either give few particles or none at all as in the case of C and O, or give particles of much less range than the adjacent odd numbered elements. The differences between the ranges of even-odd elements become much less marked for elements heavier than phosphorus.

This obvious difference in velocity of expulsion of the H nuclei from even and odd elements is a matter of great interest. Such a distinction can be paralleled by other observations of an entirely different character. Harkins has shown that elements of even atomic number are much more abundant in the earth's crust than elements of odd atomic number. In his study of Isotopes, Aston has shown that in general odd numbered elements have only two isotopes differing in mass by two units, while even numbered elements in some cases contain a large number of isotopes. This remarkable distinction between even and odd elements cannot but excite a lively curiosity, but we can at present only speculate on its underlying cause.

VELOCITY OF ESCAPE OF HYDROGEN NUCLEI

We have seen that the experiments of Bieler on the scattering of _α_ rays by aluminum and magnesium indicate that a powerful attractive force comes into play very close to the nuclei of these atoms. If this be the case, the forces of attraction and repulsion must balance at a certain distance from the nucleus. Outside this critical point the forces on a positively charged body are entirely repulsive. Certain important consequences follow from this general view of nuclear forces. Suppose, for example, that, due to a collision with a swift _α_ particle, a hydrogen nucleus is liberated from the nuclear structure. After passing across the critical surface, it will acquire energy in passing through the repulsive field. It is clear, on this view, that the energy of a charged particle after escape from the atom cannot be less than the energy acquired in the repulsive field; consequently we should expect to find evidence that there is a minimum velocity of escape of a disintegration particle. We have obtained definite evidence of such an effect both in aluminum and sulphur by examining the absorption of H nuclei from these elements. The number of scintillations for a thin film was found to be nearly constant for absorption between 7 and 12 cms., but falls off rapidly for greater thicknesses. This is exactly what is to be expected on the views outlined. No doubt the limiting velocity varies somewhat for the different elements, but a large amount of experiment will be required to fix this limit with accuracy. From these results it is possible to form a rough estimate of the potential of the field at the critical surface, and this comes out to be about 3 million volts for aluminum. The value for sulphur is somewhat greater. This brings out in a striking way the extraordinary smallness of the nuclei of these elements, for it can be calculated that the critical surface cannot be distant more than 6 x 10^-13 cm. from the centre of the nucleus. These deductions of the critical distance are in excellent accord with those made by Bieler from observations of the scattering of _α_ particles.

Another important consequence follows. It is clear that an _α_ particle fired at the nucleus will not be able to cross this critical surface and thus be in a position to produce disintegration, unless its velocity exceeds that corresponding to the critical potential. In an experiment made a few years ago, we found that the number of H nuclei liberated from aluminum fell off rapidly with diminution of the velocity of the _α_ particle and was too small in number to detect when the range of the _α_ particle was less than 4.9 cms. This corresponds to the energy of an _α_ particle falling between about 3 million volts--a value in good accord with that calculated from the escape of H nuclei.

Further experiments are required with other elements to test if this relation between the minimum velocity of H nuclei and the minimum velocity of the _α_ particle to produce disintegration holds generally; but the results as far as they go are certainly very suggestive.

It is of interest to note that these results afford a definite proof of the nuclear conception of the atom and give us some hope that we may determine the magnitude of the critical potential for a number of the light elements.

EVOLUTION OF NUCLEI

In concluding, I would like to make a few remarks of a more speculative character dealing with the fundamental problem of the origin and evolution of the elements from the two fundamental building units, the positive and negative electrons. It must be confessed that there is little information to guide us with the exception of our knowledge of the nuclear charges and masses of the various species of elements which survive to-day. It has always been a matter of great difficulty to imagine how the more complex nuclei can be built up by the successive additions of protons and electrons, since the proton must be endowed with a very high speed to approach closely to the charged nucleus. I have already discussed in this paper the evidence that powerful attractive forces varying very rapidly with the distance are present close to the nuclear structure and it seems probable that these forces must ultimately be ascribed to the constituent proton. In such a case it may be possible for an electron and proton to form a very close combination, or neutron, as I have termed it. The probable distance between the centre of this doublet is of the order of 3 x 10^-13 cm. The forces between two neutrons would be very small except for distance of approach of this order of magnitude, and it is probable that the neutrons would collect together in much the same fashion as a number of small movable magnets would tend to form a coherent group held together by their mutual forces.

In considering the origin of the elements, we may for simplicity suppose a large diffused mass of hydrogen which is gradually heated by its gravitational condensation. At high temperatures the gas would consist mainly of free hydrogen nuclei and electrons, and some of these would in course of time combine to form neutrons, emitting energy in the process. These neutrons would collect together in nuclear masses of all kinds of complexity. Now the tendency of the groups of neutrons would be to form more stable nuclear combinations, such as helium nuclei of mass four, and possibly intermediate stages of masses two and three. Energy would be emitted in these processes probably in the form of swift surplus electrons which were not necessary for the stability of the system. In a sense, all these nuclear masses would be radioactive, but some of them in their transformation may reach a stable configuration which would represent the nucleus of one of our surviving elements. If we suppose that nuclear masses over a wide range of mass can be formed before serious transformation occurs, it is easy to see how every possible type of stable element will gradually emerge. If we take the helium nucleus as a combining unit which emits in its formation the greatest amount of energy, we should ultimately expect many of the neutrons in a heavy nucleus to form helium nuclei. These helium nuclei would tend to collect together and form definite systems and it seems not unlikely that they will group themselves into orderly structures, analogous in some respects to the regular arrangement of atoms to form crystals, but with much smaller distances between the structural units. In such a case, some of the elements may consist of a central crystal type of structure of helium nuclei surrounded by positive and negatively charged satellites in motion round this central core. Assuming that such orderly arrangements of helium nuclei are possible, it is of interest to note that the observed relations between atomic charge and atomic mass for the elements can be approximately obtained on a very simple assumption. Suppose that helium nuclei form a point centred cubic lattice with an electron at the centre of a crystal unit of eight helium nuclei. A few of the possible types of grouping are given in the following table, with corresponding masses and nuclear charges. The structure 4. 3. 2. means a rectangular arrangement with sides containing 4. 3. 2. nuclei respectively. It will thus contain 24 helium nuclei, have a mass 96, and will contain 6 intranuclear electrons. Its nuclear charge will therefore be 48 - 6 = 41.

Structural arrangement of Calculated Calculated Known element of helium nuclei nuclear charge Mass equal charge

3. 2. 2. 22 48 Ti 48

3. 3. 2. 32 72 Ge 74, 72, 70

3. 3. 3. 46 108 Pd 106.7

4. 2. 2. 29 64 Cu 63.35

4. 3. 2. 42 96 Mo 96

4. 3. 3. 60 144 Nd 144

4. 4. 3. 78 192 Pt 195

While the agreement is far from perfect for all these structures, there is a general accord with observation. If we take the view that some of these structures can grow by the addition of satellites, there is room for adjustment of masses and to include the intervening elements. This point of view is admittedly very speculative and there may well be other types of structure involved. At the same time, the general evidence suggests that there are some basal structures on which the heavier atoms are progressively built up. The failure of the whole number rule for the mass of isotopes, observed in some cases by Aston, _e.g._, between tin and xenon, certainly supports such a conception. From a study of the artificial disintegration of the elements we have seen that carbon and oxygen represent very stable structures probably composed of helium nuclei. It is possible that oxygen nuclei, for example, may be the structural basis of some of the elements following oxygen, but our information is at present too meagre to be at all certain on this point.

I think, however, it will be clear from this lecture what a difficult but fascinating problem is involved in the structure of nuclei. Before we can hope to make much advance, it is essential to know more of the nature of the forces operative close to protons and electrons, and we may hope to acquire much information by a detailed study of the scattering of swift _α_ rays and _β_ rays by nuclei. Fortunately, there is now a number of distinct lines of attack on this problem, and from a combination of the results obtained we may hope to make steady, if not rapid, progress in the solution of this, the greatest problem in Physics.