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.