May 13, 1922] 



NA TURE 



615 



liberated from the elements are deflected by a strong 



magnetic field. By the adoption of special methods, 



it has been found possible to compare the amount of 



deflexion of these particles with that shown by the 



swift H-atoms produced when a-particles pass through 



ordinary hydrogen. It was found that the particles 



from nitrogen were deflected to about the same extent 



as the H-particles from hydrogen, and behaved in all 



respects like swift H-atoms carrying a positive charge. 



It seemed likely from the first that the corresponding 



particles from fluorine, phosphorus, and aluminium 



would also prove to be H-atoms liberated from the 



nuclei at speeds depending on the nature of the element 



and the velocity of the impinging particle. This has 



been confirmed in recent experiments by Dr. Chadwick 



and myself by a method similar to that employed for 



nitrogen. The bending of the particles in a magnetic 



field was determined for an absorption greater than 



32 cm. of air, in order that the experiments should not 



be complicated by the possible presence of hydrogen 



contamination in the material under examination. 



The experiments were not easy on account of the small 



number of particles present under the experimental 



conditions and it was found necessary to devise a 



special microscope with a large field of view to carry 



out the investigation. The experiments were all in 



accord witli the view that the particles from fluorine, 



phosphorus, and aluminium are swift atoms of hydrogen, 



and we may conclude that in each case an H-atom is 



liberated from the nuclei of these elements. 



The maximum speed of ejection of the H-atom from 

 the different elements can be estimated approximately 

 by assuming that the law connecting the velocity and 

 range of the a-particle holds also for the H-atom, 

 namely, that the velocity is proportional to the cube 

 root of the range. It has been calculated^ and also 

 confirmed by experiment, that the maximum speed 

 communicated to a free H-atom by a head-on collision 

 with an a-particle of velocity V is i-6F, while its range 

 in air is about 28 cm. Consequently, the maximum 

 velocity of the H-atom from nitrogen, which has a 

 range 40 cm., is i-87, and that from aluminium, with 

 a range of 90 cm., 2-377. The a-particle communicates 

 0-64 of its energy to a free H-atom in a direct collision, 

 and it can be calculated that all H-atoms which have 

 a range greater than about 56 cm. are projected with 

 energy greater than that of the bombarding a-particle. 

 In the case of aluminium, the maximum energy of the 

 H-atom is 1-4 times that of the incident a-particle. 

 This is a very interesting result, showing that in some 

 cases there is actually a gain of energy as a result of 

 the disintegration of the aluminium nucleus. We must 

 therefore conclude that at any rate for all collisions in 

 which the liberated H-atom has a range greater than 

 56 cm. of air, a part of the energy of the H-atom is 

 derived from the disintegrated nucleus. This is 

 analogous in some respects to the well-known gain of 

 energy in the escape of an a-particle from a radioactive 

 nucleus. 



It must be borne in mind that the amount of dis- 

 integration effected by the a-particles is on an excess- 

 ively minute scale. When a particle from radium-C 

 passes through aluminium, it probably passes through 

 the electronic structure of about 100,000 atoms, but 

 only about two a-particles in every million get near 



NO. 2741, VOL. 109] 



enough to the inner nucleus to effect the liberation of 

 an H-atom. We know that the collected a-particles 

 from I gram of radium give rise to 163 cubic mm. of 

 helium per year. If we suppose that all the a-particles 

 from I gram of radium were fired into aluminium, 

 the amount of hydrogen liberated by the disintegration 

 of the aluminium nuclei could not be more than i/iooo 

 of a cubic millimetre per year. The amount of 

 hydrogen liberated under possible experimental condi- 

 tions is thus almost beyond the means of detection 

 by ordinary chemical methods. It has only been 

 possible to study the disintegration by the use of such 

 a delicate method that each H-atom set free produces 

 a visible scintillation on a zinc sulphide screen. 



Mechanism of Disintegration, 

 From a study of radioactivity, it has been surmised 

 that the a-particle or helium nucleus of mass 4 is one 

 of the units of which the atoms are built up. The 

 experiments referred to in this lecture gave the first 

 definite proof that the hydrogen nucleus also is one 

 of the units of the structure of some of the lighter 

 elements. It is of interest to note that H-atoms are 

 only liberated from elements the atomic masses of 

 which are given by 4«4-2 or 4^ + 3, where « is a 

 whole number. Elements like carbon and oxygen, 

 the atomic masses of which are given by 4n, are not 

 affected. This is shown in the following table : — 



elements are built up of helium nuclei of mass 4 and 

 hydrogen nuclei. In order to account for the liberation 

 of an H-atom from these elements, it is natural to 

 suppose that the H-nuclei are satellites of the main 

 nucleus. If the satellite is not too close to the latter, 

 the a-particle in a close collision is able to give the 

 satellite sufficient energy to allow it to escape from 

 the system. It is to be anticipated that the H-satellites 

 are closer to the nucleus in the case of aluminium 

 than in the case of nitrogen, and that consequently 

 more energy is necessary in the case of aluminium to 

 effect their release. It is of interest to note that the 

 chance of liberating a swift H-atom from nitrogen is 

 not more than 1/20 of the chance of setting a free 

 H-atom in corresponding motion. This indicates 

 that it is probably only within certain prescribed 

 hmits of velocity of the satellite and position with 

 regard to the central nucleus that the liberation of 

 the satellite is possible. 



We have already referred to the fact that the H- 

 atoms from aluminium appear to be released in all 

 directions. Actually, however, the velocity in the 

 backward direction of the a-particles is distinctly less 

 than in the forward direction. Such a result at first 

 suggests that the a-particle acts the part of a detonator 

 to the aluminium nucleus and that the energy of the 

 escaping fragments is mainly derived from the nucleus. 

 I think, however, that the following explanation is more 

 probable and in better agreement with experiment. 



