It was found that, whereas, using hydrogen, the free protons are always thrown in the forward direction—a large proportion di rectly forward—with the other elements the expelled protons pro ceed in all directions, although the range of the forward ones always exceeds that of the particles proceeding in the backward direction. This indicates that in the mechanism of release of the proton from the nucleus the nuclear forces play a large part: the proton is not just knocked out of a passive nucleus by the passage of the a-particle, as an apple might be knocked from a bough by a tennis racket, but rather the close passage of the a-particle releases some trigger action, the particle being expelled at the expense of nuclear energy. Rutherford and Chadwick sug gested a mechanism in which the expelled proton is assumed to be revolving as a satellite before the expulsion initiated by the a-particle, and the difference of forward and backward velocity is expressed in terms of the path of the struck satellite within the nuclear system before expulsion. Against this Pettersson has put forward what he calls an explosion hypothesis, which supposes that the a-particle starts a disturbance by which the proton is shot out in a random direction with a definite velocity relative to the nucleus, which is itself thrown forward. The experimental evidence is insufficient to enable very much to be urged in favour of either hypothesis.
Particular interest attaches to the case of aluminium. The protons expelled in the forward direction from this element have a range of 90 cm. in air, as contrasted with a range of 7 cm. for the a-particles themselves, or 29 cm. for a free proton thrown straight forward by impact of an a-particle. The energy of the long-range proton from aluminium is considerably in excess of that of the a-particle which produces it, involving an actual gain of energy as the result of the collision, independent of the fate of the a-particle. There has in this case been a considerable re lease of nuclear energy—considerable in relation to amounts of energy involved in the motions of the particles. The actual excess of energy of expelled proton over energy of incident a-particle is, however, only of the order of 5X ergs, so that it would re quire some ten million million impacts to produce a gain of one calorie. As successful impacts are very rare, the gain of energy is not a matter of immediate practical use.
The fact that the protons are expelled at right angles to the path of the incident a-particles, and also in a retrograde direc tion, has proved of great service in the investigation of elements for expelled protons. One great difficulty of these experiments is to distinguish between protons due to hydrogen contaminations (either occluded gas or solid or liquid hydrogen compound) and protons originating in complex nuclei. Now whereas the long
range protons from hydrogen contamination are all thrown in a forward direction, those from complex nuclei, released by a trigger action of the passing a-particle, are, on the whole, discharged in much the same numbers in all directions. If, therefore, observa tions are made at right angles to the path of the incident a-par ticle, or in a direction making an acute angle (on the side near the source) with the path, the protons observed will be those that originate in the complex nuclei. This method was used by Rutherford and Chadwick, and also by Kirsch and Pettersson. As a result Rutherford and Chadwick gave the elements already named as those which show nuclear disintegration under the bombardment of a-particles. They never found evidence of nuclear disintegration with carbon or beryllium. Kirsch and Pettersson, however, have found protons of 3 cm. range with carbon, and also protons with beryllium, copper, and nickel, using the retrograde method for these elements, and later added to the list ten elements between titanium and iodine. In fact, these workers and their school seem to have seen far more scintilla tions than other workers, for even in the case of aluminium the number of protons which they give is five times as many as those observed by Rutherford and Chadwick under similar conditions. The discrepancies have not yet been explained. It may be taken as certain that with the elements in Rutherford and Chadwick's list nuclear disintegration can be produced by the bombardment with a-particles, but for other elements final decision must be reserved.
Decisive confirmation of the disruption of the nitrogen nucleus by an a-particle has been supplied by photographs of ray tracks taken with the Wilson Cloud Chamber (q.v.). By this method Blackett has taken a very large number of pictures of the passage of a-particles through nitrogen, each picture showing, from two different directions, a bundle of many ray tracks. Altogether 23,0o0 photographs were taken, showing some 270,000 tracks of a-particles of range 8 and 6 cm. and 145,0o0 tracks of range 5 cm.
from a source of Thorium As a result 8 tracks were tained showing the expulsion of a proton, the path of the proton being visible as a long thin beaded track, quite different in ap pearance from the tracks of an a-particle or struck nucleus. (See Plate I., fig. 6, WILSON CLOUD CHAMBER.) The proportion of a-particles producing disintegration agrees well with Rutherford and Chadwick's estimate from scintillations. Among the eight disruptions recorded one shows a proton ejected backwards, a further confirmation of the scintillation results.