The scattering of alpha particles at small angles by helium

Abstract

It has been shown by Mott on the basis of the wave mechanics, that in the case of collisions between identical particles the scattered particles should interfere with the projected particles travelling in the same direction. When α -particles are scattered in helium, if the scattered α -particles and projected helium nuclei of similar velocity are identical in all respects, there will be interference between the two streams of particles. For collisions in which the particles act upon each other with forces varying as the inverse square of the distance between them, the interference results in the scattering intensity varying above and below the classical value and rising to double the classical numbers at 45º. At small angles the scattering predicted by the quantum mechanics does not differ greatly from that given by the classical theory. An experiment carried out by Chadwick showed quite definitely that for sufficiently slow α -particles the amount of scattering at 45º was double that of the classical theory. For these α-particles of low velocity the results showed that the forces varied very little from Coulomb forces; hence it was evident that the discrepancy was due to the failure of the classical theory. The scattering of slow α -particles by helium has also been investigated by Blackett and Champion by means of an expansion chamber. The observed scattering was in good agreement with the wave mechanical scattering. These experiments verify the assumption upon which Mott s theory is based, namely, that it is impossible to distinguish between an α-particle and a nucleus of helium travelling at the same velocity. Thus the helium nucleus has no spin or vector quantity associated with it; its field of force is perfectly spherical.