On the analysis of α-ray photographs

Abstract

The beautiful method devised by Mr. C. T. R. Wilson of making visible the tracks of ionising particles, by the condensation of water drops on the ions left in their trail, has been used in this investigation to study the passage of α -particles through gases. As is well known the tracks of these particles appear in these cloud photographs to be very nearly straight except for occasional sudden bends due evidently to collision with the nuclei of the atoms of gas. These bends occur very much more frequently towards the end of the range where the velocity of the particles is low. The theory of the occurrence of such close collisions between the α -particles and the nuclei, based upon the assumption of a force varying inversely as the square of their distance apart, has been given by Rutherford and Darwin and verified by Geiger and Marsden, Chadwick and others. These investigations confirmed in general the hypothesis but indicated a departure from the law of force when the two particles approach very close. Now it is also certain that at sufficiently great distances from the nucleus, the effective force on the particle must be different from that given by the inverse square law owing to the partial or total screening of the nucleus by the electrons. An investigation of the field at such greater distances can be done most sensitively by using particles of such a low velocity that they will not approach closer to the nucleus than the distance considered. As no particles have been detected with certainty with velocities less than about 0.4 v 0 , where v 0 is the initial velocity of the α -particles from radium c , by either the photographic or scintillation methods, it is necessary to make use of the Wilson expansion system.