The scattering of cosmic ray particles in metal plates

Using an ordinary cloud chamber, Anderson (1933) measured the average angle of scattering of cosmic-ray particles of energy up to 3 x 108 e-volts in a lead plate of thickness 1·1 cm. Williams (1936) pointed out that, in such experiments, the scattering is multiple, and that the observed values agreed with the theoretical predictions, assuming the particles to be electrons. Recently Neddermeyer and Anderson (1937) have made some new measure­ments using a counter-controlled cloud chamber and a 1 cm. platinum plate. No numerical results are given, but the observed scattering of particles of energy up to 5 x 108 e-volts, which appears to be the limit of the energy measurements, seems in rough agreement with the earlier results. Using the counter-controlled cloud chamber already described by Blackett (1936) and by Blackett and Brode (1936), the multiple scattering of cosmic-ray particles of energy up 9 x 109 e-volts in lead and copper plates has been measured. In order to make such measurements possible, it is necessary to reduce as far as possible the distortion of the tracks in the chamber. The technique by which this can be achieved has been described by Blackett and Wilson (1937) in connexion with the measure­ment of the energy loss of rays in traversing metal plates. In fact, the photographs taken for the measurement of the energy loss were, amongst others, found suitable for the measurement of the scattering. The angle of scattering was measured by means of a goniometer eyepiece attached to a travelling microscope. The cross-wires were set tangentially to the track, first on one side and then on the other side of the plate. The mean of the two measured angles of deflexion, obtained from the two stereoscopic photographs, was taken as giving the projection on the plane of the chamber of the actual angle of scattering.